Canopy adjustment in Pinus edulis as a response to experimental manipulations of water availability

Background/Question/Methods

Climate change predictions for the southwestern United States indicate more severe, prolonged drought events in the near future.  To understand the drought responses of woody species in a semi-arid woodland, we used a two-step allometric approach to non-destructively estimate the response of canopy leaf area (LA) of Pinus edulis trees subjected to four years of rainfall manipulation at the Sevilleta LTER in New Mexico, USA. First, we collected four twigs (0.3-0.7 cm basal diameter) from each of five trees in drought, irrigation, and untreated plots at our study site.  With these samples we developed treatment-specific estimates of the LA per twig basal area for this size class, which bears most of canopy LA in P. edulis. Second, we developed an allometric relationship describing the number of twigs in the 0.3-0.7 cm size class as a function of the basal diameter of large branches.  We estimated canopy LA for each tree in each treatment by measuring basal diameter of all primary branches, calculating the number of 0.3-0.7 cm twigs and applying the treatment specific allometric functions to yield LA borne on these twigs. We validated this method by comparison with direct measurements of LA from destructively harvested branches away from treatment plots.

Results/Conclusions

Our water treatments produced clear differences in the amount of whole-tree leaf area.  Our results indicate an average increase of 20% in the amount of leaf area present on irrigated trees, and a 50% average decrease in whole-tree leaf area for droughted trees in comparison to ambient controls.  The canopy acts as a barrier between the tree and the environment, with many physiological processes functioning in relation to canopy size.  Canopy adjustments of the magnitude shown in our study are likely to impact processes including, but not limited to photosynthesis, respiration, transpiration, and sapflow.