Measuring and modelling carbon dioxide and water vapour exchange over a temperate broad‐leaved forest during the 1995 summer drought

Abstract

Forests in the south-eastern United States experienced a prolonged dry spell and above-normal temperatures during the 1995 growing season. During this episode, nearly continuous, eddy covariance measurements of carbon dioxide and water vapour fluxes were acquired over a temperate, hardwood forest. These data are used to examine how environmental factors and accumulating soil moisture deficits affected the diurnal pattern and magnitude of canopy-scale carbon dioxide and water vapour fluxes. The field data are also used to test an integrative leaf-to-canopy scaling model (CANOAK), which uses micrometeorological and physiological theory, to calculate mass and energy fluxes. When soil moisture was ample in the spring, peak rates of net ecosystem CO2 exchange (A'pj occurred around midday and exceeded 20 ;Umol m~^ s"'. Rates of A',; were near optimal when air temperature ranged between 22 and 25°C. The accumulation of soil moisture deficits and a co-occurrence of high temperatures caused peak rates of daytime carbon dioxide uptake to occur earlier in the morning. High air temperatures and soil moisture deficits were also correlated with a dramatic reduction in the magnitude of N^. On average, the magnitude of N,i decreased from 20 to 7 ^umol m"^ s"' as air temperature increased from 24 to 30°C and the soil dried. The CANAOK model yielded accurate estimates of canopyscale carbon dioxide and water vapour fluxes when the forest had an ample supply of soil moisture. During the drought and heat spell, a cumulative drought index was needed to adjust the proportionality constant of the stomata] conductance model to yield accurate estimates of canopy CO2 exchange. The adoption of the drought index also enabled the CANOAK model to give improved estimates of evaporation until midday. On the other hand, the scheme failed to yield accurate estimates of evaporation during the afternoon.

Energy balance storage terms and big-leaf evapotranspiration in a mixed deciduous forest

“…The heat stored by net carbon assimilation is illustrated in figure 2e, and corresponds well with values found by other authors [3,4]. A clear diurnal course was observed.…”

Inverse estimation of Vcmax, leaf area index, and the Ball‐Berry parameter from carbon and energy fluxes

“…Although actual leaf growth, C and N translocation, and plant water relations represented by these leaf characteristics change continuously in response to changing internal and external conditions [Tanaka et al, 2002], micrometeorological data used to estimate these parameters are also subject to mea- surement errors. Some authors have addressed this measurement uncertainty by estimating ecosystem properties (i.e., light response curves, respiration rates) from flux measurements averaged across several days [Baldocchi, 1997]. Instead of averaging the flux measurements, we fitted single values of the parameters across a period of several days.…”

Dynamics of isotopic exchange of carbon dioxide in a Tennessee deciduous forest

“…EC fluxes were calculated as F = p<w'C'>, where F represents flux of sensible heat, latent heat, or CO2, p is air density, w is vertical wind velocity, C is CO2 mixing ratio (or temperature or water vapor), <> represents Reynolds averaging, and the primes represent deviation from the Reynolds average [e.g., Baldocchi et al, 1988]. Mean CO2 mixing ratios were measured at 36, 18, 10, and 0.75 m to compute canopy CO2 storage as described by Baldocchi [1997]. Eddy covariance and storage fluxes are reported as 30-min averages.…”