Global Ecology and Conservation (Jun 2023)
Seasonal variation of environment and conspecific density-dependence effects on early seedling growth of a tropical tree in semi-arid savannahs
Abstract
Different mechanisms have been proposed to explain species coexistence and maintainance of local species diversity in tree communities. Among the proposed mechanisms, negative density dependence theory (NDD) predicts that seedling growth rate declines at high conspecific densities. However, local environmental conditions and seasonality may also modulate the strength of NDD. We used data from dry and wet seasons in West African semi-arid savannas to explore the importance of NDD and environmental heterogeneity in predicting seedling growth of Afzelia africana Sm, a threatened species with high economic and cutural values. First, we analysed variation of seedling growth in dry and wet seasons, and between vegetation types (tree and shrub savannas). Next, we assessed the effects of environmental heterogeneity (soil type, slope and elevation) and biotic factors (conspecific/heterospecific seedling and adult densities, leaf fungal infection, herbivory) on seedling growth in dry and wet seasons, while controlling for initial seedling size. We predicted that seedling growth would be higher in the wet than in the dry season, and in shrub savannas than in tree savannas due to variation in stand density and intensity of competitive biotic interactions. We also hypothesized that slope and negative density dependence would influence seedling growth and that, the density-dependent effects would vary with environmental heterogeneity and seasonality. Results showed two distinct patterns of seedling growth primarily driven by seasonality: a higher growth rate in the wet season due to water availability and a smaller growth rate during the dry season, possibly because seedlings develop adaptation to drought and fire at the expense of their growth. Growth rate was also higher on flat and gentle slope. Density-dependent effects on seedling growth were only observed with conspecific adults. Seasonality modulated the strength of the density-dependent effect, which was significantly negative in the wet season and nonsignificant in the dry season. This study suggests that A. africana early seedling growth is modulated by interacting effects of seasonality, terrain slope and adult conspecific density. Campaigns for reforestation with A. africana in the semi-arid areas will require increased attention on the seedlings during the long periods of drought.
