Testing metabolic ecology theory for allometric scaling of tree size, growth and mortality in tropical forests

240

291

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...

Section: Empirical Results and Theoretical Extensionsmentioning

confidence: 99%

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

Enquist

West

Brown

2009

240

291

“…The irrelevance of size structure in determining the metacommunity SAD provides a theoretical underpinning for the success of NBT in predicting dispersal-limited, local SADs in tropical forests, where variation in demographic rates is strongly correlated with body size (16).…”

Section: Discussionmentioning

confidence: 99%

“…We assume that this variation in demographic rates depends on size alone, and is not linked to species identity, an approach that is closely related to the philosophy of allometric scaling theory (8,(22)(23)(24): individuals of a given size play by the same rules, regardless of species identity. Given the strong evidence that demographic rates in nature are correlated with size (8,16), this synthesis of size variation with demographic stochasticity may be thought of as adding a crucial extra layer of biological realism to NBT.…”

Section: T He Introduction Of Neutral Biodiversity Theory (1) (Nbt)mentioning

confidence: 99%

“…NBT proposes that community-level patterns are primarily determined by the effects of demographic stochasticity, and that a detailed knowledge of the traits of and interactions between individuals comprising the community is irrelevant. It has been argued that the predictions of NBT are uninformative of process (10-12), and it has been demonstrated that the assumptions underpinning NBT are often manifestly violated: in the tropical forests where the theory has found striking success (1,13,14), there is a huge variation in demographic rates of individuals (15,16). Although there has been much debate (6,17,18) about the importance of neutral, stochastic processes, one common principle has emerged (17-20): the need for a unified, theoretical framework both to quantify the effects of demographic stochasticity relative to other forces, and to generate a broader range of predictions more closely tied to process.…”

mentioning

confidence: 99%

See 1 more Smart Citation

An integrative framework for stochastic, size-structured community assembly

O’Dwyer

Lake

Ostling

et al. 2009

We present a theoretical framework to describe stochastic, sizestructured community assembly, and use this framework to make community-level ecological predictions. Our model can be thought of as adding biological realism to Neutral Biodiversity Theory by incorporating size variation and growth dynamics, and allowing demographic rates to depend on the sizes of individuals. We find that the species abundance distribution (SAD) is insensitive to the details of the size structure in our model, demonstrating that the SAD is a poor indicator of size-dependent processes. We also derive the species biomass distribution (SBD) and find that the form of the SBD depends on the underlying size structure. This leads to a prescription for testing multiple, intertwined ecological predictions of the model, and provides evidence that alternatives to the traditional SAD are more closely tied to certain ecological processes. Finally, we describe how our framework may be extended to make predictions for more general types of community structure. ignited an ideological debate in community ecology, challenging the viewpoint that deterministic forces play the dominant role in shaping patterns of biodiversity (2-9). NBT proposes that community-level patterns are primarily determined by the effects of demographic stochasticity, and that a detailed knowledge of the traits of and interactions between individuals comprising the community is irrelevant. It has been argued that the predictions of NBT are uninformative of process (10-12), and it has been demonstrated that the assumptions underpinning NBT are often manifestly violated: in the tropical forests where the theory has found striking success (1,13,14), there is a huge variation in demographic rates of individuals (15,16). Although there has been much debate (6,17,18) about the importance of neutral, stochastic processes, one common principle has emerged (17-20): the need for a unified, theoretical framework both to quantify the effects of demographic stochasticity relative to other forces, and to generate a broader range of predictions more closely tied to process.In this article, we integrate the effects of demographic stochasticity with ontogenetic variation in the size of individuals (21), and allow the demographic rates of individuals to explicitly depend on their size. We assume that this variation in demographic rates depends on size alone, and is not linked to species identity, an approach that is closely related to the philosophy of allometric scaling theory (8,(22)(23)(24): individuals of a given size play by the same rules, regardless of species identity. Given the strong evidence that demographic rates in nature are correlated with size (8, 16), this synthesis of size variation with demographic stochasticity may be thought of as adding a crucial extra layer of biological realism to NBT.From these ingredients we derive a functional differential equation to describe ecological communities, and use its analytical solution to answer three key questions. First, are our predict...

“…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.…”

mentioning

confidence: 99%

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

Deng

Zuo

Wang

et al. 2012

There is general agreement that competition for resources results in a tradeoff between plant mass, M, and density, but the mathematical form of the resulting thinning relationship and the mechanisms that generate it are debated. Here, we evaluate two complementary models, one based on the space-filling properties of canopy geometry and the other on the metabolic basis of resource use. For densely packed stands, both models predict that density scales as M −3/4 , energy use as M 0 , and total biomass as M 1/4 . Compilation and analysis of data from 183 populations of herbaceous crop species, 473 stands of managed tree plantations, and 13 populations of bamboo gave four major results: (i) At low initial planting densities, crops grew at similar rates, did not come into contact, and attained similar mature sizes; (ii) at higher initial densities, crops grew until neighboring plants came into contact, growth ceased as a result of competition for limited resources, and a tradeoff between density and size resulted in critical density scaling as M −0.78 , total resource use as M −0.02 , and total biomass as M 0.22 ; (iii) these scaling exponents are very close to the predicted values of M −3/4 , M 0 , and M 1/4 , respectively, and significantly different from the exponents suggested by some earlier studies; and (iv) our data extend previously documented scaling relationships for trees in natural forests to small herbaceous annual crops. These results provide a quantitative, predictive framework with important implications for the basic and applied plant sciences.allometric scaling | energy equivalence | plant energetics | self-thinning T he structure and dynamics of plant populations and communities often reflect the interacting consequences of three fundamental processes: (i) competition for resources, (ii) the effect of body size on resource use, and (iii) the effect of plant density on growth and mortality (1-4). 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 (4-11), where the thinning process is complicated by effects of shading and other factors on asymmetries in resource supply and resulting growth and mortality rates (11-16). 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. Studies of such simplified agricultural systems have led to theoretical and empirical selfthinning relationships that characterize the temporal trajectory of decreasing population density as a function of increasing plant size as stands develop under conditions of resource limitation and competition (17)(18)(19). These exhibit a characteristic phenomenology in which plants grow with minimal mortality until they reach a size-dependent critical density, ...