Global maps and factors driving forest foliar elemental composition: the importance of evolutionary history

Impacts of leaf traits on vegetation optical properties in Earth system modeling

Quantifying surface energy and carbon budgets is essential for projecting Earth's climate. Earth System Models (ESMs) typically simulate land surface processes based on plant functional types (PFTs), neglecting the diversity of plant functional traits or characteristics (PFCs; e.g., chlorophyll content and leaf mass per area). Here, we demonstrate substantial differences in modeled leaf optical properties (LOP) and surface albedo between traditional PFT-based and PFC-based approaches, particularly in tropical and boreal forests. We configure the canopy radiative transfer scheme in the Community Earth System Model using PFC-based LOP. This new configuration produces lower shortwave surface albedo in the tropics but higher albedo in boreal regions (>5 W m-2 radiative flux differences), and a weaker tropical but stronger boreal carbon sink. Through land-atmosphere coupling, the PFC-based configuration further alters atmospheric processes, leading to different temperature, cloud cover, and precipitation patterns. Our findings highlight the need to move beyond traditional PFT-based approaches in ESMs.

Coordinated variations in leaf and root biogeochemical niches

The biogeochemical niche (BN) hypothesis posits that each species has a specific elemental composition. However, the BN of roots and its interaction with leaf BN have largely been neglected until now across diverse environmental conditions. We investigated the relationships between the elemental compositions of leaves and roots, phylogeny, and environmental variables, as well as the connection between leaf and root BN. We analyzed the concentrations of carbon, nitrogen, phosphorus, potassium, calcium, and magnesium in the leaves and roots of 12 394 individuals from 1238 species. Consistent with the BN hypothesis, despite significant differences in elemental concentrations and their ratios between leaves and roots, we observed strong legacy (phylogenetic + species) signals in the species-specific elemental compositions. This finding confirms that the elemental compositions of leaves and roots can contribute to identifying species niches. Our study revealed a higher phylogenetic conservatism for BN in leaves than in roots and provided evidence of a tight association between the species-specific BN of leaves and roots. Our results underscore the broad applicability of the BN hypothesis across diverse species and biomes and demonstrate the critical role of evolutionary legacy in driving coordinated dynamics in both above- and belowground ecological niches.

Canopy functional trait variation across Earth's tropical forests

Tropical forest canopies are the biosphere's most concentrated atmospheric interface for carbon, water and energy1,2. However, in most Earth System Models, the diverse and heterogeneous tropical forest biome is represented as a largely uniform ecosystem with either a singular or a small number of fixed canopy ecophysiological properties3. This situation arises, in part, from a lack of understanding about how and why the functional properties of tropical forest canopies vary geographically4. Here, by combining field-collected data from more than 1,800 vegetation plots and tree traits with satellite remote-sensing, terrain, climate and soil data, we predict variation across 13 morphological, structural and chemical functional traits of trees, and use this to compute and map the functional diversity of tropical forests. Our findings reveal that the tropical Americas, Africa and Asia tend to occupy different portions of the total functional trait space available across tropical forests. Tropical American forests are predicted to have 40% greater functional richness than tropical African and Asian forests. Meanwhile, African forests have the highest functional divergence-32% and 7% higher than that of tropical American and Asian forests, respectively. An uncertainty analysis highlights priority regions for further data collection, which would refine and improve these maps. Our predictions represent a ground-based and remotely enabled global analysis of how and why the functional traits of tropical forest canopies vary across space.

Soil resource acquisition strategy modulates global plant nutrient and water economics

Natural selection favors growth by selecting a combination of plant traits that maximize photosynthetic CO2 assimilation at the lowest combined carbon costs of resource acquisition and use. We quantified how soil nutrient availability, plant nutrient acquisition strategies, and aridity modulate the variability in plant costs of nutrient acquisition relative to water acquisition (β). We used an eco-evolutionary optimality framework and a global carbon isotope dataset to quantify β. Under low soil nitrogen-to-carbon (N : C) ratios, a mining strategy (symbioses with ectomycorrhizal and ericoid mycorrhizal fungi) reduced β by mining organic nitrogen, compared with a scavenging strategy (symbioses with arbuscular mycorrhizal fungi). Conversely, under high N : C ratios, scavenging strategies reduced β by effectively scavenging soluble nitrogen, compared with mining strategies. N2-fixing plants did not exhibit reduced β under low N : C ratios compared with non-N2-fixing plants. Moisture increased β only in plants using a scavenging strategy, reflecting direct impacts of aridity on the carbon costs of maintaining transpiration in these plants. Nitrogen and phosphorus colimitation further modulated β. Our findings provide a framework for simulating the variability of plant economics due to plant nutrient acquisition strategies in earth system models.

Cones and consequences: the false dichotomy of conifers vs broad-leaves has critical implications for research and modelling

In plant science research and modelling, particularly from the northern hemisphere, the terms 'needle-leaved' and 'conifer' along with 'broad-leaved' and 'angiosperm' are often used synonymously, creating the false dichotomy that conifers are needle-leaved and angiosperms are broad-leaved. While these equivalences may be largely correct in the temperate northern hemisphere, they do not hold true in equatorial and southern hemisphere forests. Confounding needle-leaved conifers and broad-leaved angiosperms present significant issues in empirical research and modelling. Here, we highlight the likely origins and impacts of misusing conifer-related terminology, the misinterpretation that ensues and its implications. We identify the issue of a focus on Pinaceae and coin the term 'Pinaceae panacea' to describe this. We provide recommendations for future research: from standardising the use of definitions to shifting away from using Pinaceae as a model group for all conifers.

Divergent responses of plant multi-element coupling to nitrogen and phosphorus addition in a meadow steppe

The intricate biogeochemical cycling of multiple elements plays a pivotal role in upholding a myriad of ecosystem functions. However, our understanding of elemental stoichiometry and coupling in response to global changes remains primarily limited to plant carbon: nitrogen: phosphorus (C: N: P). Here, we assessed the responses of 11 elements in plants from different functional groups to global changes. Investigating the stoichiometric ratios and interrelationships of these elements in response to global change is crucial for advancing our understanding of nutrient cycling dynamics in ecosystems. We found that N deposition induced stoichiometric imbalances in Gramineae, leading to a reduction in elemental coupling. This disruption in elemental coupling could potentially affect plant growth and ecosystem functioning. However, leguminous plants, which possess specialized nitrogen fixation mechanisms, were unaffected, suggesting that their ability to independently regulate N may help them maintain stable nutrient ratios despite external N inputs. These findings highlight functional differences among plant groups in their response to global changes, with important implications for ecosystem resilience and nutrient dynamics. In summary, these diverse responses underscore the importance of understanding the underlying mechanisms to be able to better predict the future trajectory of terrestrial biogeochemical cycles under global N enrichment.

Unraveling the drivers of optimal stomatal behavior in global C3 plants: A carbon isotope perspective

Understanding the drivers of stomatal behavior is critical for modeling terrestrial carbon cycle and water balance. The unified stomatal optimization (USO) model provides a mechanistic linkage between stomatal conductance (gs) and photosynthesis (A), with its slope parameter (g1) inversely related to intrinsic water use efficiency (iWUE), providing a key proxy to characterize the differences in iWUE and stomatal behavior. While many studies have identified multiple environmental factors influencing g1, the potential role of evolutionary history in shaping g1 remains incompletely understood. Leaf organic matter 13C discriminations (Δ13C) can be applied to estimate g1 over timescales from days to whole growing season. However, most applications assume that mesophyll conductance (gm)-a critical parameter in the Δ13C model-is infinite, due to limited information. Here, we incorporated new insight of gm to allow for more realistic parameterization of this variable, and subsequently to enable improved estimation of g1 based on a global bulk leaf Δ13C dataset comprising 2215 observations of 1521 species that span major biomes. Our analysis revealed a significant phylogenetic signal in g1 values, which differed among phylogenetic groups. Through a Bayesian phylogenetic linear mixed model, we found that species and phylogeny together explained 36.63 % of g1 variance, a contribution comparable to that of the environmental factors (44.59 %). Our findings uncovered for the first time that environmental factors, species-level and phylogenetic effects jointly shape g1 variability, thereby contributing to a more comprehensive understanding of optimal stomatal behavior in the context of global environmental change.

Temperature and leaf form drive contrasting sensitivity to nitrogen deposition across European forests

Raised emissions of biologically reactive nitrogen (N) have intensified N deposition, enhancing tree productivity globally. Nonetheless, the drivers of forest sensitivity to N deposition remain unknown. We used stem growth data from 62,000 trees across Europe combined with N deposition data to track the effects of air temperature and precipitation on tree growth's sensitivity to N deposition and how it varied depending on leaf form over the past 30 years. Overall, N deposition enhanced conifer growth (until 30 kg N ha-1 yr-1) while decreasing growth for broadleaved angiosperms. Lower temperatures led to higher growth sensitivity to N deposition in conifers potentially exacerbated by N limitation. In contrast, higher temperatures stimulated growth sensitivity to N deposition for broadleaves. Higher precipitation equally increased N deposition sensitivity in all leaf forms. We conclude that air temperature and leaf form are decisive in disentangling the effect of N deposition in European forests, which provides crucial information to better predict the contribution of N deposition to land carbon sink enhancement.

Global patterns and determinants of the initial concentrations of litter carbon components

Plant litter is an important carbon (C) and nutrient pool in terrestrial ecosystems. The C components in plant litter are important because they regulate plant litter decomposition rate, but little is known on the global patterns and determinants of their concentrations in freshly fallen plant litter. Here, we quantified the concentrations of leaf litter C components (i.e., carbohydrate, polyphenol, tannin, and condensed tannin) with 864 measurements from 161 independent publications. We found that (1) the mean concentrations of leaf litter carbohydrate, polyphenol, tannin and condensed tannin were 27.7, 6.08, 8.84 and 5.7 %, respectively; (2) the concentrations of leaf litter C components were affected by taxonomic division, mycorrhizal association, life form, and/or leaf shedding strategy; (3) soil property had similar impacts on the concentrations of the four C compounds, while the influence of mean annual temperature and precipitation varied; and (4) elevation had opposing effects on carbohydrate and polyphenol concentrations, but not on that of tannin and condensed tannin, and only carbohydrate concentration was strongly affected by absolute latitude. In general, our results clearly show the global patterns and drivers of the concentrations of litter C compounds, providing new insights into the role of litter decomposition in global C dynamics.

The spatial variations and driving factors of C, N, P stoichiometric characteristics of plant and soil in the terrestrial ecosystem

Carbon(C), nitrogen(N), and phosphorus(P) are crucial elements in the element cycling in the terrestrial ecosystems. In the past decades, the spatial patterns and driving mechanisms of plant and soil ecological stoichiometry have been hot topics in ecological geography. So far, many studies at different spatial and ecological scales have been conducted, but systematic review has not been reported to summarize the research status. In this paper, we tried to fill this gap by reviewing both the spatial variations and driving factors of C, N, P stoichiometric characteristics of plant and soil at regional to large scale. Additionally, we synthesized researches on the relationships between plant and soil C, N and P stoichiometric characteristics. At the global scale, plant C, N, P stoichiometric characteristics exhibited some trends along latitude and temperature gradient. Plant taxonomic classification was the main factor controlling the spatial variations of plant C, N and P stoichiometric characteristics. Climate factor and soil properties showed varying impacts on the spatial variations of plant C, N, P stoichiometric characteristics across different spatial scales. Soil C, N, P stoichiometric characteristics also varied along climate gradient at large scale. Their spatial variations resulted from the combined effects of climate, topography, soil properties, and vegetation characteristics at regional scale. The spatial pattern of soil C, N, P stoichiometric characteristics and the driving effects from environmental factors could be notably different among different ecosystems and vegetation types. Plant C:N:P was obviously higher than that of soil, and there existed a positive correlation between plant and soil C:N:P. Their trends along longitude and latitude were similar, but this correlation varied significantly among different vegetation types. Finally, based on the issues identified in this paper, we highlighted eight potential research themes for the future studies.

Global patterns and controlling factors of tree bark C : N : P stoichiometry in forest ecosystems consistent with biogeochemical niche hypothesis

Bark serves crucial roles in safeguarding trees physically and chemically, while also contributing to nutrient cycling and carbon sequestration. Despite its importance, the broader biogeographical patterns and the potential factors influencing bark C : N : P stoichiometry in forest ecosystems remain largely unknown. In this study, we compiled a comprehensive dataset comprising carbon (C), nitrogen (N), and phosphorus (P) concentrations in bark with 1240 records from 550 diverse forest sites to systematically analyze the large-scale patterns and the factors controlling bark C : N : P stoichiometry. The geometric means of bark C, N, and P concentrations were found to be 493.17 ± 1.75, 3.91 ± 0.09, and 0.2 ± 0.01 mg g-1, respectively. Correspondingly, the C : N, C : P, and N : P mass ratios were 135.51 ± 8.11, 3313.19 ± 210.16, and 19.16 ± 0.6, respectively. Bark C : N : P stoichiometry exhibited conspicuous latitudinal trends, with the exception of N : P ratios. These patterns were primarily shaped by the significant impacts of climate, soil conditions, and plant functional groups. However, the impact of evolutionary history in shaping bark C : N : P stoichiometry outweigh climate, soil, and plant functional group, aligning with the biogeochemical niche (BN) hypothesis. These finding enhance our understanding of the spatial distribution of bark nutrient stoichiometry and have important implications for modeling of global forest ecosystem nutrient cycles in a changing environment.

Global patterns of plant functional traits and their relationships to climate

Plant functional traits (FTs) determine growth, reproduction and survival strategies of plants adapted to their growth environment. Exploring global geographic patterns of FTs, their covariation and their relationships to climate are necessary steps towards better-founded predictions of how global environmental change will affect ecosystem composition. We compile an extensive global dataset for 16 FTs and characterise trait-trait and trait-climate relationships separately within non-woody, woody deciduous and woody evergreen plant groups, using multivariate analysis and generalised additive models (GAMs). Among the six major FTs considered, two dominant trait dimensions-representing plant size and the leaf economics spectrum (LES) respectively-are identified within all three groups. Size traits (plant height, diaspore mass) however are generally higher in warmer climates, while LES traits (leaf mass and nitrogen per area) are higher in drier climates. Larger leaves are associated principally with warmer winters in woody evergreens, but with wetter climates in non-woody plants. GAM-simulated global patterns for all 16 FTs explain up to three-quarters of global trait variation. Global maps obtained by upscaling GAMs are broadly in agreement with iNaturalist citizen-science FT data. This analysis contributes to the foundations for global trait-based ecosystem modelling by demonstrating universal relationships between FTs and climate.

Environmental versus phylogenetic controls on leaf nitrogen and phosphorous concentrations in vascular plants

Global patterns of leaf nitrogen (N) and phosphorus (P) stoichiometry have been interpreted as reflecting phenotypic plasticity in response to the environment, or as an overriding effect of the distribution of species growing in their biogeochemical niches. Here, we balance these contrasting views. We compile a global dataset of 36,413 paired observations of leaf N and P concentrations, taxonomy and 45 environmental covariates, covering 7,549 sites and 3,700 species, to investigate how species identity and environmental variables control variations in mass-based leaf N and P concentrations, and the N:P ratio. We find within-species variation contributes around half of the total variation, with 29%, 31%, and 22% of leaf N, P, and N:P variation, respectively, explained by environmental variables. Within-species plasticity along environmental gradients varies across species and is highest for leaf N:P and lowest for leaf N. We identified effects of environmental variables on within-species variation using random forest models, whereas effects were largely missed by widely used linear mixed-effect models. Our analysis demonstrates a substantial influence of the environment in driving plastic responses of leaf N, P, and N:P within species, which challenges reports of a fixed biogeochemical niche and the overriding importance of species distributions in shaping global patterns of leaf N and P.

Foliar elementome and functional traits relationships identify tree species niche in French Guiana rainforests

Biogeochemical niche (BN) hypothesis aims to relate species/genotype elemental composition with its niche based on the fact that different elements are involved differentially in distinct plant functions. We here test the BN hypothesis through the analysis of the 10 foliar elemental concentrations and 20 functional-morphological of 60 tree species in a French Guiana tropical forest. We observed strong legacy (phylogenic + species) signals in the species-specific foliar elemental composition (elementome) and, for the first time, provide empirical evidence for a relationship between species-specific foliar elementome and functional traits. Our study thus supports the BN hypothesis and confirms the general niche segregation process through which the species-specific use of bio-elements drives the high levels of α-diversity in this tropical forest. We show that the simple analysis of foliar elementomes may be used to test for BNs of co-occurring species in highly diverse ecosystems, such as tropical rainforests. Although cause and effect mechanisms of leaf functional and morphological traits in species-specific use of bio-elements require confirmation, we posit the hypothesis that divergences in functional-morphological niches and species-specific biogeochemical use are likely to have co-evolved.

CO2 fertilization contributed more than half of the observed forest biomass increase in northern extra-tropical land

The existence of a large-biomass carbon (C) sink in Northern Hemisphere extra-tropical ecosystems (NHee) is well-established, but the relative contribution of different potential drivers remains highly uncertain. Here we isolated the historical role of carbon dioxide (CO2 ) fertilization by integrating estimates from 24 CO2 -enrichment experiments, an ensemble of 10 dynamic global vegetation models (DGVMs) and two observation-based biomass datasets. Application of the emergent constraint technique revealed that DGVMs underestimated the historical response of plant biomass to increasing [CO2 ] in forests (β Forest Mod ) but overestimated the response in grasslands (β Grass Mod ) since the 1850s. Combining the constrainedβ Forest Mod (0.86 ± 0.28 kg C m-2  [100 ppm]-1 ) with observed forest biomass changes derived from inventories and satellites, we identified that CO2 fertilization alone accounted for more than half (54 ± 18% and 64 ± 21%, respectively) of the increase in biomass C storage since the 1990s. Our results indicate that CO2 fertilization dominated the forest biomass C sink over the past decades, and provide an essential step toward better understanding the key role of forests in land-based policies for mitigating climate change.

Variability and limits of nitrogen and phosphorus resorption during foliar senescence

Foliar nutrient resorption (NuR) plays a key role in ecosystem functioning and plant nutrient economy. Most of this recycling occurs during the senescence of leaves and is actively addressed by cells. Here, we discuss the importance of cell biochemistry, physiology, and subcellular anatomy to condition the outcome of NuR at the cellular level and to explain the existence of limits to NuR. Nutrients are transferred from the leaf in simple metabolites that can be loaded into the phloem. Proteolysis is the main mechanism for mobilization of N, whereas P mobilization requires the involvement of different catabolic pathways, making the dynamics of P in leaves more variable than those of N before, during, and after foliar senescence. The biochemistry and fate of organelles during senescence impose constraints that limit NuR. The efficiency of NuR decreases, especially in evergreen species, as soil fertility increases, which is attributed to the relative costs of nutrient acquisition from soil decreasing with increasing soil nutrient availability, while the energetic costs of NuR from senescing leaves remain constant. NuR is genetically determined, with substantial interspecific variability, and is environmentally regulated in space and time, with nutrient availability being a key driver of intraspecific variability in NuR.

Patterns and controls of foliar nutrient stoichiometry and flexibility across United States forests

Plant element stoichiometry and stoichiometric flexibility strongly regulate ecosystem responses to global change. Here, we tested three potential mechanistic drivers (climate, soil nutrients, and plant taxonomy) of both using paired foliar and soil nutrient data from terrestrial forested National Ecological Observatory Network sites across the USA. We found that broad patterns of foliar nitrogen (N) and foliar phosphorus (P) are explained by different mechanisms. Plant taxonomy was an important control over all foliar nutrient stoichiometries and concentrations, especially foliar N, which was dominantly related to taxonomy and did not vary across climate or soil gradients. Despite a lack of site-level correlations between N and environment variables, foliar N exhibited intraspecific flexibility, with numerous species-specific correlations between foliar N and various environmental factors, demonstrating the variable spatial and temporal scales on which foliar chemistry and stoichiometric flexibility can manifest. In addition to plant taxonomy, foliar P and N:P ratios were also linked to soil nutrient status (extractable P) and climate, especially actual evapotranspiration rates. Our findings highlight the myriad factors that influence foliar chemistry and show that broad patterns cannot be explained by a single consistent mechanism. Furthermore, differing controls over foliar N versus P suggests that each may be sensitive to global change drivers on distinct spatial and temporal scales, potentially resulting in altered ecosystem N:P ratios that have implications for processes ranging from productivity to carbon sequestration.