Regional scale tree mortality after drought events is a common occurrence and is predicted to increase with climate warming. Isolating the mechanism of tree mortality, however, is challenging. Our study addresses two hypotheses regarding regional piñon pine (Pinus edulis) mortality that occurred after the 2002 drought in the southwestern USA. Hypothesis one – hydraulic failure – posits that drought directly kills trees by desiccation induced by xylem cavitation. The second hypothesis– carbon starvation – posits that drought results in depletion of carbon reserves, through low photosynthesis or direct water potential constraints on resin production, which in turn predisposes trees to mortality from insect attack. Our objectives are to quantify insect attacks and resistance mechanisms at an experimental drought study. The experimental study started in 2007 and is located at the Sevilleta LTER in central New Mexico and is the site of ongoing, collaborative tests of the hydraulic mechanisms underlying mortality. It includes four 40 x 40 m treatment plots replicated in three blocks. Treatments consist of ambient, removal of 50% ambient annual precipitation, irrigation to produce 150% addition of ambient annual precipitation, and an infrastructure control that measures the impact of precipitation removal/addition equipment without changing the annual amount.

Results/Conclusions

Measurements of insect resistance mechanisms including twig and bole resin flow and needle toughness, and insect surveys were conducted for one year pretreatment (2007) through two years post-treatment (2008-2009). Measured insect resistance characteristics have been similar among treatments to date. In August 2008, however, 13% of all trees were attacked by bark beetles (Ips confusus). 75% of these trees were located in the water removal treatment with the remaining 25% in the water addition treatment. This result suggests that the experimental drought predisposed some trees to bark beetle attacks. To evaluate our hypotheses, results from this study will be compared with results from a concurrent regional study which will use a combination of approaches including SEM imaging of tracheid pit aspiration, presence of bark beetle attacks; measurements of radial growth, xylem resin duct density and size, and Δ¹³ C discrimination for dead versus live trees.  Sampling of the regional field sites began in 2009. In addition, comparison of the measured insect resistance characteristics to hydraulic tests conducted by our collaborators at the Sevilleta site is allowing a holistic examination of the mechanisms of regional vegetation mortality.