Impacts of tree allometry on structure, composition, functioning and competitive interactions in savanna ecosystems on the northern Australian tropical transect

Aim: Allometric relationships, which describe plant growth patterns shaped by environmental conditions, reflect functional trade-offs and represent key functional traits that optimise adaptation, resource acquisition, stress tolerance and competition. Here, we assess how these allometric relationships and associated functional trade-offs drive ecosystem structure, functioning and competitive interactions between plant functional types (PFTs) in savanna ecosystems. Location: Rainfall gradient, North Australian Tropical Transect (NATT), Australia. Time Period: 1901–2022. Taxon: Tree species of Northern Australia. Methods: Using quantile regression, we established adaptive allometric relationships among diameter at breast height, tree height, crown radius and crown volume. These relationships were integrated into a dynamic vegetation model to simulate tree growth and competitive interactions in local patches across the broader savanna landscape. The model was validated using observed biomass, height, leaf area index and productivity data from six flux tower sites across the NATT. A neighbour removal experiment was conducted to analyse PFT performance under varying competitive pressure, expressed as a competitive index. Results: The results demonstrate that incorporating adaptive allometric relationships improved the model's ability to represent vegetation dynamics and productivity. Tall Eucalyptus PFTs exhibited competitive dominance in high rainfall areas, while Acacia and other deciduous species thrived under drier conditions. The neighbour removal experiment revealed that competition strongly influenced PFT performance, with carbon mass production varying significantly between stand types. Tall Eucalyptus PFTs showed little response to neighbour removal, while other PFTs benefitted strongly from neighbour removal. The competitive index of PFTs increased significantly with rainfall, indicating stronger competition under wetter conditions. Main Conclusions: Our findings suggest that savanna ecosystems are shaped by complex interactions between growth conditions, functional traits and adaptive strategies for coping with competition and stress. These interactions are reflected in allometric relationships and the associated trade-offs in plant growth strategies, which vary across different rainfall gradients.