Net CO2 and H2O fluxes of terrestrial ecosystems

Using 139 flux studies, we addressed the variability of net ecosystem surface assimilation (A(smax)), net ecosystem surface respiration (R-smax), as well as net surface evapotranspiration (E-smax) among and within vegetation types. While forests and C-3 crops, particularly in the northern hemisphere, have been preferentially investigated, information on tropical forests, C-4 grasslands or wetlands is rather limited. Almost no data are available for disturbed sites. Despite large variations within a vegetation type, enclosure studies tended to give highest A(smax) rates compared to micrometeorological techniques. Excluding enclosure studies, we tested the effect of stand age and leaf area index (LAI) on net ecosystem gas exchange. For grasslands, A(smax) increased by 7 mu mol m(-2) s(-1) per unit LAI, for C-4 crops by 11 mu mol m(-2) s(-1) and for coniferous forests by 0.9 mu mol m(-2) s(-1) per unit LAI. In contrast, A(smax) of broad-leaved forests and C-3 crops as well as R-smax stayed m(-2) s(-1) constant over a wide range of LAI. A(smax) and R-smax of forests were lowest in young stands (< 20 years old) and highest in stands of age 38-80 years. A(smax) of old forests (> 160 years) was within the same range as those of 30- to 80-year-old forests, and always higher than those of regenerating stands. R-smax seemed to decrease with age. A(smax) increased linearly with ecosystem surface conductance for all vegetation types (r(2) = 0.65). A(smax) of forests and grasslands was closely related to E-smax (r(2) = 0.87), with a slope of 0.082 mu mol CO2 m(-2) s(-1)/mmol H2O m(-2) s(-1). The results clearly illustrated where gaps in our knowledge exist and how ecosystem properties affect the capacity of net ecosystem gas exchange.