Woody-tissue respiration for Simarouba amara and Minquartia guianensis, two tropical wet forest trees with different growth habits.

We measured CO2 efflux from stems of two tropical wet forest trees, both found in the canopy, but with very different growth habits. The species were Simarouba amara, a fast-growing species associated with gaps in old-growth forest and abundant in secondary forest, and Minquartia guianensis, a slow-growing species tolerant of low-light conditions in old-growth forest. Per unit of bole surface, CO2 efflux averaged 1.24 μmol m-2 s-1 for Simarouba and 0.83 μmol m-2s-1 for Minquartia. CO2 efflux was highly correlated with annual wood production (r 2=0.65), but only weakly correlated with stem diameter (r 2=0.22). We also partitioned the CO2 efflux into the functional components of construction and maintenance respiration. Construction respiration was estimated from annual stem dry matter production and maintenance respiration by subtracting construction respiration from the instantaneous CO2 flux. Estimated maintenance respiration was linearly related to sapwood volume (39.6 μmol m-3s-1 at 24.6° C, r 2=0.58), with no difference in the rate for the two species. Maintenance respiration per unit of sapwood volume for these tropical wet forest trees was roughly twice that of temperate conifers. A model combining construction and maintenance respiration estimated CO2 very well for these species (r 2=0.85). For our sample, maintenance respiration was 54% of the total CO2 efflux for Simarouba and 82% for Minquartia. For our sample, sapwood volume averaged 23% of stem volume when weighted by tree size, or 40% with no size weighting. Using these fractions, and a published estimate of aboveground dry-matter production, we estimate the annual cost of woody tissue respiration for primary forest at La Selva to be 220 or 350 g C m-2 year-1, depending on the assumed sapwood volume. These costs are estimated to be less than 13% of the gross production for the forest.