Snowmelt from forested, mountainous environments in the western United States is a critical regional water resource for streamflow and ecological productivity. These landscapes are undergoing rapid changes from the combined effects of forest fires, insect infestation and climate change. Numerous observational studies demonstrate that trees control snowpack accumulation and ablation over scales of tens of metres. Representing forest heterogeneity in models is important for understanding how changes in climate and vegetation cover affect the snowpack; yet, many snow models simplify a forest into two categories: canopy covered and non-canopy covered. We combine existing parameterizations of mass and energy fluxes within a new three-dimensional framework informed by Airborne Laser Swath Mapping (ALSM)-derived canopy maps and evaluated with ALSM-derived snow depth maps to explicitly simulate snow cover in relation to heterogeneous canopy. Model results capture much of the observed snow variability depicted in the 1-m ALSM-derived snow depth maps. Observations and modelled results identify open areas <15 m from tree canopies as having more snow and more snow variability than areas >15 m from tree canopies, and modelled results predict that open areas <15 m from tree canopies have 30-40% more net snow water input than areas that are underneath tree canopies and 10-25% more net snow water input than areas that are >15 m from tree canopies. Furthermore, 1-m simulations give higher estimates for net snow water input than coarser resolution simulations, mainly in areas with fewer trees. These results suggest the importance of explicitly representing canopy edges in snow models.

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Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO sink. However, such analyses are poorly constrained by measured CO exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water-limited Southwest region of North America with observed ranges in annual precipitation of 100-1000 mm, annual temperatures of 2-25°C, and records of 3-10 years (150 site-years in total). Annual fluxes were integrated using site-specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production (NEP) varying from -350 to +330 gCm across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest-dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production (GEP), and ecosystem respiration (R ) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration (ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and R were negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross-site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS-based models captured only 20-30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO exchange may be up to 3-5 times larger than current estimates.

Recent bark beetle epidemics have caused regional-scale tree mortality in many snowmeltdominated headwater catchments of western North America. Initial expectations of increased streamflow have not been supported by observations, and the basin-scale response of annual streamflow is largely unknown. Here we quantified annual streamflow responses during the decade following tree die-off in eight infested catchments in the Colorado River headwaters and one nearby control catchment. We employed three alternative empirical methods: (i) double-mass comparison between impacted and control catchments, (ii) runoff ratio comparison before and after die-off, and (iii) time-trend analysis using climatedriven linear models. In contrast to streamflow increases predicted by historical paired catchment studies and recent modeling, we did not detect streamflow changes in most basins following die-off, while one basin consistently showed decreased streamflow. The three analysis methods produced generally consistent results, with time-trend analysis showing precipitation was the strongest predictor of streamflow variability (R 2 5 74-96%). Time-trend analysis revealed post-die-off streamflow decreased in three catchments by 11-29%, with no change in the other five catchments. Although counter to initial expectations, these results are consistent with increased transpiration by surviving vegetation and the growing body of literature documenting increased snow sublimation and evaporation from the subcanopy following die-off in water-limited, snow-dominated forests. The observations presented here challenge the widespread expectation that streamflow will increase following beetle-induced forest die-off and highlight the need to better understand the processes driving hydrologic response to forest disturbance. Special Section: Disturbance HydrologyKey Points:Streamflow did not increase as predicted Three empirical methods produced consistent results Weak, variable streamflow response is consistent with recent process literature Supporting Information:Supporting Information S1 (2015), Recent tree die-off has little effect on streamflow in contrast to expected increases from historical studies, Water Resour. Res., 51, 9775-9789,

Global modeling efforts indicate semiarid regions dominate the increasing trend and interannual variation of net CO2 exchange with the atmosphere, mainly driven by water availability. Many semiarid regions are expected to undergo climatic drying, but the impacts on net CO2 exchange are poorly understood due to limited semiarid flux observations. Here we evaluated 121 site‐years of annual eddy covariance measurements of net and gross CO2 exchange (photosynthesis and respiration), precipitation, and evapotranspiration (ET) in 21 semiarid North American ecosystems with an observed range of 100 – 1000 mm in annual precipitation and records of 4–9 years each. In addition to evaluating spatial relationships among CO2 and water fluxes across sites, we separately quantified site‐level temporal relationships, representing sensitivity to interannual variation. Across the climatic and ecological gradient, photosynthesis showed a saturating spatial relationship to precipitation, whereas the photosynthesis–ET relationship was linear, suggesting ET was a better proxy for water available to drive CO2 exchanges after hydrologic losses. Both photosynthesis and respiration showed similar site‐level sensitivity to interannual changes in ET among the 21 ecosystems. Furthermore, these temporal relationships were not different from the spatial relationships of long‐term mean CO2 exchanges with climatic ET. Consequently, a hypothetical 100‐mm change in ET, whether short term or long term, was predicted to alter net ecosystem production (NEP) by 64 gCm−2 yr−1. Most of the unexplained NEP variability was related to persistent, site‐specific function, suggesting prioritization of research on slow‐changing controls. Common temporal and spatial sensitivity to water availability increases our confidence that site‐level responses to interannual weather can be extrapolated for prediction of CO2 exchanges over decadal and longer timescales relevant to societal response to climate change.

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Seasonally, snow‐covered forests are a critical source of water in the Western United States and are subject to major disturbances, including fire, harvest, disease and insect‐caused mortality, that have relatively unknown effects on water availability. In this study, we investigated changes in winter season snow accumulation and ablation in a forest following the Las Conchas fire in the Jemez Mountains of New Mexico. We investigated two competing sets of processes that should determine the peak annual snowpack prior to snowmelt: (1) reduced interception by forest canopy results in greater new snow accumulation and (2) increased winter season ablation of the snowpack results in reduced peak snowpack volumes. These processes were evaluated with approximately 800 spatially distributed manual observations of new snow, 1500 manual observations of peak snowpack, and light detection and ranging‐derived snow depth, vegetation and terrain datasets collected prior to the fire. A single snowfall event yielded significantly larger snow depths in the post‐burn area versus the unburned area (p < 0.001), with 25% to 45% interception in the unburned area and near zero in the post‐burn area. Conversely, the peak snowpack depths were significantly larger in the unburned area compared with the post‐burn area (mean of 55 and 47 cm, respectively), despite nearly identical peak snowpacks prior to the fire (72 and 72 cm, respectively). The lack of strong vegetation controls led to less variability at peak snowpack in the post‐burn area and a shift towards topographically controlled variability, caused by differences in elevation and aspect, occurring at larger spatial scales. The unburned area had roughly 10% more water available for melt than the post‐burn area, with winter season ablation reducing snowpacks by nearly 50% prior to melt in the post‐burn area. The relative importance of shortwave radiation to the snowpack energy balance and sublimation suggests that the 10% reductions in peak snow water storage found in these north‐facing areas could be a conservative estimate for winter season ablation following fire. Further work is necessary to assess the role that topography plays in altering water partitioning following forest disturbance and the potential implications for ecological health and downstream water resources. Copyright © 2013 John Wiley & Sons, Ltd.

Global-scale studies indicate that semiarid regions strongly regulate the terrestrial carbon sink.However, we lack understanding of how climatic shifts, such as decadal drought, impact carbon sequestration across the wide range of structural diversity in semiarid ecosystems. Therefore, we used eddy covariance measurements to quantify how net ecosystem production of carbon dioxide (NEP) differed with relative grass and woody plant abundance over the last decade of drought in four Southwest U.S. ecosystems. We identified a precipitation "pivot point" in the carbon balance for each ecosystem where annual NEP switched from negative to positive. Ecosystems with grass had pivot points closer to the drought period precipitation than the predrought average, making them more likely to be carbon sinks (and a grass-free shrubland, a carbon source) during the current drought. One reason for this is that the grassland located closest to the shrubland supported higher leaf area and photosynthesis at the same water availability. Higher leaf area was associated with a greater proportion of evapotranspiration being transpiration (T/ET), and therefore with higher ecosystem water use efficiency (gross ecosystem photosynthesis/ET). Our findings strongly show that water availability is a primary driver of both gross and net semiarid productivity and illustrate that structural differences may contribute to the speed at which ecosystem carbon cycling adjusts to climatic shifts.For much of the last two decades, drought conditions have prevailed over the semiarid southwestern United States [Dai, 2013]. This "21st century drought" has varied in severity and location across these years resulting in a myriad of regional challenges for natural and human-dominated systems, such as water shortages, forest mortality, and wildfires [e.g., Barnett and Pierce, 2008;Breshears et al., 2005;McAuliffe and Hamerlynck, 2010;Westerling and Bryant, 2008]. Climate models have suggested the characteristics of this drought, including the prevalence of reduced winter precipitation and higher air temperatures, may represent a "new normal" [McAfee and Russell, 2008] and that this region will experience even more severe and protracted drought in the future [Cayan et al., 2010]. We do not know how the current drought, let alone predicted increased desiccation, has altered carbon exchange in this region [Archer and Predick, 2008; van der Molen et al., 2011].