Tree architecture and scaling rules: Functional Branch Analysis (FBA), above and belowground

Trees come in various shapes, grow at different rates and interact with their neighbours during development. Yet, many of the properties of an individual tree can be predicted if we know the diameter of its stem. The relationship between this diameter and properties such as tree height, tree biomass, leaf area and harvestable timber are called 'scaling rules' or. Empirical allometric scaling equations (the most generic form is Y = a D ) for tree biomass Y on the basis of stem diameter D are often used in forest inventories and assessment of carbon and nutrient stocks in vegetation. They are based on cutting selected trees and obtaining destructive measurements to relate to the stem diameter. When shifting from plantation forestry to mixed forestry or multi-species agroforestry systems, however, shortcuts to the empirical approach are desirable. Certain regularities in the development of tree form are captured in 'fractal branching' models; such models can provide a transparent scheme for deriving tree specific scaling rules on the basis of easily observable, non-destructive methods. Apart from total tree biomass, the models can provide rules for total leaf area, relative allocation of current growth to leaves, branches, stem or litter, or the ratio of green to brown projection area that modulates tree-crop interactions in savanna.

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