1. The elevational gradient in plant defense (EGPD) hypothesis posits that natural enemy pressures increase positively alongside temperature across elevational climatic gradients, thereby selecting for enhanced defenses at lower elevations while leaving plants less defended at higher elevations. Phylogenetically constrained tests of this hypothesis in tree populations are exceedingly rare. Nevertheless, the presumed presence of poorly-defended trees has been invoked as an important driver of recent pest outbreaks at higher elevations than historically common.
2. Tree age, growth rate, and size have all been correlated with levels of tree defenses. Thus, we sought to disentangle the interacting influences of these properties from possible elevational climatic effects on monoterpene composition, concentrations, and diversity of constitutive resin within three widespread pines (Pinus contorta, Pinus ponderosa, and Pinus flexilis) across a 1532 m elevational transect in the Rocky Mountains, Colorado, USA.
3. Collectively, elevation and tree properties were relatively weak predictors of defenses in P. ponderosa and P. flexilis, but explained ~ 75% of variation in monoterpene concentration and ~ 50% of diversity in P. contorta.
4. Increasing tree age had the greatest positive influence on monoterpene concentration and diversity in P. contorta, while increasing tree size had a negative influence suggesting a potential lifelong tradeoff between tree growth and defense. Elevation had a significant, negative influence on monoterpene concentration but little to no effect on monoterpene diversity in P. contorta.
5. Overall, we found some support for the EGPD hypothesis within P. contorta, but no support within P. ponderosa or P. flexilis. Our results suggest the presence of divergent conifer defense allocation strategies and drivers even among congeners growing in shared environments. An improved understanding of the controls on tree defenses, particularly possible influences of climate-based drivers, is necessary for predicting forest pest dynamics under global change scenarios.