Impacts of Deer Browse on Managed Grasslands, Sanford Farm Nantucket

Transcription

1 Impacts of Deer Browse on Managed Grasslands, Sanford Farm Nantucket Internal Project Progress Report June 2017 Jennifer Karberg, Research Program Supervisor Project Background Nantucket Island hosts the highest density of white-tailed deer in Massachusetts, ~45-55 deer per square mile as estimated by the Massachusetts Division of Fisheries and Wildlife. Large, unrestricted deer populations can have strong negative impacts on native flowering plant communities. Studies, primarily in forests in New England and the Midwest, have shown dramatic reductions in flowering plant diversity, decreases in rare plant species and increases in some non-native invasive plants such as garlic mustard in areas where deer are not excluded. The impacts deer browsing have on plant communities is very easily studied through simple deer exclosure experiments. Tall exclosure fences designed to exclude deer from an area for 5-10 years can allow native species to germinate and flourish in the absence of deer browse pressure (Figure 1). Figure 1: Dramatic example of vegetation diversity within a fenced forest understory in New England. In 2011, NCF s Science and Stewardship Department designed and implemented a study to understand what effect browse by deer has on plant community composition in managed sandplain grasslands on

2 Nantucket. The majority of deer browse impact studies have occurred in forested areas with very little research in grasslands. On Nantucket we know that deer densities are high across the island and we have observed deer browse within our grasslands. We have particularly observed browse impacts on the rare New England Blazing Star (Liatris scariosa var. novae-angliae) as well as other flowering plants found primarily within out grasslands. Nantucket s sandplain grasslands are disturbance dependent ecosystems, vegetation composition depends on and is driven by disturbance events such as prescribed fire, mowing and grazing. Given high deer densities on Nantucket and observed impacts of deer browse on disturbance dependent sandplain grassland plant species, NCF s Science staff decided to investigate what effect deer browsing might be have on these rare and unique plant communities. Project Purpose The purpose of this research project is to examine the effect of deer browse on plant community composition in managed sandplain grasslands on Nantucket. We examined how the exclusion of deer from a fenced research plot influences plant species composition, diversity and abundance. In addition, we attempted to identify species that are preferentially browsed by deer, with a particular focus on rare plant species. The results of this research will help inform our understanding of the impact of deer on the natural plant communities of Nantucket. Methods Study Site NCF s Sanford Farm property is composed of open, managed grasslands and low shrublands interspersed with unmanaged shrub wetlands. The open areas of this property have been managed by dormant season mowing for the past 30+ years, with some prescribed fire management taking place between 1986 and 1995, although not in the particular location of this study. Mowing is one of the management tools NCF uses to maintain and promote rare sandplain grassland and heathland habitats and cessation of mowing in an area often leads to an increase in the spread and dominance of woody shrubs and trees. The study site was located within a managed, grassy portion of Sanford Farm (Figure 2) and selected as an area large enough for this study with a visual uniformity of slope, vegetation, and soils as well as a lower visual density of woody shrubs and a higher visual density of rare sandplain grassland species, particularly Blue-eyed grass (Sisyrinchium fuscatum) and Bushy rock-rose (Crocanthemum dumosum) (the latter of which has since been delisted). Additionally, established deer trails were noted near the selected study site. During the course of this study, the research area continued to be managed using dormant season mowing to allow us to examine the effect of deer exclusion in addition to mowing as a management tool in sandplain grasslands and heathlands.

3 Figure 2: Deer Browse study location within NCF s Sanford Farm property. Two research plots, one open control (South) and one permanent exclosure (North). Research Plot Establishment In June 2011, two large 50 x 50-m research plots were established at Sanford Farm (Figure 2). Plot corners were created using ArcGIS and then marked in the field using capped rebar and flags. During the first growing season in 2011, both plots remained un-exclosed, to conducted pre-treatment vegetation sampling prior to altering deer access. In winter 2012, the North research plot was randomly selected for exclosure and a permanent deer exclosure fence was erected by NCF s Properties Maintenance staff (Figure 3) while the South research plot was retained as an unfenced control. This fence was designed to completely exclude deer and to limit the presence of rabbits within the plot. The only way to completely eliminate rabbits from the study area would be to dig the fence at least 6-8inches into the ground. We decided not to completely eliminate rabbits for two reasons: 1) Digging a permanent fence into the ground would create a large amount of soil disturbance and thereby drastically alter this unique area of conservation land, likely permanently changing the landscape even after the removal of the fence from the study area and 2) from a management standpoint, deer populations can be drastically reduced if this proves beneficial to native plant populations, while rabbit populations are more difficult, expensive and time-intensive to manage. This also reduced the need to attempt to differentiate deer browse from rabbit browse in the field, which can prove difficult,

4 particularly on grasses and finer scale forbs. For the purposes of our analysis, any browse observed in the permanent exclosure was attributed to rabbits. Figure 3: Construction of the Deer Browse exclosure at Sanford Farm by NCF s Property Maintenance crew. Sampling Plot Establishment Vegetation community composition and vegetation browse were measured in 1-m 2 plots located within each large research plot. Using ArcGIS, we established 50 randomly located sampling plots within each large research plot. In the field, we navigated to each point which was used to establish the SE corner of the 1-m 2 plot. Using a compass, a 1-m 2 plot frame was aligned with magnetic north and each plot was permanently marked with pins and orange painted washers in the NW and SE corners and pins only in the NE and SW corners. We attached metal tags to the SE corner pin with Large Plot # and Sampling Plot # for identification and tied orange flagging around pins to aid in relocation. During sampling, the SE and NW corner of each small plot were marked with pin flags which were then removed at the end of sampling to allow for dormant season mowing. Vegetation Sampling At each sampling plot (n=100) we aligned a 1-m 2 plot frame with the pinned corners. We identified all graminoids, forbs, and woody species to the species level (where possible) and recorded the percent cover of each species within the plot using modified percent cover categories (Table 1). Using the same percent cover categories, we estimated functional group coverage as well as bare ground and litter. To document browse, we recorded the percent browse of each species within the following categories: (0-25% of the individuals of a particular species in the plot were browsed, 25-50% browsed, 50-75% and %). Lastly, we counted the number of stems for each individual forb species and recorded the number of browsed vs. un-browsed stems.

5 Table 1: Percent Cover Categories for Vegetation Sampling Abundance Code Percent Cover Range Sampling Code Percent Cover Range 2 0-1% % 3 1-2% % 4 2-5% % % % Analysis We conducted a non-metric Multidimensional Scaling Analysis (nmds) based on a Bray-Curtis dissimilarity matrix to compare the vegetation composition between the Control plot (South plot) and the Exclosure plot (North plot) after 5 years (R Core Team 2013). An analysis of similarity (ANOSIM) using the Bray-Curtis index of similarity was conducted to examine differences in the similarity of each plot and a SIMPER (Similarity Percentages) analysis was conducted to examine the average contribution of individual species to the average dissimilarity between plots using Primer 7 (Plymouth Routines in Multivariate Ecological Research. Vegetation composition differences between plots was examined at both the functional group level and the individual species level. Additionally, we examined patterns in browse over time in both research plots. Results and Discussion Vegetation Community Changes Prior to constructing the deer exclosure, the Control plot contained significantly less litter/duff ground cover compared to the Exclosure plot. No other significant difference was observed between the Exclosure and Control plots in functional group composition (% woody, graminoid, forb, bare ground). Five years after treatment (2016), no significant difference existed between functional group composition of the research plots. Whatever was initially driving a difference in litter composition between the plots was likely an environmental and not a management factor. The Exclosure and Control plots showed very little difference in individual plant species composition prior to the construction of the deer exclosure, and this did not significantly change 5 years post exclosure construction. The ANOSIM showed moderate to low levels of dissimilarity between the two plots even after 5 years of excluding deer browse. In 2016, the South and North plots had an average dissimilarity of 52%, primarily attributable to increased Little Bluestem grass (Schizachyrium scoparium) and Sickle-leaved golden-aster (Pityopsis falcata) in the Control plot and increased Pennsylvania sedge (Carex pensylvanica) and Northern dewberry (Rubus flagellaris) in the Exclosure plot (Table 2).

6 Table 2: Results of the SIMPER analysis of dissimilarity between the South and North plot in Species Contribution (%) Cumulative (%) Avg. Abundance Avg. dissimilarity Avg. Dissimilarity = South Plot North Plot Little bluestem Pennsylvania sedge Northern dewberry Sickle-leaved golden-aster Lowbush blueberry Bayberry Virginia rose Huckleberry Toothed white-topped aster Sweet vernal grass The small differences in individual species observed between the Exclosure and Control plots could be indicative of slow shifts in vegetation cover as a result of the absence of deer browse, particularly an increase in the woody dewberry and the clonal Pennsylvania sedge. The observed in plant species in each plot over time are not strong enough to date to draw distinctive conclusions. The one trend we expected to document which has not yet occurred is the release or recruitment of more flowering plants as a result of the exclusion of deer browse. This can be due to a number of factors. First, this area has been intensively subjected to deer browse for numerous decades and a seed bank of desirable forb species may no longer exist at Sanford Farm. A study of management and species composition differences between Ram Pasture and Sanford Farm in 2012 showed that there were significantly fewer sandplain grassland flowering plants in Sanford Farm (Karberg, Internal NCF Report, 2012). This difference was attributed to the effects of long term dormant season mowing, which does not appear to promote flowering plant persistence. A second reason for the lack of desirable forb recruitment within the exclosure area may be due to the deer exclosure only being in place for five years, which may not be enough time to allow successful recruitment and establishment of forb species. Vegetation Browse A main objective of this study was to determine what plant species are directly impacted by deer browse at a long-term managed grassland. Over the course of the study, browse was only observed on a few species, the majority of which were grasses with minor browse on flowering plants. Interestingly, the browse observed on grasses was similar between both the exclosure and the non-exclosure (Figure 3). This suggests that the majority of browse seen at Sanford Farm, at least in these open grasslands during the growing season, can be attributed to rabbit browse. Rabbits were not excluded by the exclosure. The only plant significantly more browsed outside of the exclosure (by deer) was Bushy aster, which was not abundantly present at the site.

7 PERCENT OF TOTAL BROWSED Browse Intensity within Both Large Plots South (Open) North (Exclosure) 0 Figure 3: Browse intensity within both large plots. The only species browsed more significantly outside of the exclosure (definitely by deer) was Bushy aster. All other species were browsed within and outside of the exclosure (presumably by rabbits) and included significant grass species. Broad Conclusions This preliminary analysis of vegetation composition within the deer browse exclosure at Sanford Farm indicated that deer browse is not having a significant impact on plant species composition within this long-term managed grassland. The browse that was observed at this site, primarily on grass species, can be attributed to rabbits, since the same level of browse was observed both within and outside of the deer exclosure. The lack of change in vegetation composition within the deer exclosure may be due to the fact that there is a depauperate seed bank of desirable flowering species at this site in Sanford Farm. This conclusion is supported by property monitoring conducted at Sanford Farm and Ram Pasture in 2012, which indicated a much lower occurrence of desirable flowering plants in the annually, dormant-season mown Sanford Farm when compared to adjacent Ram Pasture which has experienced varied mowing and prescribed fire treatment. Future Research Significant staff time and resources went into the development and maintenance of the deer browse fence and this study design. Given the significant findings of this study, there are more interesting and useful management questions that can now be asked using the study design already in place.

8 Of highest interest and importance would be looking at the impacts of seed and/or transplant addition of desirable flowering plant species both within and outside of the deer exclosure. Strong interest exists in maintaining and enhancing current sandplain grasslands on Nantucket. This location would provide an ideal location to determine if seed and/or transplant addition of desirable flowering plant species to a low diversity grassland will provide an opportunity to enhance this habitat. Although deer browse was not significant in this current study that was likely due to a lack of nutritious flowering plants. In other areas of the island, flowering forbs have been observed as being highly browsed. Establishing a flowering plant restoration project within the deer exclosure may help us understand if and how deer browse may impact this kind of restoration. Utilizing the deer browse exclosure will allow us to understand the potential success of such a restoration effort without the added expense and staff time of performing large scale seeding and transplanting enhancement that may not be successful given deer densities on island. A study such as this will greatly inform Property Conservation Management Plans at Sanford Farm and other locations on Nantucket that contain this habitat type.