Characteristics of whitebark pine (Pinus albicaulis) growth & defense in disturbance-prone, high-elevation, montane ecosystems of the northern Rocky Mountains

Whitebark pine (Pinus albicaulis) is a high-elevation conifer, recognized as a foundation species due to the numerous ecological benefits it provides in subalpine environments. In whitebark pine and other conifers, resin-based defenses have long been recognized as the primary mechanism by which trees respond to bark beetle attacks and several studies have linked resin duct properties to survivorship during periods of increased beetle activity. Utilizing a unique dataset of whitebark pine collected on the Flathead Indian Reservation in northwestern Montana, we set out to investigate the following research questions:

(1) Are there differences in physiology (tree growth and resin duct anatomy) between trees that persisted through recent mountain pine beetle outbreaks and trees that died?

(2) Does constitutive resin chemistry differ between whitebark and co-occurring lodgepole pine and are there relationships between tree growth, resin duct anatomy and resin chemistry?

(3) Does competition influence constitutive resin chemistry in either whitebark or lodgepole pine? and

(4) Is whitebark pine growth and/or resin duct anatomy constrained by warmer and/or regionally drier conditions?

We found that whitebark pine trees that have persisted through recent stand-level disturbance produced fewer but larger resin duct structures with greater duct area compared to trees that died. We also detected important differences in the chemical composition of resin between whitebark and lodgepole pine that generally support field observations, whereby under endemic scenarios mountain pine beetle preferentially select lodgepole pine, while under outbreak scenarios, beetles successfully colonize whitebark pine trees. We found complex relationships between tree growth, resin duct anatomy and constitutive resin chemistry that present beetles with many permutations of resin-based defenses, while competition, particularly with Engelmann spruce (Picea engelmannii) can further influence constitutive resin chemistry. Lastly, we found that whitebark pine across our study sites are experiencing increased growth and defense under warmer and regionally drier conditions. Whitebark pine at our study sites exhibit differing strategies in the allocation of resources toward growth and defense. Our results support the idea that maintaining genetic variability promotes diverse response strategies to a complex array of biophysical stressors that might leave a species vulnerable to extinction across its range.