Higher treefall rates on slopes and waterlogged soils result in lower stand biomass and productivity in a tropical rain forest

Abstract

Summary 1.Relationships between tropical rain forest biomass and environmental factors have been determined at regional scales, e.g. the Amazon Basin, but the reasons for the high variability in forest biomass at local scales are poorly understood. Interactions between topography, soil properties, tree growth and mortality rates, and treefalls are a likely reason for this variability. 2. We used repeated measurements of permanent plots in lowland rain forest in French Guiana to evaluate these relationships. The plots sampled topographic gradients from hilltops to slopes to bottomlands, with accompanying variation in soil waterlogging along these gradients. Biomass was calculated for >175 tree species in the plots, along with biomass productivity and recruitment rates. Mortality was determined as standing dead and treefalls. 3. Treefall rates were twice as high in bottomlands as on hilltops, and tree recruitment rates, radial growth rates and the abundance of light-demanding tree species were also higher. 4. In the bottomlands, the mean wood density was 10% lower than on hilltops, the basal area 29% lower and the height:diameter ratio of trees was lower, collectively resulting in a total woody biomass that was 43% lower in bottomlands than on hilltops. 5. Biomass productivity was 9% lower in bottomlands than on hilltops, even though soil Olsen P concentrations were higher in bottomlands. 6. Synthesis. Along a topographic gradient from hilltops to bottomlands there were higher rates of treefall, which decreased the stand basal area and favoured lower allocation to height growth and recruitment of light-demanding species with low wood density. The resultant large variation in tree biomass along the gradient shows the importance of determining site characteristics and including these characteristics when scaling up biomass estimates from stand to local or regional scales.

Identifying climatic drivers of tropical forest dynamics

“…5). This confirms the observation of Ferry et al (2010) and the hypothesis that waterlogged soils in space or in time are risky for trees. Moreover, during the rainy season, strong rainfall events often come with strong winds that may accelerate this process (Toledo et al, 2011).…”

“…Rain may also be responsible for water saturation, a phenomenon that is far less studied but that can have an effect on tree mortality or growth. For instance, Ferry et al (2010) underlined a higher mortality rate in waterlogged areas. Inter-annual variations of rain quantities can lead to more or less waterlogged soils, independent of their topographical location, implying instability that can cause cascading tree falls.…”

Herbivory, growth rates, and habitat specialization in tropical tree lineages: implications for Amazonian beta‐diversity

“…This result correlates with the higher soil resources found in these habitats and rapid rates of turnover of stems in these forests due to seasonal flooding (Ferry et al 2010, Baraloto et al 2011). However, higher LPR was not accompanied by higher HR.…”

Contrasting taxonomic and functional responses of a tropical tree community to selective logging

“…Second, the principal topographic features, hilltops and slopes, differ in their taxonomic composition. Bottomland forests also occur at Paracou, but as they were little impacted by logging activities (Ferry et al 2010) they are not considered here. We therefore stratified our study design such that each habitat was sampled once within each of the six combinations of two topographic positions (hilltop and slope) and three taxonomic blocks, for a total of 18 samples.…”