Interactive effects of deer and non-native plant invasion on the herb layer of suburban forests
Forest plant communities in the suburban landscape face joint challenges from overabundant deer and invasion by multiple non-native plant species. Deer can have direct negative effects on plants in the herb layer by herbivory and trampling, and indirect negative effects by facilitating the invasion of non-natives, if deer avoid them as food. We studied change in species richness in New Jersey, with a factorial experiment in three suburban forests with high deer pressure. We combined deer treatments (excluded or not) with addition treatments of two common forest invaders (Microstegium vimineum, MIVI; Alliaria petiolata, ALPE; both; or neither) in a total of 103 4x4 m plots. The experiment is conducted by one professor and changing teams of 4-8 undergraduates. The students begin with no knowledge of the flora. Accurate identifications must be done non-destructively and rapidly, so that the census is completed in a short time frame. We developed a training method that builds from field teaching by the professor, to peer teaching, to peer pairs. We rely heavily on smart phone technology, which allows students in different locations to check an identification in real time with the professor, or to make a record of an unknown species for later identification when the professor is at the plot. The students become adept at identifying most species (over 130).
Species richness increased by an average of two species per plot from Fall 2012 to Fall 2014. Deer exclusion from March 2013 - October 2014 had no significant effects on species richness of native herbaceous plants, native woody plants, or non-native woody plants. ALPE addition resulted in sparse establishment, while MIVI established dense stands in many plots. Species richness of both native herbs and native woody species (tree and shrub species) increased by 2-3 species on average in plots without MIVI added, but very little with MIVI added (ANOVA, P<0.0001 for both herbs and woody), regardless of fencing (fencing x addition, NS). Species richness of non-native woody plants (mostly shrubs) was unaffected by any of the invasive addition treatments, but increased more (by 1 species) in the forest that started out with the most species (P<0.0001). Bottom-up suppression by MIVI invasion of the natural increase in native species richness contrasts with the lack of any top-down influence by overabundant deer, especially for woody species, which are subject to deer browse all year.