Effects of beetle kill on Colorado mountain stream ecosystems

In the past decade, bark beetles have caused widespread tree mortality across approximately 50 million hectares of coniferous forest from Alaska to Mexico. Tree mortality potentially could affect stream ecosystems through changes in solar irradiance, amount of runoff, concentrations of nitrogen and phosphorus fractions, quantity and quality of organic carbon, and periphyton biomass. In 2009 a spatially distributed study (July-October) was conducted on 53 watersheds, and in 2010 a seasonal study (April-October) was conducted on 19 watersheds. Both sets of studies were designed to show the effects of watershed characteristics (elevation, slope, aspect, runoff, geographic location, and beetle induced tree mortality) on stream characteristics. The watersheds ranged from non-detectable to ~60% tree mortality (90% mortality of large trees).;Numerous watershed studies from the literature indicate that forest disturbances (e.g., logging, extreme storms) affect biogeochemistry. For example, Rhoades et al. (2013) showed that nitrate export increases after a disturbance in proportion (∼ 400%) to nitrate export prior to a disturbance. The present study shows that the effects of an MPB infestation on stream ecosystems are minimal. Watershed characteristics other than tree mortality explain the variability of dissolved inorganic nitrogen (DIN) and phosphorus. Only small decreases in runoff during the early fall were detected in watersheds with high tree mortality. Mountain pine beetles attack lodgepole heterogeneously across time and space, and this, in conjunction with rapid forest regeneration in N-limited Colorado forests led to increased watershed uptake of nutrients following tree mortality. Thus, DIN and phosphorus export and runoff differed from studies of other kinds of disturbance. Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) did increase significantly during summer low flows, but not sufficiently to have significant ecological effects. Increases in DOC and DON likely stem from the decomposition of dead lodgepole needles and roots. Solar irradiance and periphyton biomass did not respond to increases in tree mortality because lodgepole are not the dominant species in the riparian zone. Because beetle kill, unlike harvesting or extreme storms, is not accompanied by physical damage to soils or subcanopy vegetation, the watershed compensatory responses protect streams from biogeochemical changes.