Stem CO2 fluxes in two Sahelian shrub species (Guiera senegalensis and Combretum micranthum)

1. Measurements are presented of CO2 flux from woody stems of two shrub species in the Sahelian zone of West Africa (Guiera senegalensis and Combretum micranthum). Measurements were made on excised stems and in situ. 2. An experiment suggested. that the respiration rate was closely related to the stem surface area but showed little relationship with stem volume, and was therefore associated with the cambium and phloem rather than the sapwood. This contrasts with several studies in which sapwood volume appeared to be the dominant component and the difference is attributed to the comparatively small diameter of stems in the shrubs studied here. 3. Measurements were made of the response of stem CO2 flux to photosynthetic photon flux density (Q) and gave evidence of corticular photosynthesis in the stems. 4. The response of stem respiration to temperature was determined in situ. The data were analysed using a variant on the standard exponential relationship with temperature, R = (R-om + R-og) exp(k T), where R is respiration rate, R-om is maintenance respiration rate at 0 degrees C, R-og is growth respiration rate at 0 degrees C, k is a temperature coefficient and Tis temperature. Data from the dry season were assumed to represent maintenance respiration and used to define R-om and k. The R-og term thus quantifies the increase in respiration during the wet season beyond this baseline level, presumably attributable to growth. Values of R were 0.054 and 0.074 mu mol m(-2) s(-1) in G. senegalensis and C. micranthum, respectively, whilst R-og ranged between 0.061 and 0.96 mu mol m(-2) s(-1) during the wet season. 5. At ecosystem scale On an annual basis, stem respiration represented 17% of leaf photosynthesis, whilst corticular photosynthesis was at most 11.1% of stem respiration.