Black locust (Robinia pseudoacacia): The effect of secondary plant chemistry on insect herbivores

The nitrogen fixing, pioneer tree species, Robinia pseudoacacia, was studied under both experimental and natural conditions to evaluate the changes in levels of carbon based secondary metabolites. The experimental aspect utilized a plantation of Robinia in Greene County, Georgia. The objective was to examine the effects of three treatments (nitrogen fertilization, irrigation and a combination of both fertilization and irrigation). In the second aspect of the study herbivory and carbon-based secondary chemistry of Black locust was examined in different aged openings in the southern Appalachians.;Nitrogen and water ammendments had little effect on the phenolic levels of Robinia. A nitrogen fixing species would not be expected to respond to nutrient and water ammendments since the physiology of nitrogen and water uptake is closely linked to the uptake of carbon through the energy provided by photosynthesis. The examination of levels of phenolics in seven tree tissues provided a unique data set on whole tree distribution of secondary chemicals. The phenolic levels in the leaves were a magnitude higher than levels in the bark, bole, branches, fine and main roots.;The secondary chemistry and herbivory from Robinia leaf and bole tissue exhibited significant changes in different successional openings (3-30 yrs). Levels of total phenolics and condensed tannins in locust leaves decreased until mid-succession (11-13 yrs) after which they gradually increased. In contrast, leaf hydrolyzable tannin concentrations increased significantly during early succession, were reduced from 13-17 yr and then rose again in later successional stages. Bole phenolics generally exhibited mid-successional peaks (8-11 yr) before gradually increasing later in succession (13-30 yr). Sun leaves from Robinia had significantly higher levels of phenolic chemistry and lower leaf area damaged by insect herbivores than did shaded leaves from the same trees. Insect leaf herbivory and locust stem borer attacks peaked between 8-13 years.;Robinia leaves generally contain their lowest levels of secondary chemicals and experience their highest herbivory during mid-succession (years 8-13). The mid-successional years (8-13 yr) are when black locust is becoming crown closed (40% of the trees), N-fixation is reduced and the trees experience natural thinning due to shading. Trees under these multiple stress factors have reduced photosynthesis due to reduced leaf area resulting from shading. As the tree's carbon fixation declines, the energy costly nitrogen fixing bacteria (Rhizobium) cannot be supported by Robinia and nitrogen fixation declines. Insect herbivory is highest during mid-succession which further reduces the critical leaf area. Locust trees enter a death spiral at this time when additional stresses, such as the locust stem borer accelerate the self-thinning process and cause the death of large numbers of locust trees.