The ongoing epidemic of bark beetles (BB) and their associated xylem blocking blue-stain (BS) fungi is unprecedented in Rocky Mountain subalpine forests. Besides the mountain pine BB (Dendroctonus ponderosae) and associated BS (Ophiostoma spp.) in lodgepole pine (Pinus contorta); high rates of infection are observed by related BB and BS in Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). As this epidemic continues, we have two main questions 1) what is the first order effect of BB and BS on the temporal and spatial extent of evapotranspiration and 2) what are the higher order impacts on greenhouse gases (GHG) emissions such as carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4)? To answer these questions, our studies occur in forests composed of lodgepole pine at ~2750 m and spruce-fir located at ~3,200 m elevation. Measurements include eddy covariance, tree sap flux and soil GHG (lodgepole pine forest only). Two sampling strategies are used: 1) monitoring net ecosystem CO2 and H2O exchange as the outbreak unfolds in both forest types, and, (2) monitoring additional soil and vegetation parameters in lodgepole pine stands of known history of BB and BS infestation.
Results/Conclusions
The first order impacts of the beetles and fungi on the spruce-fir forest are slow taking two years to kill the trees likely due to the cooler, high elevation location. Lodgepole pine forests experience over 50 % reduction in transpiration rates per tree within the first month following the infection before the needles turn red and the sapwood is stained blue. The higher order effects in lodgepole pine are equally dramatic starting with a seasonal soil moisture increase of up to 100%. In addition, soils beneath the mature stand infested for two years respired 50% less CO2, assimilated 63% less CH4, and produced 133% more N2O compared to a regenerating stand at the sapling phase. These ecosystem responses indicate decoupling of tight terrestrial C and N cycling. Although the ecosystem level impacts are not as dramatic when scaled up to the eddy covariance footprint, which still includes uninfested stands, our stand level measurements allow predictions of near-future larger scale impacts. Further investigation will test scaling from plot to ecosystem using stable isotopes of water sources and vapor, and investigate the subsequent impact of forest succession on water cycling and GHG emissions.
