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.