Abstract
The sizes and shapes of tree crowns are of fundamental importance in ecology, yet understanding the forces that determine them remains elusive. A cardinal question facing ecologists is the degree to which general and non-specific versus ecological and context-dependent processes are responsible for shaping tree crowns. Here, we test this question for the first time across diverse tropical ecosystems. Using trees from 20 plots varying in elevation, precipitation, and ecosystem type (savanna-forest transitions) across the paleo- and neo-tropics, we test the relationship between crown dimensions and tree size. By analyzing these scaling relationships across environmental gradients, biogeographic regions, and phylogenetic distance, we extend Metabolic Scaling Theory (MST) predictions to include how local selective pressures shape variation in crown dimensions. Across all sites, we find strong agreement between mean trends and MST predictions for the scaling of crown size and shape, but large variation around the mean. While MST explained approximately half of the observed variation in tree crown dimensions, we find that local, ecosystem, and phylogenetic predictors account for the half of the residual variation. Crown scaling does not change significantly across regions, but does change across ecosystem types, where savanna tree crowns grow more quickly with tree girth than forest tree crowns. Crowns of legumes were wider and more voluminous than those of other taxa. Thus, while MST can accurately describe the central tendency of tree crown size, local ecological conditions and evolutionary history appear to modify the scaling of crown shape. Importantly, our extension of MST incorporating these differences accounts for the mechanisms driving variation in the scaling of crown dimensions across the tropics. We present allometric equations for the prediction of crown dimensions across tropical ecosystems. These results are critical when scaling the function of individual trees to larger spatial scales or incorporating the size and shape of tree crowns in global biogeochemical models.
Highlights
The sizes and shapes of tree crowns are conspicuous and fundamental attributes of the organisms that comprise the structure of Earth’s forests
Metabolic Scaling Theory (MST) does not make specific predictions for crown depth, but as we show in Supplementary Material, uniform Euclidean assumptions about crown shapes lead to the same prediction for crown depth as for crown radius: αdepth = 0.67
standardized major axis (SMA) results largely confirm LMM results, and details are included in Supplementary Material
Summary
The sizes and shapes of tree crowns are conspicuous and fundamental attributes of the organisms that comprise the structure of Earth’s forests. MST, for example, does not directly address the roles of niche or ontogeny, and competitive convergence and sphere packing models steer further away from ecological and physiological perspectives, given their absence of environmental and evolutionary factors. In this respect, these more recent perspectives might be classified as “general” theories, in contrast to “environmental” and “ecological” theories that privilege variation attributable to abiotic conditions in the former case, and species, niche, and life history strategy in the latter (e.g., Sapijanskas et al, 2014). In accordance with our classification, we refer to “general” and “ecological” classes of theories below
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