The elevational gradient in plant defense (EGPD) hypothesis posits that natural enemy pressures increase 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 are uncommon, with tests focused on defenses of mature trees in natural settings being exceedingly rare. In the absence of this information, predicting the spatiotemporal dynamics of forest pests that preferentially attack mature trees is rendered more difficult. Tree properties such as age, growth rate, and size have all been correlated with levels of tree defenses against insect pests. Thus, we sought to disentangle the interacting influences of these properties from possible elevational climatic effects on monoterpene concentrations, diversity, and intraspecific dissimilarity of constitutive resin within three widespread pines (Pinus contorta, Pinus ponderosa, and Pinus flexilis) across a 1530-m elevational transect in the Rocky Mountains, Colorado, USA. Overall, we found some support for the EGPD hypothesis within all three of the pine species studied. However, while elevation and tree properties were strong predictors of the variation in monoterpene concentration and moderate predictors of variation in diversity and dissimilarity in P. contorta, they were a relatively weak set of predictors in P. ponderosa and P. flexilis. Significant, (negative) elevational effects were found for total monoterpene concentration in P. contorta and monoterpene diversity in P. ponderosa, while elevation had a significant negative effect on monoterpene concentration and diversity, and a positive effect on dissimilarity in P. flexilis. Nevertheless, tree age, size, and/or growth measures had significant influences on one or more monoterpene measures in all three of the pine species indicating a need to parse effects of these factors within studies of tree defenses across environmental gradients. Our results offer some support for the EGPD hypothesis while suggesting the presence of a variable hierarchy of drivers of pine defense and resistance across congeners in shared environments—a situation that presents challenges for predicting spatiotemporal patterns in secondary chemistry across species from a common set of potential drivers.
Ferrenberg, S., J. M. Langenhan, S. A. Loskot, L. M. Rozal, and J. B. Mitton. 2017. Resin monoterpene defenses decline within three widespread species of pine (Pinus) along a 1530-m elevational gradient. Ecosphere 8(10):e01975. 10.1002/ecs2.1975