Ontogenetic changes in tree insect herbivory and foliar traits in dry and wet southern beech Patagonian forests

MAZÍA, C. N.; GARIBALDI, L. A.; CHANETON, E. J.; KITZBERGER, T.

San Carlos de Bariloche

Congreso; VI Southern Connection Congress; 2010

As plants pass through successive life stages, marked changes occur in apparency, structural complexity, growth rate and nutritional quality, which can all influence plant susceptibility to herbivory. Several models were proposed to account for predictable herbivory patterns with plant ontogeny. Based on different assumptions, existing models make contrasting predictions encompassing linear and non-linear herbivore responses to composite changes in host apparency, nutrient content and defence levels among age-size classes. Yet, there have been few tests of the effect of ontogeny on herbivory, especially in forest ecosystems. We examined leaf damage by different insect guilds for three age-size classes of Nothofagus pumilio trees, in dry (800 mm/yr) and wet (3000 mm/yr) forests of northern Patagonia, Argentina. In each forest, we sampled the canopy of saplings (< 1m tall), juveniles (3-4 m) and adult trees (>10 m), and measured physical and chemical leaf traits for each plant (n = 14). Early in the growing season, leaf area damaged was higher in saplings and juveniles than in adult trees in both forests, and remained so later in the season in the wet forest. In contrast, in the dry forest, insect damage markedly increased towards the end of the season on adult trees, which blurred earlier differences among age classes. There were significant, habitat-specific differences in foliar traits among tree classes, but these could not account for herbivory patterns neither in the wet nor in the dry forest. Overall, we found partial support for the hypothesis that insect herbivory declines as trees grow from a deep understorey layer into the upper forest canopy. However, ontogenetic changes in tree folivory appeared to depend on the forest habitat.