A theory of demographic optimality in forests

Area of Science:

• Ecology
• Climate Science
• Forestry

Background:

• Dynamic Global Vegetation Models (DGVMs) face uncertainty in carbon sink projections due to unknown parameters.
• Observed forest inventories in Amazonia and the USA exhibit similar tree-size distributions, indicating a potentially simpler underlying ecological principle.
• Demographic Equilibrium Theory (DET) relates tree-size distribution curvature to mortality-to-growth ratios.

Purpose of the Study:

• To extend Demographic Equilibrium Theory (DET) by incorporating recruitment limitations.
• To introduce and explore the concept of Demographic Optimality (DO) in forest ecosystems.
• To reconcile observed forest structures and reproductive strategies with theoretical ecological models.

Main Methods:

• Extended DET to include recruitment limited by competitive exclusion.
• Developed the Demographic Optimality (DO) concept, identifying simultaneous maxima for tree density and biomass.
• Integrated DO with empirical mortality-to-growth ratios from US and Amazonian forests.

Main Results:

• The model predicts an optimal allocation of approximately one-eighth of primary productivity to reproduction, consistent with observations.
• A key prediction is that seed mortality decreases with increasing seed size, counteracting the advantage of numerous small seeds.
• Demographic Optimality (DO) successfully explains common tree-size distribution shapes across diverse forest types.

Conclusions:

• Demographic Optimality (DO) provides a unifying framework for understanding forest structure and function.
• The model's predictions align with empirical data on reproductive allocation and seed size-mortality relationships.
• This approach offers a more robust method for projecting future land carbon sinks compared to complex DGVMs.