Load Bearing Capacity of Understory Treelets of a Tropical Wet Forest

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Abstract
The crown load required to bend over understory treelets was measured in a tropical lowland forest of northeastern Costa Rica. The study provided the first direct measure of margins of safety against elastic instability in trees. Doubling or tripling the initial crown weight caused substantial bending in adults of three understory species. These results were interpreted as indicating that actual trunk diameters were only 1.3-1.4.times. the minimum diameter required to prevent instability. Saplings of two overstory species also had low stability safety factors, but had lower ratios of crown mass to trunk mass than did the understory species. Higher safety factors were measured in treelets of an open field species. Low safety factors allow understory plants to attain greater height for a given support cost, but mechanical failure is more likely to occur. Evidence of past instability was noted in two of the understory species. The observed stability safety factors were lower than those reported for canopy trees which are subject to additional wind forces.

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