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1. Water, not carbon, drives drought‐constraints on stem terpene defense against simulated bark beetle attack in Pinus edulis

   https://doi.org/10.1111/nph.20218  

   Malone, Shealyn C; Thompson, R Alex; Chow, Pak S; de_Oliveira, Celso R; Landhäusser, Simon M; Six, Diana L; McCulloh, Katherine A; Adams, Henry D; Trowbridge, Amy M (January 2025, New Phytologist)

   Summary Drought predisposes forest trees to bark beetle‐induced mortality, but the physiological mechanisms remain unclear. While drought‐induced water and carbon limitations have been implicated in defensive failure and tree susceptibility, evidence demonstrating how these factors interact is scarce.We withheld water from mature, pottedPinus edulisand subsequently applied a double‐stem girdle to inhibit carbohydrate transport from the crown and roots. Within this isolated segment we then elicited a defense response by inoculating trees with a bark beetle‐fungal symbiont (Ophiostomasp.). We quantified local mono‐ and sesquiterpenes (MST), nonstructural carbohydrates (NSC), and pressure potential of the inner bark.Both drought‐stressed and watered trees had similar NSC concentrations just before inoculation and depleted NSC similarly following inoculation, yet MST induction (i.e. increased concentration and altered composition) was constrained only in drought‐stressed trees. Thus, NSC consumption was largely unrelated tode novoMST synthesis. Instead, stoichiometric calculations show that induction originated largely from stored resin. Watered trees experiencing higher pressure potentials consistently induced higher MST concentrations.We demonstrate the importance of preformed resin toward an induced MST response in a semi‐arid conifer where drought‐constraints on defense occurred through biophysical limitations (i.e. reduced turgor hindering resin transport) rather than through substrate limitation. 

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2. Resin-based defenses in Pinus edulis are only reduced after long-term drought

   https://doi.org/10.1093/treephys/tpag004  

   Peltier, Drew_M P; Malone, Shealyn C; McIntire, Cameron D; Thompson, R Alex; Pinzon-Navarro, Sara; Pereverzeva, Ksenia A; Richardson, Andrew D; McDowell, Nathan G; Adams, Henry D; Carbone, Mariah S; et al (February 2026, Tree Physiology)

   Epron, Daniel (Ed.)

   Abstract 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. 

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3. Hotter droughts alter resource allocation to chemical defenses in piñon pine

   https://doi.org/10.1007/s00442-021-05058-8  

   Trowbridge, Amy M.; Adams, Henry D.; Collins, Adam; Dickman, Lee Turin; Grossiord, Charlotte; Hofland, Megan; Malone, Shealyn; Weaver, David K.; Sevanto, Sanna; Stoy, Paul C.; et al (December 2021, Oecologia)

   Abstract Heat and drought affect plant chemical defenses and thereby plant susceptibility to pests and pathogens. Monoterpenes are of particular importance for conifers as they play critical roles in defense against bark beetles. To date, work seeking to understand the impacts of heat and drought on monoterpenes has primarily focused on young potted seedlings, leaving it unclear how older age classes that are more vulnerable to bark beetles might respond to stress. Furthermore, we lack a clear picture of what carbon resources might be prioritized to support monoterpene synthesis under drought stress. To address this, we measured needle and woody tissue monoterpene concentrations and physiological variables simultaneously from mature piñon pines ( Pinus edulis ) from a unique temperature and drought manipulation field experiment. While heat had no effect on total monoterpene concentrations, trees under combined heat and drought stress exhibited ~ 85% and 35% increases in needle and woody tissue, respectively, over multiple years. Plant physiological variables like maximum photosynthesis each explained less than 10% of the variation in total monoterpenes for both tissue types while starch and glucose + fructose measured 1-month prior explained ~ 45% and 60% of the variation in woody tissue total monoterpene concentrations. Although total monoterpenes increased under combined stress, some key monoterpenes with known roles in bark beetle ecology decreased. These shifts may make trees more favorable for bark beetle attack rather than well defended, which one might conclude if only considering total monoterpene concentrations. Our results point to cumulative and synergistic effects of heat and drought that may reprioritize carbon allocation of specific non-structural carbohydrates toward defense. 

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4. Decades‐old carbon reserves are widespread among tree species, constrained only by sapwood longevity

   https://doi.org/10.1111/nph.20310  

   Peltier, Drew_M P; Carbone, Mariah S; Ogle, Kiona; Koch, George W; Richardson, Andrew D (February 2025, New Phytologist)

   Summary Carbon reserves are distributed throughout plant cells allowing past photosynthesis to fuel current metabolism. In trees, comparing the radiocarbon (Δ¹⁴C) of reserves to the atmospheric bomb spike can trace reserve ages.We synthesized Δ¹⁴C observations of stem reserves in nine tree species, fitting a new process model of reserve building. We asked how the distribution, mixing, and turnover of reserves vary across trees and species. We also explored how stress (drought and aridity) and disturbance (fire and bark beetles) perturb reserves.Given sufficient sapwood, young (< 1 yr) and old (20–60+ yr) reserves were simultaneously present in single trees, including ‘prebomb’ reserves in two conifers. The process model suggested that most reserves are deeply mixed (30.2 ± 21.7 rings) and then respired (2.7 ± 3.5‐yr turnover time). Disturbance strongly increased Δ¹⁴C mean ages of reserves (+15–35 yr), while drought and aridity effects on mixing and turnover were species‐dependent. Fire recovery inSequoia sempervirensalso appears to involve previously unobserved outward mixing of old reserves.Deep mixing and rapid turnover indicate most photosynthate is rapidly metabolized. Yet ecological variation in reserve ages is enormous, perhaps driven by stress and disturbance. Across species, maximum reserve ages appear primarily constrained by sapwood longevity, and thus old reserves are probably widespread. 

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5. Climate indirectly modulates tree survival of spruce beetle attacks via effects on constitutive and induced secondary metabolites

   https://doi.org/10.3389/fpls.2026.1801237  

   Khedive, Ehsan; Fathi-Moghanloo, Saeideh; Davis, Thomas Seth (April 2026, Frontiers in Plant Science)

   Lyu, Lixin (Ed.)

   Despite many years of research, it remains challenging to determine why some trees survive bark beetle attacks while others do not. Current theory suggests that survival is predicated by interactions between tree defense systems and climatic drivers, but such interactions are poorly resolved at the scale of individual trees. Using a widespread conifer species (Engelmann spruce,Picea engelmannii) and a lethal phloem-feeding herbivore (North American spruce bark beetle,Dendroctonus rufipennis) as a study system, we characterized tree responses to mass attack across five populations representing a climate gradient in a field experiment. Over the course of a growing season, we measured variation in monoterpene concentrations in the phloem of 100 trees, half of which were challenged with mass attack using pheromone baits. Study trees were visited two years after baiting to record survival, and match survival to constitutive and induced monoterpenes. Our results revealed that regional climate correlated with constitutive, pre-attack concentration of monoterpenes in spruce phloem: a 1 kPa increase in vapor pressure deficit correlated with a doubling of monoterpene concentration. Higher constitutive phloem monoterpene concentrations predicted tree survival: a one-fold increase in concentration was associated with a three-fold increase in the odds of survival. Regional climate did not significantly affect the magnitude of induced response; however, early induction of monoterpenes in phloem distal to attack sites was associated with an average of twofold increase in survival odds, suggesting rapidity of monoterpene induction is a key resistance trait. These findings extend our understanding of climate-defense relationships in this system and indicate that Engelmann spruce tree populations in warmer and drier sites may have traits associated with higher constitutive resistance. 

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