The role of biotic and abiotic stressors in Jeffrey pine susceptibility to bark beetle

Background/Question/Methods Mature Jeffrey pine (Pinus Jeffreyi 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 (ET) demands. 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 Jeffrey pine beetle (Dendroctonus jeffreyi Hopk.) included resin quality and exudation flow rates, and xylem pressure potential, as well as phloem tissue quality.

Results/Conclusions

Tree drought stress was not correlated to latitude, e.g., the southern populations were not necessarily more drought stressed that northern populations with significantly lower ET. Local precipitation patterns dominated the level of physiological tree drought stress that developed at each of the 5 sites. ET and soil water deficit drove physiological tree drought stress north of 37 N latitude, but only soil water deficits affected tree drought south of 37 N. In general, individual trees within high density stands developed greater physiological tree drought stress and were more susceptible to bark beetle attack. However, in dense stands with high average, annual rates of basal area increments, trees did not develop physiological drought stress, and were not attacked by Jeffrey pine beetle. Physiological tree drought stress as measured in the canopy was correlated to phloem pressure potential, a specific signature of resin quality, inversely correlated to exudation flow rates, and positively correlated to bark beetle and wood borer (Buprestidae) attack in Jeffrey pine.