Insect defoliator outbreaks and environmental heterogeneity in Nothofagus forests in the Patagonian Andes

Recent changes in the spatiotemporal patterns of herbivorous insect outbreaks have stimulated considerable public concern and research activity in the context of potential ecological impacts of global warming trends. While changes in insect outbreak dynamics appear to be occurring worldwide, research has been mainly focused on northern hemisphere temperate and boreal forests. In the temperate forests of the southern Andes, Nothofagus pumilio, one of the dominant tree species of the region, experiences severe defoliation caused by caterpillars of the Ormiscodes genus (Lepidoptera: Saturniidae). This study examines the spatial and temporal patterns of Ormiscodes outbreaks in relation to regional scale climatic variability and in the context of the spatial heterogeneity of the environment. My research approach combines dendroecological reconstructions of outbreaks on Nothofagus pumilio forests with the identification and analysis of spatial patterns of outbreaks at the landscape scale using remote sensing and Geographic Information Systems (GIS), as well as a laboratory experiment on environmental influences on Ormiscodes amphimone performance and consumption rate. This research was conducted in two disjunct study areas, distinguished by climatic differences, in the Patagonian Andes at c. 41°S (northern Patagonia) and 49°S (southern Patagonia). While in northern Patagonia the frequency of Ormiscodes outbreaks since the late 1970s is apparently not unprecedented compared to the previous c. 130 years, in southern Patagonia outbreak frequency increased significantly in the late 20th century. I found significant associations between outbreaks and variability in the Southern Annular Mode (SAM), the El Nino-Southern Oscillation (ENSO), and phase combinations of these climate modes. Although relationships between Ormiscodes outbreaks and climate proved to be complex, defoliation events are generally associated with drier and warmer than average growing seasons. These associations suggest that the positive trend in the SAM (linked to regional warming) and the increased incidence of ENSO (opposite phases linked to warm and dry conditions) episodes predicted for the 21st century will likely promote outbreaks in Patagonia. However, these probable climatic influences on outbreak occurrence are contingent on the sensitivity of Nothofagus and Ormiscodes to temperature and precipitation along environmental gradients. Spatially, forests of N. pumilio are more susceptible to Ormiscodes outbreaks than lower elevation forests of other Nothofagus species (N. dombeyi or N. antarctica). Additionally, stands located at intermediate elevations and on gentle slopes (< 15°) are also more susceptible to defoliation than higher and lower elevation stands located on high angle slopes. Finally, stands growing in areas with intermediate to high precipitation, relative to the distribution of Nothofagus along the precipitation gradient, are more susceptible to Ormiscodes attack than are drier areas. Laboratory experiments with O. amphimone larvae indicate that higher temperature and foliage from mesic N. pumilio stands favor O. amphimone performance and consumption rate, which is consistent with the spatiotemporal patterns documented at broader scales. Overall, this research constitutes the first systematic study examining the associations of insect outbreaks with environmental heterogeneity at multiple scales in the Nothofagus forests of Patagonia. In addition, this work improves a preliminary understanding of how climate variability affects complex ecological processes involving herbivorous insect impacts on forest productivity and other ecological processes in Patagonian Nothofagus forests.