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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. 

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. 

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. 

Abstract Increases in the frequency and severity of climate-sensitive disturbances like wildfire, drought, and insect outbreaks pose an imminent threat to Earth’s forests, including their status as a net carbon sink. Forest treatments including thinning and prescribed fire are increasingly viewed as key tools for mitigating disturbance impacts, but their efficacy in stabilizing carbon stocks and reducing mortality, especially for drought and insect outbreaks, remains uncertain. Moreover, we have limited understanding of whether the moderating effect of forest treatments provides a net benefit to vegetation carbon stocks, or if the initial carbon loss required to implement treatments outweighs potential reductions in carbon losses during subsequent disturbance. Here we conduct a systematic meta-analysis of published literature to understand how thinning, prescribed fire, and combined treatments impact survival and carbon stocks following wildfires, droughts, and insect outbreaks. We found that treatments improved survival following wildfires, but had only marginal impacts on survival following drought and insect outbreaks. While thinning had a modest positive effect on carbon stocks following wildfire, treatments generally reduced carbon stocks following drought and had no impact following insect outbreaks. Overall, our findings suggest that the benefits of forest treatments for vegetation carbon stocks are limited, especially following drought and insect disturbances. These findings have important policy implications for carbon credit programs. 

Summary The partitioning of photosynthate among various forest carbon pools is a key process regulating long‐term carbon sequestration, with allocation to aboveground woody biomass carbon (AGBC) in particular playing an outsized role in the global carbon cycle due to its slow residence time. However, directly estimating the fraction of gross primary productivity (GPP) that goes to AGBC has historically been difficult and time‐consuming, leaving us with persistent uncertainties.We used an extensive dataset of tree‐ring chronologies co‐located at flux towers to assess the coupling between AGBC and GPP, calculate the fraction of fixed carbon that is allocated to AGBC, and understand the drivers of variability in this fraction.We found that annual AGBC and GPP were rarely correlated, and that annual AGBC represented only a small fraction (c. 9%) of fixed carbon. This fraction varied considerably across sites and was driven by differences in stand density and site climate. Annual AGBC was suppressed byc. 30% during drought and remained below average for years afterward.These results imply that assumptions of relatively stationary allocation of GPP to woody biomass and other plant tissues could lead to systematic biases in modeled carbon accumulation in different plant pools and thus in carbon residence time. 

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. 

Over the past three decades, assessments of the contemporary global carbon budget consistently report a strong net land carbon sink. Here, we review evidence supporting this paradigm and quantify the differences in global and Northern Hemisphere estimates of the net land sink derived from atmospheric inversion and satellite-derived vegetation biomass time series. Our analysis, combined with additional synthesis, supports a hypothesis that the net land sink is substantially weaker than commonly reported. At a global scale, our estimate of the net land carbon sink is 0.8 ± 0.7 petagrams of carbon per year from 2000 through 2019, nearly a factor of two lower than the Global Carbon Project estimate. With concurrent adjustments to ocean (+8%) and fossil fuel (−6%) fluxes, we develop a budget that partially reconciles key constraints provided by vegetation carbon, the north-south CO₂gradient, and O₂trends. We further outline potential modifications to models to improve agreement with a weaker land sink and describe several approaches for testing the hypothesis.