The global biogeography of tree leaf form and habit

Abstract

Understanding what controls global leaf type variation in trees is crucial for comprehending their role in terrestrial ecosystems, including carbon, water and nutrient dynamics. Yet our understanding of the factors influencing forest leaf types remains incomplete, leaving us uncertain about the global proportions of needle-leaved, broadleaved, evergreen and deciduous trees. To address these gaps, we conducted a global, ground-sourced assessment of forest leaf-type variation by integrating forest inventory data with comprehensive leaf form (broadleaf vs needle-leaf) and habit (evergreen vs deciduous) records. We found that global variation in leaf habit is primarily driven by isothermality and soil characteristics, while leaf form is predominantly driven by temperature. Given these relationships, we estimate that 38% of global tree individuals are needle-leaved evergreen, 29% are broadleaved evergreen, 27% are broadleaved deciduous and 5% are needle-leaved deciduous. The aboveground biomass distribution among these tree types is approximately 21% (126.4 Gt), 54% (335.7 Gt), 22% (136.2 Gt) and 3% (18.7 Gt), respectively. We further project that, depending on future emissions pathways, 17-34% of forested areas will experience climate conditions by the end of the century that currently support a different forest type, highlighting the intensification of climatic stress on existing forests. By quantifying the distribution of tree leaf types and their corresponding biomass, and identifying regions where climate change will exert greatest pressure on current leaf types, our results can help improve predictions of future terrestrial ecosystem functioning and carbon cycling.

Fig. 1. Global coverage of forest inventory locations (GFBi data) and plot-level leaf-type proportions.

a, A total of 9,781 forest inventory plots (green points) were used for geospatial modelling of forest leaf types. b, Number of plots in relation to their proportion of evergreen vs deciduous and broadleaved vs needle-leaved individuals.

Fig. 2. The global distribution of forest leaf types.

a, The global distribution of tree leaf type as predicted by a random forest model built from area-based leaf-type data (see Methods). Pixels are coloured in the red, green and blue spectrum according to the percentage of total tree basal area occupied by broadleaved evergreen, broadleaved deciduous and needle-leaved tree types, as indicated by the ternary plot. Needle-leaved evergreen and needle-leaved deciduous forests were combined due to the low global coverage of needle-leaved deciduous trees. b–e, Predicted relative coverage of each leaf type from random forest models. Ref. was used to mask non-forest areas. b, Broadleaved evergreen coverage. c, Broadleaved deciduous coverage. d, Needle-leaved evergreen coverage. e, Needle-leaved deciduous coverage.

Fig. 3. Variable importance of environmental covariates on forest leaf-type variation.

a,b, Cumulative importance of the first six principal components of climate, soil, topographic and vegetation covariates in the variation of leaf habit (a) and leaf form (b). c,d, Variable importance of selected environmental features on variation in leaf habit (c) and leaf form (d). Bars in c and d represent the mean ± 95% CI; relative importance based on the 10 best random forest models (n = 10; see Methods). Area-based leaf-type proportions were used to represent forest (plot-level) leaf-type variation.

Fig. 4. The global proportion of evergreen broadleaved, deciduous broadleaved, needle-leaved evergreen and needle-leaved deciduous trees.

The relative proportions of trees that occur within tropical, temperate, boreal and arid regions are shown as separate pie charts for each leaf type.

Fig. 5. Forested areas where future climates may no longer support prevailing leaf types.

If a pixel’s forest area was predominantly (>60%) covered by one leaf type, it was classified as that specific leaf type. Pixels where no leaf type exceeded 60% coverage were classified as mixed forest. To determine the relative proportion of each leaf type per plot, we considered the basal area of individual trees (area-based leaf type). Coloured pixels on the map indicate areas that, by the end of the century (2071–2100), will face climate conditions that currently support a different forest type. The future climate conditions were represented using three climate change scenarios: low-emission (SSP1–RCP2.6; a,b), business-as-usual (SSP3–RCP7; c,d) and high-emission (SSP5–RCP8.5; e,f) for the period 2071–2100. Panels a, c and e show the present forest types. In contrast, panels b, d and f show the type of forest expected under the projected future climate of each respective pixel.