Intra- and interspecific variation for summer precipitation use in pinyon-juniper woodlands

In the arid southwest of North America, winter precipitation penetrates to deep soil layers, whereas summer "monsoon" precipitation generally wets onl y surface layers. Use of these spatially separated water sources was determ ined for three dominant tree species of the pinyon-juniper ecosystem at six sites along a gradient of increasing summer precipitation in Utah and Ariz ona. Mean summer precipitation ranged from 79 to 286 mm, or from 18% to 60% of the annual total across the gradient. We predicted that, along this sum mer rainfall gradient, populations of dominant tree species would exhibit a clinal off-on response for use of water from upper soil layers, responding at particular threshold levels of summer precipitation input. This predict ion was largely supported by our observations of tree water source use over a two-year period and from irrigation experiments. Hydrogen and oxygen stable isotope ratios (deltaD and delta O-18) of tree x ylem water were compared to that of precipitation, groundwater, and deep an d shallow soil water to distinguish among possible tree water sources. delt aD-delta O-18 relationships and seasonal xylem water potential changes reve aled that trees of this ecosystem used a mixture of soil water and recent p recipitation, but not groundwater. During the monsoon period, a large propo rtion of xylem water in Pinus edulis and Juniperus osteosperma was from mon soon precipitation, but use of this precipitation declined sharply with dec reasing summer rain input at sites near the regional monsoon boundary in Ut ah. Quercus gambelii at most sites along the gradient used only deep soil w ater even following substantial inputs of summer rain. Populations of Querc us at sites with the highest average summer precipitation input, however, p redominantly used water in upper soil layers from recent summer rain events . Soil temperature correlated with patterns of summer precipitation use acr oss the gradient; high soil temperatures north of the monsoon boundary may have inhibited surface root activity for some or all of the three tree spec ies. Irrigation experiments with deuterium-labeled water revealed that Quercus g ambelii in northern Arizona and southern Utah did not use water from surfac e layers. In contrast, Juniperus osteosperma at these sites responded signi ficantly to the irrigations: between 37% and 41% of xylem water originated from irrigations that wetted only the top 30 cm of soil. Responses by Pinus edulis to these irrigations were variable; uptake of labeled water by this species was greater in September at the end of the summer than during the hot midsummer period. Inactivity of Pinus roots in midsummer supports the h ypothesis that root activity in this species is sensitive to soil temperatu re. Seasonal patterns of leaf gas exchange and plant water potential correspond ed to the seasonality of rainfall at different sites. However, no correlati on between a species' ability to use summer rainfall and its tolerance to w ater deficits at the leaf level was found. Midday stomatal conductance (g(s )) for Pinus needles approached zero at predawn water potentials near -2 MP a, whereas g(s) in Quercus and Juniperus declined to zero at -2.8 and -3.7 MPa, respectively. The relationship between photosynthesis (A) and g(s) was similar among the three species, although Quercus maintained higher overal l rates of gas exchange and tended to operate higher on the A/g(s) curve th an the two conifers. At sites in eastern Arizona where Quercus fully used m oisture from summer rains, leaf gas exchange characteristics were similar t o those of Pinus and Juniperus.