• Understanding controls over plant–atmosphere CO 2 exchange is important for
quantifying carbon budgets across a range of spatial and temporal scales. In this
study, we used a simple approach to estimate whole-tree CO 2 assimilation rate
(A Tree ) in a subalpine forest ecosystem.
• We analysed the carbon isotope ratio (d 13 C) of extracted needle sugars and
combined it with the daytime leaf-to-air vapor pressure deficit to estimate tree
water-use efficiency (WUE). The estimated WUE was then combined with obser-
vations of tree transpiration rate (E) using sap flow techniques to estimate A Tree .
Estimates of A Tree for the three dominant tree species in the forest were combined
with species distribution and tree size to estimate and gross primary productivity
(GPP) using an ecosystem process model.
• A sensitivity analysis showed that estimates of A Tree were more sensitive to
dynamics in E than d 13 C. At the ecosystem scale, the abundance of lodgepole pine
trees influenced seasonal dynamics in GPP considerably more than Engelmann
spruce and subalpine fir because of its greater sensitivity of E to seasonal climate
variation.
• The results provide the framework for a nondestructive method for estimating
whole-tree carbon assimilation rate and ecosystem GPP over daily-to weekly time
scales.
