A Tale of Two Forests: Simulating Contrasting Lodgepole Pine and Spruce Forest Water and Carbon Fluxes Following Mortality from Bark Beetles

Abstract:

In recent decades, bark beetle infestation in western North America has reached epidemic levels. The resulting widespread forest mortality may have profound effects on present and future water and carbon cycling with potential negative consequences to a region that relies on water from montane and subalpine watersheds. We simulated stand-level ecosystem fluxes of water and carbon at two bark beetle-attacked conifer forests in southeast Wyoming, USA. The lower elevation site dominated by lodgepole pine (Pinus contorta) was attacked by mountain pine beetle (Dendroctonus ponderosae) during 2008-2010. The high elevation Engelmann spruce (Picea engelmannii) dominated site was attacked by the spruce beetle (Dendroctonus rufipennis) during roughly the same time period. Both beetle infestations resulted in >60% canopy mortality in the footprint of eddy covariance towers located at each site. However, carbon and water fluxes responses to mortality depended on the forest type. Using data collected at the sites, we scaled simulated plant hydraulic conductivity by either percent canopy mortality or loss of live tree basal area during infestation. We also simulated a case of no beetle attack. At the lodgepole site, the no-beetle model best fit the data and showed no significant change in growing season carbon flux and a 15% decrease in evapotranspiration (ET). However, at the spruce site, the simulation that tracked canopy loss agreed best with observations: carbon flux decreased by 72% and ET decreased by 31%. In the lodgepole stand, simulated soil water content agreed with spatially distributed measurements that were weighted to reflect overall mortality in the tower footprint. Although these two forest ecosystems are only 20 km apart, separated by less than 300m in elevation, and have been impacted by similar mortality agents, the associated changes in carbon and water cycling are significantly different. Beetle effects on hydrologic cycling were greatest at high elevation due to slow succession, which is the predominant regional water source from higher snowpack amounts. The surprising lack of bark beetle impacts at the lodgepole pine site may be due to faster than expected succession as suggested by vegetation measurements.