• Energy Research

The carbon sink potential of peatlands depends on the balance of carbon uptake by plants and microbial decomposition. The rates of both these processes will increase with warming but it remains unclear which will dominate the global peatland response. Here we examine the global relationship between peatland carbon accumulation rates during the last millennium and planetary-scale climate space. A positive relationship is found between carbon accumulation and cumulative photosynthetically active radiation during the growing season for mid- to high-latitude peatlands in both hemispheres. However, this relationship reverses at lower latitudes, suggesting that carbon accumulation is lower under the warmest climate regimes. Projections under Representative Concentration Pathway (RCP)2.6 and RCP8.5 scenarios indicate that the present-day global sink will increase slightly until around AD 2100 but decline thereafter. Peatlands will remain a carbon sink in the future, but their response to warming switches from a negative to a positive climate feedback (decreased carbon sink with warming) at the end of the twenty-first century.

SummaryGlobal vegetation and land‐surface models embody interdisciplinary scientific understanding of the behaviour of plants and ecosystems, and are indispensable to project the impacts of environmental change on vegetation and the interactions between vegetation and climate. However, systematic errors and persistently large differences among carbon and water cycle projections by different models highlight the limitations of current process formulations. In this review, focusing on core plant functions in the terrestrial carbon and water cycles, we show how unifying hypotheses derived from eco‐evolutionary optimality (EEO) principles can provide novel, parameter‐sparse representations of plant and vegetation processes. We present case studies that demonstrate how EEO generates parsimonious representations of core, leaf‐level processes that are individually testable and supported by evidence. EEO approaches to photosynthesis and primary production, dark respiration and stomatal behaviour are ripe for implementation in global models. EEO approaches to other important traits, including the leaf economics spectrum and applications of EEO at the community level are active research areas. Independently tested modules emerging from EEO studies could profitably be integrated into modelling frameworks that account for the multiple time scales on which plants and plant communities adjust to environmental change.

AbstractTwo simplifying hypotheses have been proposed for whole‐plant respiration. One links respiration to photosynthesis; the other to biomass. Using a first‐principles carbon balance model with a prescribed live woody biomass turnover, applied at a forest research site where multidecadal measurements are available for comparison, we show that if turnover is fast the accumulation of respiring biomass is low and respiration depends primarily on photosynthesis; while if turnover is slow the accumulation of respiring biomass is high and respiration depends primarily on biomass. But the first scenario is inconsistent with evidence for substantial carry‐over of fixed carbon between years, while the second implies far too great an increase in respiration during stand development—leading to depleted carbohydrate reserves and an unrealistically high mortality risk. These two mutually incompatible hypotheses are thus both incorrect. Respiration is not linearly related either to photosynthesis or to biomass, but it is more strongly controlled by recent photosynthates (and reserve availability) than by total biomass.

Leaf size, climate, and energy balance Why does plant leaf size increase at lower latitudes, as exemplified by the evolutionary success of species with very large leaves in the tropics? Wright et al. analyzed leaf data for 7670 plant species, along with climatic data, from 682 sites worldwide. Their findings reveal consistent patterns and explain why earlier predictions from energy balance theory had only limited success. The authors provide a fully quantitative explanation for the latitudinal gradient in leaf size, with implications for plant ecology and physiology, vegetation modeling, and paleobotany. Science , this issue p. 917

Contents Summary 507 I. Introduction 507 II. The return on investment approach 508 III. CO2 response spectrum 510 IV. Discussion 516 Acknowledgements 518 References 518 SummaryLand ecosystems sequester on average about a quarter of anthropogenic CO2 emissions. It has been proposed that nitrogen (N) availability will exert an increasingly limiting effect on plants’ ability to store additional carbon (C) under rising CO2, but these mechanisms are not well understood. Here, we review findings from elevated CO2 experiments using a plant economics framework, highlighting how ecosystem responses to elevated CO2 may depend on the costs and benefits of plant interactions with mycorrhizal fungi and symbiotic N‐fixing microbes. We found that N‐acquisition efficiency is positively correlated with leaf‐level photosynthetic capacity and plant growth, and negatively with soil C storage. Plants that associate with ectomycorrhizal fungi and N‐fixers may acquire N at a lower cost than plants associated with arbuscular mycorrhizal fungi. However, the additional growth in ectomycorrhizal plants is partly offset by decreases in soil C pools via priming. Collectively, our results indicate that predictive models aimed at quantifying C cycle feedbacks to global change may be improved by treating N as a resource that can be acquired by plants in exchange for energy, with different costs depending on plant interactions with microbial symbionts.

Plant functional traits (FTs) determine the survival strategies of plants and their adaptations to the environment, which affect the structure and productivity of vegetation. However, global patterns of many FTs remain uncertain. Currently available global maps of FTs generated by different upscaling approaches show considerable divergence. Potentially different trait responses could be induced by climate change in herbaceous, evergreen and deciduous taxa. Better understanding of these variations should improve our ability to predict global trait patterns and the consequences of global environmental change for vegetation.We compiled a global data set for 18 FTs, including 42,676 species from 89,478 natural vegetation plots. All FTs in the data set have community-weighted mean values for each plot; seven also have species-mean values. We grouped the species into non-woody, woody deciduous and woody evergreen categories according to their life form and leaf phenology. Then we calculated community-weighted mean values of the seven FTs having species-mean records for the three plant groups. We selected three bioclimatic variables: a moisture index (MI, representing plant-available moisture), mean temperature of the coldest month (MTCO, representing winter cold), and mean growing season temperature (MGST, representing summer warmth) to create a three-dimensional global climate space and define global climate classes. Principal Component Analysis (PCA) was used to estimate the main functional continua on which FTs converge for each plant group. Redundancy Analysis (RDA) was used to describe the extent to which variation in trait combinations can be explained by bioclimatic variables. Correlations between each trait and bioclimatic variables for the three plant groups were described by Generalized Additive Models (GAMs). We used the GAMs to visualise the trait distribution in global climate space for all seven FTs, group by group. We finally fitted new comprehensive GAMs considering bioclimatic variables and remotely-sensed global cover of the three plant groups in order to predict global patterns for all 18 FTs at 0.1° spatial resolution.Bioclimatic variables explain more trait variance for woody than non-woody plants. Two trait combinations are common to all plant groups: one is plant height (H) – diaspore mass (DM), positively associated with seasonal temperatures; the other is leaf mass per unit area (LMA) – leaf nitrogen content per unit area (Narea), decreasing with moisture availability. Stem specific density (SSD) of non-woody plants is correlated with the LMA–Narea axis, but SSD of woody evergreen plants is correlated with the H–DM axis. For woody deciduous plants, SSD is correlated with leaf nitrogen content per unit mass (Nmass). Leaf area (LA) is positively correlated with all bioclimatic variables and shows variation in all plant groups that is independent of other traits. FTs within the same trait combination tend to present similar patterns in the global climate space. GAMs based on bioclimatic variables and vegetation cover explain up to three-quarters (on average about a half) of global trait variation.Our study reveals universal relationships among traits and between traits and climates, highlights certain key differences between within non-woody, woody deciduous and woody evergreen taxa, and produces high-resolution global maps for plant functional traits.