Evapotranspiration partitioning in a semiarid woodland: Ecohydrologic heterogeneity and connecitvity of vegetation patches

Partitioning evapotranspiration into its evaporation and transpiration components is critical for understanding ecohydrologic processes in dry lands. Existing partitioning estimates, however, have not adequately accounted for the heterogeneity associated with woody plant canopy patches and inter canopy patches so characteristic of dry land ecosystems. We measured water contents, stable isotopes (δ ²H and δ ¹⁸O), Cl ^-, and NO ₃ ^- from core samples collected during an intense drought in canopy and inter canopy patches in a semiarid, piñon-juniper [Pinus edulis Engelm.- Juniperus monosperma (Engelm.) Sarg.] woodland in northern New Mexico to assess patch-scale heterogeneity and evapotranspiration partitioning. Soil zone residence times based on Cl ^- ranged from 6 to 37 yr, highlighting the long time scale of percolation in these woodlands. The average NO ₃ ^- concentration was nearly seven times lower in canopy patches, indicating substantial biogeochemical heterogeneity. Average δ ²H values from shallow soil (<0.1 m) were 11 to 17‰ lower in canopy patches, suggesting lower soil evaporation losses compared with inter canopy patches; however, significantly larger Cl ^- inventories in canopy patches indicate up to four to six times more total evapotranspiration. Taken together, lower evaporation and greater evapotranspiration suggest that canopy patches have substantially larger transpiration rates and lower evaporation/transpiration ratios than inter canopy patches. Our results support a basic but untested conceptual model of patch connectivity where woody plants utilize substantial amounts of inter canopy water that has been redistributed from inter canopy to canopy patches via hydraulic gradients created by root uptake a finding not generally modeled but potentially relevant to globally extensive patchy-structured drylands.