Abstract
High-elevation, snow-dependent, semiarid ecosystems across southwestern United States are expected to be vulnerable to climate change, including drought and fire, with implications for various aspects of the water cycle. To that end, much less is known about the dynamics of transpiration, an important component of the water cycle across this region. At the individual-tree scale, transpiration is estimated by scaling mean sap flux density by the hydroactive sapwood area (SA). SA also remains a key factor in effectively scaling individual tree water-use to stand level. SA across large spatial scales is normally established by relating SA of a few trees to primary size measures, e.g., diameter at breast height (DBH), tree height (H), or canopy diameter (CD). Considering the importance of SA in scaling transpiration, the primary objective of this study was therefore to establish six species-specific (aspen, maple, white fir, ponderosa pine, Douglas fir, Englemann spruce) allometric relationships between SA and three primary size measures (DBH, CD, or H) across two high-elevation, snow-dependent, semiarid ecosystems in New Mexico and Arizona. Based on multiple statistical criteria (coefficient of determination, index of agreement, Nash–Sutcliffe efficiency) and ease of measurement in the forest, we identified DBH as the primary independent variable for estimating SA across all sites. Based on group regression analysis, we found allometric relationships to be significantly (p < 0.05) different for the same species (ponderosa pine, Douglas-fir) across different sites. Overall, our allometric relationships provide a valuable database for estimating transpiration at different spatial scales from sap flow data in some of our most vulnerable ecosystems.
Highlights
High-elevation, snow-dependent, semiarid forest ecosystems across the southwestern United States are vulnerable to climate change (Williams et al 2010; Allen et al 2015), with significant implications for the associated coupled human–natural systems (Seager et al 2007)
diameter at breast height (DBH) accounted for more than 90% of the variation in sapwood area (SA) using both the power (Eq 1) and log (Eq 4) functions for ponderosa pine in New Mexico (Fig. 2c, Table 2, Table S2)
DBH was the best estimator of SA for maple at Arizona, with both power (Table 2) and log functions (Table S3) providing similar coefficient of determination (Fig. 3a)
Summary
High-elevation, snow-dependent, semiarid forest ecosystems across the southwestern United States are vulnerable to climate change (Williams et al 2010; Allen et al 2015), with significant implications for the associated coupled human–natural systems (Seager et al 2007). These include changes in the hydrologic inputs to these ecosystems including alteration in the timing of the peak streamflow runoff and changes in precipitation frequency and magnitude (Barnett et al 2005). Scaling of sap flux density to the canopy and landscape level to quantify transpiration is crucial for addressing critical questions with regard to impact of climate change and with regard to “forest water use and potential water conservation on ecosystem-scale processes” (Warren et al 2011)
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