Seedling resistance, tolerance and escape from herbivores: insights from co‐dominant canopy tree species in a resource‐poor African rain forest

Abstract

Summary Although plants can reduce the impacts of herbivory in multiple ways, these defensive traits are often studied in isolation and an understanding of the resulting strategies is incomplete. In the study reported here, empirical evidence was simultaneously evaluated for the three main sets of traits available to plants: (i) resistance through constitutive leaf traits, (ii) tolerance to defoliation and (iii) escape in space, for three caesalpiniaceous tree species Microberlinia bisulcata, Tetraberlinia bifoliolata and T. korupensis, which co‐dominate groves within the lowland primary rain forest of Korup National Park (Cameroon). Mesh cages were placed around individual wild seedlings to exclude insect herbivores at 41 paired canopy gap and understorey locations. After following seedling growth and survival for c. 2 years, caged and control treatments were removed, leaves harvested to determine nutrient and phenolic concentrations, leaf mass per area estimated, and seedling performance in gaps followed for a further c. 2 years to quantify tolerance to the leaf harvesting. The more nutrient‐rich leaves of the weakly shade‐tolerant M. bisulcata were damaged much more in gaps than the two strongly shade‐tolerant Tetraberlinia species, which had higher leaf mass per area and concentrations of total phenols. Conversely, the faster‐growing M. bisulcata was better able to tolerate defoliation in terms of height growth (reflushing capacity), but not at maintaining overall leaf numbers, than the other two species. Across gaps, insect‐mediated Janzen–Connell effects were most pronounced for M. bisulcata, less so for T. korupensis, and not detectable for T. bifoliolata. The three species differed distinctly in their secondary metabolic profiles. Taken together, the results suggested a conceptual framework linking the three sets of traits, one in which the three co‐dominant species adopt different strategies towards herbivore pressure depending on their different responses to light availability. This study is one of the first in a natural forest ecosystem to examine resistance to, tolerance of, and escape from herbivory among a group of co‐occurring tropical tree species.