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Abstract Climate change has increased the frequency and severity of drought and large wildfire events across western North America. Despite the increasing concurrence of drought and wildfire events and the importance of forests as a global carbon sink, the impacts of fire on tree drought and carbon acquisition traits are not well understood, particularly on multi‐year time‐scales.In 2022–2024, we leveraged a natural experiment at a large 2018 wildfire in southwestern Colorado, comparing leaf and xylem functional traits related to drought resistance and carbon acquisition in burned and unburned ponderosa pine, quaking aspen, subalpine fir, and Engelmann spruce trees.Relative to unburned trees of the same species, we found reduced xylem vulnerability to embolism (P₅₀) in burned ponderosa pine and subalpine fir; decreased leaf heat tolerance (T₅₀) in burned quaking aspen and ponderosa pine; and increased investment in leaf structural over photosynthetic components (leaf C:N isotopic ratio) in burned quaking aspen, subalpine fir, and Engelmann spruce.In contrast to previous studies, our results suggest that wildfire positively impacts functional traits related to drought resistance and water movement in surviving burned trees. However, generally negative impacts of wildfire were found with respect to leaf physiological and photosynthetic traits, suggesting divergent water and carbon responses to fire. Read the freePlain Language Summaryfor this article on the Journal blog. 

ABSTRACT Nature‐based climate solutions in Earth's forests could strengthen the land carbon sink and contribute to climate mitigation, but must adequately account for climate risks to the durability of carbon storage. Forest carbon offset protocols use a “buffer pool” to insure against disturbance risks that may compromise durability. However, the extent to which current buffer pool tools and allocations align with current scientific data or models is not well understood. Here, we use a tropical forest stand biomass model and an extensive set of long‐term tropical forest plots to test whether current buffer pool contributions are adequate to insure against observed disturbance regimes. We find that forest age and disturbance regime both influence necessary buffer pool sizes. In the majority of disturbance scenarios in a major carbon registry buffer pool tool, current buffer pools are substantially smaller than required by carbon cycle science. Buffer pool tools and estimates urgently need to be updated to accurately assess disturbance regimes and climate change impact on disturbances based on rigorous, open scientific datasets for nature‐based climate solutions to succeed. 

Summary Human‐caused climate change is predicted to bring more frequent droughts and higher temperatures in the western United States, which threaten ecologically important trembling aspen forests.We used ring‐specific vulnerability curves of aspen branches along two climate gradients to determine whether damages to pit membranes accumulate as the xylem ages.We found that rings older than 3 yr have a significant decline in hydraulic conductivity, especially at average summer water potentials for the species. These differences were not due to differences in the diameter of the vessels, but a difference in how much xylem was active between rings older than 3 yr and 1 yr, suggesting the presence of accumulated damage to pit membranes impairing water transport.Vulnerability to embolism differs across ring age and between wetter and drier populations, underscoring that damages due to drought may accumulate to lethal levels if the xylem does not acclimate to climate change in newer growth. 

Abstract Cyclonic storms, or hurricanes, are expected to intensify as ocean heat energy rises due to climate change. Ecological theory suggests that tropical forest resistance to hurricanes should increase with forest age and wood density. However, most data on hurricane effects on tropical forests come from a limited number of well‐studied long‐term monitoring sites, restricting our capacity to evaluate the resistance of tropical forests to hurricanes across broad environmental gradients.In this study, we assessed whether forest age and aridity mediate the effects of hurricanes Irma and Maria in Puerto Rico, Vieques and Culebra islands. We leveraged functional trait data for 410 tree species, remotely sensed measurements of canopy height and cover, along with data on forest stand characteristics of 180 of 338 forest monitoring plots, each covering an area of 0.067 ha. The plots represent a broad mean annual precipitation (MAP) gradient from 701 to 4598 mm and a complex mosaic of forest age from 5 to around 85 years since deforestation.Hurricanes resulted in a 25% increase in basal area mortality rates, a 45% decrease in canopy height and a 21% reduction in canopy cover. These effects intensified with forest age, even after considering proximity to the hurricane path. The links between forest age and hurricane disturbances were likely due the prevalence of tall canopies.Tall forest canopies were strongly linked with low community‐weighted wood density (WD). These characteristics were on average more common in moist and wet forests (MAP >1250 mm). Conversely, dry forests were dominated by short species with high wood density (WD > 0.6 g cm⁻³) and did not show significant increases in basal area mortality rates after the hurricanes.Synthesis. Our findings show that selection towards drought‐tolerant traits across aridity gradients, such as short stature and dense wood, enhances resistance to hurricanes. However, forest age modulated responses to hurricanes, with older forests being less resistant across the islands. This evidence highlights the importance of considering the intricate links between ecological succession and plant function when forecasting tropical forests’ responses to increasingly strong hurricanes. 

Abstract Metrics to quantify regulation of plant water status at the daily as opposed to the seasonal scale do not presently exist. This gap is significant since plants are hypothesised to regulate their water potential not only with respect to slowly changing soil drought but also with respect to faster changes in air vapour pressure deficit (VPD), a variable whose importance for plant physiology is expected to grow because of higher temperatures in the coming decades. We present a metric, the stringency of water potential regulation, that can be employed at the daily scale and quantifies the effects exerted on plants by the separate and combined effect of soil and atmospheric drought. We test our theory using datasets from two experiments where air temperature and VPD were experimentally manipulated. In contrast to existing metrics based on soil drought that can only be applied at the seasonal scale, our metric successfully detects the impact of atmospheric warming on the regulation of plant water status. We show that the thermodynamic effect of VPD on plant water status can be isolated and compared against that exerted by soil drought and the covariation between VPD and soil drought. Furthermore, in three of three cases, VPD accounted for more than 5 MPa of potential effect on leaf water potential. We explore the significance of our findings in the context of potential future applications of this metric from plant to ecosystem scale. 

Structural overshoots, where biomass is overallocated to tree leaf area compared to sapwood area, could result in lethal stress during droughts. Climate change may alter climatic cues that drive leaf area production, such as temperature and precipitation, as well as seasonal dynamics that underlie summer rainfall due to the North American Monsoon (NAM). Combined, this could lead to temporal mismatches between leaf area‐driven water demand and availability, and increased drought‐induced mortality events.We used leaf area to sapwood area ratios to investigate the prevalence of overshoots and whether overshoots increase drought‐induced mortality. We measured populations of aspen spanning the northern transition zone of the NAM during and following severe droughts.We observed increased overshoots and drought‐induced mortality in southern latitude populations that rely more on summer monsoon rainfall. Changes in convective activity from low snowpack the preceding winter may be a climatic driver of heightened summer monsoon rainfall in the region and therefore may also trigger increased production of leaf area during wetter summers.Our results suggest that an overshoot of leaf area to sapwood area (A_L:A_S) ratios is associated with drought‐induced tree mortality and highlight that climate‐change driven alterations to the NAM could have major consequences for tree species' acclimation to environmental change. Read the freePlain Language Summaryfor this article on the Journal blog. 

Plant functional traits hold the potential to greatly improve the understanding and prediction of climate impacts on ecosystems and carbon cycle feedback to climate change. Traits are commonly used to place species along a global conservative-acquisitive trade-off, yet how and if functional traits and conservative-acquisitive trade-offs scale up to mediate community and ecosystem fluxes is largely unknown. Here, we combine functional trait datasets and multibiome datasets of forest water and carbon fluxes at the species, community, and ecosystem-levels to quantify the scaling of the tradeoff between maximum flux and sensitivity to vapor pressure deficit. We find a strong conservative-acquisitive trade-off at the species scale, which weakens modestly at the community scale and largely disappears at the ecosystem scale. Functional traits, particularly plant water transport (hydraulic) traits, are strongly associated with the key dimensions of the conservative-acquisitive trade-off at community and ecosystem scales, highlighting that trait composition appears to influence community and ecosystem flux dynamics. Our findings provide a foundation for improving carbon cycle models by revealing i) that plant hydraulic traits are most strongly associated with community- and ecosystem scale flux dynamics and ii) community assembly dynamics likely need to be considered explicitly, as they give rise to ecosystem-level flux dynamics that differ substantially from trade-offs identified at the species-level.