Regional rainfall and local topography jointly drive tree community assembly in lowland tropical forests of New Caledonia

• Published in: Journal of vegetation science 2019-09, Vol.30 (5), p.845-856
• Main Authors: Blanchard, Grégoire, Munoz, François, Ibanez, Thomas, Hequet, Vanessa, Vandrot, Hervé, Girardi, Jérémy, Birnbaum, Philippe, Duarte, Leandro
• Format: Article
• Language: English
• Subjects: Assembly; Biodiversity; Biodiversity and Ecology; Botanics; Communities; Composition; Drought; Drought resistance; Dry matter; Ecological effects; Ecology, environment; economic spectrum; Economics; Ecosystems; environmental filtering; Environmental Sciences; Landscape; Leaf area; Life Sciences; Moisture content; New Caledonia; Precipitation; Rain; Rainfall; Resource availability; Resource conservation; Ridges; Species; Statistical analysis; Systematics, Phylogenetics and taxonomy; Topography; tree community; Tropical forests; Vegetal Biology; water availability; Wood
• ISSN: 1100-9233; 1654-1103
• DOI: 10.1111/jvs.12781

Aim To understand how variations in precipitation and topographic wetness influence tree community assembly at both regional and landscape scales in tropical forests. Location New Caledonia (SW Pacific). Methods We sampled 40 tree communities in 0.04‐ha plots laid along topographic gradients within two landscapes with contrasting precipitation. Within a dry (2,500 mm/year) landscape we used the Topographic Wetness Index (TWI) to sample communities in topographic position with low (e.g., ridges) and high (e.g., valleys) water accumulation. For each sampled species, we measured five functional traits involved in drought resistance and resource acquisition (wood density, leaf area, leaf specific area, leaf dry matter content, and leaf thickness). We first examined trait covariation across species. We then analysed how the functional composition of communities varied between landscapes (according to precipitation) and within landscapes (according to TWI), using trait‐based statistics and null models. Results We identified two ecological trade‐offs driving trait variation across species: (i) one opposing high hydraulic efficiency to drought resistance, related to a wood economic spectrum; and (ii) the other opposing resource acquisition to resource conservation, related to a leaf economic spectrum. Across landscapes, species with drought resistance strategies were favoured at lower precipitation. Within landscapes, drought resistant species were selected under low TWI in the dry landscape, while low TWI increased the abundance of species with conservative strategies in the wet landscape. Conclusions Precipitation and topography jointly shape the functional composition of tree communities. At low precipitation, hydric constraints prevailed on ridges and upslopes by filtering drought resistant strategies along the wood economic spectrum. Contrastingly, higher precipitation relaxed the hydric constraints and resource availability became a primary driver of changing strategies along the leaf economic spectrum. Thus, the landscape scale influence of topography on processes driving community assembly and functional composition critically depends on the regional climatic context. Using a nested sampling scheme and functional trait data, we investigate the joint influence of precipitation and topography on the assembly of tropical trees. With low precipitation, topography drives environmental filtering of drought resistant species. With high