Summer water source partitioning by pinon and juniper: Quantifying water source and total water use by two Great Basin tree species in central Nevada

Water use by single-leaf pinon (Pinus monophylla) and Utah juniper (Juniperus osteosperma) was measured at two sites in a mountain range in central Nevada during the summer of 2012 using thermal dissipation sapflow techniques. The sites were distinguished as a relatively flat "valley" site at the floor of the canyon, and an "upland" east-facing hillslope. Small (~11.9cm) and medium (~23.9cm) diameter size classes were measured for each species, representing the variation found in natural pinon-juniper stands. Sapflow was measured for medium pinon at both sites in 2011. Precipitation for this area was below the 29 year average of 668 mm in 2012, and above average in 2011. When sapflux was scaled to actual tree size, juniper used more water on average at both sites than pinon. Additionally, water use by small pinon at the valley site was significantly lower than other pinons over the sample period. Pinon and juniper have been shown to reduce effective precipitation by increasing rainfall interception and thus reducing the amount of water that would be available for shrubs and grasses. During larger rain events these trees generate stemflow that is funneled to the base of the tree where it infiltrates the soil via preferential flow paths along roots. This leads to a significant increase in soil moisture that may be available to the tree to relieve plant stress during dry summer months. Current ecological theory predicts stemflow should be beneficial to the plant that generated the stemflow, but this has not been previously tested. We used isotopically labeled water and simulated stemflow generated from a 20 mm rainstorm to test the theory that trees that generate stemflow benefit from this water source. This experiment was conducted during the summer in both a wet and a dry year. Results determined that stemflow is taken up by pinon and juniper, though the addition and use of this water was not sufficient for relieving plant stress during a dry summer. However, pinon did experience an alleviation of water stress in a wet summer with the addition of stemflow. Differences in water uptake patterns illustrate differences in rooting characteristics between pinon and juniper, as well as differences in drought tolerance. Determining the fate of stemflow is necessary for understanding how these trees may alter the water balance in semiarid ecosystems to their advantage, in order to out-compete shrubs and grasses and increase their distribution.