Increasing tree size across Amazonia

Abstract

Climate change and increasing availability of resources such as carbon dioxide are modifying forest functioning worldwide, but the effects of these changes on forest structure are unclear. As additional resources become available, for example, through CO₂ fertilization or nitrogen deposition, large trees, with greater access to light, may be expected to gain further advantages. Conversely, smaller light-suppressed trees might benefit more if their light compensation point changes, while bigger trees may be the most negatively impacted by increasing heat and drought. We assessed recent changes in the structure of Earth's largest tropical forest by analysing 30 years of Amazonian tree records across 188 mature forest plots. We find that, at a stand level, trees have become larger over time, with mean tree basal area increasing by 3.3% per decade (95% CI 2.4; 4.1). Larger trees have increased in both number and size, yet we observed similar rates of relative size gain in large and small trees. This evidence is consistent with a resource-driven boost for larger trees but also a reduction in suppression among smaller trees. These results, especially the persistence and consistency of tree size increases across Amazonian forest plots, communities and regions, indicate that any negative impacts of climate change on forests and large trees here have so far been mitigated by the positive effects of increased resources.

Fig. 1. Potential impacts of growth stimulation and climate change on forest structure.

a, Expected change in tree size distribution under different hypotheses. D, diameter at breast height. b, Direction of anticipated changes in key tree size descriptors compared to the original forest. Winners-take-all hypothesis: If increase in resources benefits the largest trees, asymmetric competition for light will increase, leading to greater light suppression in the understorey. This will increase the mean tree size but will not affect the median size, potentially decrease stem numbers (N), increase the scale parameter (scale) and raise the Gini coefficient. Carbon-limited benefit hypothesis: If CO₂ stimulates growth in understorey trees, improving their carbon balance, smaller trees will grow more, increasing recruitment in smaller size classes. This will raise stem numbers, decrease median tree size (with little effect on the mean), increase the Gini coefficient and decrease the scale parameter. Shared benefits hypothesis: If increase in resources benefits all trees equally, we expect an increase in mean and median tree sizes and number of stems, a larger scale parameter and no change in the Gini coefficient. Large trees lose hypothesis: If increasing heat, drought, lightning or wind disproportionately impact the mortality of large trees, the mean tree size would decrease, median size would remain unchanged, stem numbers would decline and the scale parameter would lower, with lower inequality (greater Gini coefficient).

Fig. 2. Spatial trends of mean tree size and the scale parameter across Amazonian forests.

a,b, Distribution of annual trends of mean tree BA (a) and scale parameter (b) per inventory plot across Amazonia. Trends represent the slope of a linear regression fit to mean tree size and scale parameter within each inventory plot. These vary from −7.1 × 10⁻⁴ to 2.2 × 10⁻³ m² yr⁻¹ and −1.1 to 3.7 yr⁻¹ for mean tree size and scale parameter, respectively. Mean tree size changed on average by 1.45 × 10⁻⁴ m² yr⁻¹ across all plots, 3.3% gain per decade compared to initial mean size of 4.78 × 10⁻² m². The average change in scale parameter was 0.4 yr⁻¹, 3.8% per decade compared to initial mean scale parameter of 103. Arrows show the magnitude and direction of trends at each plot location, with blue arrows showing increasing trends and red arrows showing declining trends.

Fig. 3. Changes in stem BA distribution between 1990 and 2010.

Data are plotted for 30 ha of forest across 22 plots, all censused before 1990 and after 2010, and illustrate an increase in the frequencies and size (shown by the stem BA) of the largest stems.

Fig. 4. Changes in mean stem BA, scale parameter and stem numbers across mature Amazonian forests.

Left: individual plot-level linear trends in size structure parameters across the full interval each plot was censused for. For visualization purposes, only 92 of the 188 plots are included, with the most strongly weighted plots based on area and monitoring period length included. Positive trend lines are coloured blue, and negative trend lines are coloured red. Right: annual rate of change of size structure parameters. Red vertical lines show the overall bootstrapped mean (solid lines) and 95% CI (dashed lines). Blue lines are positioned at 0, that is, no change.

Fig. 5. Histograms of linear slopes of absolute and relative change in tree size in Amazon plots as a function of time within tree size classes.

Comparison between trends in tree size within different size classes (D < 200 mm, D = 200–399 mm and D ≥ 400 mm). Red solid line and dashed lines represent bootstrapped mean and 95% CI, zero is shown by the red line. To help visualize trends in absolute terms, plots that show annual rates of change < −0.002 m² yr⁻¹ or > 0.002 m² yr⁻¹ are omitted from the graph (59 plots, D ≥ 400 mm; 1 plot, D = 200–399 mm). Note that although the increase in tree size is more evident within large trees (D ≥ 400 mm) in absolute terms, the trends in size are similar in relative terms regardless of the size class.

Extended Data Fig. 1. Spatial distribution and trends of forest structure across Amazonian forests.

Distribution of mean (a) and median tree size (b), measured in basal area terms, and scale parameter (c) calculated per inventory plot across Amazonia.

Extended Data Fig. 2. Changes in median stem BA and gini coefficient across mature Amazonian forests.

Left: individual plot-level linear trends in size structure parameters across the full interval each plot was censused for. For visualisation purposes, only 92 of the 188 plots are included, with the most strongly weighted plots based on area and monitoring period length included. Positive trends lines are coloured blue, and negative trend lines are colored red. Right: Annual rate of change of size structure parameters. Red vertical lines show the overall bootstrapped mean (solid lines) and 95% CI (dashed lines). Blue lines are positioned at zero, that is no change.

Extended Data Fig. 3. Changes in forest structure for the four Amazonian biogeographic regions.

Histograms of change in mean stem BA, the scale parameter, and stem numbers per ha by biogeographic region, showing consistent directional change in size distribution parameters across all four regions. Red vertical lines show the overall bootstrapped mean (solid lines) and 95% CI (dashed lines). Blue lines are positioned at zero, that is no change.

Extended Data Fig. 4. Histograms of linear slopes of absolute and relative change in mean stem BA in Amazon plots between different canopy strata.

Red solid line and dashed lines represent bootstrapped mean and 95% CI, while the blue line shows 0. Canopy stratum was defined using the Ideal Tree Distribution (ITD) model (5). Note that trends in tree size for overstorey and understorey differ in absolute but not in relative terms.

Extended Data Fig. 5. Trends in plot-level wood density.

(a) Histogram showing the distribution of rates of change in plot-level mean wood density. Overall trends in wood density are not significantly different from zero (mean = 1.51 ×10⁻⁵, 95% CI = -7.28 x 10⁻⁵; 1.01 x 10⁻⁴). (b) Relationship between trend in mean tree size and trend in mean wood density (two-sided t-test, t = 7.32, p-value = 0.16, R² = 0.005). If forests were under late successional recovery, an increase in mean tree size would be associated with an increase in wood density.

Extended Data Fig. 6. Effect of initial values of structural parameters on their rates of change.

(a) Relationship between initial mean tree size and rate of change in mean tree size (two-sided t-test, t = 1.8, p-value = 0.89, R² = 0) (b) Relationship between initial scale parameter and rate of change in scale parameter (two-sided t-test, t = 2.7, p-value = 0.21, R² = 0.003). If forests were under successional recovery, we would expect the increase in size to be more pronounced in forests with smaller initial mean tree size and lower scale parameter values.