PS 15-206 - Sap flow of Single Leaf Piñon and Utah Juniper in a Great Basin woodland: a six-year dataset to quantify tree water use at the stand-scale

Abstract: Our objective was to quantify long-term water use dynamics of single-leaf piñon (Pinus monopylla) and Utah juniper (Juniperus osteosperma). Sap flux density was measured using thermal dissipation sap flux techniques at an instrumented watershed in the Central Great Basin in Nevada. Three sites were instrumented: a flat valley site, a west aspect hillslope site, and an east aspect hillslope site and measured continuously for six years (2013-2018). Total sapwood area and stand density data were used to scale sap flux density to the whole-tree and stand-level scales. Precipitation for the period encompassed a four-year drought, two wetter than average years and an average year. Tree water use from these three sites was averaged to get representative estimates of water use for these species at a landscape-scale. Average daily sap flow for 29 piñons and 16 junipers ranged from 0.5 to 43.5 L d^-1 and 0.6 to 33.2 L d^-1 for piñon and juniper, respectively. Average daily sap flow per unit basal area ranged from 0.00055 to 0.11 L cm^-2 d^-1 and 0.0030 to 0.10 L cm^-2 d^-1 for piñon and juniper, respectively. In general piñon was more highly regulated by declines in soil moisture relative to juniper. At the stand-level scale total piñon and juniper water use ranged from 0.0076 to 1.45 mm d^-1 and 0.0052 to 0.18 mm d^-1 for piñon and juniper, respectively. Total stand-scale transpiration for both species combined ranged from 0.035 to 1.61 mm d^-1. Although whole-tree water use and water use per basal area was similar in magnitude for both species, the stand-level water use of juniper was an order of magnitude less than piñon. Total annual tree transpiration for the site ranged from 144.7 to 213.6 mm, where the transpiration fraction of water year precipitation showed a negative relationship with increasing precipitation (r²=0.70), indicating that these trees are partially decoupled from precipitation and are likely accessing water stored at deeper depths. The transpiration fraction of water year precipitation ranged from 0.46 to 0.91. These data represent the longest continuously measured time-series in a piñon and juniper woodland in the Great Basin, and to our knowledge no studies have been conducted previously on single-leaf piñon. These data suggest that piñon and juniper woodlands are a significant contributor to total evapotranspiration across a range of precipitation inputs.