The ponderosa pine forests and piñon-juniper woodlands of the southwestern US have recently experienced unusually high amounts of drought-related tree mortality, with greater amounts at the species’ lower elevation ecotone. Trees often adjust hydraulic or morphological characteristics in response to water stress. We hypothesized that, compared with trees at high elevations, trees at low elevations would exhibit (1) lower vulnerability to cavitation (VC), (2) lower predawn water potential (ΨPD) but similar midday water potential (ΨMD), (3) lower leaf mass per area (LMA), and (4) greater sapwood-to-leaf area ratio, or Huber value. For both ponderosa pine (Pinus ponderosa) and piñon pine (P. edulis), we selected one upper and one lower ecotone site along an elevation gradient in northern Arizona, with the lower and upper site for the respective species being the same location. The ponderosa-piñon pine site also included Utah juniper (Juniperus osteosperma). Comparisons between elevations were made for both pine species of root and shoot VC using the air-injection and the centrifuge methods, ΨPD and ΨMD during the driest annual period for two years, LMA, and Huber value.
Results/Conclusions
For all species and sites, roots were more vulnerable to cavitation than shoots. For ponderosa pine, VC did not differ between elevations for both roots and shoots. For piñon pine, VC was lower at low elevation than high elevation for both tissues. Root VC was similar for both pines where they co-occurred at the ponderosa-piñon site, but ponderosa pine shoots were less vulnerable than piñon pine. Utah juniper roots and shoots were less vulnerable than the pines. Ponderosa pine ΨPD showed distinct drought-stress at low elevation compared to high elevation (2005: ΨPD= –1.3 and –0.8 MPa, P=0.0009; 2006: ΨPD= –1.9 and –0.8 MPa, P<0.0001; low and high elevations, respectively), and ΨMD had a similar pattern (P=0.007). Piñon pine ΨPD showed a similar but less pronounced pattern (P=0.055), while ΨMD was less negative at the high elevation site (2005: ΨMD= –2.2 and –1.9 MPa, P=0.037; 2006: ΨMD= –2.1 and –1.7 MPa, P<0.0001; low and high elevations, respectively). LMA did not differ between elevations for either species, but the Huber value was greater at low elevation for both species (ponderosa pine, P=0.0003; piñon pine, P=0.055) because of less leaf area. Lower Huber value at low elevation likely reduced water loss and cavitation, especially for ponderosa pine, while piñon pine adjusted to increased drought-stress via a reduced VC.