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… More

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Stable stratification of the nocturnal lower boundary layer inhibits convective turbulence, such that tur-bulent vertical transfer of ecosystem carbon dioxide (CO2), water vapor (H2O) and energy is driven bymechanically forced turbulence, either from frictional forces near the ground or top of a plant canopy,or from shear generated aloft. The significance of this last source of… More

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tThe partitioning of net ecosystem exchange of CO2(NEE) into photosynthesis and respiration can be chal-lenging and is often associated with assumptions that yield unknown amounts of uncertainty, therebyhindering model development. This occurs because we are inferring two pieces of information fromone equation and measurement, NEE. While there are multiple methods for partitioning NEE, each hasunique… More

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In evergreen conifers, where the foliage amount changes little with season, accurate detection of the underlying “photosynthetic phenology” from satellite remote sensing has been difficult, presenting challenges for global models of ecosystem carbon uptake. Here, we report a close correspondence between seasonally changing foliar pigment levels, expressed as chlorophyll/carotenoid ratios, and evQ: 13 ergreen photosynthetic… More

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Ecosystem models often perform poorly in reproducing interannual variability in carbon and water fluxes, resulting in considerable uncertainty when estimating the land-carbon sink. While many aggregated variables (growing season length, seasonal precipitation, or temperature) have been suggested as predictors for interannual variability in carbon fluxes, their explanatory power is limited and uncertainties remain as to… More

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The global terrestrial carbon sink offsets one-third of the world’s fossil fuel emissions, but the strength of this sink is highly sensitive to large-scale extreme events. In 2012, the contiguous United States experienced exceptionally warm temperatures and the most severe drought since the Dust Bowl era of the 1930s, resulting in substantial economic damage. It… More

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Eddy-covariance measurements are widely used to develop and test parameterizations of land- atmosphere interactions in earth system models. However, a fundamental challenge for model-data comparisons lies in the scale mismatch between the eddy-covariance observations with small (10−1–101 km2) and temporally varying flux footprint, and the continuous regional-scale (102–104 km2) gridded predictions made in simulations. Here,… More

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The Finite-Element Tree-Crown Hydrodynamics model version 2 (FETCH2) is a tree-scale hydrodynamic model of transpiration. The FETCH2 model employs a finite difference numerical methodology and a simplified single-beam conduit system. It explicitly resolves xylem water potential throughout the tree’s vertical extent. Empirical equations relate water potential within the stem to stomatal conductance of the leaves… More

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Many ecophysiological and biogeochemical processes respond rapidly to changes in biotic and abiotic conditions, while ecosystem-level responses develop much more slowly (e.g., over months, seasons, years, or decades). To better understand the role of the slow responses in regulating interannual variability in NEE, we partitioned NEE into two major ecological terms—gross primary productivity (GPP) and… More

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