Model Trees

How big is a tree, is a similar question to how long is a piece of string? But how big should it be seems to be just as intangible. Species is the first thing that comes to mind, although the quality of the soil, the rainfall, sunlight and temperature come fast behind.

A great deal of effort has already been expended on computer modeling of the growth of trees and the density of forests. Most of this is either very expensive in computer time or too specific. Either way, there is always room for one more computer model to add to the debate.

The new one is by Kempes et al and makes use a standard mathematical tree – a fractal design with a series of scaling rules for local meteorology and the flow of water through the structure (1). The more favorable the energy budget, the bigger the dendritic superstructure of the tree, and hence, the taller it grows.

Of course, a pre-requisite is good local information. Species can be allowed for if average heights are scaled in. The model is remarkably successful at predicting the maximum heights across the various regions of the US.

If the local meteorology is changed, to account for some trait such as global warming, a concomitant variation in tree size is seen. An added bit of information is the density of the cells controlling the water flow in the canopy along with the density of the canopy.

 This in turn can be used to calculate the reflectivity of the canopy, carbon sequestration and all sorts of scary information that most of us will ignore and then look surprised. Rather like the changes in the local climate in southern Somalia due to the decimation of the acacia forest to satisfy the charcoal export trade. Some joined up thinking about all our futures is long overdue.


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