Contrasting global effects of woody plant removal on ecosystem structure, function and composition

Abstract

Encroachment of woody plants into savannas and grasslands has increased markedly over the past century due to global changes in climate and intensified land use disturbance (e.g., grazing, fire). Removal of woody plants is mostly used globally to attempt to reinstate open woodlands and grasslands to increase forage plant production for livestock. However, there is still considerable controversy over the effectiveness of different removal programs and a global synthesis of removal impacts on ecosystem processes is still lacking, limiting our ability to provide ecologically-based advice on how best to manage woody encroachment. We used a global meta-analysis to explore the effects of woody plant removal on ecosystems. Analyses of 263 publications revealed that the overall effect of removal varied among different ecosystem response variables, with increases in composition (e.g., grass richness), reductions in structure (e.g., biocrust cover, woody plant cover and density), but no effects on function (e.g., increases in grass biomass, which compensated for reductions in soil roughness and shrub biomass). The outcomes of woody plant removal depended strongly on environmental context and woody plant traits, with removal more effective in mesic areas, but varied depending on both aboveground and belowground traits of the plants (e.g. plant shape, root types). Effectiveness of woody plant removal was relatively short-lived (i.e. within 5 years), but legacy effects on ecosystem function were generally large, negative, and lasted for up to 10 years. Our results highlight the wide disparity in removal outcomes, and reinforce the notion that the impacts of removal are strongly context dependent, vary with treatment methods, and generally ecologically undesirable in the long term. As climate changes, woody plant removal will become less effective due to drier climates and increased woody expansion. Treatment methods should be targeted to specific management goals (e.g., pastoral production or ecosystem conservation), and particular ecosystem outcomes (e.g. ecosystem structure, or function or composition) to improve the efficiency of woody removal in global savannas under the changing climate.