Figure 1.
Geographical gradients of species composition and underlying patterns of species distribution.
Plot scores on the first three compositional axes of a correspondence analysis of the plots (n = 56,445)×species (n = 31) abundance matrix were mapped (A, C, E). Variance explained by each axis is given in brackets. Solid lines represent country borders, names of main cities are indicated on the third map. Barplots give species scores across each compositional axis (B, D, F), with bar shading indicating the four species groups with contrasting distribution patterns that were identified by a cluster analysis (Fig. 2). Light and dark grey bars indicate species associated with the positive or negative end of the first compositional axis, respectively, while black and white bars indicate the two pioneer species, i.e. Lophira and Musanga, respectively, that both formed separate single-species groups.
Table 1.
Focal tree species, their overall abundance and frequency, and their functional traits.
Figure 2.
Cluster dendrogram grouping species with similar distribution patterns.
A hierarchical cluster analysis on the Euclidean distances between species scores and an average agglomeration method was used to identify groups of species according to their distribution patterns, i.e. position across the compositional axes. The grey boxes indicate the cut-off level used to identify the four groups.
Table 2.
Results of spatial regression models relating environmental and historical factors to compositional axes.
Figure 3.
Compositional and structural differences among forests growing on different geological substrates.
Plot scores on the first compositional axis were used to describe the main variation in species composition (A). Structural differences were assessed as the % deviation from mean plot basal area (B). Significant differences between substrates at P<0.001 in paired Wilcoxon tests are indicated by different letters.
Figure 4.
Functional differences among species associated with different geological substrates.
Species scores on the first compositional axis were used as an indicator of species association with the geological substrate. Relationships between species scores and functional traits were assessed for leaf phenology (A), shade tolerance (B), wood density (C), and maximum annual growth rate (D). Different lower case letters above the boxplots indicate significant differences (P<0.05) in paired comparisons using Wilcoxon tests. Regression lines were plotted for quantitative traits. Symbol shading indicates the four species groups with contrasting distribution patterns: light and dark grey symbols indicate species positively or negatively associated with the sandstone substrate, respectively, while black and white symbols indicate the two pioneer species Lophira and Musanga.