Mountain pine beetle epidemic changes ecosystem flux controls of lodgepole pine

Background/Question/Methods

Disturbances to an ecosystem allow tests of our understanding of processes that control biogeochemical cycling. An ongoing mountain pine beetle and associated xylem-blocking blue-stain fungi epidemic is causing widespread, yet spatially heterogeneous mortality of mature trees in conifer forests across the mountains of western North America and this mortality is changing controls over carbon, water and energy fluxes. Previous work has shown large changes to the energy budgets of these ecosystems with a biological flux control of stomata component remaining unlike more well-studied fire and clear-cutting disturbances with no significant plant canopy remaining. In this disturbance surviving trees leave a heterogeneous canopy and surviving trees are still actively photosynthesizing. This work will build on those results by using eddy covariance data from three (2009-2011) growing seasons. The study site in the Medicine Bow Mountains in Southeast Wyoming had first records of beetles in 2007 and mortality is currently near 80% of lodgepole pine trees in the tower footprint. We use standard atmospheric boundary layer measurements to monitor carbon, water and energy fluxes to quantify the changing controls of those fluxes. We hypothesize that controls of ecosystem fluxes will transition from a biologically controlled system, i.e. canopy conductance dominated by stomata and respiration dominated by autotrophic, to a mixed biologically and physically controlled state during the outbreak and that regulation of ecosystem fluxes by canopy conductance decreases with increasing tree mortality in the tower footprint.

Results/Conclusions

Our data shows that energy budget closure was 64% in 2009, 59% in 2010, and 74% in 2011. Low energy balance closure was associated with heterogeneity of live canopy in the middle of the beetle outbreak. A_max (μmole m−2 s−1) extracted from a a Michaelis Menten light response curve was 19.5 in 2009 and decreased further to 19.2 and 17.6 in 2010 and 2011 respectively. The water use efficiency of the ecosystem declined from 2.6 to 2.1 (water μmole/ carbon μmole) throughout the outbreak as stomatal control of carbon and water flux declined and evaporation became more important. Ecosystem models that attempt to capture the change in carbon, water and energy fluxes during ecological disturbances such as the ongoing bark beetle outbreak must account for changing fluxes and controls as a function of mortality in the ecosystem. Our work shows that predictions of ecosystem carbon, water and energy fluxes must account for canopy heterogeneity caused by the mortality disturbance from insects common to these montane forests.