Tree and shrub species in suburban forest understories may be subject to chronic herbivory from overabundant white-tailed deer. An undocumented potential consequence of this pressure may be shifts in secondary metabolite production associated with defense and stress responses, with relevance for plant communities if resource allocation patterns also shift, and if responses differ among species, e.g. natives and invasive non-natives. We aimed to learn whether several species that had been protected from deer exhibited different metabolomic profiles compared to those exposed to deer. Leaf tissue was collected in a suburban New Jersey forest on 27 July 2018, in 6-7 unfenced plots and in 6-7 plots fenced since 2013 (three plants/plot/species). Presence/absence of current deer browse was noted for unfenced plants. Tissue was immediately frozen in liquid nitrogen and kept frozen at -80oC until extraction. It was ground to fine powder in liquid nitrogen, vortex-mixed with cold chloroform:methanol:water (1:2:1) buffer (20 minutes), and centrifuged at 15000g (20 minutes). The supernatant was lyophilized until dry and resuspended in 70% methanol. After centrifugation, the supernatant was used for untargeted metabolomic analysis by LC-MS/MS. Spectral data were converted to mzXML files, pre-processed using XCMS and CAMERA packages, and statistical analyses were performed on Metaboanalyst.
Results/Conclusions
We tested the native species Nyssa sylvatica and Lindera benzoin, and the invasive non-native Rosa multiflora. For all species, the metabolomic profiles were different between fenced vs. unfenced plots and for plants with and without current browse signs. In each case, the fenced and unfenced groups separated into distinct clusters both on the principal component analysis and partial least square discriminant analysis plots. Despite the inherent qualitative difference in metabolite accumulation among these species, we found responses predicted by common metabolic pathways that are affected in all plant species, including the trans-farnesyl diphosphate (FPP) biosynthesis pathway. Interestingly, the FPP pathway is known to play an important role in the synthesis of sequiterpenes and sterols, and affects jasmonic acid mediated defense responses. These results add to ecological knowledge of the important effects of high deer densities in forest plant communities; exposure to chronic deer browse appears to cause metabolome-wide changes in plants, including defense-related metabolites. Further study is now needed to determine how common these metabolomic shifts are among forest species, how the intensity of deer pressure may influence metabolomic responses, which metabolites are most affected, and what the ecological consequences are at the physiological, population, and community levels.