Contribution of coarse woody debris to ecosystem respiration in an old-growth tropical rain forest

Background/Question/Methods

Coarse woody debris (CWD; diameter > 10 cm) is an important component of the carbon stored in tropical forests. Respiration from decomposing CWD in tropical forests has been found to produce considerable CO₂ emissions to the atmosphere. The objectives of our study were (1) to quantify the respiration rates from CWD, (2) to identity the driving abiotic and biotic variables of CWD respiration, and (3) to evaluate the contribution of CWD to ecosystem respiration in an old-growth tropical rain forest. The study was conducted on old alluvial terraces (Oxisols) at La Selva Biological Station, Costa Rica. Coarse woody debris was stratified by decay class and bole diameter. Coarse woody debris respiration rates were measured using closed dynamic chambers. The chambers were connected to an infrared gas analyzer. Samples from each bole were taken and analyzed for wood density, nutrient, and lignin content.

Results/Conclusions

At our study site, the sum of fresh, partially and fully decomposed CWD contains approximately 33 % as much carbon as in the live trees aboveground. Bole diameter had no influence on CWD respiration rates. Average per unit bole surface CO₂ efflux ranged from 2.0 µmol m^-2 s^-1 (fresh CWD) to 2.8 µmol m^-2 s^-1 (fully decomposed CWD). Although not significant, CWD respiration rates were negatively correlated with CWD density and with CWD lignin:N ratio. In contrast, CWD respiration rates tended to be positively related to wood moisture. At La Selva, respiration from CWD (per unit surface area) is after soil respiration (2.8 to 3.6 µmol m^-2 s^-1) the second important respiration flux. The surface area based measurements of CWD respiration will be up scaled to the stand level using the estimation of total surface area of CWD. Stand level data will be used to assess the contribution of CWD to overall ecosystem respiration measured by eddy covariance technique.