Background
Water-use efficiency (WUE) represents the coupling of forest carbon and water. Little is known about the responses of WUE to thinning at multiple spatial scales. The objective of this research was to use field measurements to understand short-term effects of two thinning treatments (T1: 4,500 stems ha-1; and T2: 1,100 stems ha-1) and the control (C: 27,000 stems ha-1) on WUE (the ratio of leaf photosynthesis to leaf transpiration, or tree growth to tree transpiration, or net stand above-ground biomass (AGB) accumulation to stand transpiration) and the intrinsic WUE (WUEi, the ratio of leaf photosynthesis to stomatal conductance or the net stand AGB accumulation to canopy conductance) in a 16-year old natural lodgepole pine forest. Leaf-level measurements were conducted in 2017, while tree- and stand-level measurements were conducted in both 2016 (the normal year) and 2017 (the drought year).
Results
There was no significant effect of thinning on the tree- and stand-level WUE in 2016, while in 2017, only T2 exhibited significantly higher tree-level WUE (0.63 mm2 kg-1) than the C (0.06 mm2 kg-1), and the stand-level WUE values were significantly higher in the thinned stands, with the means of 0.34, 0.61 and 0.7 kg m-3 for the control, T1 and T2, respectively. Stand-level WUEi was, however, significantly higher in the unthinned stands than in the thinned stands. In contrast, the leaf-level WUE values were not significantly different from each other, while the leaf-level WUEi was statistically higher in C than in the thinned stands. Thinning changed the WUE-VPD relationships at tree and stand levels, but not at the leaf level.
Conclusions
The effects of thinning on forest carbon and water coupling differed with the spatial scales and the metrics (WUE or WUEi) of water use efficiency. Lacking consistent reponses of WUE metrics to thinning treatments across the spatial scales suggests that caution must be exercised when transferring and modeling WUE from one spatial scale to others. Both tree-level and stand-level WUE values in the more heavily thinning stands were significantly promoted under the drough condition, demonstrating that thinning can improve WUE and consequently support forests to cope with the drought effects.