The exchange of carbon dioxide in tropical forest

This study investigated the structure of, and carbon dioxide fluxes at, a 'primary' rain forest in SW Amazonia, Brazil (PRF), and a disturbed secondary rain forest in SW Cameroon (SRF). The total above-ground biomass and leaf area index (LA! ) at PRF were respectively 220 (±95% c.l. 48) Mg ha' and 4.0 (±95% c.l. 0.7) m 2 m 2, and at SRF 90 (±95% c.l. 9.4) Mg ha' and 4.4 (±95% c.l. 0.9) m2 m 2 . A novel method was devised to quantify the vertical profile in LA! : SRF was distinguished from PRF by a higher concentration of leaf area near the ground. Three methods were used to determine the flux of CO 2 from soil and gave overall agreement (static and dynamic chambers, and eddy covariance). The mean soil efflux in PRF and SRF was respectively 5.5 tmol m 2 s' (±95% c.l. 0.2; n = 42) and 4.5 jimol m 2 s' (±95% c.l. 0.2; n = 178) at 20 24 °C. The temperature response was higher in PRF than SRF (Q,0 = 2.3 vs 1.9). Soil efflux rates were also obtained from cerrado vegetation in central Brazil, where the efflux was 3.2 tmol m 2 s' (±95% c.l. 0.2; n = 10) and the Q10 1.6, at 16 -23 °C. Heterogeneity in emissions was higher in SRF than in PRF and could be described by a non-linear model incorporating the variables: soil temperature, organic carbon and total nitrogen (r2 = 0.82). Carbon was the most important variable determining respiration in SRF; soil moisture was not limiting. There was no observable effect of season on efflux rates in either rain forest, but a decline occurred in cerrado during the dry season. Effluxes of CO2 were measured from stems and branches of diameter 0.002 in 1.6 in in 24 species in PRF and 17 species in SRF; emission rates were 0.1 3.3 tmol m 2 s' with a Q,0 of 1.8 in PRF, and 0.2 5.2 tmol m 2 s' with a Q10 of 1.6 in SRF. Bark temperatures ranged between 18 °C and 28 °C. Maintenance respiration was 80% and construction respiration 20% of total woody tissue respiration (Re) in SRF. A functional model described the relationship between R, and diameter in SRF better than a purely empirical one (r2 = 0.66). A novel method was devised to estimate sap CO 2 concentrations which in SRF were 1.2 11.0 mmol dm 3 for Distemonanthus benthamianus and Musanga cecropioides. Sap CO2 levels were sensitive to sap pH, and represented 1 30% of cuvettemeasured leaf photosynthesis. Maintenance leaf respiration (Rm) was measured through the vertical profile during the night in PRF and SRF. Rm increased with height at both sites: -0.2 in PRF vs -0.3 in SRF at 1.5 m, and -0.5 in PRF vs -0.9 in SRF at 26 in (values normalised to 22 °C, units: imol m 2 s'). Leaf nitrogen and potassium concentrations (N,,. f and P,) declined with height in the canopy. Pl,d concentrations were higher in SRF than PRF where P,. f appeared to limit respiration. R. was not significantly related to N,,. f or P,,,d on a mass basis, but was strongly correlated with these variables on an area basis, partly as a consequence of variation in specific leaf area. A molar ratio, Rm :Ni explained variation in R. with height. A biochemical model was fitted to measurements of leaf photosynthesis (A,) made throughout the vertical profile of the SRF canopy. Stomatal conductance (g,) was highest in the morning at the canopy-top, and declined during the day; maximum g, was 0.3 1.0 mol m 2 s'. Al followed the same pattern, though a delayed peak was observed near the ground. A ma,, was 11 14 tmol m 2 s' at the canopy-top and 6 tmol m 2 s' near the ground; cx, the quantum efficiency, varied from 0.04 0.06 mol mol'. Fitted photosynthetic parameters varied with height and were significantly related on an area basis to N, and P,. Forest stand respiration models were made for both sites by component summation. For SRF, a multilayer model of photosynthesis was constructed using leaf level photosynthetic parameters, and compared with a 'big-leaf' one parameterised using eddy covariance measurements. Respiration estimates were combined with both to retrieve net forest assimilation at 0.7 0.9 jimol m 2 s'; a model estimated an annual carbon sink for PRF during 1992/3 of 0.3 . tmol m 2 s' (0.9±0.2 Mg ha 1 yr'; Grace et al., 1995, Science, 270, 778-780). Both forests were highly sensitive to CO 2 concentration and temperature. The temperature sensitivity of effluxes from soil was found to determine strongly the sink-source strength of both forests and should remain a focus for further work.