Axiomatic Plant Ecology: Reflections Toward a Unified Theory for Plant Productivity

Abstract

Starting from an axiom that sunlight is the fundamental energy source for green plants, we derived some theorems in plant ecology. We began by reviewing the Monsi-Saeki model, which was the first to relate the structure of canopy and productivity. Optimum leaf area (and thus leaf biomass) were predicted from the Monsi-Saeki model, which also introduced the concept of constant final yield per unit land area. A relationship between total biomass (y) and plant number (n) per unit land area was derived by combining the principles of constant final yield and logistic plant growth, which is based upon the diminishing return of total individual photosynthesis due to self-shading, An analogous equation was obtained in the analysis of cumulative plant biomass (Y) against cumulative plant number (N) within a stand. Mass-number relationships among stands (y-n) and within a stand (Y-N) were revealed to be the same under one-sided competition for light. The self-thinning line is the point where individual plant’s growth becomes zero on the translocation of a Y-N relationship through time. Self-thinning is expected to occur due to the death of the smallest plants shaded by larger plants. The Monsi-Saeki modeling framework was reoriented considering leaf longevity, which is the optimal timing to replace individual leaves to maximize carbon gain of the plant. Under canopy ergodic hypothesis, which supposes space-time equivalence in the performance of leaves, leaf longevity can be used to circumvent the difficulties in the scaling from leaf to canopy. Gross primary production then can be estimated using functional leaf longevity together with the mean labor time of a leaf, two measures of the time during the leaf span when it can be photosynthetically active. Finally, if leaf longevity is used in a species-specific normalization constant, plant productivity can be described as an allometric function of plant mass. In that case, the relative growth rate of plants can be shown to have an inverse relationship to leaf longevity.