Water Use Efficiency as a Means for Up Scaling Carbon Fluxes from Leaf to Stand

Abstract:

Estimation of carbon fluxes of small forest stands is needed in order to adequately assess the effect of variable stand conditions and of different management strategies. Such estimations may not be possible using micrometeorological methods such as the eddy covariance technique (EC), as large areas are required with homogeneous land use, management and species composition. Earlier findings show that the leaf scale carbon uptake and water use ratio (water use efficiency, WUE) of beech (Fagus Sylvatica, L.) is homogenous within the canopy only depending on air humidity and light conditions (Linderson et al., 2012). This finding enables estimations of the canopy carbon uptake from its water use as estimated by sap flow measurements and thus to assess the individual tree carbon uptake and its variability.

In this study, the methodology developed for beech is tested for Norway spruce (Picea abies, L.) and further developed to comprise longer time scales (days to seasons) using existing leaf flux measurements from the Skogaryd ecosystem field research station (www.fieldsites.se). The shoot gas exchange was measured once every half hour at several heights in the canopy between 2007 and 2010, using automated chambers tracking ambient meteorological conditions. Air temperature, humidity and PAR were measured simultaneously and adjacent to the shoots. The VPD normalized WUE is assessed as the ratio between the carbon uptake and the conductance, where conductance is estimated from the measured transpiration divided by VPD.

Preliminary results, using data from May to September and 6-18h to make the spruce and beech measurements comparable, show that the leaf scale VPD normalized WUE for spruce reaches light saturation at low PAR (on average 250 μmolm^-2s^-1), compared to beech (on avg. 500 μmolm^-2s^-1). For light saturating conditions, WUE is also higher for spruce (avg. 9 mmolmol^-1hPa) than for beech (avg. 5 mmolmol^-1hPa). These results indicate that spruce has a different water saving strategy than beech. The results will be further analyzed for within-canopy variability.

Linderson, M-L. et al, 2012: Up-scaling of water use efficiency from leaf to canopy as based on leaf gas exchange relationships and the modeled in-canopy light distribution. Agricultural and Forest Meteorology 152:201-211.