Briffa et al. (1998), https://doi.org/10.1038/35596 published a seminal paper on the reduced sensitivity of annual tree growth to temperature across Northern Hemisphere treeline stands. By averaging tree-ring chronologies to sub-continental means, they found decade-long trends in maximum latewood density (MXD) progressively diverging from observed warming temperatures since the 1960s. This divergence challenges the reliability of the proxy, but the lack of an intercontinental network of up-to-date MXD chronologies extending into the 21st century hindered large-scale evaluations of the phenomenon, leaving it unresolved. Here, we introduce nine new MXD chronologies along the North American Rocky Mountains between 38 degrees and 69 degrees N and analyze their trends after applying novel approaches to preserve low-frequency variability. When following the original Briffa et al. (1998), https://doi.org/10.1038/35596 methodology, the divergence between increasing temperatures and MXD chronologies reaches offset values greater than 1 degrees C by 2020 CE. However, divergence markedly decreases and even disappears entirely when MXD chronologies are (a) based on high-replication data sets including differently old trees, (b) detrended using signal-free age-dependent splines instead of Hugershoff curves, and (c) calibrated against optimum season instead of April-September temperatures. MXD chronologies north of 60 degrees N exhibit stable relationships with regional summer temperatures on interannual to multi-decadal timescales, but at the southern sites, a lack of high-frequency proxy-target coherency is evident starting in the second half of the 20th century. This study emphasizes the importance of careful site and target selection, sampling design, and chronology development for overcoming the divergence problem and reconstructing summer temperatures from MXD data in North America.

Floodplain forests along the Upper Mississippi River System (UMRS) are highly influenced by the human-altered river flow regime. More intense and frequent floods are inducing population decline in less flood-tolerant tree species. Northern pecan (Carya illinoinensis (Wangenh. K.Koch)) is showing increasing defoliation and mortality with a lack of regeneration. We investigated the biometric and site factors related to the decline in C. illinoinensis by recording species composition, health status, diameter at breast height, and tree location for a total of 15 plots along the UMRS floodplain. Additionally, we collected increment cores to study the disturbance regime, and the effects of climate and river flow on radial growth. We applied a principal component analysis and a mixed effect regression model to determine the factors related to C. illinoinensis health at site and individual tree level, respectively. Two major disturbances occurred during the 20th century (in 1965 and 1993), both related to extraordinary discharges in the river that led to significant tree mortality in the overstory. C. illinoinensis exhibited growth release after the disturbances, however, the incidence of recruitment for this species was very scarce during the 20th century. Our analysis revealed that healthy C. illinoinensis are predominately located in the southern portion of our study area, which is characterized by less frequent extraordinary floods and less stand competition. We thus suggest C. illinoinensis-oriented management efforts, especially in the northern extent of the species range, to prevent C. illinoinensis extirpation and subsequent loss of tree diversity in the UMRS.

Droughts and tropical cyclones are two well-known hazards that can interact in dynamic ways. Now, research shows that rainfall from tropical cyclones shortens and weakens droughts in coastal regions but not in a uniform way.

Droughts and tropical cyclones (TCs) are among the deadliest and costliest natural hazards and are expected to intensify in the twenty-first century due to anthropogenic climate change. The Hurricane Region of the Americas (HRA), an area frequently affected by TCs and droughts, is home to some of the most vulnerable countries to these hazards and the impacts of climate change worldwide. While TCs and drought have been extensively studied separately, little research has been conducted on their interplay in the HRA, particularly in areas lacking quality long-term climate data. In this work, we analyze the impact of TCs on drought characteristics, such as severity and duration, in the HRA from 1983 to 2024. We use high-resolution gridded climate data and a variety of drought metrics to conduct our analysis. We find that, while TCs on average contribute to 4%-15% of annual and seasonal mean precipitation across the region, they also ameliorated or terminated drought at least once in similar to 60% of the HRA in a single month in 1983-2024. As TCs continue to get wetter and droughts become more severe in the 21st century, it is critical to understand their interactions for water resource management and climate adaptation.

Glacier Peak, a remote dacitic-andesitic stratovolcano in the North Cascade Range of Washington State, has an explosive eruptive history that has substantially impacted regional climate and ecosystems. Although the last confirmed eruption occurred between 3550 BCE and 200 CE, tephrochronology and records of Indigenous knowledge suggest a recent event around 1700 CE ± 100 years. Sparse historical records and limited chronological precision make dating this event challenging. In this study, we use long-lived Tsuga mertensiana [(Bong.) Carr.] trees to reconstruct 547 years of summer temperatures using latewood blue intensity (LWBI) as a temperature proxy. Our results reveal a significant positive relationship (r2 = 0.64, p < 0.0001) between LWBI values and March–September mean temperatures. A superposed epoch analysis confirms that the chronology captures cooling responses to well-established global volcanic eruptions, with tropical eruptions showing a one-year lag and extratropical eruptions occurring in the event year. Of the four years at or below the 1st percentile, a 1696 cooling anomaly ( -2.7 ^∘ C) was the third coldest year of the past five centuries. This anomaly occurs within a broader interval of late-17th century Northern Hemisphere cooling linked to clustered volcanic forcing, including the 1693 Hekla eruption and a proposed mid-1690s tropical eruption, and therefore cannot be unequivocally attributed to a single source. However, statistical analysis of LWBI values, combined with Indigenous accounts, tephra evidence, and the spatially confined nature of the anomaly relative to surrounding Cascades chronologies, suggests an additional localized cooling influence potentially consistent with the most recent eruption of Glacier Peak. These results improve understanding of volcanic impacts on Pacific Northwest climate and provide new insight into pre-anthropogenic temperature variability. More broadly, this study demonstrates the value of tree-ring proxies, when integrated with geological and traditional knowledge records, for extending and refining historical reconstructions of volcanic activity.

Oak (Quercus) masting, the phenomenon in which reproduction is highly variable from year to year, is ecologically important, causing population shifts for diverse organisms within associated ecosystems. The resource dynamics underpinning this variable reproduction are still not fully understood, in part due to limited record lengths. Here, we use a 16-year acorn record to examine the relationship between growth and acorn production at the individual tree level for two species of oak, Quercus alba and Quercus velutina, in southern Indiana. We measured five growth variables: total ring width, latewood width, earlywood width, average earlywood vessel area, and number of earlywood vessels per ring. We correlated acorn production and growth variables with monthly maximum temperature, maximum vapor pressure deficit, and precipitation. Following that, using linear mixed effect models, we modeled growth (total ring and latewood width) using seasonal climate variables, acorn production, and individual tree random effects. Growth and acorn production responded to climate during different periods of time. At the population level, there were no strong relationships between growth and reproduction in either species. However, linear mixed-effects models incorporating individual tree random effects revealed some evidence of trade-offs between growth and reproduction in latewood width for Q. velutina, but not for Q. alba. This suggests divergent resource allocation strategies whereby Q. velutina reallocates resources from growth to reproduction, whereas Q. alba conserves resources across years for reproduction. Consequently, the term "masting" likely oversimplifies and obscures the varied reproductive behaviors even among species within the same genus.

Montane tree species are undergoing accelerated elevational range shifts in response to climate warming, yet the mechanisms driving interspecific variation remain elusive. Here we show that hydraulic traits predict opposing biogeographic responses of montane trees to warming and drought. By integrating hemispheric-scale dendrochronological records from 45 species (121,743 individuals), global observations of elevational range shifts from 102 species and hydraulic trait data for 11 functional attributes, we demonstrate that climate-sensitive species tended to rapidly track warming into higher elevations, whereas taxa resistant to these stresses were poised to have expanded downslope. In addition, elevational dependencies in drought sensitivity changed over time for nearly one-third of the species, revealing dynamic reorganization of climate–growth relationships across mountain gradients. These findings establish leaf- and stem-level hydraulic traits as a fundamental predictor of range dynamics, providing a mechanistic basis for forecasting the reorganization of montane forests under continued climate change. The authors integrate dendrochronological records with range shifts to show that shifts in montane tree species are tightly linked to tree hydraulic properties. While climate-sensitive species track warming to higher elevations, stress-resistant species expand downslope.

The magnitude of the terrestrial carbon sink remains a key uncertainty in future climate projections, in part due to poorly understood links between carbon uptake and its allocation to woody biomass in vegetation. Here, in this study, we show that photosynthesis and aboveground growth occur asynchronously across diel to seasonal scales in eight North American oak species. Across 137 tree ring sites, current-year annual growth was insensitive to climate variability after midsummer despite 26 to 36% of annual gross primary productivity (GPP) occurring during this period. Hourly GPP flux and growth measurements at four sites spanning seven site years further demonstrate that wood formation ceases earlier than photosynthesis and is restricted to periods of low atmospheric aridity and temperature. This photosynthesis-growth decoupling intensifies with interannual variability in vapor pressure deficit (r = 0.86, P < 0.05), suggesting that by assuming tight coupling between photosynthesis and woody biomass, current earth system models may overestimate long-term carbon sequestration in forests.

Summer maximum temperatures () in the Sierra Nevada have risen rapidly since the turn of the 20th century, especially above 1,500 m where trends in the south exceed 3 degrees C century-1. To place this warming into context, we developed a 504-year reconstruction of growing-season (April-September) (1520-2023 CE) from blue-intensity and maximum latewood-density data at nine high-elevation conifer sites. The model explains 60% of instrumental variance () and shows that the 20th-21st centuries were the warmest of the past five. The warmest year is 2021 (+2.38 degrees C), while four of the five coldest years coincide with major volcanic eruptions. Since 1980, mean summer increased 1.14 degrees C (; 0.026 degrees C yr-1), concurrent with declining PDSI and a threefold rise in compound hot-dry-fire years. Dynamic regression suggests a shift from snowpack-buffered to temperature-dominated soil-moisture regimes after 1900. These results show that post-1980 warming and unprecedented compound extremes mark a new era of temperature-driven ecological vulnerability in the Sierra Nevada.

Rising concentrations of atmospheric CO2 (ca) increase plant photosynthesis (An) and reduce stomatal conductance (gs). This increases the intrinsic water-use efficiency (iWUE = An / gs), a major proxy of tree adaptation to climate change. However, whether an increase in iWUE leads to a concomitant increase in tree growth remains in dispute, prompting interest in theoretical links between iWUE and tree productivity. Here using an optimality theory for kinetics of stomatal aperture, we establish an envelope delineating maximal relative increases in tree productivity that can be inferred/expected from relative increases in iWUE. The resulting expressions are used to interpret relations between iWUE (an observable proxy) and tree growth (the target variable), using available experimental data from manipulation experiments and tree-ring isotopes. While rising ca increases iWUE, proportional increases in tree growth are unlikely given ameliorating environmental (for example, rising atmospheric dryness) and anatomical/physiological (for example, tree height) influences.

Streamflow variability is a critical component of water availability across the South Atlantic-Gulf (SAG) water resource region of the United States, yet long-term coherence among basins remains poorly understood. We developed independent May-July streamflow reconstructions for the Roanoke River (South Atlantic), Pascagoula River (southern Mississippi Basin), and St. Johns River (northern and central Florida) spanning 1100-2015 CE. Each reconstruction is highly skillful (RE = 0.39-0.62; CE = 0.39-0.62) and explains 51-63% of observed variance. Across the 916-year record, only five droughts affected all three basins simultaneously, yet three occurred since 2000 (2006, 2007, 2011). These 21st-century droughts were broader and more spatially coherent than comparable events in 1491 and 1587. Basin-to-basin comparisons reveal shared low-flow years were most frequent between the Pascagoula and St. Johns Rivers (15 events), followed by Roanoke-Pascagoula (12) and Roanoke-St. Johns (9). When the St. Johns River experienced low [high] flows, the Pascagoula River had a 40% [48%] likelihood of concurrent extremes-the highest regional coherence observed. Low-flow events for individual basins lasted 2-3 years on average, with the longest drought persisting 26 years on the St. Johns (1459 CE). For both the Pascagoula and St. Johns River, return intervals based on observational streamflow records underestimated the recurrence frequency of extreme events like the early-2000s low flow event by 100-500 years. In contrast, observationally based return intervals on the Roanoke overestimate the length of time between the driest events, indicating severe droughts are more likely to occur than previously thought. These findings illustrate the risk of multi-basin droughts in the SAG region, particularly for the closely linked Pascagoula and St. Johns basins. Future drought planning and water-resource management must account not only for drought in individual basins, but the complex effects of synchronous hydrologic drought across the region.

Increasing drought pressure under anthropogenic climate change may jeopardize the potential of tropical forests to capture carbon in woody biomass and act as a long-term carbon dioxide sink. To evaluate this risk, we assessed drought impacts in 483 tree-ring chronologies from across the tropics and found an overall modest stem growth decline (2.5% with a 95% confidence interval of 2.2 to 2.7%) during the 10% driest years since 1930. Stem growth declines exceeded 10% in 25% of cases and were larger at hotter and drier sites and for gymnosperms compared with angiosperms. Growth declines generally did not outlast drought years and were partially mitigated by growth stimulation in wet years. Thus, pantropical forest carbon sequestration through stem growth has hitherto shown drought resilience that may, however, diminish under future climate change.

Summer circulation and moisture patterns in the Southeast United States are controlled by the position of the North Atlantic subtropical high. In a warming climate, the subtropical high is projected to strengthen and expand west, but there remains uncertainty regarding its variability and linkages to natural drivers. Here, we use a tree-ring network across the Southeast United States to reconstruct the relative intensity of the pressure gradient across the subtropical high’s western flank over the past 870 years. Variations in the flank’s position and the pressure gradient have been a major driver of the hydroclimate—including creating a Southeast-Caribbean moisture dipole—since 1140 CE. We document a significant increase in flank positional variability since 1900 CE, with westward migrations becoming more extreme. Likewise, major volcanic eruptions cause a multiyear period of westward positioning, leading to distinct regional moisture gradients. Our record highlights important changes in flank behavior, which has important implications for water resource management in a warming world.

Tree-growth patterns encode valuable information about forest dynamics, ecological processes, and environmental changes. However, extracting this information requires precise visualization of tree-ring boundaries. While species with distinct tree-ring boundaries (e.g., conifers and ring-porous hardwoods) have been extensively studied, diffuse-porous and semi-ring-porous species remain understudied, despite their ecological importance. Addressing this knowledge gap requires improved tree-ring visualization techniques for these challenging species to enable their reliable measurement and crossdating. In this study, we evaluated the effectiveness of various sample preparation and imaging techniques for enhancing tree-ring visibility in 62 temperate tree species sensu lato from East Asia and North America. We compared three preparation methods: increment cores surfaced with a core microtome with and without chalk, and double-stained thin sections. These were combined with four imaging techniques: visible light microscopy and fluorescence microscopy using ultraviolet (UV), green fluorescent protein A (GFPA), and red fluorescent protein 1 (RFP1) filters. The results demonstrated that double-stained thin sections under visible light yielded the clearest tree-ring boundaries, followed by chalked surfaced cores under visible light. Among the fluorescence techniques, UV fluorescence imaging outperformed GFPA and RFP1 fluorescence. Although double-stained thin-section preparation requires greater expertise, it is a reliable, relatively fast, and cost-effective approach that can expand the scope of dendrochronological studies and support broader applications in forest management, climate research, and biodiversity conservation.

Tree rings have long provided critical insights into past temperature variability and extremes, helping to contextualize recent warming trends. In recent decades, blue intensity (BI), a light-based reflectivity method, has become a widely used and cost-effective tool for estimating densiometric ring growth, often yielding stronger temperature signals than radial width measurements. However, certain aspects of BI remain underexplored, particularly the potential of a carryover effect from transition wood blue intensity (TBI) on latewood blue intensity (LWBI). In this study, we analyzed Tsuga canadensis (eastern hemlock) to determine whether removing the carryover effect of TBI improves the climate signal embedded within LWBI. Across 14 sites, TBI was significantly correlated with LWBI (mean r = 0.42, p < 0.01), suggesting a carryover effect. By removing the influence of TBI, the adjusted LWBI (LWBIa) reduced spring correlations (March-April; mean Delta =-0.05) while increasing correlations with August temperatures (mean Delta = +0.04) and growing season average Tmax (May-September; mean Delta = +0.04). LWBIa outperformed DeltaBI, with 93 % of sites showing improved correlations with August Tmax compared to 57 % for DeltaBI. However, LWBIa reduced correlations for September, limiting improvements in late summer averages (August-September). These findings demonstrate that adjusting LWBI by accounting for the carryover effect of TBI can improve temperature sensitivity for August and the growing season, potentially enhancing the accuracy of BI-based reconstruction models. Further studies are needed to evaluate this adjustment across other species and regions to fully assess its broader applicability.

Forest composition is changing, yet the consequences for terrestrial carbon cycling are unclear. In the eastern United States, water-demanding "mesophytic" tree species are replacing "xerophytic" oaks (Quercus spp.) and hickories (Carya spp.), raising concerns that forest productivity will become increasingly sensitive to more frequent and severe drought conditions predicted for the region. However, we have a limited understanding of the extent to which the mortality risk of xerophytes versus mesophytes is coordinated with their growth sensitivity during drought. Here, we evaluated growth and mortality dynamics for 20 abundant eastern United States tree species following a severe drought in the summer of 2012. We synthesized data from ~4500 forest inventory plots and used an approach that quantified relative drought responses between co-located trees to minimize impacts from environmental heterogeneity. We found that mesophytes were just as likely to perish as co-occurring xerophytes but were more sensitive to drought in terms of diminished growth. These findings suggest that xerophytic decline is likely to lead to reduced carbon uptake during drought and that management efforts to conserve oak-hickory stands will be decisive to sustain the carbon mitigation potential of these forests. However, we also found that growth-mortality relationships differed between functional groups. Among xerophytes, growth and survival during drought were decoupled. Among mesophytes, there was a high degree of coordination, where species that experienced greater mortality also experienced greater growth reductions. Therefore, mesophytes with high growth sensitivity to water deficits are likely to be the most vulnerable to drought-driven die-off events moving forward.

Contextualizing current increases in Northern Hemisphere temperatures is precluded by the short instrumental record of the past ca. 120 years and the dearth of temperature-sensitive proxy records, particularly at lower latitudes south of <50 °N. We develop a network of 29 blue intensity chronologies derived from tree rings of Tsuga canadensis (L.) Carrière and Picea rubens Sarg. trees distributed across the Mid-Atlantic and Northeast USA (MANE)---a region underrepresented by multi-centennial temperature records. We use this network to reconstruct March-September air temperatures back to 1461 CE based on a model that explains 62% of the instrumental temperature variance from 1901-1976 CE. Since 1998 CE, MANE summer temperatures are consistently the warmest within the context of the past 561 years exceeding the 1951-1980 mean +1.3 °C. Temperature variability across MANE is linked with the position and intensity of the Northern Hemisphere polar jet stream (NHJ), which triggers regional warm and cool extremes, respectively. The new network reveals a strong regional temperature response to large volcanic eruptions, with the most severe temperature departure of -1.05 °C following the eruption of Mount Tambora in 1815 CE. Expanding the MANE network to the west and south and combining it with existing temperature-sensitive proxies across North America is an important next step toward producing a gridded temperature reconstruction field for North America.

Forests around the world are experiencing changes due to climate variability and human land use. How these changes interact and influence the vulnerability of forests are not well understood. In the eastern United States, well-documented anthropogenic disturbances and land-use decisions, such as logging and fire suppression, have influenced forest species assemblages, leading to a demographic shift from forests dominated by xeric species to those dominated by mesic species. Contemporarily, the climate has changed and is expected to continue to warm and produce higher evaporative demand, imposing stronger drought stress on forest communities. Here, we use an extensive network of tree-ring records from common hardwood species across ~100 sites and ~1300 trees in the eastern United States to examine the magnitude of growth response to both wet and dry climate extremes. We find that growth reductions during drought exceed the positive growth response to pluvials. Mesic species such as Liriodendron tulipifera and Acer saccharum, which are becoming more dominant, are more sensitive to drought than more xeric species, such as oaks (Quercus) and hickory (Carya), especially at moderate and extreme drought intensities. Although more extreme droughts produce a larger annual growth reduction, mild droughts resulted in the largest cumulative growth decreases due to their higher frequency. When using global climate model projections, all scenarios show drought frequency increasing substantially (3-9 times more likely) by 2100. Thus, the ongoing demographic shift toward more mesic species in the eastern United States combined with drier conditions results in larger drought-induced growth declines, suggesting that drought will have an even larger impact on aboveground carbon uptake in the future in the eastern United States.

Vapour pressure deficit (VPD) is a critical measure of the atmospheric demand for water and can be used to assess short-term and seasonal drought. To provide for probabilistic comparisons of VPD across space and time, we develop a Standardized Vapor Pressure Deficit Index (SVPDI). Similar to the way that other standardised drought indices are used, SVPDI allows for the analysis and comparison of changes in VPD across regions with different base level VPD values. It also should be useful for analysing impacts on vegetation that has varying levels of adaptation to high VPD. We use 1-, 3-, 6- and 12-month timescales for the development of SVPDI and show that the gamma distribution is superior to other zero-limited probability distributions for analysing VPD and, therefore, for calculating SVPDI. Then, focusing on the short-term variations at the 1- and 3-month timescales, we show how SVPDI has changed globally from 1958 to 2023 and how those changes differ from those of the commonly used Standardized Precipitation Evaporation Index (SPEI). We find that SVPDI shows more widespread drying conditions that also are larger in magnitude compared to those of SPEI. Although the two indices are moderately well correlated across the terrestrial surface, we discover that they are more decoupled in humid and arid regions compared to dry sub-humid and semi-arid regions. Using four locations that have recently experienced severe drought, we find that SVPDI generally showed longer drought duration and more severe drought events in the last decade when compared to SPEI.