Spatial variation in plant resistance and tolerance in successional habitats

Plants can evolve resistance traits to reduce herbivore damage and/or tolerance traits to minimize the negative fitness effects of herbivore damage.  If plant defense against herbivory is influenced by aspects of the environment, then resistance and tolerance traits should be non-randomly distributed within heterogeneous habitats.  For long-lived perennial plants, the environment and thus selection pressures for resistance and tolerance to herbivory may also change during succession.  Studies examining the patterns, causes and consequences of spatial variation in resistance and tolerance within habitats are currently lacking.  We quantified the spatial distribution of Solidago canadensis resistance, tolerance, and fitness traits, and herbivore damage within three early- and three late-successional fields and identified environmental variables that may influence distribution patterns.  In late-successional fields, the majority of environmental variables (e.g., neighboring plant composition, light availability, and soil moisture) and putative defensive traits (e.g., leaf toughness, specific leaf area, and relative growth rate) were strongly spatially autocorrelated.  In early-successional fields, these traits were significantly more random in their distributions.  In both early- and late-successional fields, the composition of neighboring plants had a positive effect on herbivore damage and defense traits, and a negative effect on plant fitness.  These data suggest that plant stress due to competition with neighboring species may be a primary determinant of the within-field distribution of plant defenses.