Reduction of Tree Water Stress by Large Herbivores in an African Savanna

ABSTRACT

Large mammalian herbivores (LMH) play complex roles in shaping tree cover and composition in savannas by suppressing trees through browsing and reducing competition from neighbouring trees and understory vegetation. Where LMH are declining, release from browsing and intensification of competition has unpredictable physiological consequences. To test how LMH removal affects tree growth and water stress, we combined 16 years of growth data with measurements of leaf water potential and gas exchange from a herbivore‐exclusion experiment in Kenya. Across tree species, LMH exclusion increased growth but elevated water stress, particularly under dry conditions. Moreover, the tree species with the most acquisitive water‐use strategy was most susceptible to increased water stress and reduced carbon assimilation under exclusion. These findings indicate that although LMH limit tree growth, they also reduce water stress. Ultimately, interspecific differences in tree physiology will affect how savanna communities respond to interacting pressures from climate change and LMH declines.