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Lawns as a landcover change substantially alter evapotranspiration, CO2, and energy exchanges and are of rising importance considering their spatial extent. We contrast eddy covariance (EC) flux measurements collected in the Denver, Colorado, USA metropolitan area in 2011 and 2012 over a lawn and a xeric tallgrass prairie. Close linkages between seasonal vegetation development, energy fluxes, and net ecosystem exchange (NEE) of CO2 were found. Irrigation of the lawn modified energy and CO2 fluxes and greatly contributed to differences observed between sites. Due to greater water inputs (precipitation + irrigation) at the lawn in this semi-arid climate, energy partitioning at the lawn was dominated by latent heat (LE) flux. As a result, evapotranspiration (ET) of the lawn was more than double that of tallgrass prairie (2011: 639(±17) mm vs. 302(±9) mm; 2012: 584(±15) mm vs. 265(±7) mm). NEE for the lawn was characterized by a longer growing season, higher daily net uptake of CO2, and growing season NEE that was also more than twice that of the prairie (2011: −173(±23) g C m−2 vs. -81(±10) g C m−2; 2012: −73(±22) g C m−2 vs. -21(±8) g C m−2). During the drought year (2012), temperature and water stress greatly influenced the direction and magnitude of CO2 flux at both sites. The results suggest that lawns in Denver can function as carbon sinks and conditionally contribute to the mitigation of carbon emissions - directly by CO2 uptake and indirectly through effects of evaporative cooling on microclimate and energy use.

We recently noticed a mistake in Fig. 1b of Burns et al. (2011).

The eddy covariance technique, which is used in the determination of net ecosystem CO₂ exchange (NEE), is subject to significant errors when advection that carries CO₂ in the mean flow is ignored. We measured horizontal and vertical advective CO₂ fluxes at the Niwot Ridge AmeriFlux site (Colorado, USA) using a measurement approach consisting of multiple towers. We observed relatively high rates of both horizontal $(F_{{\\rm{hadv}}} )$ and vertical $(F_{{\\rm{vadv}}} )$ advective fluxes at low surface friction velocities ${\\rm{(u}}_* )$ which were associated with downslope katabatic flows. We observed that $F_{{\\rm{hadv}}} $ was confined to a relatively thin layer (0-6 m thick) of subcanopy air that flowed beneath the eddy covariance sensors principally at night, carrying with it respired CO₂ from the soil and lower parts of the canopy. The observed $F_{{\\rm{vadv}}}$ came from above the canopy and was presumably due to the convergence of drainage flows at the tower site. The magnitudes of both $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ were similar, of opposite sign, and increased with decreasing ${\\rm{u}}_*$, meaning that they most affected estimates of the total CO₂ flux on calm nights with low wind speeds. The mathematical sign, temporal variation and dependence on ${\\rm{u}}_*$ of both $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ were determined by the unique terrain of the Niwot Ridge site. Therefore, the patterns we observed may not be broadly applicable to other sites. We evaluated the influence of advection on the cumulative annual and monthly estimates of the total CO₂ flux $(F_c )$, which is often used as an estimate of NEE, over six years using the dependence of $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ on ${\\rm{u}}_*$. When the sum of $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ was used to correct monthly $F_c$, we observed values that were different from the monthly $F_c$ calculated using the traditional ${\\rm{u}}_*$-filter correction by -16 to 20 g C.m‾².mo‾¹; the mean percentage difference in monthly $F_c$ for these two methods over the six-year period was 10%. When the sum of $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ was used to correct annual $F_c$ we observed a 65% difference compared to the traditional ${\\rm{u}}_*$-filter approach. Thus, the errors to the local CO₂ budget, when $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ are ignored, can become large when compounded in cumulative fashion over long time intervals. We conclude that the \"micrometeorological\" (using observations of $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$) and \"biological\" (using the ${\\rm{u}}_*$ filter and temperature vs. $F_c$ relationship) corrections differ on the basis of fundamental mechanistic grounds. The micrometeorological correction is based on aerodynamic mechanisms and shows no correlation to drivers of biological activity. Conversely, the biological correction is based on climatic responses of organisms and has no physical connection to aerodynamic processes. In those cases where they impose corrections of similar magnitude on the cumulative $F_c$ sum, the result is due to a serendipitous similarity in scale but has no clear mechanistic explanation.

Air temperature T a , specific humidity q, CO₂ mole fraction χ c , and three-dimensional winds were measured in mountainous terrain from five tall towers within a 1 km region encompassing a wide range of canopy densities. The measurements were sorted by a bulk Richardson number Ri b . For stable conditions, we found vertical scalar differences developed over a “transition” region between 0.05 < Ri b < 0.5. For strongly stable conditions (Ri b > 1), the vertical scalar differences reached a maximum and remained fairly constant with increasing stability. The relationships q and χ c have with Ri b are explained by considering their sources and sinks. For winds, the strong momentum absorption in the upper canopy allows the canopy sublayer to be influenced by pressure gradient forces and terrain effects that lead to complex subcanopy flow patterns. At the dense-canopy sites, soil respiration coupled with wind-sheltering resulted in CO₂ near the ground being 5-7 μmol mol⁻¹ larger than aloft, even with strong above-canopy winds (near-neutral conditions). We found Ri b -binning to be a useful tool for evaluating vertical scalar mixing; however, additional information (e.g., pressure gradients, detailed vegetation/topography, etc.) is needed to fully explain the subcanopy wind patterns. Implications of our results for CO₂ advection over heterogenous, complex terrain are discussed.

A significant fraction of Earth consists of mountainous terrain. However, the question of how to monitor the surface–atmosphere carbon exchange over complex terrain has not been fully explored. This article reports on studies by a team of investigators from U.S. universities and research institutes who carried out a multiscale and multidisciplinary field and modeling investigation of the CO₂ exchange between ecosystems and the atmosphere and of CO₂ transport over complex mountainous terrain in the Rocky Mountain region of Colorado. The goals of the field campaign, which included ground and airborne in situ and remote-sensing measurements, were to characterize unique features of the local CO₂ exchange and to find effective methods to measure regional ecosystem–atmosphere CO₂ exchange over complex terrain. The modeling effort included atmospheric and ecological numerical modeling and data assimilation to investigate regional CO₂ transport and biological processes involved in ecosystem–atmosphere carbon exchange. In this report, we document our approaches, demonstrate some preliminary results, and discuss principal patterns and conclusions concerning ecosystem–atmosphere carbon exchange over complex terrain and its relation to past studies that have considered these processes over much simpler terrain.

(1) The short-term canopy photosynthesis and water-use efficiency of a deciduous forest were measured with the eddy correlation method. Canopy photosynthetic rates were strongly dependent on photosynthetically active radiation and were highly correlated with transpiration. These rates were not correlated with vapour pressure deficit, air temperature and stomatal resistance since limiting conditions were not encountered. Water-use efficiency was found to be dependent on vapour pressure deficit but was independent of net radiation. (2) Measurements of canopy photosynthetic rates and water-use efficiency were compared with theoretical estimates. Canopy photosynthesis was computed by coupling the leaf photosynthesis model of Marshall & Biscoe (1980) with the canopy radiative transfer model of Baldocchi & Hutchison (1986). Considering the simplicity of the canopy photosynthesis model, the overall agreement between measured and computed values was reasonable. Water-use efficiency measurements agreed well with values computed with the model of Sinclair, Tanner & Bennett (1984). (3) Canopy-atmosphere gas exchange rates were lower than values commonly observed over agricultural crops. This was partly because deciduous forest is aerodynamically rougher than agricultural crops and deciduous trees have a greater canopy stomatal resistance than crops. The ratio between atmospheric CO2uptake and canopy photosynthesis was different from that commonly measured over agricultural crops. This is because the floor of a deciduous forest has more detritus than an agricultural crop, which allows greater CO2efflux from the forest floor.

Lake‐atmosphere CO2 flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake‐water CO2 storage and with flux predictions from boundary‐layer and surface‐renewal models. Over a 3‐yr period, lake‐atmosphere exchanges of CO2 were measured over 5 weeks in spring, summer, and fall. Observed springtime CO2 efflux was large (2.3–2.7 umol m‐2 s‐1) immediately after lake‐thaw. That efflux decreased exponentially with time to less than 0.2 umol m‐2 s−1 within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO2 concentrations, lake CO2 storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (−0.17 μmol m‐2 s1) to the lake's surface, while late fall flux was trendless and smaller (−0.0021 μmol m‐2 s−1). Large springtime efflux afforded an opportunity to make direct measurement of lake‐atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air‐water gas‐transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake‐water storage change in CO2, agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundary‐layer and surface‐renewal models. Comparison of measured and modeled transfer velocities for CO2, along with measured and modeled cumulative CO2 flux, indicates that in most instances the surface‐renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary‐layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.

A significant fraction of Earth consists of mountainous terrain. However, the question of how to monitor the surface-atmosphere carbon exchange over complex terrain has not been fully explored. This article reports on studies by a team of investigators from U.S. universities and research institutes who carried out a multiscale and multidisciplinary field and modeling investigation of the CO sub(2) exchange between ecosystems and the atmosphere and of CO sub(2) transport over complex mountainous terrain in the Rocky Mountain region of Colorado. The goals of the field campaign, which included ground and airborne in situ and remote-sensing measurements, were to characterize unique features of the local CO sub(2) exchange and to find effective methods to measure regional ecosystem-atmosphere CO sub(2) exchange over complex terrain. The modeling effort included atmospheric and ecological numerical modeling and data assimilation to investigate regional CO sub(2)

Lake-atmosphere CO2flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake-water CO2storage and with flux predictions from boundary-layer and surface-renewal models. Over a 3-yr period, lake-atmosphere exchanges of CO2were measured over 5 weeks in spring, summer, and fall. Observed springtime CO2efflux was large (2.3-2.7 μmol m-2s-1) immediately after lake-thaw. That efflux decreased exponentially with time to less than 0.2 μmol m-2s-1within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO2concentrations, lake CO2storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (-0.17 μmol m-2s-1) to the lake's surface, while late fall flux was trendless and smaller (-0.0021 μmol m-2s-1). Large springtime efflux afforded an opportunity to make direct measurement of lake-atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air-water gas-transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake-water storage change in C02, agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundary-layer and surface-renewal models. Comparison of measured and modeled transfer velocities for CO2, along with measured and modeled cumulative CO2flux, indicates that in most instances the surface-renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary-layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.