Soil water-holding capacity and monodominance in Southern Amazon tropical forests

Background and aims We explored the hypothesis that low soil water-holding capacity is the main factor driving the monodominance ofBrosimum rubescensin a monodominant forest in Southern Amazonia. Tropical monodominant forests are rare ecosystems with low diversity and high dominance of a single tree species. The causes of this atypical condition are still poorly understood. Some studies have shown a relationship between monodominance and waterlogging or soil attributes, while others have concluded that edaphic factors have little or no explanatory value, but none has accounted for soil-moisture variation other than waterlogging. This study is the first to explicitly explore how low soil water-holding capacity influences the monodominance of tropical forests. Methods We conducted in situ measurements of vertical soil moisture using electrical resistance collected over 1 year at 0-5; 35-40 and 75-80 cm depths in aB. rubescensmonodominant forest and in an adjacent mixed-species forest in the Amazon-Cerrado transition zone, Brazil. Minimum leaf water potential (psi min) of the seven most common species, includingB. rubescens, and soil water-holding capacity for both forests were determined. Results The vertical soil moisture decay pattern was similar in both forests for all depths. However, the slightly higher water availability in the monodominant forest and psi min similarity betweenB. rubescensand nearby mixed forest species indicate that low water-availability does not cause the monodominance. Conclusions We reject the hypothesis that monodominance ofB. rubescensis primarily determined by low soil water-holding capacity, reinforcing the idea that monodominance in tropical forests is not determined by a single factor.