Fractionation of carbon isotopes by plants during CO ₂ uptake and fixation (Δ _leaf ) varies with environmental conditions, but quantitative patterns of Δ _leaf across environmental gradients at the global scale are lacking. This impedes interpretation of variability in ancient terrestrial organic matter, which encodes climatic and ecological signals. To address this problem, we converted 3,310 published leaf δ ¹³ C values into mean Δ _leaf values for 334 woody plant species at 105 locations (yielding 570 species-site combinations) representing a wide range of environmental conditions. Our analyses reveal a strong positive correlation between Δ _leaf and mean annual precipitation (MAP; R ² = 0.55), mirroring global trends in gross primary production and indicating stomatal constraints on leaf gas-exchange, mediated by water supply, are the dominant control of Δ _leaf at large spatial scales. Independent of MAP, we show a lesser, negative effect of altitude on Δ _leaf and minor effects of temperature and latitude. After accounting for these factors, mean Δ _leaf of evergreen gymnosperms is lower (by 1–2.7‰) than for other woody plant functional types (PFT), likely due to greater leaf-level water-use efficiency. Together, environmental and PFT effects contribute to differences in mean Δ _leaf of up to 6‰ between biomes. Coupling geologic indicators of ancient precipitation and PFT (or biome) with modern Δ _leaf patterns has potential to yield more robust reconstructions of atmospheric δ ¹³ C values, leading to better constraints on past greenhouse-gas perturbations. Accordingly, we estimate a 4.6‰ decline in the δ ¹³ C of atmospheric CO ₂ at the onset of the Paleocene-Eocene Thermal Maximum, an abrupt global warming event ∼55.8 Ma.

Give It Time: Experimental ecological studies in recent years have provided a great deal of insight into how species diversify and influence ecosystem properties, but in most cases the experiments have been relatively brief (up to ∼5 years). Reich et al. (p. 589 ; see the Perspective by Cardinale ) performed two 13- and 15-year grassland experiments and found that the effects of plant species richness on community-level processes like biomass production tend to be saturating at early stages but that those impacts grow stronger and more linear as experiments run longer. Stronger influences through time were largely driven by increasing amounts of “complementarity” among species, and these trends were correlated with greater expression of functional diversity in multispecies assemblages. Thus, the effects of diversity grow stronger through time as species gain more and more opportunity to vary in their use of the limiting biological resources in their environment, which emphasizes the functional importance of maintaining diversity in ecosystems.

Much research focuses on increasing carbon storage in mineral-associated organic matter (MAOM), in which carbon may persist for centuries to millennia. However, MAOM-targeted management is insufficient because the formation pathways of persistent soil organic matter are diverse and vary with environmental conditions. Effective management must also consider particulate organic matter (POM). In many soils, there is potential for enlarging POM pools, POM can persist over long time scales, and POM can be a direct precursor of MAOM. We present a framework for context-dependent management strategies that recognizes soils as complex systems in which environmental conditions constrain POM and MAOM formation.

The relationship between plant diversity and productivity in grasslands could depend, partly, on how diversity affects vertical distributions of root biomass in soil; yet, no prior study has evaluated the links among diversity, root depth distributions, and productivity in a long‐term experiment. We used data from a 12‐year experiment to ask how plant species richness and composition influenced both observed and expected root depth distributions of plant communities. Expected root depth distributions were based on the abundance of species in each community and two traits of species that were measured in monocultures: root depth distributions and root‐to‐shoot ratios. The observed proportion of deep‐root biomass increased more than expected with species richness and was positively correlated with aboveground productivity. Indeed, the proportion of deep‐root biomass explained variation in productivity even after accounting for legume presence/abundance and greater nitrogen availability in diverse plots. Diverse plots had root depth distributions that were twice as deep as expected from their species composition and corresponding monoculture traits, partly due to interactions between C₄ grasses and legumes. These results suggest that the productivity of diverse plant communities was partly dependent on belowground plant interactions that caused roots to be distributed more deeply in soil.