Scaling relationships for woody tissue respiration in two tropical rain forests

Abstract The relationship between gross primary productivity (GPP) and net primary productivity (NPP) is not fully understood. One of the uncertainties relevant to this issue is the magnitude of woody tissue respiration. Although some data exist for temperate and boreal zones, measurements of woody tissue respiration in tropical forests are sparse. We made in situ chamber measurements of woody tissue respiration in two tropical rain forests, one in the Brazilian Amazon (Reserva Jarú) and one in Central Cameroon (Mbalmayo Reserve). We made measurements on a wide range of species at each site and over a range of stem diameters from 0·02 to 1·4 m. The rate of efflux of carbon dioxide (CO 2 ) from bark at 25 °C, R t , varied from 0·1 to 5·2 µ mol m −2 s −1 across the two sites, and the efflux was related to both volume and surface area components of the measured stem sections. The temperature response in R t was slightly higher at Jarú than at Mbalmayo, with Q 10 values of 1·8 (± 0·1 SE) and 1·6 (± 0·1 SE), respectively. A log–log regression showed that R t was significantly related to stem diameter, D ( P < 0·001; r 2 = 0·58–0·62) and was significantly higher at Mbalmayo than at Jarú ( P < 0·001), but that the rate of increase in R t with stem diameter, D , was similar between sites. At the Mbalmayo site, tree growth measurements made over a 4 month period were used to make two estimates of the maintenance ( R m ) and construction ( R c ) components of respiration embedded in R t . The two methods agreed closely, suggesting that R m was approximately 80% of R c at this site. R m could be strongly related to D using a sigmoidal relationship that described both surface area and volume components as sources of respiratory CO 2 ( r 2 = 0·71). This functional model was combined with inventory, growth and climate data for the Mbalmayo site to make a first estimate of annual above‐ground woody tissue respiration, R A , which was 257 (± 18 SE) g C m −2 year −1 . This value corresponds to approximately 10% of GPP, slightly lower than that found for another tropical rain forest, but higher than for temperate forests. When combined with data from six other sites in tropical, temperate and boreal settings, a very strong relationship was found between R A and leaf area index (LAI), and between R A /GPP and LAI ( P < 0·001, r 2 = 0·98). This indicates that R A exerts an appreciable constraint on NPP and that this constraint varies closely with LAI across widely differing types of woody vegetation.