Plant size influences specific leaf area in palms: a case for the diminishing returns hypothesis

Body size is essential in determining an organism's functional performance and metabolic requirements, influencing biological processes from organisms to ecosystems. Metabolic scaling theory integrates the size-metabolism relationship, yet most research overlooks intraspecific trait variation due to ontogeny. Specific leaf area (SLA) is a critical functional trait that reflects investment on photosynthetic tissues relative to leaf construction costs. SLA influences photosynthetic capacity and growth rates and varies across life stages. While plants exhibit interspecific differences in acquisitive (high SLA, fast growth) and conservative (low SLA, slow growth) strategies, the diminishing returns hypothesis suggests that these strategies are shaped by the proportion of supporting structures that develop over the plant´s lifespan (intraspecific variation), predicting a negative SLA relationship with increasing size. In tropical environments, palms are ecologically important yet still understudied in functional traits. Here, we examine the relationship between SLA and size in six neotropical understory and canopy palm species (236 individuals). Results showed higher SLA in understory species and a negative SLA-size relationship across most species. SLA inversely correlated with leaf thickness and leaf water content. ANCOVA models explained substantial SLA variation related to palm size, with species-specific differences in regression slopes. These findings underscore the importance of considering inter- and intraspecific SLA variation and ontogenetic changes. Understanding the trade-off between acquisitive and conservative strategies within the context of the diminishing returns hypothesis offers insights into plant growth strategies and their ecological implications, which is essential for predicting plant adaptation to environmental gradients.