Tree physiological basis for Jeffrey pine susceptibility to Jeffrey pine beetle (Dendroctonus jeffreyi Hopk.)

Physiological drought stress triggers a cascade of responses that, depending on the level of stress experienced, may or may not predispose the tree to a successful bark beetle attack. One of the first physiological responses to drought stress is stomatal closure. Under conditions of bright sunlight, there is an excess of photons exciting electrons in the leaf photosystem, less diffusion of CO₂ into the leaf, and the photosynthetic enzyme RUBISCO increasingly mistakes O₂ for CO₂, but is able to free up the photosynthetic pigment to receive another photon. When O₂ accepts the electron, a highly reactive oxygen free radical is formed, which is strongly oxidizing. In response to within-leaf oxidation, α-alanine is produced, which in turn travels to the phloem where jasmonate is upregulated. Jasmonate stimulates resin production in the tree bole.

In this study, the hypotheses tested were 1) under moderate physiological drought stress, resin production is stimulated and turgor potential of the bole is sufficient to express the resin, but 2) under severe tree drought stress, jasmonate is highly upregulated, but there is insufficient turgor potential in the bole cambium to express the resin with high enough flow to act as a physical barrier to bark beetle attack. In order to test this, mature Jeffrey pine (Pinus Jeffreyi Grev & Balf.) along a 480 km N-S latitudinal gradient was studied on the eastern slope of the Sierra Nevada over three years with differing levels of soil moisture and evapotranspirational deficits. At each of 5 locations, stand density and distance to nearest neighbor were used as proxies for biotic stress (competition). Tree attributes used to determine susceptibility to bark beetle included quantitative measures of physiological drought stress, resin quality and exudation flow rates, and whether or not trees were attacked by Jeffrey pine beetle (Dendroctonus Jeffreyi Hopk.).

At the northern-most site (Lassen National Forest), trees in dense stands were more physiologically stressed than in thinned stands. At the southern 4 sites (Tahoe, Inyo, Sequoia, and San Bernardino NF), there was no difference in tree drought stress between dense and thinned stands. Jeffrey pine that was attacked was significantly closer to another single tree (e.g., higher tree-tree competition), but had fewer trees within its sphere of influence (e.g., in lower density stands). Physiological tree drought stress as measured in the canopy was correlated to lower turgor potential in bole phloem, a specific signature of resin quality, and low resin exudation flow. In this study of 530 trees, 9% of the trees were attacked by Jeffrey pine beetle, primarily in the year following drought; 7% of the attacks occurred in thinned stands. There was no trend of increasing mortality with decreasing latitude. Jeffrey pine beetle may be able to detect the differences in resin quality observed in drought-stressed trees.