Document: RIC-3-39-21

Growth rate as a function of diameter in tropical trees: Data from the BCI large forest plot.

CONDIT, R.*

Smithsonian Tropical Research Institute, 34002, USA ¹

Abstract:
Growth rate was calculated for > 200000 trees in the 50 ha plot on Barro Colorado Island (BCI). Shade-tolerant species invariably had low growth when stem diameter (dbh) was < 4 cm, mostly < 1 mm/yr. Growth rate increased with dbh, and leveled off (sometimes decreased) at between 3 and 12 mm/yr, depending on the species. Pioneer species had higher growth rates as saplings, so that growth was either independent of dbh or increased slightly with dbh. I tested models for predicting the relation between growth and dbh. Familiar growth models are based on the difference between photosynthesis and respiration. With both P and R modeled as power functions of dbh, this leads to a difference between two power functions: , where g is growth, s is dbh, and a-d are parameters. Unfortunately, this function could not be fit to growth data because the parameters are highly correlated (that is, there is not a unique solution for a given set of data). But I have sufficient data to estimate the allometric relationship between photosynthesis and dbh: Allometric functions for crown area vs. dbh, for tree height vs. dbh, and for light availability vs height. The first two were fit for 81 trees in the BCI plot, and the third is available from a canopy height census carried out every 5 m throughout the forest. The relation between photosynthesis and dbh was calculated by multiplying estimated crown area by light available at the estimated height, for a given dbh. This reduced the growth equation given above to 3 parameters, and these were estimated for the 81 species. This generated, for each, an estimate of the exponent d, relating whole-tree respiration to dbh. Finally, I attempted to test whether the onset of reproduction could be detected in the growth function, by fitting a model for growth rate in trees smaller than reproductive size, and then for trees larger than reproductive size; however, no consistent decline in growth around reproductive size could be detected.

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This abstract is being presented at: 8:45 AM in session:
Oral Session #1: Plant Carbon Allocation.