Recent CO2 rise has modified the sensitivity of tropical tree growth to rainfall and temperature

Mutual inhibition effects of elevated CO2 and climate change on global forest GPP

The impact of CO2 fertilization on enhancing global forest gross primary productivity (GPP) is acknowledged, but its interaction with climate factors-air temperature (Tem), precipitation (Pre), vapor pressure deficit (VPD), and radiation (Rad)-remains unclear. In this study, global forest GPP trends from 1982 to 2018 were examined using BEPS, NIRv, FLUXCOM, and revised EC-LUE datasets, with interannual trends of 5.618 (p < 0.01), 5.831 (p < 0.01), 0.227, and 6.566 g C m-2 yr-1 (p < 0.01), respectively. Elevated CO2 was identified as the primary driver of GPP trends, with the dominant area ranging from 51.11% to 90.37% across different GPP datasets. In the NIRv and revised EC-LUE datasets, the positive impact of CO2 on GPP showed a decrease of 0.222 g C m-2 yr-1, while the negative impact of Rad increased by 0.007 g C m-2 yr-1. An inhibitory relationship was found between the actual effects of elevated CO2 and climate change on GPP in most forest types. At lower latitudes, Tem primarily constrained CO2 fertilization, while at higher latitudes, VPD emerged as the key limiting factor. This was mainly attributed to the potential trade-off or competition between elevated CO2 and climate change in influencing GPP, with strategic resource allocation varying across different forest ecosystems. This study highlights the significant inhibitory effects of elevated CO2 and climate change on global forest GPP, providing insights into the dynamic responses of forest ecosystems to changing environments.

Disentangling the impact of climate change, human activities, vegetation dynamics and atmospheric CO2 concentration on soil water use efficiency in global karst landscapes

Soil Water Use Efficiency (SWUE), which quantifies the carbon gain against each unit of soil moisture depletion, represents an essential ecological parameter that delineates the carbon-water coupling within terrestrial ecosystems. However, the spatiotemporal dynamics of SWUE, its sensitivity to environmental variables, and the underlying driving mechanisms across various temporal scales in the global karst region are largely uncharted. This study utilized the sensitivity algorithm of partial least squares regression, partial differential equations, and elasticity coefficients to investigate the characteristics of SWUE variations across different climatic zones in the global karst region and their responsiveness to environmental variables. Moreover, the study quantified the individual contributions of climate variability, atmospheric carbon dioxide concentration, human activities, and vegetation changes to SWUE variations. The results indicated that SWUE across different climatic zones in the global karst region demonstrated an increasing trend from 2000 to 2018, with the most notable improvement observed in the humid zone. SWUE presented regular distribution and variation characteristics across different latitudinal zones at a monthly scale. The sensitivity of SWUE to precipitation was significantly higher compared to its responsiveness to other environmental factors. Additionally, the trend in SWUE's sensitivity to precipitation demonstrated the most significant change. The sensitivity of SWUE to various environmental factors and the trend of this sensitivity in the arid zone revealed significant variation compared to other climatic zones. Gross primary productivity and soil moisture were identified as the intrinsic factors influencing SWUE changes, contributing 16 % and - 84 %, respectively. Climate variability and human activities were identified as the primary exogenous factors contributing to the increase in SWUE, accounting for 76 % and 16 %, respectively. This study advances the understanding of carbon-water coupling in karst regions, providing significant insights into the ecological management of global karst environments amidst climate variations.

Effects of simulated climate change conditions of increased temperature and [CO2] on the early growth and physiology of the tropical tree crop, Theobroma cacao L

Despite multiple studies of the impact of climate change on temperate tree species, experiments on tropical and economically important tree crops, such as cacao (Theobroma cacao L.), are still limited. Here, we investigated the combined effects of increased temperature and atmospheric carbon dioxide concentration ([CO2]) on the growth, photosynthesis and development of juvenile plants of two contrasting cacao genotypes: SCA 6 and PA 107. The factorial growth chamber experiment combined two [CO2] treatments (410 and 700 p.p.m.) and three day/night temperature regimes (control: 31/22 °C, control + 2.5 °C: 33.5/24.5 °C and control + 5.0 °C: 36/27 °C) at a constant vapour pressure deficit (VPD) of 0.9 kPa. At elevated [CO2], the final dry weight and the total and individual leaf areas increased in both genotypes, while the duration for individual leaf expansion declined in PA 107. For both genotypes, elevated [CO2] also improved light-saturated net photosynthesis (Pn) and intrinsic water-use efficiency (iWUE), whereas leaf transpiration (E) and stomatal conductance (gs) decreased. Under a constant low VPD, increasing temperatures above 31/22 °C enhanced the rates of Pn, E and gs in both genotypes, suggesting that photosynthesis responds positively to higher temperatures than previously reported for cacao. However, dry weight and the total and individual leaf areas declined with increases in temperature, which was more evident in SCA 6 than PA 107, suggesting the latter genotype was more tolerant to elevated temperature. Our results suggest that the combined effect of elevated [CO2] and temperature is likely to improve the early growth of high temperature-tolerant genotypes, while elevated [CO2] appeared to ameliorate the negative effects of increased temperatures on growth parameters of more sensitive material. The evident genotypic variation observed in this study demonstrates the scope to select and breed cacao varieties capable of adapting to future climate change scenarios.

Differences in leaf gas exchange strategies explain Quercus rubra and Liriodendron tulipifera intrinsic water use efficiency responses to air pollution and climate change

Trees continuously regulate leaf physiology to acquire CO₂ while simultaneously avoiding excessive water loss. The balance between these two processes, or water use efficiency (WUE), is fundamentally important to understanding changes in carbon uptake and transpiration from the leaf to the globe under environmental change. While increasing atmospheric CO₂ (iCO₂ ) is known to increase tree intrinsic water use efficiency (iWUE), less clear are the additional impacts of climate and acidic air pollution and how they vary by tree species. Here, we couple annually resolved long-term records of tree-ring carbon isotope signatures with leaf physiological measurements of Quercus rubra (Quru) and Liriodendron tulipifera (Litu) at four study locations spanning nearly 100 km in the eastern United States to reconstruct historical iWUE, net photosynthesis (A_net ), and stomatal conductance to water (g_s ) since 1940. We first show 16%-25% increases in tree iWUE since the mid-20th century, primarily driven by iCO₂ , but also document the individual and interactive effects of nitrogen (NO_x ) and sulfur (SO₂ ) air pollution overwhelming climate. We find evidence for Quru leaf gas exchange being less tightly regulated than Litu through an analysis of isotope-derived leaf internal CO₂ (C_i ), particularly in wetter, recent years. Modeled estimates of seasonally integrated A_net and g_s revealed a 43%-50% stimulation of A_net was responsible for increasing iWUE in both tree species throughout 79%-86% of the chronologies with reductions in g_s attributable to the remaining 14%-21%, building upon a growing body of literature documenting stimulated A_net overwhelming reductions in g_s as a primary mechanism of increasing iWUE of trees. Finally, our results underscore the importance of considering air pollution, which remains a major environmental issue in many areas of the world, alongside climate in the interpretation of leaf physiology derived from tree rings.

How drought events during the last century have impacted biomass carbon in Amazonian rainforests

During the last two decades, inventory data show that droughts have reduced biomass carbon sink of the Amazon forest by causing mortality to exceed growth. However, process-based models have struggled to include drought-induced responses of growth and mortality and have not been evaluated against plot data. A process-based model, ORCHIDEE-CAN-NHA, including forest demography with tree cohorts, plant hydraulic architecture and drought-induced tree mortality, was applied over Amazonia rainforests forced by gridded climate fields and rising CO₂ from 1901 to 2019. The model reproduced the decelerating signal of net carbon sink and drought sensitivity of aboveground biomass (AGB) growth and mortality observed at forest plots across selected Amazon intact forests for 2005 and 2010. We predicted a larger mortality rate and a more negative sensitivity of the net carbon sink during the 2015/16 El Niño compared with the former droughts. 2015/16 was indeed the most severe drought since 1901 regarding both AGB loss and area experiencing a severe carbon loss. We found that even if climate change did increase mortality, elevated CO₂ contributed to balance the biomass mortality, since CO₂ -induced stomatal closure reduces transpiration, thus, offsets increased transpiration from CO₂ -induced higher foliage area.

Declining tree growth rates despite increasing water-use efficiency under elevated CO2 reveals a possible global overestimation of CO2 fertilization effect

Though rising atmospheric CO₂ concentrations (C_a) harm the environment and society, they may also raise photosynthetic rates and enhance intrinsic water-use efficiency (iWUE). Numerous short-term studies have investigated tree growth under elevated CO₂ (eCO₂) conditions, but no long-duration study has investigated eCO₂ impacts on tree growth and iWUE under natural conditions. Utilizing a new dendrochronological experimental design in a heavily-touristed nature preserve in Southwest China (Jiuzhaigou National Nature Reserve), we compared tree growth (e.g., basal area increment) and iWUE in two biophysically and environmentally similar valleys with contrasting anthropogenic activities. Trees in the control valley with ambient CO₂ benefited from increasing C_a, possibly due to the CO₂ fertilization effect and optimal environmental conditions. However, trees in the treatment valley with intensive tourism experienced comparatively higher localized eCO₂ and growth rate declines. While iWUE increased (1959–2017) in the control (25.3%) and treatment sites (47.8%), declining tree growth rates in the treatment site was likely because comparatively extreme CO₂ exposure levels encouraged stomatal closures. As the first long-term study investigating eCO₂ impacts on tree growth and iWUE under natural conditions, we demonstrate that increased forest iWUE is unlikely to overcome negative drought stress and rising temperature impacts. Thus, forest potential for mitigating eCO₂ and global climate change is likely overestimated, particularly under dry temperate conditions.

Increasing sensitivity of dryland vegetation greenness to precipitation due to rising atmospheric CO2

Water availability plays a critical role in shaping terrestrial ecosystems, particularly in low- and mid-latitude regions. The sensitivity of vegetation growth to precipitation strongly regulates global vegetation dynamics and their responses to drought, yet sensitivity changes in response to climate change remain poorly understood. Here we use long-term satellite observations combined with a dynamic statistical learning approach to examine changes in the sensitivity of vegetation greenness to precipitation over the past four decades. We observe a robust increase in precipitation sensitivity (0.624% yr⁻¹) for drylands, and a decrease (−0.618% yr⁻¹) for wet regions. Using model simulations, we show that the contrasting trends between dry and wet regions are caused by elevated atmospheric CO₂ (eCO₂). eCO₂ universally decreases the precipitation sensitivity by reducing leaf-level transpiration, particularly in wet regions. However, in drylands, this leaf-level transpiration reduction is overridden at the canopy scale by a large proportional increase in leaf area. The increased sensitivity for global drylands implies a potential decrease in ecosystem stability and greater impacts of droughts in these vulnerable ecosystems under continued global change.

Changes in vegetation responses to precipitation may be hydroclimate dependent. Here the authors reveal contrasting trends of vegetation sensitivity to precipitation in drylands vs. wetter ecosystems over the last 4 decades and identify increased CO2 as a major contributing factor.

Paired analysis of tree ring width and carbon isotopes indicates when controls on tropical tree growth change from light to water limitations

Light and water availability are likely to vary over the lifespan of closed-canopy forest trees, with understory trees experiencing greater limitations to growth by light and canopy trees greater limitation due to drought. As drought and shade have opposing effects on isotope discrimination (Δ13C), paired measurement of ring width and Δ13C can potentially be used to differentiate between water and light limitations on tree growth. We tested this approach for Cedrela trees from three tropical forests in Bolivia and Mexico that differ in rainfall and canopy structure. Using lifetime ring width and Δ13C data for trees of up to and over 200 years old, we assessed how controls on tree growth changed from understory to the canopy. Growth and Δ13C are mostly anti-correlated in the understory, but this anti-correlation disappeared or weakened when trees reached the canopy, especially at the wettest site. This indicates that understory growth variation is controlled by photosynthetic carbon assimilation due to variation in light levels. Once trees reached the canopy, inter-annual variation in growth and Δ13C at one of the dry sites showed positive correlations, indicating that inter-annual variation in growth is driven by variation in water stress affecting stomatal conductance. Paired analysis of ring widths and carbon isotopes provides significant insight in what environmental factors control growth over a tree's life; strong light limitations for understory trees in closed-canopy moist forests switched to drought stress for (sub)canopy trees in dry forests. We show that combined isotope and ring width measurements can significantly improve our insights in tree functioning and be used to disentangle limitations due to shade from those due to drought.

Elevated CO2 moderates the impact of climate change on future bamboo distribution in Madagascar

The distribution of bamboo is sensitive to climate change and is also potentially affected by increasing atmospheric CO₂ concentrations due to its C3 photosynthetic pathway. Yet the effect of CO₂ in climate impact assessments of potential changes in bamboo distribution has to date been overlooked. In this study, we proposed a simple and quantitative method to incorporate the impact of atmospheric CO₂ concentration into a species distribution modeling framework. To do so, we implemented 10 niche modeling algorithms with regionally downscaled climatic variables and combined field campaign observations. We assessed future climate impacts on the distribution of an economically and ecologically important and widely distributed bamboo species in Madagascar, and examined the effect of increasing CO₂ on future projections. Our results suggested that future climatic changes negatively impact potential bamboo distribution in Madagascar, leading to a decline of 34.8% of climatic suitability and a decline of 63.6 ± 3.2% in suitable areas towards 2100 under RCP 8.5. However, increasing atmosphere CO₂ offsets the climate impact for bamboo, and led to a smaller reduction of 19.8% in suitability and a potential distribution expansion of +111.6 ± 9.8% in newly suitable areas. We also found that the decline in climatic suitability for bamboo was related to increasing monthly potential evapotranspiration of the warmest quarter and minimum temperature of the warmest month. Conversely, the decreasing isothermality and increasing precipitation of the warmest quarter contributed to projected increase in bamboo-suitable areas. Our study suggested that elevated CO₂ may mitigate the decrease in climatic suitability and increase bamboo-suitable areas, through enhancing water use efficiency and decreasing potential evapotranspiration. Our results highlight the importance of accounting for the CO₂ effect on future plant species distributions, and provide a mechanistic approach to do so for ecosystems constrained by water.

Joint effects of climate, tree size, and year on annual tree growth derived from tree-ring records of ten globally distributed forests

Tree rings provide an invaluable long-term record for understanding how climate and other drivers shape tree growth and forest productivity. However, conventional tree-ring analysis methods were not designed to simultaneously test effects of climate, tree size, and other drivers on individual growth. This has limited the potential to test ecologically relevant hypotheses on tree growth sensitivity to environmental drivers and their interactions with tree size. Here, we develop and apply a new method to simultaneously model nonlinear effects of primary climate drivers, reconstructed tree diameter at breast height (DBH), and calendar year in generalized least squares models that account for the temporal autocorrelation inherent to each individual tree's growth. We analyze data from 3811 trees representing 40 species at 10 globally distributed sites, showing that precipitation, temperature, DBH, and calendar year have additively, and often interactively, influenced annual growth over the past 120 years. Growth responses were predominantly positive to precipitation (usually over ≥3-month seasonal windows) and negative to temperature (usually maximum temperature, over ≤3-month seasonal windows), with concave-down responses in 63% of relationships. Climate sensitivity commonly varied with DBH (45% of cases tested), with larger trees usually more sensitive. Trends in ring width at small DBH were linked to the light environment under which trees established, but basal area or biomass increments consistently reached maxima at intermediate DBH. Accounting for climate and DBH, growth rate declined over time for 92% of species in secondary or disturbed stands, whereas growth trends were mixed in older forests. These trends were largely attributable to stand dynamics as cohorts and stands age, which remain challenging to disentangle from global change drivers. By providing a parsimonious approach for characterizing multiple interacting drivers of tree growth, our method reveals a more complete picture of the factors influencing growth than has previously been possible.

Interannual variability of vegetation sensitivity to climate in China

Many studies have assessed the relative sensitivity of ecosystems to climate change, and even optimized climate states from long-term averages to infer short-term changes, but how ecosystem sensitivity and its relationships with climate variability vary over time remains elusive. By combining the vegetation sensitivity index (VSI) and a 15 year moving window, we analyzed interannual variability in spatiotemporal patterns of vegetation sensitivity to short-term climate variability and its correlations with climatic factors in China over the past three decades (1982-2015). We demonstrated that vegetation sensitivity shows high spatial heterogeneity, and varies with vegetation type and climate region. Generally, vegetation in the southwest and mountainous regions was more sensitive, especially coniferous forests and isolated shrubland patches. Comparatively, vegetation in dry regions was less sensitive to climate variability than in wetter climates. Due to frequent climate variability in the early 1990s, a large increase in the VSI was detected in 1996. Significant increases in the interannual variability of vegetation sensitivity were observed in greater than 23.7% of vegetated areas and decreases in only 4.2%. Solar radiation was the dominant climate driver of vegetation sensitivity, followed by temperature and precipitation. However, climate controls are not invariable across a range of climatic conditions, such as precipitation exerted an increasing influence on changes of vegetation sensitivity. Quantitative analyses of ecosystem sensitivity to climate variability such as ours are vital to identify which regions and vegetation are most vulnerable to future climate variability.