Contrasting response of high and mid elevation Rocky Mountain conifer forests to a massive bark beetle outbreak: The role of plant hydraulics, spatial variability and successional pathways
Bark beetles have increased tree mortality across western North America due to past land use interacting with climate change. Mountain pine and spruce beetles have attacked mid-elevation lodgepole pine and higher elevation Engelmann spruce dominated forests, respectively. The mechanism of mortality is hydraulic failure due to xylem occlusion by beetle-carried blue stain fungi, which causes the trees to die from symptoms that are the same as extreme drought. As the mortality event began in the late 90s to mid-2000s, the hypothesized effects on ecosystem processes and services were 1) lower water use from xylem occlusion, 2) less carbon uptake from limited canopy gas exchange, 3) increased nitrogen cycling from increased litterfall and soil moisture and 4) increased streamflow and organic N and C loading at the watershed scale from the first three consequences.
In many cases the stand-scale effects were as predicted with transpiration falling by 10-35% in proportion to the occluded xylem, carbon uptake declining by > 50% due to lack of canopy gas exchange and nitrogen cycling increasing from elevated litter inputs and stimulated organic matter decomposition. Some stands, especially mid-elevation lodgepole pine did not follow these trends because of residual vegetation taking advantage of the increased resources from the dead trees and rapid succession of new grasses, shrubs and tree seedlings. In these stands, the overall water and carbon exchange to the atmosphere was not reduced at the stand scale and nitrogen cycle stimulations were balanced by gaseous N losses. The succession of new vegetation was taking place at the same time as the trees were dieing. In high elevation spruce stand, the recovery was more rapid than expected with many ecosystem processes returned to pre-mortality levels within several years. At the scale of small to medium-sized watersheds, the impact of mortality was not detectable in stream flow due to the spatial and temporal scale muting of the mortality signal as time and spatial scales increase. The net result is that the dramatic visual impact of bark beetle mortality is not reflected directly in ecosystem processes due to compensating mechanisms. Current ecosystem and watershed models miss these compensating mechanisms with increasing scale and thus over predict the impact of bark beetle mortality.