Fig. 1: Overview of the global tree allometry database.
a Geographic distribution of the allometric data (n = 374,888 individual trees belonging to 1914 species). Individual tree records were aggregated in 200 × 200 km grid cells (mean number of trees per grid cell = 742). The map was obtained from the Natural Earth database (https://www.naturalearthdata.com) and is displayed using a Robinson projection (EPSG:54030). Relationships between each tree’s stem diameter and its b height (H), c crown diameter (CD) and d crown aspect ratio (CAR) are shown on a logarithmic scale. CAR is defined as the ratio between CD and H, with values lower than 1 indicating a vertical crown profile (H > CD) while values greater than 1 corresponding to a horizontal crown profile (CD > H). Points are coloured according to the aridity index value assigned to each tree based its geographic coordinates, with larger values corresponding to drier conditions (shown in red). Graphical illustration of the approach used to generate size-standardized estimates of e tree height (HRESID), f crown diameter (CDRESID) and g crown aspect ratio (CARRESID) for each tree species. Regression lines are predicted values obtained by fitting a linear model to the entire dataset (grey points). By comparing predicted and observed value of H, CD and CAR, we quantified how much each species departs, on average, from this general trend and identified ones with greater (blue points) or smaller H, CD and CAR values (red points) than expected given their stem diameters. This approach is conceptually similar to generating species-level predictions of H, CD and CAR at a fixed size (e.g., D = 30 cm), but avoids the need to arbitrarily select a size at which to compare species.
