Water-use efficiency as a means of modelling net assimilation in boreal forests

Although the processes governing photosynthesis are well understood, scalin g from shoot to canopy in coniferous forests is complex. Development of dif ferent sap-flow techniques has made it possible to measure transpiration of whole trees and thereby also of whole canopies. There is a strong link bet ween photosynthesis and transpiration, For which reason it would be interes ting to test whether measurements of canopy transpiration could also be use d to estimate canopy photosynthesis. As a first step towards this, water-us e efficiency (WUE) was studied at branch and canopy scales on the basis of branch gas-exchange measurements, with half-hourly and daily temporal resol ution. Half-hourly and daily WUE at both branch and canopy scales showed a strong dependency on vapour-pressure deficit (deltae). Branch photosynthesi s modelled from branch transpiration and deltae mimicked well measured bran ch photosynthesis. Also, modelled photosynthesis, scaled to canopy and comp ared to net forest CO2 exchange measured by the eddy-covariance technique, occasionally showed good agreement. In spite of these seemingly promising r esults, there was a difference in the response to deltae between branches a nd between years, which needs to be better understood.