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The mountain pine beetle (MPB) epidemic in western North America is generating growing concern associated with aesthetics, ecology, and forest and water resources. Given the substantial acreage of prematurely dying forests within Colorado and Wyoming (~two million acres in 2008), MPB infestations have the potential to significantly alter forest canopy, impacting several aspects of the local water and land‐energy cycle. Hydrologic processes that may be influenced include canopy interception of precipitation and radiation, snow accumulation, melt and sublimation, soil infiltration and evapotranspiration. To investigate the changing hydrologic and energy regimes associated with MPB infestations, we used an integrated hydrologic model coupled with a land surface model to incorporate physical processes related to energy at the land surface. This platform was used to model hillslope‐scale hydrology and land‐energy changes throughout the phases of MPB infestation through modification of the physical parameterisation that accounts for alteration of stomatal resistance and leaf area indices. Our results demonstrate that MPB infested watersheds will experience a decrease in evapotranspiration, an increase in snow accumulation accompanied by earlier and faster snowmelt and associated increases in runoff volume and timing. Impacts are similar to those projected under climate change, yet with a systematically higher snowpack. These results have implications for water resource management because of higher tendencies for flooding in the spring and drought in the summer. Copyright © 2011 John Wiley & Sons, Ltd.

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Unprecedented insect-induced tree mortality has been observed globally and while hydrologic and biogeochemical changes have been recorded, alterations to terrestrial microbial communities, which influence as well as respond to these shifts, are not well understood. The objective of this work was to better understand how bacterial communities are coupled to perturbations in biogeochemically-relevant soil physicochemical parameters resulting from beetle-induced tree death. To this end, soils beneath trees across a beetle-kill spectrum were contrasted in the central Rocky Mountains at both heavily impacted (Chimney, 85% mortality) and moderately impacted (Niwot, 18% mortality) field sites. Soil organic matter (OM), dissolved organic carbon (DOC) and dissolved organic nitrogen were all significantly altered when contrasting soils under healthy versus beetle-attacked trees at Chimney. Bacterial alpha diversity measurements were found to increase with tree death and beta diversity measures showed significant clustering with relation to tree phase. The site was characterized by a significantly higher relative abundance of bacterial clades under healthy trees that were correlated to OM and DOC concentrations. In contrast, compositional changes in soil bacterial communities and edaphic parameters associated with tree phase were not observed at the less impacted Niwot site. Our findings reveal a coupled response between shifts in organic carbon cycling and the bacterial assemblage as a result of large-scale, beetle-induced tree mortality with implications for heterotrophic respiration in near-surface soils and suggests a possible dependence on the level of forest mortality before manifestation.

The recent bark beetle epidemic across western North America may impact water quality as a result of elevated organic carbon release and hydrologic shifts associated with extensive tree dieback. Analysis of quarterly municipal monitoring data from 2004 to 2014 with discretization of six water treatment facilities in the Rocky Mountains by extent of beetle impact revealed a significant increasing trend in total organic carbon (TOC) and total trihalomethane (TTHM) production within high (≳50% areal infestation) beetle-impacted watersheds while no or insignificant trends were found in watersheds with lower impact levels. Alarmingly, the TTHM concentration trend in the high impact sites exceeded regulatory maximum contaminant levels during the most recent two years of analysis (2013-14). To evaluate seasonal differences, explore the interplay of water quality and hydrologic processes, and eliminate variability associated with municipal reporting, these treatment facilities were targeted for more detailed surface water sampling and characterization. Surface water samples collected from high impact watersheds exhibited significantly higher TOC, aromatic signatures, and disinfection byproduct (DBP) formation potential than watersheds with lower infestation levels. Spectroscopic analyses of surface water samples indicated that these heightened DBP precursor levels are a function of both elevated TOC loading and increased aromatic character. This association was heightened during precipitation and runoff events in high impact sites, supporting the hypothesis that altered hydrologic flow paths resulting from tree mortality mobilize organic carbon and elevate DBP formation potential for several months after runoff ceases. The historical trends found here likely underestimate the full extent of TTHM shifts due to monitoring biases with the extended seasonal release of DBP precursors increasing the potential for human exposure. Collectively, our analysis suggests that while water quality impacts continue to rise nearly one decade after infestation, significant increases in TOC mobilization and DBP precursors are limited to watersheds that experience extensive tree mortality.

A global phenomenon of increasing bark beetle-induced tree mortality has heightened concerns regarding ecosystem response and biogeochemical implications. Here, we explore microbial dynamics under lodgepole pines through the analysis of bulk (16S rRNA gene) and potentially active (16S rRNA) communities to understand the terrestrial ecosystem responses that are associated with this form of large-scale tree mortality. We found that the relative abundances of bulk and potentially active taxa were correlated across taxonomic levels, but at lower levels, cladal differences became more apparent. Despite this correlation, there was a strong differentiation of community composition between bulk and potentially active taxa, with further clustering associated with the stages of tree mortality. Surprisingly, community clustering as a function of tree phase had limited correlation to soil water content and total nitrogen concentrations, which were the only two measured edaphic parameters to differ in association with tree phase. Bacterial clustering is more readily explained by the observed decrease in the abundance of active, rare microorganisms after tree death in conjunction with stable alpha diversity measurements. This enables the rare fraction of the terrestrial microbial community to maintain metabolic diversity by transitioning between metabolically active and dormant states during this ecosystem disturbance and contributes disproportionately to community dynamics and archived metabolic capabilities. These results suggest that analyzing bulk and potentially active communities after beetle infestation may be a more sensitive indicator of disruption than measuring local edaphic parameters. IMPORTANCEForests around the world are experiencing unprecedented mortality due to insect infestations that are fueled in part by a changing climate. While aboveground processes have been explored, changes at the terrestrial interface that are relevant to microbial biogeochemical cycling remain largely unknown. In this study, we investigated the changing bulk and potentially active microbial communities beneath healthy and beetle-killed trees. We found that, even though few edaphic parameters were altered from beetle infestation, the rare microbes were more likely to be active and fluctuate between dormancy and metabolic activity. This indicates that rare as opposed to abundant taxa contribute disproportionately to microbial community dynamics and presumably biogeochemical cycling within these types of perturbed ecosystems.

This study investigates the isolated decomposition of spruce and lodgepole conifer needles to enhance our understanding of how needle litter impacts near-surface terrestrial biogeochemical processes. Harvested needles were exported to a subalpine meadow to enable a discrete analysis of the decomposition processes over 2 years. Initial chemistry revealed the lodgepole needles to be less recalcitrant with a lower carbon to nitrogen (C:N) ratio. Total C and N fundamentally shifted within needle species over time with decreased C:N ratios for spruce and increased ratios for lodgepole. Differences in chemistry correlated with CO2 production and soil microbial communities. The most pronounced trends were associated with lodgepole needles in comparison to the spruce and needle-free controls. Increased organic carbon and nitrogen concentrations associated with needle presence in soil extractions further corroborate the results with clear biogeochemical signatures in association with needle chemistry. Interestingly, no clear differentiation was observed as a function of bark beetle impacted spruce needles vs those derived from healthy spruce trees despite initial differences in needle chemistry. These results reveal that the inherent chemistry associated with tree species has a greater impact on soil biogeochemical signatures during isolated needle decomposition. By extension, biogeochemical shifts associated with bark beetle infestation are likely driven more by changes such as the cessation of rhizospheric processes than by needle litter decomposition.