Drought predisposes pinon-juniper woodlands to insect attacks and mortality

To test the hypothesis that drought predisposes trees to insect attacks, we quantified the effects of water availability on insect attacks, tree resistance mechanisms, and mortality of mature pinon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma) using an experimental drought study in New Mexico, USA. The study had four replicated treatments (40x40m plot/replicate): removal of 45% of ambient annual precipitation (H2O); irrigation to produce 125% of ambient annual precipitation (H2O+); a drought control (C) to quantify the impact of the drought infrastructure; and ambient precipitation (A). Pinon began dying 1yr after drought initiation, with higher mortality in the H2O treatment relative to other treatments. Beetles (bark/twig) were present in 92% of dead trees. Resin duct density and area were more strongly affected by treatments and more strongly associated with pinon mortality than direct measurements of resin flow. For juniper, treatments had no effect on insect resistance or attacks, but needle browning was highest in the H2O treatment. Our results provide strong evidence that 1 yr of severe drought predisposes pinon to insect attacks and increases mortality, whereas 3yr of the same drought causes partial canopy loss in juniper.