Plants in suburban forests of eastern North America face the dual
stressors of high white-tailed deer density and invasion by
nonindigenous plants. The combination of chronic deer herbivory and
strong competition from invasive plants could alter a plant’s stress-
and defense-related secondary chemistry, especially for long-lived
juvenile trees in the understory, but this has not been studied. We
measured foliar total antioxidants, phenolics, and flavonoids in
juveniles of two native trees, Fraxinus pennsylvanica (green ash)
and Fagus grandifolia (American beech), growing in six forests in
the suburban landscape of central New Jersey, USA. The trees grew in
experimental plots that had been subject for 2.5 years to factorial
treatments of deer access/exclosure X addition/no addition of the
nonindigenous invasive grass Microstegium vimineum (Japanese
stiltgrass). As other hypothesized drivers of plant secondary chemistry,
we also measured non-stiltgrass herb layer cover, light levels, and
water availability. Univariate mixed model analysis of the deer and
stiltgrass effects and multivariate structural equation modeling (SEM)
of all variables showed that both greater stiltgrass cover and greater
deer pressure induced antioxidants, phenolics, and flavonoids, with some
variation between species. Deer were generally the stronger factor, and
stiltgrass effects were most apparent at high stiltgrass density. SEM
also revealed that soil dryness directly increased the chemicals; deer
had additional positive, but indirect, effects via influence on the
soil; in beech PAR positively affected flavonoids; and herb layer cover
had no effect. Juvenile trees’ chemical defense/stress responses to deer
and invasive plants can be protective, but also could have a
physiological cost, with negative consequences for recruitment to the
canopy. Ecological implications for species and their communities will
depend on costs and benefits of stress/defense chemistry in the specific
environmental context, particularly with respect to invasive plant
competitiveness, extent of invasion, local deer density, and deer browse
preferences.
Effects of white-tailed deer and invasive plants on the herb layer of suburban forests
