Forage production for white-tailed deer Odocoileus virginianus is often limited in closed-canopy forests. We measured browse production and nutritional carrying capacity after prescribed burning and understory fertilization in closed-canopy hardwood stands one growing season after treatment in two physiographic regions of Tennessee. Nutritional carrying capacity estimates for prescribed burning, fertilization, and prescribed burning with fertilization were greater than in controls on the Cumberland Plateau. However, the cost per pound of forage produced after fertilization exceeded US$26. In the Coastal Plain, fertilization did not affect nutritional carrying capacity, and prescribed burning and prescribed burning with fertilization lowered nutritional carrying capacity from controls. At both sites, prescribed fire had minimal effect on soil pH or soil phosphate and potash levels. Our results suggest prescribed fire and fertilization are of limited utility for increasing browse production in closed-canopy hardwood forests.
An increasing number of nonindustrial private landowners in the eastern United States actively manage their property for wildlife (Measells et al. 2005, 2006). The majority of these landowners manage for white-tailed deer Odocoileus virginianus (hereafter deer), and the most popular land management practice is planting food plots (Schweiss and Dwyer 2008). Acreage dedicated to food plots, however, is a small fraction of the property, and practices to improve forested areas could increase nutritional carrying capacity (NCC).
Regeneration methods, such as clearcutting, and timber stand improvement practices can improve forage availability for deer (Blymyer and Mosby 1977; Miller et al. 2009). However, many landowners are not interested in harvesting their timber or removing any trees. Prescribed fire also has been used to enhance forage availability for deer in forested areas (Dills 1970); however, most work concerning use of prescribed fire for increased deer browse has followed some level of canopy removal to increase available sunlight (Masters et al. 1993; Jackson et al. 2007). Fertilization has been shown to affect production (Segelquist and Rogers 1975; Dyess et al. 1994) and nutritional quality (Wood 1986; Harlow et al. 1993) of deer forages, but data evaluating effects of fertilization in closed-canopy hardwood stands are lacking.
We are not aware of any published evaluations of the efficacy of prescribed fire and fertilization in closed-canopy hardwood forests. Evaluation of practices that do not alter the forest overstory is warranted because many landowners are interested in improving forage availability for deer without altering the forest canopy. We conducted this field study to evaluate the effects of prescribed fire, understory fertilization, and prescribed fire with understory fertilization on nutrient availability and browse production in closed-canopy hardwood stands in two distinct physiographic regions of Tennessee. Our objectives were to determine deer use of browse species and production and nutritional quality of browse after treatments.
We selected two closed-canopy hardwood stands with no recent fire histories. Stands were chosen to ensure uniformity (e.g., similar species composition, soils) within a site.
Within the Cumberland Plateau physiographic province, we selected a shortleaf pine–oak Pinus echinata–Quercus spp. stand (12.80 acre [5.18 ha]) known as Rocky River, in Sequatchie County (Figure 1).
Overstory species included scarlet oak Quercus coccinea, white oak Quercus alba, shortleaf pine, black oak Quercus velutina, and mockernut hickory Carya tomentosa. Midstory species included mockernut hickory, sassafras Sassafras albidum, sourwood Oxydendrum arboreum, blackgum Nyssa sylvatica, red maple Acer rubrum, pignut hickory Carya glabra, and flowering dogwood Cornus florida. Soils were primarily Lonewood silt loam and Lily loam that are well drained and acidic, with 2–12% slopes (Prater 2003). Site index for shortleaf pine was 70 (Prater 2003). Deer density estimates obtained with infrared-triggered cameras (Jacobson et al. 1997) indicated a minimum of 28 deer/mi2 (deer/2.59 km2).
We selected an oak–hickory stand (12.80 acre) within the Coastal Plain physiographic province at Ames Plantation in Fayette County (Figure 1). White oak, yellow-poplar Liriodendron tulipifera, southern red oak Quercus falcata, blackgum, and sweetgum Liquidambar styraciflua were common in the overstory. Midstory species included winged elm Ulmus alata, black cherry Prunus serotina, and flowering dogwood. Poison ivy Toxicodendron radicans, Japanese honeysuckle Lonicera japonica, Virginia creeper Parthenocissus quinquefolia, and supplejack Berchemia scandens were common in the understory. Soils were primarily Ruston sandy loam that are well drained and acidic, with 12–30% slopes (Flowers 1964). Site index for shortleaf pine was 50–70 (Flowers 1964). Deer density estimates obtained with infrared-triggered cameras (Jacobson et al. 1997) indicated a minimum density of 21 deer/mi2.
Sampling methodology and treatment application
We systematically located sixteen 100-yd (91.44-m) transects 100 ft (30.48 m) apart within each 12.80-acre (5.18-ha) stand during summer 2004. We measured woody leaf biomass (pounds per acre) and herbaceous forage within sixty-four 60-ft2 (5.57-m2) sampling plots systematically placed every 25 yd along each transect (Figure 2). We tallied woody browse plants within sample plots to species (stem count tally), and stems were noted as browsed or unbrowsed. We also noted browsing on herbaceous plants along the line transect. For woody vines, we used a measure of inches covered along the line transect in a regression equation to estimate total stem counts of these species from their coverage. We collected and sorted leaves of woody vegetation and all above-ground growth of herbaceous plants ≤4 ft. We placed samples in a forced-air oven at 50°C until cessation of weight loss and then weighed samples to determine dry-matter weights (grams).
After pretreatment data collection, we divided stands into four 3.2-acre (1.3-ha) sections, each containing four of the established transects (Figure 2). We collected soil samples along the four transects within each section; combined them to form a composite sample; and submitted them to the University of Tennessee Soil, Plant and Pest Laboratory for analysis of pH, phosphorus (P), and potassium (K) levels.
We burned two sections in each stand during the dormant season (Rocky River, March 30, 2005; Ames Plantation, April 5, 2005) by using low-intensity fire under the following conditions: 6–20°C, 20–40% relative humidity, wind speed of 3–6 mi/h (4.83–9.66 km/h), and a mixing height of >1,640 ft. For all controlled burns, backing fires were set initially and the remainder of the units were burned using relatively low-intensity strip-heading fires generating 6–18-in. (15.24–45.72 cm) flame heights.
We applied fertilizer in late spring 2005 (Rocky River, May 16, 2005; Ames Plantation, May 12, 2005). To avoid issues with pseudoreplication, fertilizer was applied to each individual transect (replicate) instead of across the entire burned section. Before application, we calibrated a hand spreader to ensure proper distribution for each nutrient according to pretreatment soil test results.
We fertilized four transects within one burned and one unburned section with ammonium nitrate (34–0–0 [N–P–K]) at 45 lb N/acre. Triple superphosphate (0–46–0) and muriate of potash (0–0–60) were applied to raise phosphate (P2O5) and potash (K2O) to levels where a plant response would be expected based upon soil test results. At Rocky River, we applied 72 lb (32.66 kg) phosphate/acre and 205 lb potash/acre. In the burned section at Ames Plantation, we applied 52 lb phosphate/acre and 101 lb potash/acre. For the fertilized-only transects at Ames Plantation, we applied 72 lb phosphate/acre and 131 lb potash/acre. We collected soil samples in June and August 2005 to track responses in pH, phosphate, and potash levels posttreatment.
During July and August 2005, we located plots between areas sampled in 2004 to avoid previously sampled areas. Sample plots in summer 2005 were 4 ft in width × 10 ft in length. We recorded evidence of browsing on woody plants in sample plots by using a stem tally. We collected all woody leaves and herbaceous plants ≤4 ft and sorted them by species or species groups (i.e., hickory, red oak, or white oak group). We placed samples in a forced-air oven at 50°C until cessation of weight loss and weighed to determine dry-matter weights. We combined samples of species or species groups within the same treatment into a composite sample and ground with a Wiley mill until particles passed through a 1-mm screen. We analyzed composite samples for nitrogen (N) with a LECO FP-2000 nitrogen analyzer (LECO Corp., St. Joseph, MI) by using the Dumas combustion method (method 990.03; AOAC 1998) to obtain estimates of crude protein (CP) for species or species groups. We conducted fiber analyses (neutral and acid detergent; Jung 1997) with an ANKOM 200 fiber analyzer (ANKOM Technology, Macedon, NY).
We collected browse and herbaceous forage in both years to compare production in control and treatment areas within the closed-canopy hardwood stands. Therefore, we used a completely randomized split-plot design for a mixed model analysis of variance. Fixed effects were treatment, year, and the treatment × year interaction. Random effects were transect (treatment) and sample plot (transect × treatment). Log or log + 0.5 transformations were used when necessary to address normality and homogeneity of variance. When the interaction term was significant (P < 0.05), we used the least significant difference method for mean separation. We chose 10 browse species or species groups for biomass comparisons after treatments based on deer selectivity (see description below) and contribution of each species or species group to total biomass at each site. We compared individual browse species or species group biomass among treatments by using a completely randomized design for the mixed model analysis of variance. Burn and fertilizer treatment were the treatment factors. Before using the log transformation for the 10 individual species or species groups, 0.5 was added to all biomass values to retain observations with 0 values. For testing treatment effects, we used a Bonferroni-corrected α level of 0.01 (0.10/10 species tested). When significant (P < 0.10) differences were found, we used the least significant difference method to detect differences among means.
Using pretreatment (2004) data, we calculated a selection index (Chesson index; Chesson 1978, 1983) by dividing the ratio of use and availability for a given species or species group by the sum of ratios for all species or species groups for woody plants and browse species having stem counts ≥25 (Supplemental Material, Table S1, http://dx.doi.org/10.3996/102009-JFWM-016.S1). We combined species or species groups with <25 stems into an “other” category. We could not calculate a selection index for herbaceous forage species. Cutoff values indicating no selection depended on the number of species or species group compared at each site (Ames Plantation, 1/25 = 0.04; Rocky River, 1/11 = 0.09). Values above and below these values indicate greater and lesser use, respectively, than expected at a given site.
We calculated estimates of NCC in 2005 with the explicit nutritional constraints model (Hobbs and Swift 1985). Following criteria used by Edwards et al. (2004), we estimated NCC for deer by using constraints of 12% CP and a dry matter intake of 3 lb/d. We determined nutritional values for individual browse species for each species collected within each treatment or control area. Because samples of each species within each treatment or control area were combined for nutritional analyses, we report absolute values for CP, neutral detergent fiber, and acid detergent fiber. We based browse species included in the NCC estimate upon selection indices calculated at each site. We used a completely randomized design for a mixed model analysis of variance to compare NCC estimates among treatments, with an α level of 0.05. We log-transformed data when necessary to address normality and variance problems.
Effects on soil
Soil pH remained similar across all treatments and sampling periods at both study sites (Table 1). As expected, soil phosphate and potash levels increased after fertilization treatments, but they were not influenced by prescribed fire.
Effects on forage production
Effects of treatments on forage production varied among study sites. Herbaceous forage increased in all treatments as well as controls at Rocky River from 2004 to 2005 (Table 2). At Ames Plantation, herbaceous forage was increased after prescribed fire and prescribed fire with fertilization (Table 2). Browse production at Rocky River did not increase after fertilization but did increase after prescribed fire and prescribed fire with fertilization (Table 2). No treatment increased browse production at Ames Plantation (Table 2), and there was no meaningful effect on biomass of individual browse species or species groups after treatments at either site (Table 3). Crude protein and fiber content were variable among species or species groups and treatments (Table 4).
Greenbrier Smilax spp., blackgum, and blackberry Rubus spp. were used more than expected based on availability at Rocky River. Hickory, blueberry Vaccinium spp., red maple, sourwood, sassafras, white oak group, and red oak group were used less than expected. No use was recorded for species in the red oak group. At Ames Plantation, greenbrier, supplejack, blackgum, rose Rosa spp., and winged elm were browsed more than expected based on availability. Species used less than expected based on availability included slippery elm Ulmus rubra, sugar maple Acer saccharum, blackberry, red oak group, black cherry, white oak group, eastern redbud Cercis canadensis, red maple, hickory, grape Vitis spp., Japanese honeysuckle, ash Fraxinus spp., Virginia creeper, and poison ivy. No browsing was recorded for yellow-poplar, sassafras, Carolina buckthorn Rhamnus caroliniana, common persimmon Diospyros virginiana, or devil's walkingstick Aralia spinosa.
Effect on NCC
Although no treatment effects on individual species (Table 3) or nutritional quality (Table 4) were detected, prescribed fire and fertilization increased NCC estimates at Rocky River (Table 5). Conversely, although no treatment significantly affected forage production at Ames Plantation, estimates of deer days/acre were decreased after prescribed fire.
Although others have noted changes in pH after prescribed fire (Binkley 1986; Blankenship and Arthur 1999), our results did not reveal an effect of fire on pH, which was consistent with Franklin et al. (2003). Although using ammonium nitrate fertilizers may lower pH if used annually, pH changes after infrequent fertilization are usually negligible (Fisher and Binkley 2000).
Differences in soil potash responses at Ames Plantation and Rocky River may be a result of differences in soil texture. The sandy loam at Ames Plantation contained less clay than the silt loam at Rocky River. As summer progressed, the greater clay content and cation exchange capacity of the silt loam probably contributed to the observed decline in potash ratings during the late summer sampling period because clay particles attract more of the free K ions. Plant response to fertilization can be expected to vary among different soil types.
Past research has documented increases in browse production after fertilization, but any increase is certainly buffered by available sunlight. Segelquist and Rogers (1975) and Dyess et al. (1994) reported increased production of Japanese honeysuckle after applications of lime and N fertilization, but their plots were located in cleared openings. Production of Japanese honeysuckle did not increase after fertilization at Ames Plantation. Increases in NCC estimates at Rocky River after fertilization were significant but arguably not worth the cost. Fertilizers in our study were US$0.22/lb (34–0–0), US$0.31/lb (0–46–0), and US$0.28/lb (0–0–60). Average fertilization costs for rates of N (US$28.46/acre), P (US$41.78/acre), and K (US$72.17/acre) applied totaled US$142.40/acre. The only increase in browse selected by deer after fertilization was blackberry, which increased 4.0 lb/acre at Rocky River, costing US$35.60/lb in fertilized sections. With the greatest increase of 3.5 deer days/acre (6.3 deer days/acre after prescribed burning with understory fertilization compared to 2.8 deer days/acre in control) at Rocky River and associated fertilization rates and costs (US$173.67/acre) used in our study, it would cost US$49.62 for each additional deer day.
Substantive changes in the structure and composition of understory vegetation usually necessitate several successive fires and are also influenced by season of fire and fire intensity (Brockway and Lewis 1997; Sparks et al. 1998; Hutchinson and Sutherland 2000; Peterson and Reich 2001; Glasgow and Matlack 2007; Jackson et al. 2007). Substantive changes after repeated burning are strongly correlated with increased sunlight entering the forest canopy. Our data represent the initial effect of prescribed fire in closed-canopy stands. With repeated low-intensity burning, mortality of the midstory may allow increased sunlight, which could lead to increased browse production. However, it is likely that landowners managing their property for wildlife would like to see a more timely response to their management efforts.
It is important to understand the approach we used to estimate NCC is not an absolute measure of carrying capacity. However, it does allow relative comparisons among treatments by using a biologically defensible diet-quality target using species or species groups selected by deer during the growing season. This approach is important because forage quality has a tremendous influence on available nutrition. By combining selected deer forages to average a minimum of 12% CP, our data suggested NCC was actually negatively influenced by burning at Ames Plantation, although the woody leaf biomass was not significantly decreased.
Although forage quality is important, browse species selected by deer influences NCC estimates more than increases in CP values. Although CP values in most treatments were slightly higher than those in control areas, only control areas at Ames Plantation had reductions in NCC estimates attributable to the minimum criteria for CP (12%). Managers should only use results from diet studies as general guidelines for deer use of various species and evaluate treatment effects on browse species in relation to actual deer use on specific areas.
The response of herbaceous species to treatments suggested their inclusion in NCC estimates would not have affected our results. At both study areas, sites that were burned and fertilized produced greater amounts of herbaceous forages than other treatments. However, nonpreferred species, such as American burnweed Erechtites hieraciifolia and grasses, contributed almost all of this production. On other sites with a different seedbank, a response by desirable forage species may increase NCC.
Prescribed burning and understory fertilization produced mixed effects in two physiographic regions with different soil types in Tennessee one growing season after treatment. Therefore, we caution against the use of low-intensity prescribed fire in closed-canopy stands with the objective of increasing browse for deer. Although browse production may increase during subsequent growing seasons or after additional fire prescription, we recommend some canopy reduction treatment (e.g., retention cutting and thinnings) to allow additional sunlight into the stand before burning (Healy 1997; Jackson et al. 2007), especially if a relatively quick and positive treatment effect is desired. We do not recommend understory fertilization in closed-canopy hardwood stands because plant response was minimal, and the relatively small increase makes it difficult to justify the cost of fertilization. Liming before fertilization could improve pH and nutrient availability, but application of lime in forested areas is generally not practical because of difficulty spreading lime in the woods, amount of lime needed to correct soil acidity, and associated costs. We believe money spent on liming and fertilization would be much more efficiently and effectively spent on food plot plantings.
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Table S1. Selection index value data.
Found at DOI: 10.3996/102009-JFWM-016.S1 (182 KB XLS).
Funding and support for this study were provided by the Department of Forestry, Wildlife, and Fisheries at the University of Tennessee; Hobart Ames Foundation; Sequatchie Forest and Wildlife; Quality Deer Management Association; and Tennessee Wildlife Resources Agency. Logistic support was provided by Benny Bowers, Carla Dilling, Jesus Gamboa, John Gruchy, Greg Julian, James McDonald, Larry Teague, and Shelton Whittington. We thank the Subject Editor and two anonymous reviewers, who helped improve the quality of this manuscript.
Christopher E. Shaw,* Craig A. Harper, Michael W. Black, Allan E. Houston
Shaw CE, Harper CA, Black MW, Houston AE. 2010. Initial effects of prescribed burning and understory fertilization on browse production in closed-canopy hardwood stands. Journal of Fish and Wildlife Management 1(2): 64–72; e1944-687X. doi: 10.3996/102009-JFWM-016