Abstract
Monodominant patches of forest dominated by Gilbertiodendron dewevrei are commonly found in central African tropical forests, alongside forests with high species diversity. Although these forests are generally found sparsely distributed along rivers, their occurrence is not thought to be (clearly) driven by edaphic conditions but rather by trait combinations of G. dewevrei that aid in achieving monodominance. Functional community structure between these monodominant and mixed forests has, however, not yet been compared. Additionally, little is known about nondominant species in the monodominant forest community. These two topics are addressed in this study. We investigate the functional community structure of 10 one‐hectare plots of monodominant and mixed forests in a central region of the Congo basin, in DR Congo. Thirteen leaf and wood traits are measured, covering 95% (basal area weighted) of all species present in the plots, including leaf nutrient contents, leaf isotopic compositions, specific leaf area, wood density, and vessel anatomy. The trait‐based assessment of G. dewevrei shows an ensemble of traits related to water use and transport that could be favorable for its location near forest rivers. Moreover, indications have been found for N and P limitations in the monodominant forest, possibly related to ectomycorrhizal associations formed with G. dewevrei. Reduced leaf N and P contents are found at the community level for the monodominant forest and for different nondominant groups, as compared to those in the mixed forest. In summary, this work shows that environmental filtering does prevail in the monodominant G. dewevrei forest, leading to lower functional diversity in this forest type, with the dominant species showing beneficial traits related to its common riverine locations and with reduced soil N and P availability found in this environment, both coregulating the tree community assembly.
Highlights
Tropical rain forests are complex systems with high diversity of tree species growing in three continents along the equator
Within the monodominant forest, we found lower nutrient contents (LPC 0.51 mg/g compared to 0.60 mg/g in the mixed forest, p < .001; leaf n itrogen content (LNC) 26.6 mg/g compared to 32.2 mg/g, p < .001), lower δ15N (6.2‰; compared to 7.4‰, p < .001), thicker leaves (low SLA (13.3 m2/kg; compared to 16.7 m2/kg, p < .001), high LDMC (0.41 g/g; compared to 0.37 g/g, p < .001)), higher Wood density (WD) (0.66 g/cm3; compared to 0.62 g/cm3, p < .001) combined with lower vessel density (VD) (6.9 per μm2; compared to 15.6 per μm2, p < .05) and higher Vessel diameters (VDm) (164.7 μm; compared to 112.5 μm, p < .001), and lower values for δ13C (−33.7‰; compared to −33.1‰, p < .001) combined with higher δ18O values (22.1‰; compared to 20.7‰, p < .001)
These indices show that the monodominant forest mainly differs from the adjacent mixed forest in the narrower range of its niche space, where a lower species diversity is present, which could be the result of environmental filtering (Mason et al, 2005; Villéger et al, 2008)
Summary
Tropical rain forests are complex systems with high diversity of tree species growing in three continents along the equator. Lower diversity forests are found in the form of monodominant forest, where a single canopy species constitutes ≥60% of all canopy-level trees (Connell & Lowman, 1989; Hart, 1985; Peh, Lewis, & Lloyd, 2011) Such monodominance in old-growth forests can be caused by distinct edaphic conditions (Richards, 1996), for example, in water-logged forest (Connell & Lowman, 1989; Richards, 1996) and low-nutrient forests (McGuire, 2007). We hypothesize (Hypothesis II) that other species present in this monodominant forest contain (an ensemble of) traits similar to those of G. dewevrei as they encounter the same environmental filtering and that species that do not possess these traits will be confined to the mixed forest
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