Print PageFew studies have experimentally manipulated water availability to mature trees and measured changes in insect resistance mechanisms, insect attacks, and subsequent tree survival to address the role of drought in predisposing woodlands to insect-induced mortality. Our objectives were to quantify insect attacks and tree resistance mechanisms at an experimental drought study in a piñon-juniper woodland in central New Mexico, USA. The study, initiated in Sep. of 2007, consists of four treatments applied to 40 m x 40 m plots and replicated in three blocks from 2008-2010. Treatments are: 1) ambient precipitation (A), 2) removal of 50% ambient annual precipitation (H2O-), 3) irrigation to produce 150% addition of ambient annual precipitation (H2O+), and 4) an infrastructure control that measured the impact of precipitation removal/addition equipment without changing the annual amount (CC). In piñon pine we measured insect attacks and insect resistance characteristics including bole and twig resin flow after wounding, and resin duct abundance. We also examined the carbon isotope ratio (δ13C) of recently assimilated leaf sugars and resin of piñon pine to better understand carbon sources used for constitutive bole resin production. In juniper we measured bole resin flow, insect attacks, and canopy condition.
Results/Conclusions
Treatments had no effect on piñon or juniper bole resin flow. Piñon twig resin flow was significantly lower in the H2O- and H2O+ treatments than in the A treatment on two dates. By August 2010 significantly more piñon pines (72%) in the H2O- treatment had died than in the other treatments (11, 6 and 7%, for H2O+, A, and CC, respectively). Bark and twig beetle attacks were present on 92% of dead trees. By 2010, several measures of piñon pine resin duct abundance were lowest in the H2O- treatment. Results for δ13C suggest a partial decoupling of recent carbon assimilation and constitutive bole resin of piñon pine consistent with the lack of treatment effect on bole resin flow. The H2O- treatment increased needle browning of juniper, but did not cause tree death or increase insect attacks. Our results provide strong experimental evidence that 50% less than average precipitation for 2+ years will kill many piñon pines, and will stress but not kill junipers. The amplification of drought impacts by beetle attacks on dominant trees shown with piñon pine in our study may be a harbinger of an increasing future role of lethal biotic agents to tree mortality and ecosystem change.
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