Vegetation, Megaherbivores, Man and Climate in the Quaternary and the Genesis of Closed Forests

Abstract

Plant biomass consumption by herbivores is a major influence on the structure of the vegetation canopy. In woodland and forests megaherbivores are most important. In the presence of dense populations of megaherbivores a multi-species fauna of smaller herbivores prevents the formation of a closed crown canopy. In the Tertiary and (outside the range of Homo) in the Quaternary there is always at least one species of megaherbivores on all continents and in all climates. Forests were generally more open and patchy, and while less carbon was stored in plant biomass, high herbivore biomass produced large quantities of methane.

In the tropics of the Old World, long coevolution of hominids and megaherbivores allowed some species to adapt to human hunting. Their population density, however, was reduced. As a consequence, with an increase of tree cover and canopy closure, more carbon was sequestered and less methane emitted. Since the Middle Pleistocene Homo proceeded into other climates and continents. There, all megaherbivores became extinct, and crown canopies of forested lands began to close. Since the Lower Pleistocene anthropogenic fire regimes shaped the vegetation cover outside the humid regions. It is suggested that anthropogenic changes, through extinction of megaherbivore populations and introductuion of large-scale use of fire, have changed the albedo and influenced carbon fluxes, thus triggereing climatic feedback processes of the Quaternary.