Abstract
<p><strong>Abstract.</strong> Tree allometric relationships are widely employed to estimate forest biomass and production, and are basic building blocks of dynamic vegetation models. In tropical forests, allometric relationships are often modeled by fitting scale-invariant power functions to pooled data from multiple species, an approach that fails to reflect finite size effects at the smallest and largest sizes, and that ignores interspecific differences in allometry. Here, we analyzed allometric relationships of tree height (9884 individuals) and crown area (2425) with trunk diameter using species-specific morphological and life history data of 162 species from Barro Colorado Island, Panamá. We fit nonlinear, hierarchical models informed by species traits and assessed the performance of three alternative functional forms: the scale-invariant power function, and the saturating Weibull and generalized Michaelis-Menten (gMM) functions. The relationship of tree height with trunk diameter was best fit by a saturating gMM model in which variation in allometric parameters was related to interspecific differences in sapling growth rates, a measure of regeneration light demand. Light-demanding species attained taller heights at comparatively smaller diameters as juveniles and had shorter asymptotic heights at larger diameters as adults. The relationship of crown area with trunk diameter was best fit by a power function model incorporating a weak positive relationship between crown area and species-specific wood density. The use of saturating functional forms and the incorporation of functional traits in tree allometric models is a promising approach to improve estimates of forest biomass and productivity. Our results provide an improved basis for parameterizing tropical tree functional types in vegetation models.</p>
Highlights
Allometric scaling describes how plant morphology and performance vary as a function of size, patterns that are due to size-dependent physical constraints and selective pressures (Niklas, 1994)
We address three specific questions: (i) how is interspecific variability in allometric scaling of tree height and crown area in this forest related to tree species functional traits, in particular wood density and measures of shade tolerance? (ii) How do power functions compare with various asymptotic functions in representing these species-specific allometric relationships? (iii) How does the choice of alternative tree height scaling functions affect the estimation of aboveground biomass? To answer these questions, we fitted allometric models whose parameters were related to species-specific functional traits under a Bayesian hierarchical framework, taking advantage of long-term, high-quality data from Barro Colorado, Panama
Metabolic theories based on hydraulic constraints predict a constant logarithmic scaling between tree height and trunk diameter, with an exponent close to 2/3 (Niklas and Spatz, 2004; West et al, 2009), which is inconsistent with our results that show that the community-level power function exponents differ significantly from 2/3 and that the data diverge strongly from a power function
Summary
Allometric scaling describes how plant morphology and performance vary as a function of size, patterns that are due to size-dependent physical constraints and selective pressures (Niklas, 1994). Allometric functions constitute building blocks of more complex, mechanistic forest models, including the vegetation modules of state-of-the art Earth system models (e.g., Weng et al, 2015). These functions provide a basic template for modeling carbon allocation and tree growth (Pacala et al, 1996), and differences in allometric parameters can be used to represent different species or plant functional types (PFTs, Prentice et al, 1992). Allometric relationships of tropical trees remain poorly documented when compared to temperate and boreal forest ecosystems Martínez Cano et al.: Tropical tree height and crown allometries for the Barro Colorado Nature Monument
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