STEM CO2 FLUXES IN 2 SAHELIAN SHRUB SPECIES (GUIERA-SENEGALENSIS AND COMBRETUM-MICRANTHUM)

1. Measurements are presented of CO2 flux from woody stems of two shru b 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 close ly related to the stem surface area but showed little relationship wit h stem volume, and was therefore associated with the cambium and phloe m rather than the sapwood. This contrasts with several studies in whic h sapwood volume appeared to be the dominant component and the differe nce 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 res piration to temperature was determined in situ. The data were analysed using a variant on the standard exponential relationship with tempera ture, 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 respirati on rate at 0 degrees C, k is a temperature coefficient and Tis tempera ture. Data from the dry season were assumed to represent maintenance r espiration and used to define R-om and k. The R-og term thus quantifie s the increase in respiration during the wet season beyond this baseli ne level, presumably attributable to growth. Values of R were 0.054 an d 0.074 mu mol m(-2) s(-1) in G. senegalensis and C. micranthum, respe ctively, 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.