## 4 Results and Discussion  
### Seasonal Dynamics and Inter-Annual Variability of EC GPP, PAW, VPD and API  
Growing season average EC GPP was highest in Dahra at 8.6 g C m\({}^{-2}\) d\({}^{-1}\), followed by Demokeya at 3.9 g C m\({}^{-2}\) d\({}^{-1}\) and Wankama Fallow with 3.5 g C m\({}^{-2}\) d\({}^{-1}\). EC GPP was tightly coupled with climate across all sites (Figure 3). The greening phase was dominated by a rapid decrease in VPD and an increase in the amount of PAW. Growing season EC GPP was attained across all sites when PAW was generally above 25 mm (\(\sim\)0.04 m\({}^{3}\) m\({}^{-3}\)) and when VPD was below 2 kPa, which is comparable to studies conducted in similar environments [22; 66]. The decline in PAW and the onset of high VPD during times of drought reduces plant photosynthetic capacity by causing closure of stomata to prevent moisture loss [67]. Thus, a decrease in moisture during the growing season, as happened in Dahra in 2011, where there was 42% decrease in rainfall and 46% decrease in soil moisture from the previous year, could cause a reduction in GPP. Indeed, EC GPP in Dahra during the 2011 growing season was 72% less than the previous year. A similar pattern of decline was observed in Demokeya during the 2009 growing season when there was a 25% decrease in rainfall, 14% decrease in soil moisture, and a 9% decrease in GPP from the previous year (Figure 3a). When rainfall resumes to previous levels, as happened in 2012 in Dahra (Figure 3c), the herbaceous vegetation respond rapidly to changes in the soil water regime resulting in increased C uptake [13]. It has been shown that the terrestrial biosphere can retain \"memory\" of past wet or dry conditions through soil moisture or the vegetation itself [68; 69]. In this study, the observed speed of vegetation response to the resumption of rainfall could be due to this land memory effect.  
Figure 3: Site-level 8-day data for surface and subsurface conditions in **(a)** Demokeya; **(b)** Wankama Fallow; **(c)** Dahra. The shading indicates the growing season (July–October).  
The seasonal variability of the field parameters, PAW, VPD and EC GPP, was fairly consistent across all sites (Figure 3). However, Dahra had the highest average EC GPP during the growing season (8.6 g C m\({}^{-2}\) d\({}^{-1}\)) despite having similar climatic and subsurface conditions as the other two sites (Table 1). A recent study by [PERSON] et al. (2017) provided two plausible reasons for the higher photosynthetic capacity at Dahra: (1) A total of 80% of the vegetative cover at Dahra is comprised of herbaceous C\({}_{4}\) plants, signifying a higher concentration of plants that are more productive under optimal climatic conditions (i.e., during the growing season); and (2) intense grazing disturbance causes plants to develop strategies that allocate more C to leaves thereby increasing their leaf area index and leading to increase in absorbed photosynthetically active radiation and C assimilation. A third plausible reason for the high EC GPP at Dahra is linked to the increase in relative humidity which ranges from 9% in the dry season to 63% in the growing season (Table 1). This is connected to the movement of the inter-tropical convergence zone (ITCZ) and the West African Monsoon, which transports moist air from the Gulf of Guinea to West Africa (13; 38; 71). This large influx of humid air could allow the stomata to remain open longer during the growing season, thus enabling enhanced C uptake without increased water loss through transpiration. Growing season (July-October) API in Dahra was 153% higher than Demokeya and 57% higher than Wankama Fallow, and had higher intra-seasonal variability (standard deviation = 171%). This is also probably due to West African Monsoon, which causes higher amounts of rainfall at Dahra (71; 72; 73; 74).