Carbon isotope discrimination in forest and pasture ecosystems of the Amazon Basin, Brazil

Abstract

[1] Our objective was to measure the stable carbon isotope composition of leaf tissue and CO 2 released by respiration (d r ), and to use this information as an estimate of changes in ecosystem isotopic discrimination that occur in response to seasonal and interannual changes in environmental conditions, and land-use change (forest-pasture conversion). We made measurements in primary forest and pastures in the Amazon Basin of Brazil. At the Santarém forest site, d r values showed a seasonal cycle varying from less than À29% to approximately À26%. The observed seasonal change in d r was correlated with variation in the observed monthly precipitation. In contrast, there was no significant seasonal variation in d r at the Manaus forest site (average d r approximately À28%), consistent with a narrower range of variation in monthly precipitation than occurred in Santarém. Despite substantial (9%) vertical variation in leaf d 13 C, the average d r values observed for all forest sites were similar to the d 13 C values of the most exposed sun foliage of the dominant tree species. This suggested that the major portion of recently respired carbon dioxide in these forests was metabolized carbohydrate fixed by the sun leaves at the top of the forest canopy. There was no significant seasonal variation observed in the d 13 C values of leaf organic matter for the forest sites. We sampled in pastures dominated by the C 4 grass, Brachiaria spp., which is planted after forest vegetation has been cleared. The carbon isotope ratio of respired CO 2 in pastures was enriched in 13 C by approximately 10% compared to forest ecosystems. A significant temporal change occurred in d r after the Manaus pasture was burned. Burning removed much of the encroaching C 3 shrub vegetation and so allowed an increased dominance of the C 4 pasture grass, which resulted in higher d r values.

Productivity, carbon isotope discrimination and leaf traits of trees of Eucalyptus globulus Labill. in relation to water availability

“…Schleser (1992) also found no difference between D of sunlit, upper canopy leaves and stem-wood in Fagus sylvatica. This finding is consistent with the observation of Cernusak et al (2003) that D of phloem sap sugar in stem agreed well with that expected from instantaneous gas exchange measurements on leaves at two-thirds of canopy height in several stands of E. globulus in south-western Australia, and is also consistent with the observations in Amazonian forest by Ometto et al (2002) that average D of CO 2 released from respiration was similar to that of D of the most sun exposed foliage in dominant trees. These studies show no evidence of carbon fractionation during sugar transport from leaves to stems and we agree with Schleser (1992) that such fractionation is unlikely.…”

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 1. Observations in forests along a precipitation transect in Oregon, USA

“…The model has been used extensively to interpret the carbon isotopic content of nocturnal ecosystem respiration and considerable insight about ecosystem carbon cycling is emerging from its use [Lancaster, 1990;Flanagan et al, 1996;Bowling et al, 2002;Ometto et al, 2002;Pataki et al, 2003] [28] The Keeling approach assumes that a forest with some initial background CO 2 mole fraction C b and isotopic composition d b experiences an increase in [CO 2 ] due to nocturnal respiration by all ecosystem components. Conservation of mass requires that any measured mole fraction C m will be composed of a contribution from the initial background CO 2 present (C b ) and the CO 2 added by respiration (C R ),…”

Implications of CO<sub>2</sub> pooling on δ<sup>13</sup>C of ecosystem respiration and leaves in Amazonian forest

“…The leaf therefore is a function of c i /c a ratio, which is sensitive to a variety of factors that influence the balance of stomatal conductance and assimilation rate, for example light and water availability (Pataki et al, 2003b). The carbon isotope ratio of a leaf (δ 13 C leaf ) is a measure that integrates the photosynthetic activity over the period of weeks to months during which the leaf tissue was synthesized (Ometto et al, 2002;Dawson et al, 2002;Ometto et al, 2006). In tropical forests, δ 13 C leaf is strongly correlated with the height of the leaf within the canopy.…”