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Tropical forests play a vital role in the global carbon cycle, but the
amount of carbon they contain and its spatial distribution remain
uncertain. Recent studies suggest that once tree height is accounted for
in biomass calculations, in addition to diameter and wood density, carbon
stock estimates are reduced in many areas. However, it is possible that
larger crown sizes might offset the reduction in biomass estimates in some
forests where tree heights are lower because even comparatively short
trees develop large, well-lit crowns in or above the forest canopy. While
current allometric models and theory focus on diameter, wood density, and
height, the influence of crown size and structure has not been well
studied. To test the extent to which accounting for crown parameters can
improve biomass estimates, we harvested and weighed 51 trees (11-169 cm
diameter) in southwestern Amazonia where no direct biomass measurements
have been made. The trees in our study had nearly half of total
aboveground biomass in the branches (44 ± 2 %), demonstrating the
importance of accounting for tree crowns. Consistent with our predictions,
key pantropical equations that include height, but do not account for
crown dimensions, underestimated the sum total biomass of all 51 trees by
11 to 14 %, primarily due to severe underestimates of many of the largest
trees. In our models, including crown radius greatly improves performance
and reduces error, especially for the largest trees. In addition, over the
full dataset, crown radius marginally explained more variation in
aboveground biomass (10.5 %) than height (6.0 %). Crown form is also
important: trees with a monopodial architectural type are estimated to
have 21-44 % less mass than trees with other growth patterns. Our analysis
suggests that accounting for crown allometry would substantially improve
the accuracy of tropical estimates of tree biomass and its distribution in
primary and degraded forests.
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