Stomatal conductance of West-Central Supersite vegetation in HAPEX-Sahel: measurements and empirical models

Empirical relationships were derived to describe the climatic control of stomatal conductance for four of the most common species of the HAPEX-Sahel study area. These species included a C3 shrub (Guiera senegalensis), a C3 forb (Mitracarpus scaber), a C4 grass (Digitaria gayanus) and the most common crop of the area, millet (Pennisetum glaucum), which is also a C4 species. For all four species the controlling climate variables were photosynthetically active radiation (PAR), vapour pressure deficit (VPD) and soil water potential, and their responses to these variables differed in a manner that was consistent with their different photosynthetic physiologies. Air temperature was not found to be a significant variable. Canopy average stomatal conductances were estimated from leaf-level measurements and then modelled in this paper. By relating the distribution of leaf-level stomatal conductances to light attenuation through the canopy it was shown that total PAR intercepted by the canopy, divided by leaf area index (LAI) (i.e. average PAR interception), can be used to estimate average stomatal conductance to a good approximation of the non-linear canopy integral as long as LAI variation in the measurements is small (e.g. less than 1.0). This assumes that the other environmental variables are constant through the canopy, but is significant in that it simplifies the procedure for the estimation of average stomatal conductance and canopy conductance (gc). The values of average stomatal conductance were limited by PAR availability for brief periods at sunrise and sunset and, for the two C4 species, there was evidence that shading within the canopy reduced the average values. However, the strongest diurnal variation was correlated with the increase in VPD, associated with increasing air temperature, which resulted in decreasing conductances during most of the day, following the peak in the early morning. The long-term decline in stomatal conductances following the last rainfall was associated with increasingly negative soil water potential. G. senegalensis retained its leaves through December with low but measurable stomatal conductances. Canopy conductances (gc) were estimated from average stomatal conductance and leaf area. Maximum values of gc occurred near the end of the rains, when environmental conditions were still near optimum and LAI was high.