Environmental heterogeneity and spatiotemporal variability in plant defense traits
At the scale of the local plant community, we know very little about how spatial and temporal environmental heterogeneity affects the diversity in types and levels of plant defenses. If environmental heterogeneity is an important mechanism influencing plant defense traits, then defense expression should co-vary spatially with environmental conditions and change as succession progresses. In this study, we examined how spatial heterogeneity and succession influence putative resistance and tolerance trait levels in late goldenrod Solidago altissima. We quantified the spatial distributions of herbivore damage and traits associated with resistance (leaf toughness, phenolics), tolerance (specific leaf area, relative growth rate, leaf addition rate and leaf senescence rate), and fitness (height, diameter, inflorescence biomass) of goldenrods within replicate early- and late-successional fields. Also, we characterized the local neighborhood (stem density, canopy cover, ground vegetative cover) and edaphic conditions (soil moisture, pH, N) surrounding each target ramet, and determined relationships between these environmental variables and goldenrod trait levels. The distribution of traits within fields was strongly non-random, and defense-trait levels were more strongly spatially structured (i.e. autocorrelated) in late- than in early-successional fields. Also, defense traits were most strongly correlated with aspects of the local plant neighborhood, and these relationships differed in important ways between successional stages. In late-successional fields, tolerance trait specific leaf area was positively correlated with canopy cover and negatively correlated with stem density. In early-successional fields, the relationship between ground vegetative cover and resistance (i.e. 1 - damage) was significantly stronger than in late-successional fields. A novel insight from this study is the possibility that changes in the biotic environment during succession may shift the expression of defense from a resistance to a tolerance strategy in our system. This study highlights the context dependence of plant defense trait levels, which may promote their spatial and temporal variability in heterogeneous landscapes. © 2011 The Authors. Journal compilation © 2011 Oikos.