Environmental context, species interactions, and functional traits can jointly determine invasion success or failure. One leading hypothesis posits that generalist herbivores mediate plant invasions through modifications to the abiotic and biotic environment. In eastern North American forests, white-tailed deer populations (Odocoileus virginianus) have surged well above historic densities, causing marked impacts to community structure and function. Interestingly, it has been recently appreciated that deer overabundance can be concomitant with increased population growth rates of invaders. However, the mechanisms behind deer-mediated invasions are poorly understood. Deer may facilitate plant invasion through biased foraging on natives (trophic effects) or act as ecosystem engineers to transform the abiotic understory environment that benefit invaders’ physiology (non-trophic effects). We utilized a long-term paired plot experiment in a deciduous forest in western PA that experienced recent increases in deer abundance and invasive spread of Alliaria petiolata. Comparing plant traits in the presence and absence of deer (control vs. exclusion plots), we tested 1) whether deer influence species’ leaf-level photosynthetic functions, 2) how these potential differences vary across seasons, and 3) if native and invasive species respond differently to deer impacts. Within exclusion plots we predict decreased physiological rates for Alliaria, but increases for the natives.
Alliaria showed pronounced ecophysiological differences with treatment. Mid-season (June) photosynthesis (i.e. light saturated photosynthetic rate, maximum carboxylation rate, electron transport rate) of Alliaria rosettes growing in deer access plots was significantly increased compared to rosettes in deer exclusion plots. These differences provide a mechanism for prior results that demonstrated that Alliaria populations at the site were actively spreading in deer access (λ>1) plots but declining (λ<1) in deer exclusion plots. Alliaria’s deer-mediated photosynthetic increases were expressed during months with significant leaf overstory canopy coverage (Jun-Aug, P<0.001), with no differences before overstory leaf out (May) or after canopy leaf senescence (Sept-Jan). Surprisingly, the native Maianthemum racemosum showed a similar mid-season trend (Jun-Aug), but photosynthesis did not continue into fall due to plant senescence. Another native, Trillium erectum showed no differences by treatment. Seasonal light availability (PAR) in the understory reached maximum values in control plots, suggesting deer modify understory light levels, in addition to other abiotic variables. PAR increases are likely driven by high herbivory rates on native understory species (can exceed 75% of native stems), while deer never consume Alliaria. Our results provide the first physiological evidence by which generalist herbivores alter the environment to facilitate plant invasion.