PARENT SESSION
Oral Session #63: Plant Ecology: Water Relations. Presiding: J. Cleverly.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Hall of Ideas H.

Herbivory affects plant soil moisture, plant water potential and whole-tree water use in piñon pine, Pinus edulis.

Hedlund, Megan¹, Montes-Helu, Mario¹, Hungate, Bruce¹, Koch, George¹, ¹

ABSTRACT- Herbivores are increasingly recognized as important controllers of ecosystem functions but few studies, particularly in arid systems, have addressed the consequences of herbivory for plant water relations and system-level water fluxes. On the Colorado Plateau, the piñon pine, Pinus edulis, is commonly infested with the piñon needle scale, Matsucoccus aecalyptus, resulting in a single age class of healthy needles compared to 5-8 years of needles on scale resistant trees. We have examined the effects of foliar herbivory on components of plant and soil water relations and patterns of tree-level transpiration at Sunset Crater National Monument in northern Arizona. Soil moisture and predawn water potential are significantly higher under trees with scale compared to trees uninfested by scale. Using gas exchange and sap flow methods we have found that trees with scale have higher conductance (20 −50 % increase) and reduce stomatal conductance significantly later in the day (1 to 3 hours) than trees without scale. The increased leaf-level transpiration, however, does not compensate for the loss of foliage; whole-tree water use is greater in scale-free trees. Carbon isotope analysis of needles indicate that trees infested by scale are less sensitive to interannual variation in precipitation; ¹³C varied by 1.5 ‰in scale-free trees, but by only 0.5‰in scale-infested trees across 3 years spanning a record drought. These results indicate that herbivory can significantly affect important components of ecosystem water balance.

KEY WORDS: herbivory, water relations, stomatal conductance