Savanna ecosystem structure and productivity along a rainfall gradient: the role of competition and stress tolerance mediated by plant functional traits

Abstract. Environmental gradients affect vegetation structure and ecosystem productivity. Along the northern Australia tropical transect (NATT), which transitions from tropical moist conditions in the north to arid conditions in the south, vegetation composition and structure are closely tied to rainfall patterns. We hypothesise that biotic competition and abiotic stress exhibit opposing patterns along the NATT rainfall gradient and aim to disentangle these effects on vegetation structure and productivity. Using a trait-based dynamic vegetation model, we simulated vegetation responses to varying competition and stress along the NATT. The model successfully simulated spatial variations and temporal patterns in carbon and water fluxes, where evapotranspiration and gross primary productivity decrease with rainfall along the gradient. Simulation results showed that taller and medium-sized Eucalyptus had higher carbon mass, leaf area index, and foliar projective cover at the wet end of the gradient. In contrast, Acacia and grasses were dominant at the dry end. Crown coverage shows spatial and temporal variability with rainfall, with higher variability in tree plant functional types (PFTs) crown cover in the north and more uniform in the south, while grasses have maximum coverage during the wet season in the dry end of the gradient. These patterns suggest a shift in the importance of biotic versus abiotic factors, with competition playing a more significant role in the wet region and stress becoming more influential as aridity increases in the south. Overall, our study underscores water availability as a primary driver of vegetation structure and highlights the role of competition and stress in modulating ecosystem structure, composition and productivity along the rainfall gradient.