Long-term climate influence on insect folivory in northern Patagonia forests, Argentina

Background/Question/Methods

Climate change has the potential to modify trophic interactions through changes in the abundance of resident species, creation of new niches, and novel migrations. In particular, plant-insect herbivore interactions are predicted to become stronger under projected environmental warming.

Insects are the dominant herbivores in forest ecosystems and several studies have shown the role of climatic variation in driving insect outbreaks. Few studies, in contrast, have focused on the influence of interannual climate fluctuation on endemic herbivory rates. This is surprising given that leaf area loss to insects represents a chronic stress factor and a potentially important source of forest productivity loss.

Forest ecosystem dynamics in the Patagonian Andes have been found to be highly sensitive to large-scale drivers of climatic variation (e.g. ENSO oscillations). Here we explore the potential relationship between year-to-year variation in the severity of insect folivory and climate conditions for Nothofagus pumilio forests of northern Patagonia, Argentina. We analyzed a 10-yr dataset of percentage leaf area damaged by different feeding guilds collected annually in dry/warm and wet/cold forest habitats. Through multiple regression analysis, we identified sets of temperature and rainfall variables that better explained interannual changes in folivory rates by dominant guilds.

Results/Conclusions

On average, folivory was an order of magnitude higher in dry than in wet forest stands (leaf area damage: 22% vs 2%, respectively). Damage by leaf miners, the dominant guild in dry forests, was best explained by the positive effects of mean spring temperature (slope: 0.53) and annual cumulative number of rainy days (slope: 0.77) (R² = 0.49, P = 0.003). Herbivory by leaf-tying insects, the other major guild in dry forests, increased with both mean spring (slope: 0.60) and summer temperatures (slope: 0.62) (R² = 0.81, P = 0.0001). In wet forests, consumption by the dominant leaf-chewer guild was positively associated with cumulative degree-days in early spring (slope: 0.63) and spring precipitation (slope: 0.78) (R²  = 0.58, P = 0.005).

Overall, these results indicate that annual changes in the thermal and hydric environments were both recent important drivers of insect herbivory in these forests. Our analyses broadly suggest an increase of insect folivory in dry and wet forest habitats under predicted scenarios of global warming for the Patagonian Andes. Yet, climate model predictions for decreased annual rainfall in northern Patagonia add an element of uncertainty with respect to the impact of future climatic changes on insect demography and forest canopy herbivory.