Mechanisms promoting diverse levels of plant resistance and tolerance in nature are poorly understood. We examine how spatial heterogeneity and successional changes in the environment influence resistance and tolerance trait levels in an old-field perennial, Canada goldenrod (Solidago canadensis). We quantified the spatial distributions of herbivore damage and putative resistance, tolerance, and fitness-related traits of goldenrod ramets within replicate early- and late-successional fields. Also, we characterized the local neighborhood (i.e., neighbor ramet density, canopy cover, vegetation cover) and edaphic conditions (soil moisture, pH, N) within 0.5 m of the target ramet, and determined the relationships between these environmental variables and goldenrod defense and fitness trait levels. Levels for all environmental variables and goldenrod defense and fitness traits were significantly higher in early-successional fields with the exception of canopy cover, damage, and specific leaf area (tolerance trait) which were significantly greater in late-successional fields. Defense trait levels were more similar among nearest neighbors in late- than in early-successional fields and were most strongly correlated with aspects of the local plant neighborhood. In late-successional fields, significant correlations occurred between specific leaf area and canopy cover and ramet density . In early-successional fields, there were no significant defense-neighbor correlations, but the relationship between vegetation cover and damage was significantly stronger than in late-successional fields. In both successional stages, ramet fitness traits were significantly correlated with defense traits. These data suggest that neighbor interactions (i.e., competition for resources or spatial associations) may strongly influence spatial defense patterns and may change throughout old-field succession. This study highlights the context dependence of plant defenses which may contribute to the maintenance of defense diversity in nature.
Keywords: neighbor effects, resistance, succession, Solidago canadensis, tolerance