Defining the physiological drought stress threshold for susceptibility to bark beetle: a case study for Jeffrey pine in the Sierra Nevada, California
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. Our hypotheses were that 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, resin production is highly upregulated (and attractive to beetles), but there is insufficient turgor potential in the bole cambium to express the resin. In order to test this, mature Jeffrey pine (Pinus Jeffreyi Grev & Balf.) was studied at 5 locations along a 1000 km latitudinal gradient on the eastern slope of the Sierra Nevada in years with varying site water deficits (45 to 95% of long term average annual precipitation), in dense stands and in stands that had been thinned > 20 yrs prior to the study. At each of 5 locations, pre-dawn and noon needle water potential, their delta, and needle and bole phloem turgor potential were used to define the physiological stress threshold that reduced resin flow rate and altered resin quality.
At the northern-most site (Lassen National Forest), trees in dense stands were more drought stressed than in thinned stands. At the southern 4 sites (Tahoe, Inyo, Sequoia, and San Bernardino NF), lower site available water increased tree drought stress, but there was no difference in tree stress between dense and thinned stands. Jeffrey pine that was attacked by bark beetle had greater tree to tree competition, but attacks occurred more frequently in thinned stands. Thinned stands were characterized by (dense) clumps of trees with large gaps between clumps, a pattern that was repeated across all 5 National Forests. Physiological tree drought stress as measured in the canopy was correlated to: 1) lower turgor potential in bole phloem, 2) a specific signature of resin quality, and 3) low resin exudation flow. In this study of 500+ trees, 9% were attacked by Jeffrey pine beetle, primarily in the year following droughts; 7% of the attacks occurrred in thinned stands; and there was no trend of increasing mortality with decreasing latitude.