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1. Delays in peak physiological activity may reduce resource acquisition as trees recover from wildfire

   https://doi.org/10.1002/ecs2.70325  

   Bryant, Kelsey; Lynch, Laurel; Kolden, Crystal; Stenzel, Jeffrey; Hanson, Chad; Still, Christopher; Mathias, Justin; Light, Brandon; Moon, Cooper; Hudiburg, Tara (July 2025, Ecosphere)

   Abstract Few studies have investigated how mature trees recover physiologically from wildfire damage, and none have comprehensively linked tree hydraulics with belowground function. Uncovering mechanistic links between rates of above‐ and belowground recovery is necessary for improving predictions of forest resilience and carbon dynamics following wildfire. We coupled continuous measurements of tree water flow and soil CO₂efflux with detailed physiological measurements of above‐ and belowground function following a mixed‐severity wildfire. We found that maturePinus ponderosatrees with up to 85% canopy and stem damage resumed physiological functioning by the second growing season post‐fire. However, these trees also exhibited delayed peak water uptake (relative to less‐burned trees) that coincided with summer heat and drought. Our results suggest fire damage may prevent the critical timing in which peak physiological function overlaps with optimal growing conditions (e.g., moisture and nutrient availability). As a result, we suggest the degree of root and microbial damage should be assessed along with observed aboveground damage to more effectively predict tree recovery potential. While significantly damaged trees resumed typical hydraulic function within two years, observed delays in peak water uptake could require higher water and nutrient use efficiency to maintain carbon sequestration rates. 

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2. Local carbon reserves are insufficient for phloem terpene induction during drought in Pinus edulis in response to bark beetle‐associated fungi

   https://doi.org/10.1111/nph.20051  

   Thompson, R_Alex; Malone, Shealyn_C; Peltier, Drew; Six, Diana; Robertson, Nathan; Oliveira, Jr., Celso; McIntire, Cameron_D; Pockman, William_T; McDowell, Nate_G; Trowbridge, Amy_M; et al (August 2024, New Phytologist)

   Summary Stomatal closure during drought inhibits carbon uptake and may reduce a tree's defensive capacity. Limited carbon availability during drought may increase a tree's mortality risk, particularly if drought constrains trees' capacity to rapidly produce defenses during biotic attack.We parameterized a new model of conifer defense using physiological data on carbon reserves and chemical defenses before and after a simulated bark beetle attack in maturePinus edulisunder experimental drought. Attack was simulated using inoculations with a consistent bluestain fungus (Ophiostomasp.) ofIps confusus, the main bark beetle colonizing this tree, to induce a defensive response.Trees with more carbon reserves produced more defenses but measured phloem carbon reserves only accounted forc.23% of the induced defensive response. Our model predicted universal mortality if local reserves alone supported defense production, suggesting substantial remobilization and transport of stored resin or carbon reserves to the inoculation site.Our results show thatde novoterpene synthesis represents only a fraction of the total measured phloem terpenes inP. edulisfollowing fungal inoculation. Without direct attribution of phloem terpene concentrations to available carbon, many studies may be overestimating the scale and importance ofde novoterpene synthesis in a tree's induced defense response. 

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3. Aboveground Wood Production Is Sustained in the First Growing Season after Phloem-Disrupting Disturbance

   https://doi.org/10.3390/f11121306  

   Grigri, Maxim S.; Atkins, Jeff W.; Vogel, Christoph; Bond-Lamberty, Ben; Gough, Christopher M. (December 2020, Forests)

   null (Ed.)

   Carbon (C) cycling processes are particularly dynamic following disturbance, with initial responses often indicative of longer-term change. In northern Michigan, USA, we initiated the Forest Resilience Threshold Experiment (FoRTE) to identify the processes that sustain or lead to the decline of C cycling rates across multiple levels (0, 45, 65 and 85% targeted gross leaf area index loss) of disturbance severity and, in response, to separate disturbance types preferentially targeting large or small diameter trees. Simulating the effects of boring insects, we stem girdled > 3600 trees below diameter at breast height (DBH), immediately and permanently disrupting the phloem. Weekly DBH measurements of girdled and otherwise healthy trees (n > 700) revealed small but significant increases in daily aboveground wood net primary production (ANPPw) in the 65 and 85% disturbance severity treatments that emerged six weeks after girdling. However, we observed minimal change in end-of-season leaf area index and no significant differences in annual ANPPw among disturbance severities or between disturbance types, suggesting continued C fixation by girdled trees sustained stand-scale wood production in the first growing season after disturbance. We hypothesized higher disturbance severities would favor the growth of early successional species but observed no significant difference between early and middle to late successional species’ contributions to ANPPw across the disturbance severity gradient. We conclude that ANPPw stability immediately following phloem disruption is dependent on the continued, but inevitably temporary, growth of phloem-disrupted trees. Our findings provide insight into the tree-to-ecosystem mechanisms supporting stand-scale wood production stability in the first growing season following a phloem-disrupting disturbance. 

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4. Response of Root Respiration to Warming and Nitrogen Addition Depends on Tree Species

   https://doi.org/10.1111/gcb.17530  

   Muratore, T. J.; Knorr, M. A.; Simpson, M. J.; Stephens, R. B.; Phillips, R. P.; Frey, S. D. (October 2024, Global Change Biology)

   ABSTRACT Roots contribute a large fraction of CO₂efflux from soils, yet the extent to which global change factors affect root‐derived fluxes is poorly understood. We investigated how red maple (Acer rubrum) and red oak (Quercus rubra) root biomass and respiration respond to long‐term (15 years) soil warming, nitrogen addition, or their combination in a temperate forest. We found that ecosystem root respiration was decreased by 40% under both single‐factor treatments (nitrogen addition or warming) but not under their combination (heated × nitrogen). This response was driven by the reduction of mass‐specific root respiration under warming and a reduction in maple root biomass in both single‐factor treatments. Mass‐specific root respiration rates for both species acclimated to soil warming, resulting in a 43% reduction, but were not affected by N addition or the combined heated × N treatment. Notably, the addition of nitrogen to warmed soils alleviated thermal acclimation and returned mass‐specific respiration rates to control levels. Oak roots contributed disproportionately to ecosystem root respiration despite the decrease in respiration rates as their biomass was maintained or enhanced under warming and nitrogen addition. In contrast, maple root respiration rates were consistently higher than oak, and this difference became critical in the heated × nitrogen treatment, where maple root biomass increased, contributing significantly more CO₂relative to single‐factor treatments. Our findings highlight the importance of accounting for the root component of respiration when assessing soil carbon loss in response to global change and demonstrate that combining warming and N addition produces effects that cannot be predicted by studying these factors in isolation. 

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5. Different climate sensitivity for radial growth, but uniform for tree-ring stable isotopes along an aridity gradient in Polylepis tarapacana , the world’s highest elevation tree species

   https://doi.org/10.1093/treephys/tpab021  

   Rodriguez-Caton, Milagros; Andreu-Hayles, Laia; Morales, Mariano S; Daux, Valérie; Christie, Duncan A; Coopman, Rafael E; Alvarez, Claudio; Rao, Mukund Palat; Aliste, Diego; Flores, Felipe; et al (February 2021, Tree Physiology)

   Cernusak, Lucas (Ed.)

   Abstract Tree growth is generally considered to be temperature limited at upper elevation treelines, yet climate factors controlling tree growth at semiarid treelines are poorly understood. We explored the influence of climate on stem growth and stable isotopes for Polylepis tarapacana Philipi, the world’s highest elevation tree species, which is found only in the South American Altiplano. We developed tree-ring width index (RWI), oxygen (δ18O) and carbon (δ13C) chronologies for the last 60 years at four P. tarapacana stands located above 4400 m in elevation, along a 500 km latitude aridity gradient. Total annual precipitation decreased from 300 to 200 mm from the northern to the southern sites. We used RWI as a proxy of wood formation (carbon sink) and isotopic tree-ring signatures as proxies of leaf-level gas exchange processes (carbon source). We found distinct climatic conditions regulating carbon sink processes along the gradient. Current growing-season temperature regulated RWI at northern-wetter sites, while prior growing-season precipitation determined RWI at arid southern sites. This suggests that the relative importance of temperature to precipitation in regulating tree growth is driven by site water availability. By contrast, warm and dry growing seasons resulted in enriched tree-ring δ13C and δ18O at all study sites, suggesting that similar climate conditions control carbon-source processes along the gradient. Site-level δ13C and δ18O chronologies were significantly and positively related at all sites, with the strongest relationships among the southern drier stands. This indicates an overall regulation of intercellular carbon dioxide via stomatal conductance for the entire P. tarapacana network, with greater stomatal control when aridity increases. This manuscript also highlights a coupling (decoupling) between physiological processes at leaf level and wood formation as a function of similarities (differences) in their climatic sensitivity. This study contributes to a better understanding and prediction of the response of high-elevation Polylepis woodlands to rapid climate changes and projected drying in the Altiplano. 

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