Predicting the spatial distribution of defoliator insect outbreaks in Patagonian Nothofagus forests

PARITSIS, J, J SMITH, TT VEBLEN

Boulder, CO, EE.UU.

Simposio; Building Community through Ecology and Evolution; 2009

Department of Ecology and Evolutionary Biology, University of Colorado, EE.UU:

In the Patagonian Nothofagus forests severe one-season defoliations caused by Ormiscodes spp. moths (Saturniidae) have been recorded during the last ca. 30 years but spatially explicit studies are lacking. Investigating the influences of environmental variables on the spatial occurrence of insect outbreaks is needed to understand their spatial dynamics and identify susceptible areas. Using Landsat TM images and ancillary data we analyzed the influences of key environmental variables (vegetation, topography, and precipitation) on the spatial occurrence of a severe and extensive Ormiscodes outbreak that occurred in 1986 in northwestern Patagonia. In addition, we conducted a binary logistic regression to generate a map of probabilities of outbreak occurrence. Our results show that the higher elevation forests of Nothofagus pumilio were proportionally more defoliated during the outbreak than lower elevation forests of Nothofagus species. Additionally, stands located at intermediate to high elevations and on gentle slopes were also more defoliated than expected. Finally, stands growing on areas with intermediate precipitation, relative to the distribution of N. pumilio along the precipitation gradient, were proportionally more defoliated than drier and moister areas. We provide preliminary ecological interpretations of our results that offer insights into the spatial dynamics of Ormiscodes defoliations in Patagonia.Nothofagus forests severe one-season defoliations caused by Ormiscodes spp. moths (Saturniidae) have been recorded during the last ca. 30 years but spatially explicit studies are lacking. Investigating the influences of environmental variables on the spatial occurrence of insect outbreaks is needed to understand their spatial dynamics and identify susceptible areas. Using Landsat TM images and ancillary data we analyzed the influences of key environmental variables (vegetation, topography, and precipitation) on the spatial occurrence of a severe and extensive Ormiscodes outbreak that occurred in 1986 in northwestern Patagonia. In addition, we conducted a binary logistic regression to generate a map of probabilities of outbreak occurrence. Our results show that the higher elevation forests of Nothofagus pumilio were proportionally more defoliated during the outbreak than lower elevation forests of Nothofagus species. Additionally, stands located at intermediate to high elevations and on gentle slopes were also more defoliated than expected. Finally, stands growing on areas with intermediate precipitation, relative to the distribution of N. pumilio along the precipitation gradient, were proportionally more defoliated than drier and moister areas. We provide preliminary ecological interpretations of our results that offer insights into the spatial dynamics of Ormiscodes defoliations in Patagonia.