Resin-based defenses in <i>Pinus edulis</i> are only reduced after long-term drought

Constrained carbon allocation toward secondary metabolites involved in chemical defense is a common explanation for widespread drought-related beetle-kill in conifers-we challenge the generality of this explanation. While monitoring drought stress (ψpd), we tracked both carbon reserves (non-structural carbohydrates) and chemical defenses (terpenes, phenolics, resin flow) in mature Pinus edulis Englem. trees experiencing either short-term (3-year) or a 'legacy' long-term (13-year) throughfall exclusion treatments, plus a control. We also quantified the Δ14C-age of resin to measure past allocation to current defense. While 72% of trees in short-term throughfall exclusion plots died (attacked by bark beetles, Ips confusus LeConte), mortality patterns were unrelated to throughfall exclusion intensity and all 'legacy' trees survived. We thus assessed trees in four survivorship categories: control, 'legacy', surviving, and dying trees. We found concentrations of certain defense compounds (leaf phenolics, twig monoterpenes) increased with drought stress, particularly in dying trees. In the main stem, dying trees exhibited similar terpene concentrations (94%) and phenolic concentrations (139%) relative to control trees. Compared with control trees, only 'legacy' trees had reduced stem terpenes (-49%, P < 0.05) after a decade of drought. Δ14C-age of resin could be up to 10.2 ± 0.5 years old, where the oldest resin was exuded from trees with low sugar concentrations and more negative Ψpd. Our results suggest that drought imposes a weak constraint on carbon allocation to resin-based defense. Instead, we primarily found evidence of increased concentrations of terpene and phenolic compounds under drought, even in dying trees, and only observed reductions in resin-based defenses after 10+ years of drought. Δ14C-ages demonstrate limited resin turnover and/or synthesis of resin from old reserves, suggesting that long-term drought is required to reduce resin-based defenses. Persistent allocation coupled with past investments appears to preserve or enhance concentrations of resin-based defenses even under lethal drought stress in P. edulis.