Extensions and evaluations of a general quantitative theory of forest structure and dynamics

Abstract

Here, we present the second part of a quantitative theory for the structure and dynamics of forests under demographic and resource steady state. The theory is based on individual-level allometric scaling relations for how trees use resources, fill space, and grow. These scale up to determine emergent properties of diverse forests, including size-frequency distributions, spacing relations, canopy configurations, mortality rates, population dynamics, successional dynamics, and resource flux rates. The theory uniquely makes quantitative predictions for both stand-level scaling exponents and normalizations. We evaluate these predictions by compiling and analyzing macroecological datasets from several tropical forests. The close match between theoretical predictions and data suggests that forests are organized by a set of very general scaling rules. Our mechanistic theory is based on allometric scaling relations, is complementary to ''demographic theory,'' but is fundamentally different in approach. It provides a quantitative baseline for understanding deviations from predictions due to other factors, including disturbance, variation in branching architecture, asymmetric competition, resource limitation, and other sources of mortality, which are not included in the deliberately simplified theory. The theory should apply to a wide range of forests despite large differences in abiotic environment, species diversity, and taxonomic and functional composition.allometry ͉ mortality rate ͉ plant ecology ͉ size distribution ͉ competitive thinning U nderstanding the key forces that shape the structure, function, and dynamics of ecosystems is a fundamental goal of ecology (1-6). Current approaches to plant communities focus on questions such as what allows for species coexistence (7), why tropical sites have more species than temperate ones (8), and what environmental factors determine the structure, dynamics, and species composition of communities (9). Detailed models have been developed to integrate how species-specific traits ''scale-up'' to inf luence community and ecosystem dynamics (10, 11).Here, we present a complementary but alternative approach. We use a few key principles to show how variation in resource supply together with general cross-taxa patterns of plant architecture and growth give rise to predictable emergent patterns of resource use, spatial structure, and demography. In our previous article (12) we incorporated these principles to derive the first part of a general quantitative theory for the structure and dynamics of a single-species stand. In this article we evaluate these predictions, using data from several tropical forests composed of multiple tree species including: (i) a 20-year record from Costa Rica (13); (ii) a 10-year time series from Panama; (iii) a 40-year survey from a Malaysian forest dataset; (iv) and a successional sequence of Costa Rican forests ranging from recently abandoned pasture to mature uncut forests (14) (for methodology and additional detail, see supporting information (SI) Tex...

Form, function, and evolution of living organisms

“…Interestingly, a value lower than 3=4 of the metabolic rate− mass exponent is predicted if the tree is not isotropic, i.e., its crown diameter scales sublinearly with tree height (22). The mass of the tree is not uniformly distributed but rather is concentrated in the trunk and the branches in a self-similar manner (19,20). The geometry of the organism enters through the scaling of its surface area S with volume V, S ∼ V x .…”

“…Subsequently, these organisms often grow into complex forms through independently evolved mechanisms including modularity, fractal structure, and segmentation. Despite the diversity of life forms, the approximate validity of Kleiber's law (4,6,7,(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) provides a remarkable unifying feature. This allometric scaling pertains to biological structures ranging from unicellular organisms to the tallest trees and has direct implications for characteristic rates and time scales.…”

Insights into plant size-density relationships from models and agricultural crops

“…Therefore, the density-size relationships previously documented for forests (7)(8)(9) can be generalized to a much wider range of plant sizes, growth habits, and agricultural as well as undisturbed ecosystems. Moreover, the self-thinning scaling relationship that emerges as a consequence of the tradeoff between growth and mortality in forests appears to be identical to the scaling of N crit with plant size in crops, where there is very little mortality.…”

“…Two approaches traditionally have been used to study these interactions. One focuses on theoretical models and empirical measurements of abundance, spacing, survival, mortality, and recruitment as functions of plant size in relatively undisturbed natural populations and communities, especially forests (4)(5)(6)(7)(8)(9)(10)(11), where the thinning process is complicated by effects of shading and other factors on asymmetries in resource supply and resulting growth and mortality rates (11)(12)(13)(14)(15)(16). The second approach focuses on the structure and dynamics of plants in agricultural settings (17)(18)(19)(20)(21)(22), where plants of nearly identical age grow under controlled conditions.…”

Advances, challenges and a developing synthesis of ecological community assembly theory

“…Statistical mechanics have been used to predict the relative abundance of species from trait-environment relationships [41,130]. Mechanistic models are ideally the most powerful way forward, for example, global patterns of tree form can be predicted and explained by hydraulics and thermodynamics [131], forest stand structure and dynamics can be predicted with metabolic scaling theory [132], and resource competition theory explains the (ecologically) long-term dynamics of experimental grass mixtures [133]. However, mechanistic models do not yet predict community composition in terms of species or traits.…”