Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitud... more Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO2 sink. However, such analyses are poorly constrained by measured CO2 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(-2) 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 varia...
Global modeling efforts indicate semiarid regions dominate the increasing trend and interannual v... more 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 pho...
ABSTRACT Western U.S. forest ecosystems and downstream water supplies are reliant on seasonal sno... more ABSTRACT Western U.S. forest ecosystems and downstream water supplies are reliant on seasonal snowmelt. Complex feedbacks govern forest-snow interactions in which forests influence the distribution of snow and the timing of snowmelt but are also sensitive to snow water availability. Notwithstanding, few studies have investigated the influence of forest structure on snow distribution, snowmelt and soil moisture response. Using a multi-year record from co-located observations of snow depth and soil moisture, we evaluated the influence of forest-canopy position on snow accumulation and snow depth depletion, and associated controls on the timing of soil moisture response at Boulder Creek, Colorado, Jemez River Basin, New Mexico, and the Wolverton Basin, California. Forest canopy controls on snow accumulation led to 12 to 42 cm greater peak snow depths in open versus under-canopy positions. Differences in accumulation and melt across sites resulted in earlier snow disappearance in open positions at Jemez and earlier snow disappearance in under-canopy positions at Boulder and Wolverton sites. Irrespective of net snow accumulation, we found that peak annual soil moisture was nearly synchronous with the date of snow disappearance at all sites with an average deviation of 12, 3, and 22 days at Jemez, Boulder, and Wolverton sites, respectively. Interestingly, sites in the Sierra Nevada showed peak soil moisture prior to snow disappearance at both our intensive study site and the nearby snow telemetry (SNOTEL) stations. Our results imply that the duration of soil water stress may increase as regional warming or forest disturbance lead to earlier snow disappearance and soil moisture recession in subalpine forests. This article is protected by copyright. All rights reserved.
An extended period of snow cover is a dominant climatic characteristic of montane environments in... more An extended period of snow cover is a dominant climatic characteristic of montane environments in western North America. Recent research has demonstrated that the water stored in this seasonal snow cover is the primary source not only of river discharge but also groundwater recharge and plant available water during the growing season. Although the recorded interannual variability of snow cover is quite large, ongoing changes in climate, combined with accelerating rates of forest disturbance from insects, fire, and drought, differentially are affecting the amount, timing, and partitioning of snow cover to an extent not captured in the instrumental record. A critical knowledge gap exists in predicting how these concurrent changes in climate and vegetation in topographical complex mountain environments will affect future water resources both for society and for terrestrial and aquatic ecosystems. This presentation addresses that knowledge gap through a meta analysis recent work on snowpack dynamics, runoff generation, catchment biogeochemistry, and ecosystem productivity from seasonally snow-covered forests along a gradient of snow depth and duration in the Intermountain West. Observations include long-term SNOTEL monitoring stations, CZO, LTER, and USDA observations of landsurface-atmosphere water and carbon exchange, and post disturbance observations from recently burned forests, and areas of extensive insect-induced forest mortality. Together these observations can be used to identify landscapes most at risk to climate change as well as to develop management alternatives that minimize the effects of climate change on high elevation forests and the services of water provision and carbon storage they provide
ABSTRACT Changes in both temperature and the amount and timing of precipitation have the potentia... more ABSTRACT Changes in both temperature and the amount and timing of precipitation have the potential to profoundly impact water balance in mountain ecosystems. Although changes in the amount of precipitation and potential evapotranspiration are widely considered in climate change scenarios, less attention has been given to how changes in climate or land cover may affect hydrologic partitioning and plant available water. The focus of this presentation is on how spatial transitions in ecosystem structure and temporal transitions in climate affect the fraction of precipitation potentially available to vegetation. In most temperate mountain environments winter snows are a significant fraction of annual precipitation and understanding the partitioning of snow and snow melt is critical for predicting both ecosystem water availability and stream flow under future climate scenarios. Spatial variability in net snow water input is a function of the interaction of snowfall, wind, and solar radiation with topography and vegetation structure. Integrated over larger scales these interactions may result in between 0% and 40% sublimation of winter snowfall before melt, effectively excluding this water from growing season water balance. Once melt begins, variability in the partitioning of snowmelt is driven by the rate of melt, and somewhat less intuitively, by the timing of snow accumulation the previous fall. Early accumulating snowpacks insulate soils and minimize soil frost increasing infiltration of melt the following spring. In contrast, later snowfall results in colder soils, more soil frost, reduced infiltration, increased runoff during melt, and reduced plant available water during the following growing season. This change in hydrologic partitioning, mediated by the timing of snowpack accumulation, results in lower evapotranspiration (ET) and net ecosystem exchange (NEE) the following spring. These findings suggest that abiotic controls on the partitioning of precipitation may exacerbate or attenuate the effects of climate change on mountain water balance.
This is the AmeriFlux version of the carbon flux data for the site US-Seg Sevilleta grassland. Si... more This is the AmeriFlux version of the carbon flux data for the site US-Seg Sevilleta grassland. Site Description - The Sevilleta Desert Grassland site is located within the McKenzie Flats area of the Sevilleta National Wildlife Refuge (NWR), central New Mexico. Historically, this area has been used for livestock grazing; however, the McKenzie Flats have not been grazed since 1973 and the effects of this previous grazing are considered negligible for the purposes of this study. As the name suggests, McKenzie Flats is an extensive (~130 km2), nearly flat, mixed-species desert grassland bounded on the east by Los Pinos Mountains and on the west by the Rio Grande.
ABSTRACT Stream water carbon (C) export is one important pathway for C loss from seasonally snow-... more ABSTRACT Stream water carbon (C) export is one important pathway for C loss from seasonally snow-covered mountain ecosystems and an assessment of overarching controls is necessary. However, such assessment is challenging because changes in water fluxes or flow paths, seasonal processes, as well as catchment specific characteristics play a role. For this study we elucidate the impact of: (i) changes in water flux (by comparing years of variable wetness), (ii) catchment aspect [north-facing (NF) vs. south-facing (SF)] and (iii) season (snowmelt vs. summer) on all forms of dissolved stream water C [dissolved organic C (DOC), chromophoric dissolved organic matter (CDOM) and dissolved inorganic C (DIC)] in forested catchments within the Valles Caldera National Preserve, New Mexico. The significant correlation between annual water and C fluxes (e.g. DOC r(2) = 0.83, p < 0.02) confirms annual stream water discharge as the overarching control on C efflux, likely from a well-mixed ground water reservoir as indicated by previous research. However, CDOM exhibited a dominantly terrestrial fluorescence signature (59-71 %) year round, signaling a strong riparian and near stream soil control on CDOM composition. During snowmelt, the role of water as C transporter was superimposed on its control as C reservoir, when the NF stream transported significantly more soil C (40 % DOC, 56 % DIC) than the SF stream as a result of hillslope flushing. Inter-annual variations in winter precipitation were paramount in regulating annual stream C effluxes, e.g., reducing C effluxes three-fold after a dry (relative to wet) winter season. During the warmer summer months % dissolved oxygen saturation decreased, delta C-13(DIC) increased and CDOM assumed a more microbial signature, consistent with heterotrophic respiration in the stream and riparian soils. As a result of stream C incubation and soil respiration, increased up to 12 times atmospheric values leading to substantial degassing.
Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitud... more Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO2 sink. However, such analyses are poorly constrained by measured CO2 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(-2) 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 varia...
Global modeling efforts indicate semiarid regions dominate the increasing trend and interannual v... more 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 pho...
ABSTRACT Western U.S. forest ecosystems and downstream water supplies are reliant on seasonal sno... more ABSTRACT Western U.S. forest ecosystems and downstream water supplies are reliant on seasonal snowmelt. Complex feedbacks govern forest-snow interactions in which forests influence the distribution of snow and the timing of snowmelt but are also sensitive to snow water availability. Notwithstanding, few studies have investigated the influence of forest structure on snow distribution, snowmelt and soil moisture response. Using a multi-year record from co-located observations of snow depth and soil moisture, we evaluated the influence of forest-canopy position on snow accumulation and snow depth depletion, and associated controls on the timing of soil moisture response at Boulder Creek, Colorado, Jemez River Basin, New Mexico, and the Wolverton Basin, California. Forest canopy controls on snow accumulation led to 12 to 42 cm greater peak snow depths in open versus under-canopy positions. Differences in accumulation and melt across sites resulted in earlier snow disappearance in open positions at Jemez and earlier snow disappearance in under-canopy positions at Boulder and Wolverton sites. Irrespective of net snow accumulation, we found that peak annual soil moisture was nearly synchronous with the date of snow disappearance at all sites with an average deviation of 12, 3, and 22 days at Jemez, Boulder, and Wolverton sites, respectively. Interestingly, sites in the Sierra Nevada showed peak soil moisture prior to snow disappearance at both our intensive study site and the nearby snow telemetry (SNOTEL) stations. Our results imply that the duration of soil water stress may increase as regional warming or forest disturbance lead to earlier snow disappearance and soil moisture recession in subalpine forests. This article is protected by copyright. All rights reserved.
An extended period of snow cover is a dominant climatic characteristic of montane environments in... more An extended period of snow cover is a dominant climatic characteristic of montane environments in western North America. Recent research has demonstrated that the water stored in this seasonal snow cover is the primary source not only of river discharge but also groundwater recharge and plant available water during the growing season. Although the recorded interannual variability of snow cover is quite large, ongoing changes in climate, combined with accelerating rates of forest disturbance from insects, fire, and drought, differentially are affecting the amount, timing, and partitioning of snow cover to an extent not captured in the instrumental record. A critical knowledge gap exists in predicting how these concurrent changes in climate and vegetation in topographical complex mountain environments will affect future water resources both for society and for terrestrial and aquatic ecosystems. This presentation addresses that knowledge gap through a meta analysis recent work on snowpack dynamics, runoff generation, catchment biogeochemistry, and ecosystem productivity from seasonally snow-covered forests along a gradient of snow depth and duration in the Intermountain West. Observations include long-term SNOTEL monitoring stations, CZO, LTER, and USDA observations of landsurface-atmosphere water and carbon exchange, and post disturbance observations from recently burned forests, and areas of extensive insect-induced forest mortality. Together these observations can be used to identify landscapes most at risk to climate change as well as to develop management alternatives that minimize the effects of climate change on high elevation forests and the services of water provision and carbon storage they provide
ABSTRACT Changes in both temperature and the amount and timing of precipitation have the potentia... more ABSTRACT Changes in both temperature and the amount and timing of precipitation have the potential to profoundly impact water balance in mountain ecosystems. Although changes in the amount of precipitation and potential evapotranspiration are widely considered in climate change scenarios, less attention has been given to how changes in climate or land cover may affect hydrologic partitioning and plant available water. The focus of this presentation is on how spatial transitions in ecosystem structure and temporal transitions in climate affect the fraction of precipitation potentially available to vegetation. In most temperate mountain environments winter snows are a significant fraction of annual precipitation and understanding the partitioning of snow and snow melt is critical for predicting both ecosystem water availability and stream flow under future climate scenarios. Spatial variability in net snow water input is a function of the interaction of snowfall, wind, and solar radiation with topography and vegetation structure. Integrated over larger scales these interactions may result in between 0% and 40% sublimation of winter snowfall before melt, effectively excluding this water from growing season water balance. Once melt begins, variability in the partitioning of snowmelt is driven by the rate of melt, and somewhat less intuitively, by the timing of snow accumulation the previous fall. Early accumulating snowpacks insulate soils and minimize soil frost increasing infiltration of melt the following spring. In contrast, later snowfall results in colder soils, more soil frost, reduced infiltration, increased runoff during melt, and reduced plant available water during the following growing season. This change in hydrologic partitioning, mediated by the timing of snowpack accumulation, results in lower evapotranspiration (ET) and net ecosystem exchange (NEE) the following spring. These findings suggest that abiotic controls on the partitioning of precipitation may exacerbate or attenuate the effects of climate change on mountain water balance.
This is the AmeriFlux version of the carbon flux data for the site US-Seg Sevilleta grassland. Si... more This is the AmeriFlux version of the carbon flux data for the site US-Seg Sevilleta grassland. Site Description - The Sevilleta Desert Grassland site is located within the McKenzie Flats area of the Sevilleta National Wildlife Refuge (NWR), central New Mexico. Historically, this area has been used for livestock grazing; however, the McKenzie Flats have not been grazed since 1973 and the effects of this previous grazing are considered negligible for the purposes of this study. As the name suggests, McKenzie Flats is an extensive (~130 km2), nearly flat, mixed-species desert grassland bounded on the east by Los Pinos Mountains and on the west by the Rio Grande.
ABSTRACT Stream water carbon (C) export is one important pathway for C loss from seasonally snow-... more ABSTRACT Stream water carbon (C) export is one important pathway for C loss from seasonally snow-covered mountain ecosystems and an assessment of overarching controls is necessary. However, such assessment is challenging because changes in water fluxes or flow paths, seasonal processes, as well as catchment specific characteristics play a role. For this study we elucidate the impact of: (i) changes in water flux (by comparing years of variable wetness), (ii) catchment aspect [north-facing (NF) vs. south-facing (SF)] and (iii) season (snowmelt vs. summer) on all forms of dissolved stream water C [dissolved organic C (DOC), chromophoric dissolved organic matter (CDOM) and dissolved inorganic C (DIC)] in forested catchments within the Valles Caldera National Preserve, New Mexico. The significant correlation between annual water and C fluxes (e.g. DOC r(2) = 0.83, p < 0.02) confirms annual stream water discharge as the overarching control on C efflux, likely from a well-mixed ground water reservoir as indicated by previous research. However, CDOM exhibited a dominantly terrestrial fluorescence signature (59-71 %) year round, signaling a strong riparian and near stream soil control on CDOM composition. During snowmelt, the role of water as C transporter was superimposed on its control as C reservoir, when the NF stream transported significantly more soil C (40 % DOC, 56 % DIC) than the SF stream as a result of hillslope flushing. Inter-annual variations in winter precipitation were paramount in regulating annual stream C effluxes, e.g., reducing C effluxes three-fold after a dry (relative to wet) winter season. During the warmer summer months % dissolved oxygen saturation decreased, delta C-13(DIC) increased and CDOM assumed a more microbial signature, consistent with heterotrophic respiration in the stream and riparian soils. As a result of stream C incubation and soil respiration, increased up to 12 times atmospheric values leading to substantial degassing.
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