Xylem cavitation vulnerability influences tree species' habitat preferences in miombo woodlands

Abstract

Although precipitation plays a central role in structuring Africa's miombo woodlands, remarkably little is known about plant-water relations in this seasonally dry tropical forest. Therefore, in this study, we investigated xylem vulnerability to cavitation for nine principal tree species of miombo woodlands, which differ in habitat preference and leaf phenology. We measured cavitation vulnerability (Ψ(50)), stem-area specific hydraulic conductivity (K S), leaf specific conductivity (K L), seasonal variation in predawn water potential (Ψ(PD)) and xylem anatomical properties [mean vessel diameter, mean hydraulic diameter, mean hydraulic diameter accounting for 95 % flow, and maximum vessel length (V L)]. Results show that tree species with a narrow habitat range (mesic specialists) were more vulnerable to cavitation than species with a wide habitat range (generalists). Ψ(50) for mesic specialists ranged between -1.5 and -2.2 MPa and that for generalists between -2.5 and -3.6 MPa. While mesic specialists exhibited the lowest seasonal variation in Ψ(PD), generalists displayed significant seasonal variations in Ψ(PD) suggesting that the two miombo habitat groups differ in their rooting depth. We observed a strong trade-off between K S and Ψ(50) suggesting that tree hydraulic architecture is one of the decisive factors setting ecological boundaries for principal miombo species. While vessel diameters correlated weakly (P > 0.05) with Ψ(50), V L was positively and significantly correlated with Ψ(50). Ψ(PD) was significantly correlated with Ψ(50) further reinforcing the conclusion that tree hydraulic architecture plays a significant role in species' habitat preference in miombo woodlands.