Ormiscodes amphimone Outbreak Frequency Increased Since 2000 in Subantarctic Nothofagus pumilio Forests of Chilean Patagonia

Insect outbreaks are among the largest disturbance affecting forest health, and as a consequence of global warming, their frequency can increase and their impact becomes more severe. In the southern tip of South America, massive outbreaks of the native moth Ormiscodes amphimone (Lepidoptera: Hemileucinae) have defoliated large areas of subantarctic Nothofagus pumilio forests. In 2015, the largest Ormiscodes defoliation was documented in the Southern Hemisphere in the valley of El Furioso river (Aysén Region, Chile, 46.8°S). Here, we combined tree-ring and remote sensing analysis to understand the impact of Ormiscodes outbreaks in the N. pumilio forests of this valley. We used MODIS to calculate the Enhanced Vegetation Index (EVI) to detect defoliations and to sample areas where defoliation anomalies were highly frequent (>5 anomalies) and infrequent (<5 anomalies). We developed tree-ring chronologies for each of these areas, and using a hierarchical approach, we reconstructed Ormiscodes outbreaks since 1900 in the valley. According to the EVI anomalies analysis, other outbreak events were evident in 2008 and 2011, but smaller in spatial extent than the 2015 outbreak. Using a tree-ring analysis, we confirmed these outbreaks and found that they have increased in frequency during the last decade, with four events since 2000 compared to three events between 1949 and 2000. Prior to 1949, we did not find a discernible growth or anatomical pattern that could be inferred as an outbreak event. An unprecedented, strong reduction in radial growth was evident since 2000 in the host chronology due to Ormiscodes defoliation closely resembling the steady increase in monthly maximum temperature in the study area. The patterns documented here affecting a natural forest by a native insect species inform on how climate change is disrupting natural biotic interactions, with consequences we do not fully understand on forest dynamics.