Sap flux of co-occurring species in a western subalpine forest during seasonal soil drought

Co-occurring species may utilize vastly different strategies to cope with limited water resources, particularly in areas subjected to predictable and recurring drought. While these physiological responses have commonly been measured at the leaf level, in small seedlings, and integrated in fluxes of whole stands or watersheds, sap flux measurements in large trees have become a useful tool for monitoring transpiration of individual canopies over long time periods. In this study, sap flux (J(S)) was measured with constant heat sap flow gauges for co-occuning species which have been previously evaluated at the leaf level. Measurements were taken in a subalpine stand containing most of the dominant species of the central Rocky Mountains (Pinus contorta, Abies lasiocarpa, Populus tremuloides, and Pinus flexilis). Daily J(S) values were parabolically related to daytime average atmospheric vapor pressure deficit (D) in all species, with a broad range of maximum J(S) values occurring between 1.2 and 1.8 kPa. Populus tremuloides had the greatest increases in J(S) with increasing D, while Pinus contorta showed the lowest J(S). A decrease in maximum J(S) was observed for all species later in the season when soil moisture declined from 0.35 to 0.24 m(3)/m(3) at 0-45 cm. Late-season J(S) in A. lasiocarpa decreased 50% due to stomatal closure in response to the soil moisture deficit, regardless of daily D. In contrast, the Pinus species were sensitive to D, showing larger late-season reductions in J(S) on high than on low D days. Populus tremuloides showed less sensitivity to soil moisture than the other species, with relatively high J(S) continuing late into the season and intermediate change in the response of J(S) to D with decreasing soil moisture. Stand-level estimates of transpiration by plots dominated by Pinus contorta and A. lasiocarpa (2.6 +/- 0.6 mm/d) were found to be similar to plots dominated by Populus tremuloides (2.7 +/- 0.6 mm/d) despite the nearly fourfold higher leaf area indices for the conifers.