Western US forests are experiencing high mortality from bark beetles. Western yellow pine exudes resin as a physical and chemical defense against attack. Resin production may be constrained by both carbon balance and hydraulic status. We investigated physiological drivers of resin production in Jeffrey pine across a 600 mile latitudinal range of forest stands in the upper montane, on the eastern and western slopes of the Sierra Nevada over three years with differing levels of physiological tree drought stress. Tree drought stress can cause both hydraulic and carbon limitations; the effect of both were tested for correlation with resin exudation (over a 24 h period) in early and late summer. The level of biotic stress (tree-tree competition, stand density), physiological drought stress (pre-dawn and solar noon needle water potential, needle and bole turgor potential at noon), and rates of resin exudation (in contained collection tubes) were measured on 8 trees per stand, 6-7 stands per National Forest, in six National Forests. An additional 300 trees were assessed in Sequoia National Park, CA. Additionally, for a subset of 68 trees, we performed GC-FID analysis on volatile extracts from resin samples to determine if resin quality is affected by drought stress.
Results/Conclusions
The rate of resin exudation declined with increasing level of tree drought stress experienced, with an abrupt drop in exudation occurring at bole cambium water potential values lower than -2.0 MPa. Bole cambium osmoticum, which could have a protein, carbohydrate, or energetic basis, was parabolically related to resin exudation rate: low at both low and high tissue osmotic potential, and high in the mid-range of the response, with remarkably consistent (maximum value: -1.4 MPa). The maximum resin exudation rate varied from stand to stand suggesting an environmental and/or genetic basis of variation, but the osmotic maxima was consistent across sites and seasonality. Needle osmoticum showed a threshold response, where values less than -3.5 MPa resulted in significantly reduced resin exudation rates. Resin quality (relative amounts of alkanes and terpenes) significantly varied with the level of tree drought stress. These values, either as thresholds or maxima, could be used in process-based models to predict pine susceptibility to successful bark beetle attack, as resin exudation is both a chemical and physical barrier to beetle attacks.