Wood CO₂ efflux in a primary tropical rain forest

The balance between photosynthesis and plant respiration in tropical forests may substantially affect the global carbon cycle. Woody tissue CO 2 efflux is a major component of total plant respiration, but estimates of ecosystem‐scale rates are uncertain because of poor sampling in the upper canopy and across landscapes. To overcome these problems, we used a portable scaffolding tower to measure woody tissue CO 2 efflux from ground level to the canopy top across a range of sites of varying slope and soil phosphorus content in a primary tropical rain forest in Costa Rica. The objectives of this study were to: (1) determine whether to use surface area, volume, or biomass for modeling and extrapolating wood CO 2 efflux, (2) determine if wood CO 2 efflux varied seasonally, (3) identify if wood CO 2 efflux varied by functional group, height in canopy, soil fertility, or slope, and (4) extrapolate wood CO 2 efflux to the forest. CO 2 efflux from small diameter woody tissue (<10 cm) was related to surface area, while CO 2 efflux from stems >10 cm was related to both surface area and volume. Wood CO 2 efflux showed no evidence of seasonality over 2 years. CO 2 efflux per unit wood surface area at 25° ( F A ) was highest for the N‐fixing dominant tree species Pentaclethra macroloba , followed by other tree species, lianas, then palms. Small diameter F A increased steeply with increasing height, and large diameter F A increased with diameter. Soil phosphorus and slope had slight, but complex effects on F A . Wood CO 2 efflux per unit ground area was 1.34±0.36 μmol m −2 s −1 , or 508±135 g C m −2 yr −1 . Small diameter wood, only 15% of total woody biomass, accounted for 70% of total woody tissue CO 2 efflux from the forest; while lianas, only 3% of total woody biomass, contributed one‐fourth of the total wood CO 2 efflux.