Effects of herbivory by white-tailed deer on forest vegetation in Eagle’s Crest and Spring Pond Nature Preserves, Eagle Creek Park, Indianapolis, Indiana
Prepared by: Michael A. Jenkins Associate Professor of Forest Ecology Department of Forestry and Natural Resources Purdue University West Lafayette, Indiana October 7, 2013
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Introduction Chronic herbivory by overabundant white-tailed deer (Odocoileus virginianus) populations threaten native plant communities in forests throughout eastern North America (Waller and Alverson 1997, Rooney 2001, Russell et al. 2001, Côté et al. 2004, Webster et al. 2005, Heckel et al. 2010). Chronic herbivory often drives the local extinctions of woody and herbaceous browse-sensitive species (Rooney et al. 2004, Thiemann et al. 2009), facilitates the invasion and recalcitrance of non-native invasive plants (Webster et al. 2008, Knight et al. 2009), shifts overstory composition towards non-preferred species (Tilghman 1989, Waller and Alverson 1997, Rossell et al. 2005, Jenkins 2011, Tanentzap et al. 2011), and alters below ground communities and processes (Bressette et al. 2012, Lessard et al. 2012). In natural areas managed to exclude human activities, hunting is commonly prohibited in favor of natural regulation. Under such a system, deer populations are allowed to fluctuate naturally and typically become overabundant without hunting or predation; a response that is augmented in fragmented landscapes with abundant edge habitat and/or mild winters (Leopold et al. 1947, Alverson et al. 1988, Cadenasso and Pickett 2000, Rooney 2001). Within the Midwest region of the United States, overabundant deer populations in natural areas are particularly problematic where woodlots and forests are surrounded by a matrix dominated by agriculture and exurban development (Hurley et al. 2012). In June and July of 2013, I assessed impacts of white-tailed deer herbivory in Eagle’s Crest and Spring Pond Nature Preserves located within Eagle Creek Park, Indianapolis, Indiana. Eagle’s Crest NP is a 296 (120 ha) acre forest reserve comprised of ridges and ravines. Spring Pond is a 44 acre (18 ha), poorly-drained forest located east of the Eagle Creek Reservoir. Both preserves have been closed to hunting since their establishment and concern has grown over the last two decades about negative impacts of herbivory by white-tailed deer. Methods I sampled vegetation in both nature preserves to determine: (1) severity of browse damage based upon indicator species, (2) woody species composition and stem density in forest understories, and (3) severity of browse on woody species in forest understories. Indicator species: To evaluate browse severity based upon herbaceous indicator species, I followed the methods of Webster and Parker (2000) and Webster et al. (2001). Specifically, I measured the heights of three species, jack-in-the-pulpit (Arisaema triphyllum), white baneberry (Actaea pachypoda), and sweet cicely (Osmorhiza claytonia). I measure all mature plants within variable radius plots
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(ranging from 10 to 15 m depending upon the abundance focal species) that used the same center points as permanent plots established by Dolan and Moore (2007a, 2007b; Figure 1). Jack-in-the-pulpit was abundant at Eagle’s Crest NP and fairly common at Spring Pond NP. However, I only observed the other two species at Eagle’s Crest, where they were uncommon. Therefore, I measured all individuals of these two species that I encountered both on and off the plots. I also performed systematic searches at Spring Pond NP to increase the number of jack-in-the-pulpit plants measured. I measured a total of 1276 jack-in-the-pulpit plants at Eagle’s Crest NP and 109 plants at Spring Pond NP. I measured four baneberry and seven sweet cicely plants at Eagle’s Crest NP. I also noted whether plants were flowering and/or had been browsed. The gender of jack-in-the-pulpit plants was also identified. Mean heights were calculated for each species, and adjusted percent cover (APC) was calculated using regression equations developed by Webster and Parker (2000). Jack in the pulpit: APC = 1.234(plant height) - 12.07 Sweet cicely: APC = 0.704(plant height) – 4.782 White baneberry: APC = 0.806(plant height) + 8.737 APC was used to estimate the browse damage class of each preserve based upon ranges provided by Parker and Webster (2000) and Webster et al. (2001). Understory woody species composition and stem density A nested plot design was used to sample understory woody vegetation. Stems > 1m in height (saplings) were tallied by species into two height classes (stems ≥ 1 m tall, but < 2.54 cm dbh and stems 2.1-10 cm dbh) within a 300 m2 plot (9.77 m radius). Four 4 m2 quadrats positioned four meters from plot center in cardinal directions were used to tally seedlings (stems < 1 m height) into two height classes; 0-20 cm and 21-100 cm. Mean density of saplings per hectare was calculated by height class for each sapling species. Mean density per 100 m2 was calculated by height class for each seedling species. Browse severity Browsing of woody understory stems was assessed in four 2 x 2 m quadrats placed in cardinal directions four meters from plot center. The techniques of Williamson and Hirth (1985) were
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Figure 1. (A) Eagleâ&#x20AC;&#x2122;s Crest and (B) Spring Pond Nature Preserves. Sample grid locations from Dolan and Moore (2007a, 2007b) are shown for both Preserves. Points A3, A4, B2, G3, G5, G6, H7, H8, and H9 were sampled at Eagleâ&#x20AC;&#x2122;s Crest NP. Points A1, A3, A9, C7, C8, C9, F2, and F5 were sampled at Spring Pond.
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used to quantify browse of individual twigs. Available (non-browsed) and browsed twigs (0.15 to 1.8 m above ground) were counted by species from up to eight randomly selected individuals per species. All woody species were quantitatively assessed except spicebush (Lindera benzoin), a non-preferred species and paw paw (Asimina triloba), a highly non-preferred species. Based upon field observations, I observed light and intermittent browsing of spicebush and no browsing of paw paw. Percent of twigs browsed for each species was calculated as a per plot average.
Results and discussion: Indicator species Jack-in-the-pulpit The mean height of jack-in-the-pulpit plants at Eagle’s Crest NP was 16.1 ± 0.2 cm. When this value was inserted as the independent variable into the regression equation developed by Webster and Parker (2000), APC was estimated as 7.8 %, which corresponds to a browse damage rating of severe (the lowest rating; Webster et al. 2001). The mean height of jack-inthe-pulpit plants at Spring Pond NP was 13.7 ± 0.5 cm. According to the regression equations developed by Webster and Parker (2000), this value equates to an APC of 4.8%, which also corresponds to a damage class rating of severe. This APC rivals the most severely degraded state parks examined by Webster and Parker (2000) and Webster et al. (2001). Reduced heights of forest perennials in forests with overabundant deer populations have been widely observed for palatable species (e.g. Augustine and Frelich 1998, Fletcher et al. 2001, Jenkins et al. 2007, Koh et al. 2010). However, jack-in-the-pulpit is generally viewed as unpalatable (Augustine and Jordan 1998, Heckel et al. 2010) and its reduced stature is not likely due to herbivory. Rather, deer-mediated indirect effects, including increased resistance of surface soils to water penetration and decreased leaf litter depth, have likely driven reduced height growth (Heckel et al. 2010). These indirect effects have also likely affected the demography of jack-in-the-pulpit populations in the two preserves; at Eagle’s Crest, only 1.8% of plants were flowering, while 2.8% were flowering at Spring Pond. Published rates of flowering for jack-inthe-pulpit populations in areas without overabundant deer populations range from 35-65% (Bierzychudek 1982b, Ewing and Klein 1982). While jack-in-the pulpit is not a preferred browse species, 12.5 % (3 out of 24) of flowering plants were browsed at Eagle’s Crest. In addition, two out of four female flowers were browsed at Eagle’s Crest NP (Figure 2), as was the only flowering female plant observed at Spring Pond. In jack-in-the-pulpit populations, flowering individuals are larger than nonflowering plants, and female flowering plants are larger than male flowering plants (Bierzychudek 1982a). While not preferred, deer appear to have
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selectively browsed the largest individuals within the populations of the two nature preserves. Overall, I observed few large individuals at either preserve (Figure 3).
Of all flowering jack-in-the-pulpit plants, only 16.7% at Eagleâ&#x20AC;&#x2122;s Crest NP were female. Similarly, only one out three flowering plants at Spring Pond NP was female. Jack-in-the-pulpit plants are able to switch gender in response to environmental stress, including defoliation (Bierzychudek 1984). Plants typically switch from female to male or sterile plants because flowering as a male or not flowering requires less expenditure of stored energy reserves than flowering as a female plant. The relative scarcity of flowering female plants we observed suggests that herbivory has caused most individuals from the populations of both nature preserves to revert to either sterile or male forms. Following the methods of Webster et al. (2001), I measured no plants less than 8 cm in height. Typically, such small individuals could be new seedlings and not mature plants. However, considering the very low fecundity of jack-in-the-pulpit across both reserves (only two out of 1385 plants produced fruit; 0.14%), it is unlikely that these small individuals were new seedlings.
Figure 2. Female jack-in-the-pulpit plant browsed by deer in the Eagleâ&#x20AC;&#x2122;s Crest Nature Preserve at Eagle Creek State Park.
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Figure 3. Height distributions of jack-in-the-pulpit plants sampled at Eagle’s Crest and Spring Pond Nature Preserves. Please note the different scales of the Y-axes; far fewer plants were found at Spring Pond NP.
Other indicator species Based on the seven sweet cicely plants I measured at Eagle’s Crest NP, I calculated a mean height of 33.3 ± 4.8 cm. According to the regression equation developed by Webster and Parker (2000) this value equates to an APC of 18.7%, which corresponds to a damage rating of heavy-severe. The mean height of white baneberry was 8.8 ± 1.4 (based upon four plants), which corresponds to an APC of 15.8% and a damage class rating of severe. Because of the low sample size that resulted from the relative rarity of these species, results should be viewed with greater caution than those for jack-in-the-pulpit. However, these results do support the damage class ratings calculated for jack-in-the-pulpit. The low rate of flowering I observed for
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these two species (one out of seven sweet cicely plants and none out of four baneberry plants) also supports the results observed for jack-in-the-pulpit. Understory woody species composition and stem density Eagle’s Crest Nature Preserve The lower stratum (0-20 cm height) of the seedling layer (all woody stems < 1 m height) at Eagle’s Crest NP was heavily dominated by white ash (Fraxinus americana; 49 ± 21 stems/100m2) with a lower density of sugar maple (Acer saccharum; 9 ± 6 stems/100m2). The other five species surveyed consisted of individual or scattered seedlings (Figure 4). In the upper stratum (21-100 cm), paw paw was the dominant species, followed by white ash and spicebush.
Figure 4. Density (mean ± 1 SE) by species of woody seedlings (stems < 1 m 2
height) per 100 m at Eagle’s Crest Nature Preserve.
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Paw paw and spicebush dominated the small sapling class (stems ≥ 1 m height, but ≤ 2.5 cm dbh) at Eagle’s Crest NP. Paw paw is highly unpalatable to deer and this species has been observed dominating understories of other forests under chronic heavy browse pressure (Webster and Parker 1997, Asnani et al. 2006, Jenkins 2011). While spicebush may be negatively affected by browsing (Asnani et al. 2006), other studies have observed minimal effects (Rossell et al. 2007). The large saplings (2.6-10 cm dbh) class was dominated by sugar maple and American elm (Ulmus americana). The much lower density of these two species in the seedling and small sapling classes suggest that additional stems are not recruiting into the large sapling class (Figures 5 and 6). The mean density of stems in the large sapling class was 282 stems/ha in Eagle’s Crest NP. In an old-growth forest in Ohio without a documented history of deer overabundance, Cho and Boerner (1991) observed 456 large saplings/ha in non-gap areas. When all size classes of seedlings and saplings were combined, the total understory woody stem density at Eagle’s Crest NP was 13,992 stems/ha. For comparison, Goebel and Hix (1996) observed 81,887 stems/ha in the understory of 110-129 year-old forests in southern Ohio.
Figure 5. Facing west on plot G5, Eagle’s Crest Nature Preserve, Eagle Creek State Park. The herbaceous layer is largely dominated by unpalatable species such as sedges (Carex spp.), wild ginger (Asarum canadense), wood nettle (Laportea canadensis), and wild leek (Allium tricoccum). The woody understory is dominated by unpalatable species such as paw paw and browse resistant species, such as white and green ash.
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Figure 6. Density (mean ± 1 SE) by species of woody saplings (≥ 1 m height, > 10 cm dbh) per hectare at Eagle’s Crest Nature Preserve.
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Spring Pond Nature Preserve Green ash was, by far, the dominant seedling species at Spring Pond NP in both height strata Figure 7). With the exception of spicebush, other species occurred as single or scattered individuals. The small sapling class was dominated by green ash, paw paw, and spicebush. Paw paw, green ash, and sugar maple dominated the large sapling class (Figure 8). However, the lack of sugar maple in the small sapling class suggests that continued recruitment into the large sapling class is unlikely (Figure 8).
Figure 7. Density (mean Âą 1 SE) by species of woody seedlings (stems < 1 m height) per 100 m2 at Spring Pond Nature Preserve.
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Figure 8. Density (mean ± 1 SE) by species of woody saplings (≥ 1 m height, < 10 cm dbh) per hectare at Spring Pond Nature Preserve.
Browsing of woody plants While twigs were browsed at both preserves, the frequency and severity of browsing was much greater at Spring Pond NP (Figures 9 and 10). Green ash, the dominant seedling and small sapling species at Spring Pond NP, sustained heavy browsing; 79 ± 4% of all twigs were browsed. Although green ash was heavily browsed, ash species are tolerant of browsing and may increase in importance as less-browse tolerant species begin to disappear (Rossell et al 2005). I also observed heavy browsing of dogwood (Cornus) species and multiflora rose (Rosa
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mulitiflora) at Spring Pond NP. American bladdernut (Staphylea trifolia), southern arrowwood (Viburnum dentatum), and Ohio buckeye (Aesculus glabra) also sustained heavy browsing, although these species occurred infrequently compared to other species. While little information is available about the browse preference of bladdernut and arrowwood, buckeye species are generally viewed as less preferred (Castleberry et al. 1999). Less than 40% of ironwood (Ostrya virginiana), amur honeysuckle (Lonicera maackii), and American elm twigs were browsed. Overall rates of browsing were lower at Eagle’s Crest NP. Similar to Spring Pond NP, ash species, Ohio buckeye, and American bladdernut were the most frequently browsed species at Eagle’s Crest NP.
Figure 9. Percent (mean ± 1 SE) of twigs browsed by species at Eagle’s Crest and Spring Pond Nature Preserves.
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B
A
C
Figure 10. Browse damage on twigs of (A) green ash, (b) multiflora rose, and (C) Ohio buckeye. All photos were taken in Spring Pond Nature Preserve.
Conclusions Based upon the results of this study, I conclude that both Eagleâ&#x20AC;&#x2122;s Crest and Spring Pond Nature Preserves exhibit degraded plant communities resulting from long-term chronic herbivory by white-tailed deer. Indicator species analysis based upon earlier work by Webster and Parker (2000) and Webster et al. (2001) placed both preserves in the severe impact class.
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Demographic observations of jack-in-the-pulpit suggest a shift in gender and reproductive status consistent with populations under severe environmental stress. The understories of both preserves are dominated by unpalatable and browse-resistant species. Overall, the understory stem densities of both preserves are quite low and reflect the impacts of heavy long-term browse pressure. This browse pressure is particularly pronounced at Spring Pond NP where most species exhibited browsing rates greater than 75%.
The low density of canopy species regeneration in the understory, combined with the rarity of preferred browse species in the herbaceous layer, suggest that the vegetation communities of both preserves are shifting in both composition and structure towards an alternate state defined by reduced plant species diversity and reduced recruitment of canopy trees. While reducing deer populations through controlled hunts is controversial, recent research in Indiana State Parks suggests that such hunts can successfully reduce deer abundance and allow the recovery of native plant communities (Jenkins 2011).
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