Shift in functional traits along soil fertility gradient reflects non-random community assembly in a tropical African rainforest

Background and aims - There is increasing recognition that plant traits mediate environmental influence on species distribution, justifying non-random community assembly. We studied the influence of local scale edaphic factors on the distribution of functional traits in a tropical rainforest of Cameroon with the aim to find correlations between the main edaphic gradient and community functional trait metrics (weighted mean trait, functional divergence and intraspecific variation). Methods - Within the Korup Forest Dynamics Plot (50 ha), we randomly selected 44 quadrats of 0.04 ha each, collected soils and analysed 11 topography and soil variables. Leaves were harvested from all 98 tree species found in the quadrats to calculate community trait metrics [quadrat-level weighted mean (q(k)) and functional divergence (FDiv(k))] for leaf area (LA), specific leaf area (SLA), leaf phosphorus (LPC), leaf nitrogen concentration (LNC) and nitrogen to phosphorus ratio (N:P ratio). We examined relationships between the main edaphic gradient with q(k), with FDiv(k) and with intraspecific variation and interpreted correlations as the effects of abiotic filtering and competitive interaction. Key results - Soil fertility was the main edaphic gradient and was significantly correlated with q(k) for LPC, LNC and LA and with FDiv(k) for LPC, N: P ratio, LA and SLA, confirming the influence of abiotic filtering and competitive interaction by the soil fertility gradient, respectively. For a given trait, quadrats were either over-dispersed or under-dispersed, accounting for 7-33 % of non-random trait distribution along the soil fertility gradient. Trends in intraspecific traits variation were consistently lower than quadrat-level mean traits along the soil fertility gradient. Conclusions - This study demonstrates the influence of soil fertility gradient on local scale community trait distribution and its contribution to non-random community assembly.