Ecosystem water use efficiency (WUE;  the ratio of net ecosystem productivity to evapotranspiration rate;  NEP/ET) is a complex parameter controlled by both physical and biological processes.  We took advantage of a nine year eddy flux record from neighboring forest communities with different mean ecosystem WUEs to ask the question:  can we explain differences in WUE at the ecosystem scale with a detailed examination of species differences in WUE at the leaf scale?  We analyzed approximately 7,000, 30-min averaged eddy flux observations with different wind directions and with micrometeorological variables within the same narrow margins. This allowed us to independently assess ecosystem WUE for lodgepole pine- and spruce fir-dominated forest stands.  

Results/Conclusions

Mean ecosystem WUE was 19% lower for the community in which Engelmann spruce and subalpine fir were dominant (westerly wind), compared to the community in which lodgepole pine was dominant (easterly wind).  Of that 19% difference, 8% was attributed to residual bias in the analysis that favored slightly higher ET rates from the spruce-fir dominated community.  In an effort to explain the remaining 11% difference, we used measurements of needle carbon isotope ratio, along with observations of community species composition and leaf area index (LAI) distribution.  We could only explain a small fraction of the difference in ecosystem WUE between the two communities using needle-scale measurements.  In these analyses, we distinguished two species effects – one in which different species with different inherent (physiological) WUEs compose the two communities, and one in which different species with different canopy architectures (different proportions of needles in upper versus lower canopy layers) compose the two communities.  Only the canopy architecture effect had the potential to explain differences in ecosystem WUE.  The magnitude of this effect, however, was weakened by interactions with the physiology effect – as more leaf area was shifted to lower parts of the canopy in the western spruce-fir community, with concomitant reductions in iWUE, it was also shifted to favor spruce trees, which operate at a higher iWUE compared to fir and pine.  After accounting for tree species effects, the greatest fraction of the differences in ecosystem WUE between the two communities remained unexplained; it is likely that soil fluxes, and other non-needle fluxes, account for most of the difference. Thus, while there are distinct species differences in inherent photosynthetic WUE and tree crown architecture in these two communities, these differences could not explain most of the difference in ecosystem WUE.