Assessment of the influence of livestock grazing and soil moisture on tree sap flow in savannah, Lambwe valley, Kenya

Abstract

A Kenyan humid savanna in Lambwe Valley is dominated by trees of the genus Acacia and Combretum. The ecosystem is subjected to livestock grazing at varied intensities and fluctuating rainfall amounts. Notable increase in livestock densities and declining rainfall amounts are raising concerns regarding the sustainability of the humid savanna. Sap flow measurements in mature representative trees of A. ancystoclada and C. mole were carried out to assess tree responses to varying intensities of livestock grazing and the compounding effects of fluctuating soil moisture in their natural conditions. On average, trees in the grazed fields (grazed plots) exhibited higher (P>0.0001) mean daily sap flux densities (SFD) compared to those excluded from grazing (fenced plots), amounting to 27.47 ± 8.65 g m^-2s^-1 and 20.17 ± 7.27 g m^-2s^-1, respectively. The peak daily sap flux densities were recorded around midday in both species. For both species, daily SFD was correlated (R²=0.77) with vapor pressure deficit. Seasonal changes in SFD were however positively influenced by the amount of rainfall, through changes in soil moisture. Higher SFD in the grazed plots was attributed to higher projected crown area. The diurnal variation in SFD showed that sap flow was coupled to the atmosphere with relatively low boundary layer resistance and the seasonal variation in sap flow was controlled by stomatal regulation. These findings point to the possibility that the dominant tree species in Lambwe are isohydric species.