Abstract
Understanding the dynamics of dominant tree species in tropical forests is important both for biodiversity and carbon-related issues. We focus on the Congo Basin (East of Kisangani) to investigate the respective roles of topographic/soil gradients and endogenous dynamics in shaping local variations in dominance. We used a dataset of 30 1-ha plots, in which all trees above 10 cm diameter at breast height (DBH) were censused. Soil samples were analyzed for standard pedologic variables and a digital elevation model permitted to infer topography and hydromorphy. We found that this forest is characterized by variations in the abundance of three dominant species: Petersianthus macrocarpus (P.Beauv.) Liben (PM), Gilbertiodendron dewevrei (De Wild.) J.Leonard (GD) and Julbernardia seretii (De Wild.) Troupin (JS). These variations occur independently of substratum or topography variations. At plot level, the local relative abundance never reached 50%, the threshold for monodominance proposed in the literature, but rather progressively increased to reach higher values for canopy trees (>60 cm DBH), where this threshold could be exceeded. We found no sign of shifting compositional dynamics, whereby the dominant species would switch between the canopy and the undergrowth. Our results, therefore, support the possibility of the existence of stable dominance states, induced by endogenous processes, such as biological positive feedbacks fostering monodominance. We also document a strong relation between monodominance level and alpha diversity, when giving more weight to abundant species which is expected (R² = 0.79) but also when giving more weight to rare species (R² = 0.37), showing that monodominance influences tree species richness patterns. Structural differences existed between groups, with the PM group having more (and on average smaller) stems and lighter wood on average, but paradoxically also higher biomass and basal area.
Highlights
The study of tree assemblages in tropical forests is gaining new impetus with the need to assess carbon emissions at high precision and resolution, while limiting the erosion of diversity and promoting sustainable forest management [1,2,3]
If we focus on cases with plot level dominance exceeding 10%, relative density differences between the canopy and the lower strata were on average 11.3% for G. dewevrei, 45.6% for J. seretii and 14.1% for P. macrocarpus
Using a sampling of 30 1-ha plots sampled across a 260 km2 area, we evidenced strong variations in the abundance of dominant species characterized by Gilbertiodendron dewevrei, Julbernadia seretii, Using a sampling of 30 1-ha plots sampled across a 260 km2 area, we evidenced strong variations
Summary
The study of tree assemblages in tropical forests is gaining new impetus with the need to assess carbon emissions at high precision and resolution, while limiting the erosion of diversity and promoting sustainable forest management [1,2,3]. Time can play a major role, via perturbations and succession, which can lead to a locally patchy, but overall homogeneous landscape [12] These different mechanisms need not be happening independently of each other. Biological feedback along successional gradients will alter both edaphic and (micro-) climatic conditions, leading in some cases to similar climax vegetation from different initial edaphic conditions [13]. Given this overwhelming complexity, how can we hope to glean the necessary pieces of understanding that could help in orienting management or conservation? How can we hope to glean the necessary pieces of understanding that could help in orienting management or conservation? Can a portion of the heterogeneity in lowland terra firme forests, outside blatantly extreme edaphic conditions, be considered as relatively stable and unambiguously driven either by exogenous (e.g., edaphic) or endogenous (e.g., biotic) processes, or is it all the result of chance dispersion or perturbation events and perpetually drifting [4]?
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