Buchmann, N., J.-M. Guehl, T.Barigah, and J.R. Ehleringer. 1997. Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon cycling in an Amazonian rainforest (French Guiana). Oecologia 110:120-131.

Canopy CO2 concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO2]) throughout the canopy (0.02 - 38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO2] in the upper and middle canopy dropped on average 7 - 10 mmol mol-1 below tropospheric background values due to photosynthesis. Within the main part of the canopy (> 0.7 m), [CO2] did not differ between the wet and dry seasons. In contrast, [CO2] below 0.7 m were generally higher during the dry season, resulting in larger [CO2] gradients. Supporting this observation, soil CO2 efflux was on average higher during the dry season than during the wet season; in the wet season, soil respiration rates decreased by 40 % after strong rain events, resulting in a rapid decrease of canopy [CO2] immediately above the forest floor by about 50 mmol mol-1. Temporal and spatial variations in [CO2]canopy were reflected by changes in d13Ccanopy and d18Ocanopy of CO2. Tight relationships between d13C and d18O of canopy CO2 were observed during both seasons (r2 > 0.86). The most depleted d13Ccanopy and d18Ocanopy values were measured immediately above the forest floor (d13C = -16.4 o/oo; d18O = 39.1 o/oo). Gradients in the isotope ratios of CO2 between the top of the canopy and the forest floor ranged between 2.0 o/oo and 6.3 o/oo for d13C, and between 1.0 o/oo and 3.5 o/oo for d18O. d13Cleaf and calculated ci/ca of foliage in three different positions were similar for the dry and wet seasons indicating that the canopy maintains a constant balance between photosynthesis and stomatal conductance. Plotting 1/[CO2] vs. the corresponding d13C ratios resulted in very tight, linear relationships (r2 = 0.99), with no significant differences between the two seasons suggesting negligible seasonal variability in turbulent mixing relative to photosynthesis. The intercept of these relationships that should be indicative of the d13C of respired sources was close to the measured d13C of soil respired CO2 and to the d13C of litter and soil organic matter. Carbon isotope discrimination of the entire ecosystem, DA, was calculated as 19.7 o/oo during the dry season and as 19.8 o/oo during the wet season. Canopy CO2 recycling within the main canopy (2 - 30 m), i.e., how much of the soil respired CO2 is refixed during canopy photosynthesis relative to the total respiratory flux, was estimated as 5.6 % for the dry season and as 8.4 % for the wet season. Leaf-level recycling, i.e., how much carbon in the foliage originated from respired sources, was almost zero for foliage in the upper and middle canopy, but about 3 % for understory foliage during both seasons. efflux was on average higher during the dry season than during the wet season; in the wet season, soil respiration rates decreased by 40 % after strong rain events, resulting in a rapid decrease of canopy [CO2] immediately above the forest floor by about 50 mmol mol-1. Temporal and spatial variations in [CO2]canopy were reflected by changes in d13Ccanopy and d18Ocanopy of CO2. Tight relationships between d13C and d18O of canopy CO2 were observed during both seasons (r2 > 0.86). The most depleted d13Ccanopy and d18Ocanopy values were measured immediately above the forest floor (d13C = -16.4 o/oo; d18O = 39.1 o/oo). Gradients in the isotope ratios of CO2 between the top of the canopy and the forest floor ranged between 2.0 o/oo and 6.3 o/oo for d13C, and between 1.0 o/oo and 3.5 o/oo for d18O. d13Cleaf and calculated ci/ca of foliage in three different positions were similar for the dry and wet seasons indicating that the canopy maintains a constant balance between photosynthesis and stomatal conductance. Plotting 1/[CO2] vs. the corresponding d13C ratios resulted in very tight, linear relationships (r2 = 0.99), with no significant differences between the two seasons suggesting negligible seasonal variability in turbulent mixing relative to photosynthesis. The intercept of these relationships that should be indicative of the d13C of respired sources was close to the measured d13C of soil respired CO2 and to the d13C of litter and soil organic matter. Carbon isotope discrimination of the entire ecosystem, DA, was calculated as 19.7 o/oo during the dry season and as 19.8 o/oo during the wet season. Canopy CO2 recycling within the main canopy (2 - 30 m), i.e., how much of the soil respired CO2 is refixed during canopy photosynthesis relative to the total respiratory flux, was estimated as 5.6 % for the dry season and as 8.4 % for the wet season. Leaf-level recycling, i.e., how much carbon in the foliage originated from respired sources, was almost zero for foliage in the upper and middle canopy, but about 3 % for understory foliage during both seasons.