Long-Term Impacts from Mountain Pine Beetle Outbreaks on Merchantable Biomass, Ecosystem Carbon, Surface Albedo, and Net Radiative Forcing: The Major Role of Vegetation Response

English Abstract:

Although field-based studies have highlighted the role of non-target vegetation following mountain pine beetle (MPB) outbreaks, the range of possible growth release responses has not been systematically assessed in model-based studies. The potential for the spatial distribution of mortality to modulate by itself the effects from MPB outbreaks has also been overlooked heretofore, while only one study has combined the albedo-induced cooling with the CO₂-based temperature change in order to estimate the net impact of MPB on the global climate. And even if MPB outbreaks have been recurring for millennia in western North American forests, previous studies have often spanned a few decades or less, and have never gone beyond two centuries. The main objective of our study was to contribute to the understanding of MPB consequences on forestry, ecosystem carbon cycling, and climate by modifying an existing model that computes land–atmosphere exchanges of carbon, energy, and water, and simulates dynamic changes in vegetation state and distribution. We performed various long-term simulations over different locations in British Columbia, Canada, in order to address the issues identified above and a few additional ones. We found that the growth release from the non-target vegetation can indeed play a dominant role in the strength, and even the direction, of MPB-induced changes. We also observed strong non-linearities in MPB mean effects for different outbreak severities or return intervals, along with major changes in these effects through time. Our results also support the idea that the spatial distribution of mortality may explain part of the high variability in MPB effects revealed by satellite-based studies. All these complicating factors imply that extrapolating the knowledge gained from short-term or localized studies to larger spatiotemporal scales might be misleading, so that the actual impact of the ongoing MPB outbreak cannot be estimated with confidence yet over its entire domain. For example, our results suggest that the feedback to climate change from the current MPB outbreak over British Columbia could be positive or negative. Despite this uncertainty, a simple analysis nevertheless shows that the magnitude of this impact is probably much smaller than one month of current anthropogenic CO₂ emissions.