Ecology and Conservation Status of Morro Manzanita Arctostaphylos morroensis: A Threatened Plant Endemic to Los Osos, San Luis Obispo County, California

Manzanitas, genus Arctostaphylos (Ericaceae), are iconic shrubs in the California chaparral, even where they are not numerically dominant. Nearly all taxa in this genus are found within the California Floristic Province, and 60% are local endemics (Kauffmann et al. 2021). Most of these are components of maritime chaparral, a plant community that is itself of special interest and recognized by the California Coastal Commission as an “Environmentally Sensitive Habitat Area” (Kauffmann et al. 2021). Some restricted endemic manzanita species have experienced significant range reductions and persist in stands that are fragmented and facing various threats related to human development; seven of these are listed as threatened or endangered under the U.S. Endangered Species Act.

Morro manzanita Arctostaphyos morroensis is a long-lived, perennial shrub endemic to San Luis Obispo County, southern California, USA. The species was listed as threatened under the U.S. Endangered Species Act in 1994 (USFWS 1994). It is recognized also as a 1 B.1 rare plant (seriously threatened) by the California Native Plant Society.¹ Morro manzanita is an obligate seeder, meaning that it does not resprout from a burl following fire, but rather maintains its populations solely by regenerating from soil-stored seedbanks. Previous studies suggest that fire plays a crucial role in establishment and persistence of this species (Odion and Tyler 2002; Tyler and Odion 2020).

The historical range of Morro manzanita was estimated to comprise 800 to 1,100 ha (USFWS 1994). However, at the time of listing in 1994 the area of occupancy was estimated to remain at only 340 to 360 ha (Mullany 1990)², with estimates of number of individuals ranging from 86,000 to 153,000³. Some of the data cited to describe the range of the species, particularly at the edges of the distribution, have not been verified for decades, suggesting that new surveys are warranted and could be of value. In addition, an understanding of current land use and status could help understand potential for long-term persistence of the species. For example, at the time of listing, ∼65% of the habitat was on private lands with no legal protection. The remaining 35% was on lands owned by California State Parks [CSP] and California Department of Fish and Wildlife [CDFW], and it was comprised of stands with low densities, possibly representing only 20% or less of total individuals. Information on present range and land use could provide an updated overview of threats to this species. In 1994, much of the habitat was slated for development or alteration (USFWS 1994). The identified threats were residential and urban development, including destruction and fragmentation of habitat, lack of protection on private lands and lack of management on public lands, and deterioration of habitat due to recreational activity. Other concerns were expansion of previously cultivated non-native Eucalyptus into habitat (Mullany 1990) and invasive plants, as well as stochastic extinction by virtue of the small and isolated nature of the remaining populations (USFWS 1994).

Research has been conducted on the reproductive ecology (Tyler et al. 2023), germination cues and seedbank dynamics (Tyler and Odion 2020), and early responses to prescribed burning (Odion and Tyler 2002). A comprehensive summary and evaluation of these findings could help guide present management of this species. In particular, recommendations on the role of burning are needed and require information about the longer-term recovery from prescribed fire. The studies reported in Odion and Tyler (2002) followed seedbank dynamics and seedling recruitment after fire in a 40-yr-old stand of Morro manzanita. They found that three years after the prescribed burn, seedling density was still very low and the abundance of individuals in the stand was less than half that of the pre-burn population. Their findings suggested that the stand may have experienced an overall decline after this burn. However, for a long-lived species such as Morro manzanita, postfire recovery and community succession may not be evident for decades. For this study we present recent data collected in the original burn area to report on recovery 25 yr after the prescribed fire.

The goal of this paper is to summarize and supplement the current understanding of the biology and conservation status of Morro manzanita, including its distribution, abundance, ecology, threats, and management. We review the existing literature and report new findings from recent field surveys. We conclude with recommendations for management and future study towards supporting conservation of this species and its maritime chaparral community.

For our literature review and associated discussion, we reviewed all available sources including published scientific journal articles, federal and state agency reports, descriptions in published taxonomic keys, academic theses and dissertation, and information in the California Natural Diversity Database and California Native Plant Society’s rare plant inventory. In describing the distribution of Morro manzanita, we considered a location with the species as a separate occurrence if >0.4 km from the nearest occurrence (CDFW 2024). Latitude, longitude and elevation were determined using Google Earth aerial imagery and tools, or a global positioning system (GPS) device in the field. Stated areas of properties are from records of the County of San Luis Obispo. We discuss each occurrence in sequence according to its assigned number in the California Natural Diversity Database (CDFW 2021). Common and scientific names of plants follow those used in Jepson eflora⁴. Field surveys were conducted in January, February and November 2023. We visited the reported occurrences in attempt to confirm the presence of Morro manzanita. We also surveyed additional locations that we hypothesized could have the species based on edaphic conditions (e.g., 2.2 km east of occurrence 20 in an area with Baywood fine sand).

In order to improve our understanding of postfire recovery, we resampled the stand in which a prescribed burn had been conducted in November 1998 in Montaña de Oro State Park. In that area, transects and vegetation plots had been established and sampled prior to the burn (September 1998), and after (November 1998, April 2000, April 2000, June 2001). In addition to seedbank collections and assessment of fire severity, percent cover of Morro manzanita and other plant species was recorded within 2 m radius circular plots. In March 2023, we relocated the original vegetation transects and resampled twelve of the original thirteen plots that had contained >94% cover of Morro manzanita prior to the burn. We compare these findings with those reported from the period 1 to 3 yr postfire.

Wieslander and Schreiber (1939) first named and described Morro manzanita, referencing specimens collected in 1936 and 1938 in and near Hazard Canyon south of Morro Bay, an area now within Montaña de Oro State Park. The species name was based on the locality where it occurs. The three collections, including holotype, are filed in the Vegetation Type Map Herbarium at the University of California, Berkeley (Wieslander and Schreiber 1939). Additional specimens used by Wieslander and Schreiber (1939) to verify the range are in the herbaria at Stanford University and the California Academy of Sciences. Other taxonomic descriptions, briefly summarized here, include Hoover (1970), Wells (2000), Parker et al. (2012), and Kauffmann et al. (2021).

Morro manzanita is an erect spreading shrub, generally 1 to 4 m in height (Parker et al. 2012), with some older individuals reaching 8.5 m tall (Fig. 1). This manzanita species lacks a basal burl, which is both a distinguishing taxonomic characteristic, and indicative of its postfire recovery response (Jepson 1916; Weislander and Schreiber 1939; Keeley and Zedler 1978). Other distinctive morphological traits include its bark and leaf morphologies. The grayish-brown bark on mature stems is shreddy but persistent. Its leaves are oblong to oblong-elliptic (1.5 to 3 cm long), truncate to subcordate at the base (not auriculate-clasping), with short petioles (2 to 5 mm) (Fig. 2). Notably, the leaf surfaces are unlike: dark green, shiny and lacking stomata above, while gray-tomentose on the lower surface (Hoover 1970; Wells 2000; Parker et al. 2012). Wells (1968) determined that Morro manzanita has a base chromosome number of 13 and it is diploid (2n = 26).

Morro manzanita occurs near the coast at elevations <200 m (Parker et al. 2012), primarily on stabilized sand dunes of Baywood fine sand (Fig. 3) (Carpenter and Storie 1928; Soil Conservation Service 1984; Wiegers 2009), with few outlying locations on outcrop shale or volcanic igneous substrate. The climate is Mediterranean, with cool moist winters and warm dry summers. Fog is common. Temperatures range from ∼6.5° to 23.5°C, and mean annual rainfall (recorded at Morro Bay Fire Station) is 42.1 cm, with 75% occurring between November and April.

Fog has been found to play a key role in determining the composition of maritime chaparral community structure (Vasey et al. 2014) and the physiological performance of Arctostaphylos species in particular (Vasey et al. 2012). Long-term data sets in combination with modeling suggest that there has been a significant reduction in fog since the early 1900’s (Johnstone and Dawson 2010), which could have particularly negative consequences for obligate-seeding manzanitas in maritime chaparral (Vasey et al. 2012).

Weislander and Schreiber (1939) noted that Morro manzanita was associated with chamise Adenostoma fasciculatum, black sage Salvia mellifera, California goldenbush Ericameria ericoides, and deerweed Lotus scoparius (now Acmispon glaber). To quantify descriptions of associated plant communities, Tyler and Odion (1996) surveyed mapped polygons, or areas, where Morro manzanita was shown to occur (Mullany 1990). In a subset of these (24 locations total) and over a range of manzanita cover classes, they recorded the plant species present within 100 m² circular quadrats using the relevé technique and then used two-way indicator species analysis to detect species associations. They found the vegetation in areas where living Morro manzanita occurs consists of four recognizable types: maritime chaparral, coastal scrub, pure stands of Morro manzanita, and stands of Morro manzanita dominated by coast live oak Quercus agrifolia (Tyler and Odion 1996). Much of the natural landscape in Los Osos is dominated by maritime chaparral, a relatively rich assemblage of chaparral shrubs, subshrubs, and herbaceous plants, with considerable bare ground. In maritime chaparral associations, in addition to manzanita, the dominant and indicator species of this vegetation are California goldenbush, chamise, wedgeleaf ceanothus Ceanothus cuneatus, bush monkey flower Mimulus (now Diplacus) aurantiacus, and deerweed (Tyler and Odion 1996). Although these plants occur among and immediately adjacent to Morro manzanita, they usually do not occur underneath its canopy. Areas adjacent to the manzanita also have substantial areas of bare ground, generally 30 to 50%. With openings and bare sand between shrubs, this maritime chaparral is characterized by a considerable herbaceous flora that varies significantly from year to year depending on rainfall. Where this community has been mechanically disturbed, chaparral species abundance is much lower, and coastal scrub species, such as California sagebrush Artemisia californica, black sage, coyote bush Baccharis pilularis, dune bush lupine Lupinus chamissonis, and yellow bush lupine L. arboreus, dominate, including a host of annual and perennial native herbs, and non-native species.

Embedded within these associations are patches of pure Morro manzanita stands. On level sites, such as the Elfin Forest Preserve in north Los Osos, the patches of live manzanita are relatively small, consisting of one to a few usually large individuals, with diameters at the base (db) up to 1 m, heights over 7.5 m, and canopy diameters up to ∼10 m. The sloping landscapes to the south and southwest of Los Osos, around Hazard Canyon, support large patches of nearly continuous Morro manzanita covering many hectares. Both large and small patches of pure manzanita are characterized by having few associated species, such as California manroot Marah fabaceous and bush monkey flower (Tyler and Odion 1996). Also embedded within the maritime chaparral and coastal scrub associations were Quercus dominated patches, occasionally near or adjacent to pure Morro manzanita patches. These were characterized by high relative abundances of three understory species: California manroot, fuchsia-flowered gooseberry Ribes speciosum, and poison oak Toxicodendron diversilobum. This vegetation type differs from the dense Quercus forests found on steep north facing slopes in the area (e.g., on the north-facing slope just north of the Elfin Forest). The factors influencing species composition in the community types hosting Morro manzanita have not been investigated, but likely include soil characteristics, fog frequency, patch size, time since fire, and extent of soil disturbance or mechanical clearing.

Morro manzanita is long-lived, and while maximum lifespan has not been reported in the literature, Tyler and Odion (1996) estimated stand ages using historical aerial photographs from the collection in the Map and Imagery Library at the University of California, Santa Barbara. They examined images from 1949 to 1992 to identify areas that had been cleared and/or burned. In addition, cross-sections of co-occurring wedgeleaf ceanothus were collected from the areas where stand age was estimated; this species is an obligate seeder and thus would have germinated following fire, at the same time as the manzanitas present in the stand. The annual ring counts from cross-sections confirmed the minimum stand ages based on aerial photos. In 1996, the stand ages ranged from 37 to >47 yr old, with the large tree-like (arborescent) individuals in the Elfin Forest Preserve estimated to be significantly older than 47 yr; as described above, some of these individuals are exceptionally large. Since that 1996 report, there has been only one fire in all the sites surveyed, a prescribed burn conducted in 1998 within a stand containing Morro manzanita. Thus, at present the youngest stand is 25 yr old, and the oldest stand is a minimum of 74 yr old (though most likely much older).

One of the distinctive and distinguishing characteristics of species in the genus Arctostaphylos are the “nascent” or immature inflorescences, developed many months before flowering (Jepson 1938; Keeley 1997). In Morro manzanita, these immature panicles are pendent and campanulate (Weislander and Schreiber 1939; Parker et al. 2012). Small urn-shaped flowers, which are white and occasionally tinged with pink, appear in January through March (Fig. 2).

Similar to other obligate-seeding manzanita species (Keeley 1977; Fulton and Carpenter 1979; Mahall et al. 2010), Morro manzanita produces abundant flowers. Tyler et al. (2023) recorded an average of 50 to 135 flowers per stem across a two-year period. Flower production (number of flowers per stem) varies among sites and among years with much higher (two times the average) flower production across sites in a very wet year (1998) compared to a year with below-average rainfall (1999) (Tyler et al. 2023). Water availability has been found to be related to both flower and fruit production in other manzanitas, though the timing of rainfall that influencing production varies between species. For big berry manzanita A. glauca and Eastwood manzanita A. glandulosa (Keeley 1977), as well as A. visida (Baker et al. 1982) resources (water) in the previous year appear to determine flower production. However, Mahall et al. (2010) suggested that A. glauca flower production was influenced by rainfall in both the previous year, when buds are formed, and in the present year, when flower and fruit development occur. Observations of pointleaf manzanita A. pungens (Richardson and Bronstein 2012) and Morro manzanita (Tyler et al. 2023) indicate that flower production was most strongly related to present year resources (rainfall).

Reproduction of Morro manzanita is dependent on pollinators. Tyler et al. (2023) found that when inflorescences were bagged to exclude animal pollinators, fruits were not produced. Bees are the most common pollinators of Morro manzanita, and include yellow-faced bumblebees Bombus vosnesenskii, the common anthophorid bee Anthophora urbana, halictid bees, Colletes sp., and European honey bees Apis mellifera. Other pollinators observed visiting Morro manzanita flowers include syrphid flies, monarch butterflies Danus plexxipus, bee flies Bombylius sp. and Anna’s hummingbird Calypte anna (Tyler et al. 2023). This is consistent with research on congeners that demonstrated self-incompatibility and reliance on pollinators for successful reproduction – pink bracted manzanita A. pringlei var. drupacea and A. glauca (Brum 1975) and A. pungens (Richardson and Bronstein 2012). Bees have also been found to be important pollinators of other manzanita species (Gankin and Major 1964; Brum 1975; Fulton and Carpenter 1979).

Fruit set, or the percent of flowers producing a fruit, varies among sites and years, ranging from an annual average of 10 to 18% (Tyler et al. 2023). These data were reported for two adjacent years (1998, 1999), with fruit set being consistent across years for some sites, and varying significantly (5 times higher in one year) at another site. To our knowledge, the only other reported data on fruit set in manzanitas are for A. pungens (Richardson and Bronstein 2012), in which highest values for fruit set in control/natural conditions was 36% in 1998, and no fruit set observed in the following year. Thus, although we suspect fruit set is comparatively low in Morro manzanita, longer-term data and data on other congeners are lacking in order for this to be confirmed. Seed set, as determined by viable seed to ovule ratios, has been investigated in several species of manzanita by Kelly and Parker (1991). They reported that Morro manzanita has an average of 7.3 ovules per ovary/fruit (each flower contains one ovary in the Ericaceae), and that seed set is relatively high at 73% (Kelly and Parker 1991). This suggests that once pollinated, Morro manzanita successfully produces viable seeds, and that low fruit set may indicate pollinator limitation (Tyler et al. 2023).

The fruits mature in spring-summer (Fig. 2). They are reddish-brown and spherical to slightly flattened, or depressed globose. Morro manzanita fruits are drupes, covered by a thin exocarp, and containing a dry, mealy mesocarp surrounding multiple hard stones or “nutlets” (Meyer 2008; Parker et al. 2012). They contain an average of five (Kelly and Parker 1991) to eight (Tyler and Odion 1996) generally unfused nutlets (i.e., seeds) per fruit. Fruit drop occurs in late spring to late fall, though the timing can vary annually. Tyler et al. (2023) found that the majority of fruits fell from the plants during June and early July one year (1998), and August to early October in the following year (1999).

Dropped fruits of Morro manzanita are removed quickly by predators. Tyler et al. (2023) conducted studies of removal rates by vertebrate predators, by placing trays containing mature fruits under and adjacent to shrubs in several different manzanita stands, and under multiple individual shrubs. This was carried out in two years (1998, 1999) and was initiated when natural fruit drop was first recorded. They found that in both years, predators, most likely small mammals and birds, removed a majority of fruits and did it relatively quickly. From 60 to 70% of fruits were removed within 1.5 mo in both years. High predation rates have been reported for other species of manzanita (Keeley 1977; Kelly and Parker 1990).

Fruit removal alone does not mean all seeds are eliminated from the site, as some animals may scatter-hoard or cache seeds that could be incorporated into the soil seed bank (Parker 2015; Crowe and Parker 2023). Although no field studies have been conducted to identify the species that collect, bury, or consume fruits and seeds of Morro manzanita, Tyler et al. (2023) report observations of woodrat Neotoma fuscipes, and brush rabbit Sylvilagus bachmani feces in or adjacent to fruit trays. Morro Bay kangaroo rat, Dipodomys heermanni morroensis, potentially a former scatter-hoarder of Morro manzanita, has not been observed in the wild since 1986, and may be extinct (Villablanca et al. 2021). Other rodents, such as deer mouse Peromyscus maniculatus, brush mouse P. boylii, California pocket mouse Chaetodipus californicus, as well as birds that co-occur at the sites may collect, bury, or consume Morro manzanita fruits. Trail cameras set near fruit trays would be a useful method to identify associated animal species that interact with Morro manzanita, and help to assess the potential role they play in consumption vs. caching of seeds. Previous studies have documented the important role of scatter-hoarders in developing manzanita seedbanks (Parker 2015; Crowe and Parker 2023). However, in Morro manzanita it is unknown if such a mutualistic relationship exists, and some evidence points to consumers as seed predators rather than planters. For example, sites where seed bank density is exceptionally low (Elfin Forest) have the highest rates of fruit removal (Tyler and Odion 2020; Tyler et al. 2023). In addition, relatively few Morro manzanita seeds are found in the soil away from the shrub canopies, and overall viable seed densities can be very low (Tyler and Odion 2020), suggesting that removal of a large fraction of fruits, even if some were buried and forgotten, could have a negative impact overall.

Based on seed drop, seed predation rates, and estimates of the number of seeds per fruit for Morro manzanita, Tyler et al. (2023), estimated annual seed input to the soil seed bank over two years (1998 and 1999). The relative addition of seeds across years was similar at all sites (i.e., about 1.5 times greater in 1998 compared to 1999). However, seed input varied considerably among sites, and rates appear to decline with stand age. Annual seed input was at least 4 times lower at the oldest-aged stand at the Elfin Forest (316 seeds per m² in 1998) compared to the youngest stand in Montaña de Oro State Park (1,608 seeds per m² in 1998). The intermediate-aged stand had an intermediate value with an estimated seed input of 912 seeds per m² in this same year.

Tyler and Odion (2020) examined soil seed banks in different-aged stands, predicting that seed densities would be positively correlated with stand age. Soil cores (10 cm depth) were collected under multiple shrubs across three sites. Morro manzanita seed density in the soil varied greatly among sites, from 1,326 to 62,251 seeds per m². However, contrary to expectations, the oldest had especially low seed densities (Tyler and Odion 2020). Since seed input, as described above, was particularly low in the oldest stand, seed densities in the soil seedbank may decline even further. There has been one other study (Parker and Ingalls 2022) that reported seed bank densities of Morro manzanita. In their comparative study of ten Arctostaphylos species to investigate the relationship between seed size and seed bank density, Parker and Ingalls (2022) collected Morro manzanita seed and estimated the seed bank density (for 5 cm deep cores) to be an average of 1,900 per m². Although the location of their collections is not reported, if this value is doubled to make an equivalent comparison to results reported by Tyler and Odion (2020), their finding of a seed density of ∼3,800 per m² is within the same range.

Interestingly, the striking variation in Morro manzanita soil seedbanks across different aged sites is in contrast to a previous study of change in manzanita seedbanks over time. Over a ten-year period, Keeley (1987a) found that despite the huge input of seed, estimated to be over 22 × 10⁶ seeds per ha for the obligate-seeder A. glauca, no significant change in seed density was detected in the soil seedbank. Such an equilibrium in seed densities may be a result of input balanced against loss due to mortality factors including seed predation (Keeley 1987a). In contrast, the variation noted in Tyler and Odion (2020) in stands of Morro manzanita suggests a non-equilibrium. This difference may be, in part, related to stand age. The A. glauca individuals sampled in Keeley (1987a) were in a stand that was mature at 90 to 100 yr old. The Morro manzanita stands sampled in Tyler and Odion (2020) were ∼40, >47, and >75 yr old, with accumulation of seed still occurring in the youngest stand, and net loss occurring in the oldest (potentially senescent) stand. Further study is warranted to investigate change in soil seedbanks over time. In addition, understanding the dynamics of fruits and seed accumulation in the litter layer (Fig. 2) would be useful as this may be a potential seed source for restoration efforts.

Percent viability of Morro manzanita seeds, i.e., the proportion of viable to total seeds, within the soil seed bank is very low across all stands, with an average of 4% (Tyler and Odion 2020). The oldest stand (the Elfin Forest) has exceptionally low seed viability, averaging 2%. Viability of fresh seeds has not been recorded in the literature, so rates of change with seed age are unknown.

Morro manzanita is an obligate-seeder, meaning that it does not resprout when the crown and stem are burned, and thus must re-establish from seeds in the soil seed bank. Seeds of obligate-seeders are mainly or even completely refractory (Sweeney 1956; Keeley 1987b; Keeley 1991); that is, germination is inhibited until primary dormancy is released by a specific mechanism. Fire-related cues such as heat and by-products of combustion have been identified as principal mechanisms that break seed dormancy of fire recruiters that rely on soil-stored seed (Keeley 1991). In other manzanita species, germination rates are low, but enhanced with smoke, charate, heat shock, or some combination of these treatments (Odion 2000; Keeley et al. 2005; Jurado et al. 2011). Tyler and Odion (2020) examined germination of Morro manzanita seeds in response to various cues, and confirmed that germination, while very low on average (1 to 4%), was greatest in treatments that combined heat and charred wood. However, neither heat nor charred wood alone enhanced germination (Tyler and Odion 2020). One factor responsible for low germination is that seed viability is very low – from 3 to 6%. Germination as a percentage of estimated viable seeds was found to be relatively high (23 to 100%). Unexpectedly, ∼40% of viable seeds germinated with no fire treatments. Viability and germinability of fresh Morro manzanita seed has not been investigated. Although fresh and litter-stored seeds are unlikely to contribute to post-fire seedling establishment, as they would be consumed in a burn, this seed source might be used in restoration purposes; thus, study of its viability rates and germination cues is warranted.

To assess the effects of burning on seedling establishment in Morro manzanita, a prescribed fire was conducted by California State Parks in Montaña de Oro State Park in 1998. At the time of the burn, the stand was 40 yr-old. The total area burned was ∼2.3 ha. Odion and Tyler (2002) recorded pre-burn seed densities and seed viabilities, seed mortality due to fire, and postfire germination and seedling survivorship for three years, to compare the population that established after the burn with the one present before.

Prior to the burn, there was abundant soil-stored seed, ∼11,000 seeds per m², though seed viability in this stand (and all others) was low, resulting in an average of 334 viable seeds per m² before the fire. As was expected, seed mortality through the burn was high, and following the experimental fire only about a third remained, leaving an average of 99 viable seeds per m². Germination was recorded in the first two wet seasons after the fire, though most seedlings did not survive their initial summer drought. After three years, the density of Morro manzanita seedling was less than half the estimated density of the adult shrubs present before the fire. The authors speculated that the most likely factors responsible for the low number of seedling recruits were low numbers of viable seed in the soil, and relatively high mortality of both germinants and young seedlings. They concluded that with further mortality of the remaining seedlings and no additional germination, the 40-yr-old stand may not have had an adequate seed bank to compensate for mortality and thus prevent population decline.

Post-fire succession in maritime chaparral unfolds over many years and even decades. Early stages include the first years dominated by herbaceous annual and perennial species, resprouting shrubs, and seedlings of both woody sub-shrub species and shrubs (Keeley et al. 1981; Davis et al. 1988, 1989; Tyler 1996; Odion 2000; Odion and Davis 2000; Van Dyke et al. 2001; Fox and Potts 2023). Thus, a comprehensive view of post-fire vegetation patterns in chaparral is ideally based on observations over multiple time intervals.

In order to improve our understanding of long-term post-fire recovery of Morro manzanita, we resampled the stand in which a prescribed burn had been conducted in November 1998 in Montaña de Oro State Park. In that area, transects had been established and sampled prior to the burn (September 1998), and after (November 1998, April 2000, April 2000, June 2001). In addition to seedbank collections and assessment of fire severity, percent cover of Morro manzanita and other plant species was recorded within 12.6 m² circular plots (radius 2 m). These plots were located from 0 to 10 m off the transect (distance along and from the transects was recorded) and centers marked with rebar stakes. In the three years post-fire, these plots were easily relocated. Twenty-five years later, in March 2023, we identified the locations of the original vegetation transects. Unfortunately, the precise location of the original 12.6 m² circular plots could not be verified, as the center stakes were not evident and the area along the transect was now densely vegetated. However, given the certainty of the transect locations and using the recorded distance along and from the transects for the plot center locations, we are confident that we were within a few meters of the original plot locations. We resampled twelve of the original plots that had contained >94% cover of Morro manzanita in 1998 prior to the burn. We compare these findings with those reported from the period 1 to 3 yr postfire.

In the plots that had been dominated by Morro manzanita prior to the burn, the standing vegetation was almost entirely consumed in the fire (Odion and Tyler 2002). Three years later, the percent cover of all species was still low (∼20% total) and the site was predominately bare ground (Table 1) Morro Manzanita seedlings were present but in low density, and cover of this species was still <1% on average. The plant species with the highest cover at this time were California sagebrush (7%), deerweed (5%) and coyote bush (4%) (Table 1). Based on these observations, and low manzanita seed densities in post-fire soil seedbank samples, Odion and Tyler (2002) reported concerns that the stand of Morro manzanita might not return to its pre-burn abundance or cover, and that the fire had caused a net loss to the population.

However, 25 yr post-burn we found that cover of Morro manzanita along the original transects was high, ranging from 30% to 100%, with a median of 93%, and mean of 83% cover Morro manzanita (Table 1). There was also a relatively low cover of associated species, including the obligate-seeding wedgeleaf ceanothus, which had been present at the site but was not found in these plots before the fire (Table 1). We observed that most manzanitas were flowering and/or producing fruit, indicating that individuals had reached sexual maturity and that replenishment of the soil seedbank is very likely taking place. These new findings suggest that this stand is recovering slowly but successfully from the fire, in terms of percent cover of Morro manzanita. Conclusions based on data collected in the early post-burn period had suggested otherwise because of the low density of seedlings established by the third year after fire (Odion and Tyler 2002).

For several reasons, we propose that this significant increase in cover of manzanita (from <1% to 83%) is most likely the result of growth and increase in the canopy spread of seedlings that established within the first two years following the burn. Many obligate-seeding species such as Morro manzanita have refractory seed, which is stimulated to germinate by fire. In laboratory studies, Tyler and Odion (2020) found that highest seed germination in Morro manzanita resulted from the treatment that mimicked conditions of burning – heat and application of charred wood. In addition, young individuals of Morro manzanita are uncommon in unburned areas, supporting the hypothesis that it is the direct effects of fire that stimulates a flush of germination soon after the burn. Finally, Odion and Tyler (2002) compared viable seed densities in the soil before and one week after the burn, and found that in the top 5 cm of the soil less than a quarter of the viable seed remained (54 viable seeds per m²). They hypothesized that most of the seed that survived the fire would have been stimulated to germinate, which would have further depleted the seedbank.

Alternatively, additional seed germination in the third winter following the fire could have supplemented the number of seedlings establishing in this site. If this occurred, shrub densities may have approached those in the adult stand prior to the burn, and with time and increased growth, the canopy cover would also be restored. As noted above, most of the remaining viable seed would have probably germinated within the first two years of the burn. However, a small portion may have still been present after the germination that occurred in year two. Although previous studies on manzanita seed suggest that once dormancy is broken (by heat, scarification, or combustion products) germination is imminent (Keeley 1987b; Parker and Kelly 1989; Keeley 1991), Odion and Tyler (2002) recorded seedlings that emerged in the second year following the burn. The latter indicates that there is some mechanism by which treated (i.e., burned) seed may maintain dormancy for at least one year. It is unknown if this latent condition might extend even one more year post-burn.

Our 2023 field surveys provide evidence that, contrary to the conclusions of Odion and Tyler (2002) that the former population of Morro manzanita had failed to re-establish after the fire, reestablishment of manzanita cover is occurring in this site. While this stand was relatively young (40 yr old) at the time of the prescribed fire, seedbank densities and subsequent seedling recruits may have indeed been adequate to restore cover of adults killed in the burn. These new findings highlight that while early post-fire sampling is appropriate and useful, to accurately determine the response to fire in long-lived obligate-seeding species, much longer time scales of observation, on the order of decades, are required. This recent survey also indicates that burning should not be avoided as a management tool in supporting restoration of Morro manzanita, though stand age should be an important factor in considering the appropriate use of fire for a particular area. We would recommend against burning stands that are much younger than 40 yr; another decade or so of seed input may have substantially increased the viable seed and thus potential seedling recruits in the prescribed burn stand. At this time, since nearly all stands containing Morro manzanita are at least 74 yr old, with the exception of the 2 ha area described above, prescribed fire may be one of the most effective tools in ensuring the regeneration of new individuals in aging stands. This approach may not be feasible in some sites where there is close proximity to residential development, such as the Elfin Forest Preserve, even though these oldest stands may be experiencing declining soil seedbanks over time (Tyler and Odion 2022). In such sites, exposing seedbanks to fire-related cues to stimulate germination, using small scale approaches such as burn boxes, or treating soil off-site could be suitable alternatives. Research to determine the efficacy and effectiveness of these restoration approaches are highly recommended.

Morro manzanita is restricted to a small portion of coastal area in and near Los Osos, San Luis Obispo County, California (Fig. 4 and 5). Its distribution is predominantly correlated with the Pleistocene eolian sand mapped as Baywood fine sand (Carpenter and Storie 1928; Soil Conservation Service 1984; Wiegers 2009), where there is no slope to moderate slope. Based on the distribution of Baywood fine sand in the Los Osos area (Fig. 3), the historic distribution of Morro manzanita is estimated to have covered 800 to 1,000 ha (USFWS 1994).

At the time of listing in 1994, it was estimated, based on aerial photos and surveys², that the geographic range of Morro manzanita was ∼340 to 360 ha (Tyler and Odion 1996): one half consisted of small or low-density patches in and around developed areas, and one half consisted of more continuous or more dense stands with at least 50% cover. This represented a reduction of the species’ geographic range to one third of its historical size due to removal of individuals or habitat elimination (Odion and Tyler 2002). By 2013 ∼75% of historical habitat had been converted for residential use, resulting in highly fragmented populations (USFWS 2013).

While the geographic range in 1994 was reported to be ∼350 ha, Tyler and Odion (1996) pointed out that this was an over-representation of the actual areal extent of the species because individuals are often present in low-density patches within a matrix of associated plant communities. In order to estimate the cover of Morro manzanita, Tyler and Odion (1996) recalculated to account for stands with a sparse cover of Morro manzanita having been equally weighted with stands with high cover. Using previously reported cover classes and estimated acreage of each (Mullany 1990; McGuire and Morey 1992), they estimated the area actually covered by Morro manzanita to be less than 162 ha (Tyler and Odion 1996).

Another source of information on the geographic range of the species is the California Natural Diversity Database (CDFW 2021), which includes spatial information associated with special status species and their recorded occurrences. For Morro manzanita there are six known occurrences, each with an assigned number by the California Natural Diversity Database (CDFW 2021). Their numbers are not in sequence. As knowledge of the species’ distribution improved, some previously recognized occurrences have been combined with other occurrences, maintaining the criteria that separate occurrences are >0.4 km from any other occurrence. The assigned numbers for Morro manzanita occurrences remaining are 1, 4, 9, 18, 20 and 21 (Fig. 4). USFWS (2022) estimated the area of occupancy for the six occurrences, described below, using the California Natural Diversity Database (CDFW 2021) primarily from maps dated 1980 and 1990–1992.

For occurrence 1, CNDDB (CDFW 2021) uses 28 ha as its area. Weislander and Schreiber (1939) give the location for paratype specimen (UC1334951/Ben Bolt 644/VTM14631) as “Valencia Peak”, collected 23 March 1936. They state the distribution of Morro manzanita as “sandy hills south of Morro Bay, 100–400 feet”, but Valencia Peak (Montaña de Oro State Park) is 1,345 ft/410 m elevation. Data in the pocket of the herbarium sheet give the following information: 1 mile east-northeast Valencia Peak; verbatim elevation as “400” ft; and habitat as woodland, north slope, and small type Monterey shale. California Natural Diversity Database (CDFW 2021) states “exact location unknown. mapped as best guess 1 air mile ENE of Valencia Peak”. USFWS biologist Emily Levin and California State Parks Environmental Scientist John Sayers visited Valencia Peak on 3 March and 30 March 2023, respectively. They searched the area for Morro manzanita, finding none. In addition, we followed California Natural Diversity Database and used 1.6 km east-northeast Valencia Peak. We assigned the coordinates 35.26853, −120.85569, 61 m north of Islay Creek in its valley at 109 m elevation, for this analysis. We hiked the Islay Creek Road to a point 391 m northeast of the coordinates at 77 m (253 ft) elevation, but we were prevented access from the road by dense chaparral and riparian vegetation, poison oak and rugged terrain. Bolt’s stated location with Morro manzanita is possibly along Islay Creek Road, but we were unable to find it. More likely, “Valencia Peak” or 1 mile east-northeast Valencia Peak are erroneous data. It is possible that occurrence 1 was within the boundaries of what is now occurrence 9. Alternatively, the location itself was erroneous. In her MS thesis, Mullany (1990) describes Morro manzanita south of its typical range on “a ridge of Chamise shaly loam located approximately 0.25 mile south of Valencia Peak” (our emphasis added.) She also states that this “may be the area referred to in the original description as Valencia Peak. Other individuals of A. morroensis may have been overlooked on this ridgetop because of the difficulty of examining steep slopes and impenetrable chaparral. However, it is evident this is not part of the main range.” Given Mullany’s description, we recommend that the presence of Morro manzanita south of Valencia Peak should be investigated to determine if this occurrence remains valid.

Occurrence 4 is in north Los Osos. It is comprised of the Elfin Forest Preserve (14 ha), the adjacent part (21 ha) of Morro Bay State Park, and private land (69 ha), for a total estimated area of 104 ha (CDFW 2021). It is mapped as 16 polygons, mostly according to data from 1980 and 1990 to 1992. The majority of private land are residential parcels (Fig. 5); here some Morro manzanita are incorporated into residential landscaping (CDFW 2021) but they are likely substantially reduced in number and their ecological function is unknown.

Occurrence 9 is west of Pecho Valley Road, and south of the west end of Los Osos Valley Road extending to ridges south of Hazard Canyon. This occurrence includes multiple preserve properties as well as private land. It is mapped as 15 polygons, mostly according to map data from 1980 and 1990–1992, with an estimate of >152,200 plants. Its total area is 402 ha (CDFW 2021) and comprises the largest occurrence. We recorded the location of the southmost individual, which is within Montaña de Oro State Park, at 35.28145, -120.88440.

Occurrence 18 is at coordinates 35.34346, -120.8214, in Morro Bay State Park. It is 344 m east of South Bay Blvd and 74 m north of the Live Oak Trail, almost half way up the hill, 58 m elevation on the south facing slope, and 27 m east of the dead Eucalyptus trees. We observed ∼12 individuals on 28 January 2023, with some apparent hybrids in the vicinity. We also observed here Oso manzanita A. osoensis. This is the northmost occurrence for Morro manzanita. The rocky, volcanic exposure of porphyritic dacite (igneous rock; Weigers 2009) is an unusual substrate for the species. The underlying substrate is mapped as Rock outcrop-Lithic Haploxerolls complex (Soil Conservation Service 1984). The only previous observation here was in 1989 (Mullany 1990) with one or a few individuals reported. CDFW (2021) estimates its area as 2 ha.

Occurrence 20 is at coordinates 35.31398, -120.81615, as mapped by Mullany (1990) at the eastern terminus of Freeman Lane in east Los Osos. Mullany (1990) reported a small group of plants (1 to a few individuals) in the 1980s growing with coast live oak Quercus agrifolia in dark, fine-textured soil. The dark, fine textured soil is a result of organic debris added to Baywood fine sand from the coast live oak. USFWS (2022) estimated the area of this occurrence as 2 ha. We searched for this occurrence of Morro manzanita on 28 January 2023, but without success. The occurrence is now likely extirpated. In the 1980s the terminus of Freeman Lane was 50 m northwest of the present-day terminus. House and facilities construction in 2005 probably removed the plants.

Occurrence 21 is at coordinates 35.33881, -120.81141, in Morro Bay State Park, 2.15 km east of South Bay Blvd., 68 m elevation. It is 19 m south of the Crespi Trail, on a south facing outcrop of shale (Nelson 2015). Soil Conservation Service (1984) mapped the underlying substrate as Rock outcrop-Lithic Haploxerolls complex. We visited the occurrence on 28 January 2023 and observed ∼ 20 individuals, some with flowers and growing with Oso manzanita. The only previous observation of this occurrence was by Nelson (2015), who reported many plants. The area of this occurrence is estimated as 2 ha (CDFW 2021).

One new occurrence was identified in our November 2023 field surveys. Mullany (1990) wrote that “although A. morroensis is abundant along upper slopes of the Manzanita Trail, the steep slopes and similar appearance of the codominant A. crustacea precludes estimating cover there.” We confirmed the presence of Morro manzanita along the Manzanita Trail and East Boundary Trail in Montaña de Oro State Park at three localities not recorded in CNDDB. These were: 35.29106, -120.85267; 35.28797, -120.84735; and 35.29085, -120.84406 (Fig. 4, 5). The first new locality with ∼10 individuals is 0.37 km from occurrence 9, and thus would be included in this occurrence. The latter two (eastmost) comprise a new occurrence. The second new locality is 0.59 km from occurrence 9 (thus a new occurrence), and the third with one individual is 0.66 km from occurrence 9. At the second locality, which was at the periphery of a rocky outcrop, we observed ∼12 individuals of Morro manzanita and several brittle leaf manzanita A. crustacea subsp. crustacea. The underlying soil type for all three new localities is mapped as Santa Lucia shaly clay loam (Soil Conservation Service 1984). Data for these localities were submitted through online field survey forms to CNDDB, and voucher specimens were deposited in the herbarium at UC Santa Barbara’s Cheadle Center for Biodiversity and Ecological Restoration.

Summarizing the findings based on data for all occurrences of Morro manzanita from the California Natural Diversity Database (CDFW 2021) and our field surveys, we suspect that the location of occurrence 1 was based on erroneous data, and that occurrence 20 is no longer extant. Occurrences 9 and 4 represent at least 98% of this species’ area of occupancy. The remaining three occurrences 18, 21 and our new occurrence are small but significant outlying stands at the edges of the species’ range. Finally, the estimated total area of occupancy for this species using GIS data from the CNDDB is 540 ha (this does not include our new occurrence data).

This estimate is much higher than the geographic range of ∼350 ha cited by Tyler and Odion (1996) and others². We believe the CNDDB derived figure, based primarily on maps dated 1980 and 1990 to 1992, is an overestimate for several reasons. Most significantly, the estimates for area of occupancy include land that has been converted to residential development. This is especially evident in occurrence 4 (Fig. 4) where many polygons mapped as Morro manzanita in north Los Osos are clearly lined by streets and dominated by houses (Fig. 5). This is also the case for some outlying polygons in occurrence 9 (Fig. 4) east of Morro Dunes Ecological Reserve Bayview Unit, and west of the Broderson Site (Fig. 4 and 5). In addition, occurrence 1 is likely to have been in error, meaning the 28 ha attributed to this occurrence is not correct. Instead, we suspect the records for this occurrence may be more accurately included in occurrence 9, or if it is present at a different location (south rather than north of Valencia Peak), and it is a much smaller area with likely only a few individuals, perhaps 1 ha total area. Thus, we suggest the previous estimate for the current geographic range of Morro manzanita as ∼350 ha is more accurate than the 514 ha estimated in USFWS (2022) or 540 ha estimated above. However, there is clearly a need for updated information on the presence/absence of Morro manzanita within mapped polygons to better understand the current extent of this species, especially in the fragmented patches scattered across occurrence 4. At the same time, potential exists for additional outlying locations of Morro manzanita to be found. The species should be looked for at the periphery of the known geographic range. This includes east of South Bay Boulevard and north of Los Osos Valley Road, south and east of Shark Inlet (southmost point in Morro Bay) in Montaña de Oro State Park, and south of Hazard Canyon in Montaña de Oro State Park.

In the original species recovery plan, USFWS (1998) designated four “conservation planning areas”. These areas were identified and delineated to focus conservation efforts where multiple listed species would benefit and where recovery potential was high. The criteria were sites where: 1) the distributions of Morro manzanita, Morro shoulderband snail Helminthoglypta walkeriana, and Indian Knob mountainbalm Eriodictyon altissimum overlap or are contiguous with each other, 2) “natural habitats are relatively large and unfragmented by development”, and 3) “natural habitats are in public ownership or are adjacent to areas that are already secured and are to be managed for their biological diversity” (USFWS 1998). USFWS (2013) presented an amended version with three conservation planning areas (West Pecho, South Los Osos, Northeast Los Osos), which included the Morro manzanita’s entire geographic range as known in 2013. USFWS (2022) maintained the amended version, with an updated geographic range of Morro manzanita. We show the current conservation planning areas and geographic range based on CDFW in 2021 (Fig. 4, Table 2).

Since the listing of Morro manzanita in 1994, California State Parks and CDFW have acquired substantial amounts of land in the vicinity of Morro Bay for conservation, including lands occupied by Morro manzanita. There are currently eight preserves that include significant cover of the species, and these are managed by three different agencies – California State Parks, California Department of Fish and Wildlife, and the County of San Luis Obispo (Table 2.) The preserves are distributed across the Conservation Planning Areas (Table 2). Approximately half of this area with Morro manzanita is managed by California State Parks, with their largest preserve (Montaña de Oro) included within the South Los Osos and West Pecho Conservation Areas (Fig. 4, Table 2).

Based on records from the CNDDB there is a large area (total 237 ha) on private lands that is supporting Morro manzanita in low to high cover (Table 2). A small portion of this probably accurately describes some high cover stands that are in the South Los Osos Conservation Planning Area, north of Hazard Canyon and south-southwest of the Broderson Site. However, as described above in our discussion of the CNDDB based maps of occurrences, we suggest this is a substantial overestimate because polygons in the developed portions of North Los Osos are unlikely to support even low cover of Morro manzanita at the present time. With the exception of the private lands south-southwest of the Broderson Site, the vast majority of properties supporting Morro manzanita are the named preserves (Table 2).

The first delisting criterion for Morro manzanita as stated in the recovery plan (USFWS 1998) is that 90% of existing core hectares supporting high (75 to 100%) and medium (25 to 75%) cover of Morro manzanita and 85 to 90% of low (1 to 24%) cover are secured from human-induced threats in the Conservation Planning Areas with no greater fragmentation. To assess progress toward that goal, we summarize the current status of hectares with various cover classes of Morro manzanita within the Conservation Planning Areas, and the preserves within each (Table 3). One Conservation Planning Area, South Los Osos, supports high and medium cover of Morro manzanita, and this is the largest unit at 243 ha. Seventy percent of the South Los Osos Conservation Area (170 ha), and thus 70% of hectares supporting high and medium cover of Morro manzanita is in three preserves: Montaña de Oro State Park, Morro Dunes Ecological Reserve Bayview Unit, and the Broderson Site (Table 3). The other two Conservation Planning Areas, Northeast Los Osos and West Pecho, both support Morro manzanita at low cover. These two planning areas combined total 122 ha, with 109 ha in preserves: Morro Bay State Park, Elfin Forest Preserve, Montaña de Oro State Park, and Morro Dunes Ecological Reserve Pecho Unit. Thus 89% of low cover Morro manzanita hectares is in preserves. Given these data, at this point in time the first delisting criterion appears not to have been met, as 70% rather than 90% of acreage with high and medium cover are protected. In addition, even within protected areas, Morro manzanita is not wholly secure from human-induced threats, as will be discussed below.

We note that the private land south of the Broderson Site and southwest of Cabrillo Estates supports high cover (75 to 100%) of Morro manzanita (Mullany 1990) (Fig. 4). This encompasses the most substantial portion of remaining unfragmented, intact hectares of Morro manzanita outside of preserves. Protecting these existing core stands from human-induced threats, such as through purchase and incorporation into existing preserves, would contribute greatly toward conservation of this species.

Morro manzanita was listed as threatened under the U.S. Endangered Species Act in 1994 (USFWS 1994). The three main factors reported as contributing toward vulnerability of this species were residential development, competition from non-native plants, and stochastic events that negatively impact small, isolated populations (USFW 1994). Subsequent to the original listing, additional documents have addressed existing and potential new threats. A recovery plan was prepared in 1998 (USFWS 1998), and three 5-yr status reviews have been conducted, (USFWS 2008, 2013, 2022). In the first 5-yr review (USFWS 2008), status was reported to not be markedly different than at the time of listing. Two changes, one positive and one negative, were noted at that time. One was the threat of habitat loss by development was substantially reduced due to transfer of private lands with the species to CDFW or California State Parks; however, there were no management plans directed toward Morro manzanita conservation. The other change noted was a new possible threat - altered fire regimes/fire management practices (USFWS 2008). In the second 5-yr review, the status was essentially the same as in the previous review, but with one new threat identified: climate change (USFWS 2013). All described threats are still on-going.

A continuing threat to the persistence of Morro manzanita and a primary factor in its listing is clearing of habitat for conversion to residential development. USFWS (1994) stated “the restricted range and narrow habitat requirements of A. morroensis, coupled with continuing alteration, destruction, and fragmentation of habitat, make it vulnerable to becoming endangered in the near future.” Although progress has been made in protecting maritime chaparral and coastal scrub with Morro manzanita through establishment of preserves (Table 2), threats by development on private lands remain. As described above, one of the largest remaining intact areas with high cover of Morro manzanita is on private land. Conversion of these stands to residences would be an irreversible loss of both Morro manzanita individuals and habitat capable of supporting this species. Such loss of habitat exacerbates the current negative impacts of fragmentation including reduced movement of pollinators and other associated species.

In addition, alteration of the habitat on private land adjacent to housing, due to current fire management practices, extends negative impacts into intact Morro manzanita stands. California’s new code section 51179 requires homeowners in areas at high risk of wildfire to maintain a defensible space around their homes, which is an area free of excess or dead vegetation. Ninety-five percent of occurrence 9 is an area designated very high risk, which is the most severe category. This includes the following housing estates in Los Osos: Vista Court, Cabrillo Estates, the Seascape Place/Rodman Drive area, Bayview Heights, and Marguerite Drive mobile homes area. A homeowner must maintain a combustible-free zone of 1.5 m from the house, a lean/clean/green zone within 9 m feet of the house, and reduce potential fuel within 30 m feet of the house (Kerstein 2021; Calif. Dept. Forestry Fire Prevention 2023). In 2019, a fuel break (30 m wide) was constructed around the eastern edge of Cabrillo Estates, in which most vegetation was cleared and Morro manzanita severely pruned, removing the majority of shrub canopies and removing low branches contacting the ground. CA Department of Forestry and Fire Prevention is currently proposing to extend this fuel break to encompass the Seascape Place/Rodman Drive area along Pecho Valley Road, and construct another fuel break from Cabrillo Estates eastward to the vicinity of Los Osos Oaks State Natural Reserve. Such intensive removal of manzanita biomass converts this former maritime chaparral habitat to open landscaping with denuded shrub-like specimens. The functional ecological value of these individual pruned manzanitas is unknown, though without doubt their reproductive output will be substantially reduced and habitat for associated wildlife will be altered. The original listing for Morro manzanita (USFWS 1994) acknowledged the past and future potential for such deleterious impacts stating that “in addition to direct removal of habitat, development has had secondary effects on quality of adjacent remaining habitat, such as fragmentation, deterioration of habitat due to increased recreational activity, and the introduction of non-native species.” Clearing for fuel breaks around residences that are adjacent to high cover Morro manzanita stands is another such secondary impact that poses a potential threat to the species. While maintaining defensible space is an important and valid public safety concern, it would be beneficial to explore alternatives to severe thinning of manzanitas beyond the 9 m requisite border, or to consider mitigation of impacts off-site.

It should also be noted that while establishment of preserves (Table 2) has been a critical step in reducing the extinction risk for Morro manzanita, even these protected areas are not secured from human-induced threats. For example, targeted vandalism of Morro manzanita was reported in the Elfin Forest Preserve (P. Sarafian pers. comm. 2009). Invasive species, such as Eucalyptus discussed below, are abundant in Montaña de Oro State Park and other sites. To our knowledge, of the five relevant preserves, only the Broderson Site⁵ and Elfin Forest Preserve⁶ have final management plans, though plans that include specific strategies for the restoration and long-term conservation of Morro manzanita have not yet been implemented in any preserves or their respective Conservation Planning Areas. The management plan for the Los Osos Habitat Conservation Plan Preserve System⁷, which would incorporate the two units of the Morro Dunes Ecological Reserve, does include proposed strategies for habitat-based restoration in general, as well as specific measures for Morro manzanita protection and enhancement. These actions include removal of non-native species.

Competition from non-native, invasive plant species also remains a threat. Species described in USFWS (1994) included iceplant Carpobrotus sp., veldt grass Ehrharta calcina, and Eucalyptus spp. The latter is especially problematic. Eucalyptus plantations, as well as small stands, were planted in the early 1900’s in Los Osos and within what is now Montaña de Oro State Park (Hook 1988). Based on the soils, Baywood fine sands, and adjacent vegetation, it is very likely that these were planted in sites formerly occupied by Morro manzanita (Mullany 1990). Where extensive Eucalyptus plantings abut dense Morro manzanita stands, few mature manzanitas remain under the Eucalyptus canopy and there is no regeneration there, perhaps due to competition for water or other biotic factors (Mullany 1990.) Also concerning is that expansion of Eucalyptus has been documented. In 1949 Eucalyptus covered 48.3 ha, by 1986 it had expanded to 73.5 ha (Bicknell 1990), and by 2021 it had further expanded to 141.6 ha (McFadden 2021). Finally, the extensive Eucalyptus plantations in Montaña de Oro State Park are highly flammable, and thus pose a wildfire risk. While Morro manzanita is adapted to fire, burning at increased frequencies (i.e., fire intervals under 40 yr) could lead to population declines. Dense cover of veldt grass poses a similar risk of altering fire regimes to the detriment of both Morro manzanita and other components of the maritime chaparral and dune scrub; similar impacts of invasive species have been documented across a variety of plant communities (Brooks et al. 2004). At the same time, the current Eucalyptus plantations offer opportunities for restoration and expansion of Morro manzanita. Removal of at least portions of these plantations followed by seeding or planting with Morro manzanita would allow for the re-establishment of this species into its former habitat. Potential locations for the initiation of such efforts would be the edges of Eucalyptus stands that have been recently thinned, such as along the East Cable Trail east of Pecho Valley Road in Montaña de Oro State Park (Fig. 6). Here, intact Morro manzanita stands persist at the outer edges of the plantations, and removal of 10 to 20 Eucalyptus could provide 100 m² of area to replant manzanitas, gradually reducing the area occupied by the plantations along the accessible periphery.

The third main threat identified in the original listing for Morro manzanita (USFWS 1994) was negative effects of stochastic events impacting small, isolated populations. Environmental stochastic events that could reduce abundance of Morro manzanita would include wildfires that occur at intervals too short for adequate seedbank stores to accumulate. Frequent fires have not been observed in the area, but increased spread of invasive grasses or flammable Eucalyptus species could alter the natural fire regime in this maritime climate. Demographic stochasticity refers to random fluctuations in reproduction and mortality, and in small populations these fluctuations can result in reduced growth rates (e.g., Allee effect). Although further study is warranted, Tyler and Odion (2022) found that seed from Morro manzanita in the most isolated stand, the Elfin Forest Preserve, had significantly lower seed viability compared to other stands, 2% vs 4%, respectively. They hypothesized that low seed viability and high ‘‘infertility’’ (no evidence of embryo development) at the isolated stand may have been caused by inbreeding effects. Small, fragmented plant populations are susceptible to increased genetic drift and inbreeding (Sampson et al. 2016), which compromises plant reproduction (Aguilar et al. 2006). This may be particularly true for Morro manzanita, which is dependent on localized insect pollination (Tyler et al. 2023). Studies to describe the genetic diversity within and among patches would be useful in making decisions about the appropriateness of promoting gene flow in restoration plantings by using seed or cuttings from non-adjacent stands. Recent work by Huang et al. (2022) on the genomics of A. glauca may serve as a valuable reference for understanding genetic diversity in other manzanita species.

Climate change-driven drought may present a new threat to Morro manzanita as well as other endemic species with restricted distributions in the region’s maritime chaparral. Langridge (2018) provided a comprehensive assessment of how climate change will affect California’s Central Coast, including increased maximum/minimum temperatures, uncertainty in fog, slightly increased precipitation with substantially increased variability, increased extreme rainfall events, accelerated sea level rise, increased drought, and frequent and sometimes large wildfires. Mortality and stem die-off of several large Morro manzanitas in the Elfin Forest Preserve were observed since 2015 (over a period of extended drought), which may have been associated with the extremely low rainfall (P. Sarafian pers. comm. 2021). The tolerance of Morro manzanita to climate change is unknown, however, it is a habitat specialist in the coastal zone with marine fog. Morro manzanita cannot disperse to distant locations because it has a small geographic range and endemic soil requirements.

We highlight one new potential threat for Morro manzanita: the sudden oak death pathogen Phytophthora ramorum. Lee et al. (2019) and Frankel et al. (2020) reported the sudden oak death pathogen affecting multiple species of Arctostaphylos, including Morro manzanita in the botanic gardens of the University of California Santa Cruz in 2017 (M. Garbelotto pers. comm. 2022). In 35 yr, this disease had killed more than 50,000,000 trees in California and Oregon, primarily tanoak Lithocarpus densiforus and coast live oak. Among eight species of Arctostaphylos tested for susceptibility, Morro manzanita was intermediate (Garbelotto et al. 2020). Although no infected plants of any species have been found in the wild in San Luis Obispo County, the nearest infections are 3 km north of the county line in Salmon Creek Canyon, southwest Monterey County (M. Garbelotto pers. comm. 2022), which is 72 km north of the nearest occurrence of Morro manzanita (occurrence 18). However, since 2019 the pathogen has been detected by polymerase chain reaction analysis in four streams in coastal San Luis Obispo County: Santa Rosa Creek (also known as Old Creek) 14.8 km north of occurrence 18 (6.6 km northwest of Cayucos); 34 km northwest of occurrence 18; San Simeon Creek, 38 km northwest of occurrence 18; and San Carpoforo Creek, 63 km northwest of occurrence 18. Despite intensive searches, no infected vegetation has been found in the watersheds (K. Corella pers. comm. 2023). Continued monitoring for the presence of this pathogen would be prudent since the potential consequences could be substantial.

We reviewed all available sources, reported new findings from recent field surveys, and evaluated the threats to Morro manzanita in 2023. Currently, the threats identified in the 1994 listing and subsequent 5-yr reviews are still on-going. Highest priority actions should include continued efforts to preserve intact high-cover stands, as well as coordinated conservation and research to inform management in restoring and maintaining existing stands and facilitating recruitment. Based on the information presented here we provide the following recommendations:

• (1) Conserve and protect existing stands of Morro manzanita, with an emphasis on the largest remaining intact areas with high cover of Morro manzanita.

• (2) Encourage discussion with USFWS prior to fuel reduction impacting intact Morro manzanita stands in Los Osos, such as that conducted by CA Department of Forestry and Fire Prevention.

• (3) Conduct field surveys to improve the current distribution and abundance data for Morro manzanita. This should include verifying presence/absence of Morro manzanita in isolated patches mapped across Los Osos residential areas and others, as well as recording new locations. Submit these findings to the CNDDB.

• (4) Develop and implement site-specific management plans for Morro manzanita within preserves, including success criteria for evaluating effectiveness of management.

• (5) Develop protocols for long-term restoration success of Morro manzanita. Conduct research on viability and germination requirements of freshly collected manzanita seed to aid in restoration efforts.

• (6) Identify potential restoration sites across the Conservation Planning Areas both within protected areas to direct management efforts there, and within private land to be considered within potential habitat conservation plans. Investigate options for restoring connectivity between fragmented stands, including re-establishment of associated native plant and animal species.

• (7) Conduct research to describe the genetic diversity within and among existing stands/patches. If warranted by results of genetic diversity, when planting Morro manzanita for restoration, consider introducing some individuals, generated from seed or cuttings, from non-adjacent stands to enhance gene flow and genetic diversity, especially for isolated stands.

• (8) Remove Eucalyptus and re-establish Morro manzanita where feasible in the southwest part of the range including sites along Pecho Valley Road. Potential Eucalyptus selected for removal would exclude those individuals identified as Monarch butterfly roost sites or important wind breaks.

• (9) Coordinate and share information between agencies, researchers and citizen groups including the San Luis Obispo Chapter of the California Native Plant Society, and Friends of El Moro Elfin Forest, who are involved with outreach and conservation of Morro manzanita.

• (10) Continue studies of the relationship of Morro manzanita with fire.

• (11) Conduct prescribed burns of vegetation in Los Osos to reduce the risk of wildfire. This would also benefit Morro manzanita by stimulating germination and establishment of new seedlings.

• (12) Conduct modeling to anticipate effects of climate change on distribution and abundance of Morro manzanita, including changes in temperature, precipitation, amount and extent of marine fog layer, and sea level rise.

• (13) Collect seeds of Morro manzanita for conservation seed banking.

• (14) Introduce Morro manzanita (with representative genetic diversity) into living collections at several botanic gardens.

We are grateful for the valuable contributions of the following people. Dennis Odion initiated much of the field research cited here, and without his contributions, both intellectual and on the ground, we would know far less about Morro manzanita. Daniel Meade and Max Moritz also contributed significantly to research conducted with C. Tyler. Peter Slaughter assisted with field work and contributed photographs. Katie Ferguson provided documents from the CNDDB. The following persons engaged in valuable discussion: David Chipping, Kim Corella, Catherine Darst, Katie Drexhage, Christopher Lee, Emily Levin, Debora Kirkland, Marilyn Mullany, Peter Sarafian, John Sayers, Kristie Scarazzo, Monica Stillman, Michael Walgren and Sal Zaragoza. Emily Levin and John Sayers also assisted in the field. Mark Metevier calculated hectares based on CNDDB (1980s/1990s) and other data and prepared the maps. Ken Niessen, Sarah Termondt, and two anonymous reviewers made suggestions that improved the manuscript.