Deer pressure in suburban/exurban forests and its relation to deer browse on invasive plants

Background/Question/Methods

Suburban/exurban forest herb layers are depauperate in native plants but rich in populations of non-native, invasive plants - along with intensive herbivore pressure from deer. Deer may facilitate invasion by preferring natives, but in these forests non-natives may become the most abundant food available to deer, thus changing their ecological role. We investigated whether deer feed on invasive species, and if it is related to the intensity of deer pressure. Accurately measuring deer density/impact is challenging in small forests embedded in a human-dominated landscape, so we first compared three measures of deer pressure, in six suburban/exurban forests. We estimated density by surveying pellet group accumulation in 15 112-m² plots per forest. We measured current browse on native woody twigs along three 100-m transects in each forest, and we measured chronic deer pressure with visual assessments of vertical percent cover of native shrubs, at 40 points per forest. Next, we surveyed 12 forests for abundance of and browse on the  invasives garlic mustard (Alliaria petiolata, ALPE) and Japanese stilt-grass (Microstegium vimineum, MIVI). In each forest we made nine 10-minute visual scans for browse along deer and human paths  in stands of ALPE and MIVI, and assessed deer pressure via shrub cover.   

Results/Conclusions

Measures of shrub cover and browse on twigs showed similar patterns of deer pressure; the three forests with little shrub cover generally had greater browse than the three forests with greater shrub cover. However, the pellet group survey mostly indicated higher deer density in forests with lower deer pressure. This suggests either that deer tend to avoid the open understory of highly browsed forests but, when present, browse whatever food plants are still available at a high rate, or that the pellet method does not accurately capture deer density. Both ALPE and MIVI showed signs of deer browse in the 12 forests studied Sept.-Nov., 2011. Of the 64 paths with ALPE, 17% showed browse on ALPE, and 23% of the 97 paths with MIVI showed browse on MIVI. Chronic deer pressure was positively correlated with both MIVI abundance (P=0.05, n=12) and the mean number of browsed MIVI plants observed per scan (P=0.06), but was not correlated to ALPE abundance or browse. Deer have potential to be important actors in the ecology of invasive plant species in highly deer-impacted and invaded suburban/exurban forests, through direct effects of herbivory, and in this case deer pressure influenced MIVI more than ALPE.