Fire and grazing controlling a tropical tree line: Effects of long‐term grazing exclusion in Bale Mountains, Ethiopia

Aims: Tropical tree lines are often associated with abrupt shifts in vegetation, soils and disturbance regimes, but the underlying mechanisms are poorly understood. We analysed the role of grazing, fuels and fire in maintaining a sharp tree line with flammable heathland above non-flammable forest.

Location: Bale Mountains, Ethiopia.

Methods: Grazing exclosures, repeated vegetation sampling, soil analyses and burning and sowing experiments along an altitudinal gradient with Hagenia abyssinica forest, Erica trimera forest and Erica heathland; all heavily grazed, the latter burnt on short rotation.

Results: Contrary to expectation, livestock exclusion did not increase forest fuel flammability, but instead resulted in a dense carpet of non-flammable herbs. In the heathland, livestock exclusion led to somewhat faster post-fire fuel recovery, but no major vegetation change. Seeding of tree species resulted in some seedling establishment, but notably Hagenia grew poorly in the heath...