Browsing by Author "Hsieh, Cheng-I."
Now showing 1 - 2 of 2
- Results Per Page
- Sort Options
Item Estimating the uncertainty in annual net ecosystem carbon exchange: Spatial variation in turbulent fluxes and sampling errors in eddy-covariance measurements(2006-04) Oren, Ram; Hsieh, Cheng-I.; Stoy, Paul C.; Albertson, John; McCarthy, Heather R.; Harrell, Peter; Katul, Gabriel G.Above forest canopies, eddy covariance (EC) measurements of mass (CO2, H2O vapor) and energy exchange, assumed to represent ecosystem fluxes, are commonly made at one point in the roughness sublayer (RSL). A spatial variability experiment, in which EC measurements were made from six towers within the RSL in a uniform pine plantation, quantified large and dynamic spatial variation in fluxes. The spatial coefficient of variation (CV) of the scalar fluxes decreased with increasing integration time, stabilizing at a minimum that was independent of further lengthening the averaging period (hereafter a ‘stable minimum’). For all three fluxes, the stable minimum (CV=9–11%) was reached at averaging times (τp) of 6–7 h during daytime, but higher stable minima (CV=46–158%) were reached at longer τp (>12 h) during nighttime. To the extent that decreasing CV of EC fluxes reflects reduction in micrometeorological sampling errors, half of the observed variability at τp=30 min is attributed to sampling errors. The remaining half (indicated by the stable minimum CV) is attributed to underlying variability in ecosystem structural properties, as determined by leaf area index, and perhaps associated ecosystem activity attributes. We further assessed the spatial variability estimates in the context of uncertainty in annual net ecosystem exchange (NEE). First, we adjusted annual NEE values obtained at our long‐term observation tower to account for the difference between this tower and the mean of all towers from this experiment; this increased NEE by up to 55 g C m−2 yr−1. Second, we combined uncertainty from gap filling and instrument error with uncertainty because of spatial variability, producing an estimate of variability in annual NEE ranging from 79 to 127 g C m−2 yr−1. This analysis demonstrated that even in such a uniform pine plantation, in some years spatial variability can contribute ∼50% of the uncertainty in annual NEE estimates.Item Sensitivity of stand transpiration to wind velocity in a mixed broadleaved deciduous forest(2014-04-15) Dohyoung, Kim; Oren, Ram; Oishi, A. Christopher; Hsieh, Cheng-I.; Phillips, Nathan; Novick, Kimberly A.; Stoy, Paul C.Wind velocity (U) within and above forest canopies can alter the coupling between the vapor-saturated sub-stomatal airspace and the drier atmosphere aloft, thereby influencing transpiration rates. In practice, however, the actual increase in transpiration with increasing U depends on the aerodynamic resistance (RA) to vapor transfer compared to canopy resistance to water vapor flux out of leaves (RC, dominated by stomatal resistance, Rstom), and the rate at which RA decreases with increasing U. We investigated the effect of U on transpiration at the canopy scale using filtered meteorological data and sap flux measurements gathered from six diverse species of a mature broadleaved deciduous forest. Only under high light conditions, stand transpiration (EC) increased slightly (6.5%) with increasing U ranging from ∼0.7 to ∼4.7 m s−1. Under other conditions, sap flux density (Js) and EC responded weakly or did not change with U. RA, estimated from Monin–Obukhov similarity theory, decreased with increasing U, but this decline was offset by increasing RC, estimated from a rearranged Penman–Monteith equation, due to a concurrent increase in vapor pressure deficit (D). The increase of RC with D over the observed range of U was consistent with increased Rstom by ∼40% based on hydraulic theory. Except for very rare half-hourly values, the proportion of RA to total resistance (RT) remained <15% over the observed range of conditions. These results suggest that in similar forests and conditions, the direct effect of U reducing RA and thus increasing transpiration is negligible. However, the observed U–D relationship and its effect on Rstom must be considered when modeling canopy photosynthesis.