A global database of shrub encroachment effects on ecosystem structure and functioning

Abstract

The encroachment of woody plants into grasslands, and the conversion of savannas and open woodlands into shrublands, has been widely reported during the past decade. Encroachment has generated considerable interest among ecologists worldwide, but there have been few quantitative syntheses of the effects of encroachment on ecosystem processes across large areas of the globe. Here we summarize the results of observations of the effects of encroachment by woody plants on 111 ecosystem response variables using data obtained from 1722 encroached-unencroached pairs, reported in 160 studies from North and South America, Africa, Europe, Australia, and Asia. We used an extensive review of the literature, including both published and unpublished data, to summarize available data on the effects of a change from open woodland or grassland to shrubland or closed woodland, on richness of plant and animal taxa, soil chemistry, and the status of the soil surface. Our database is restricted to arid, semi-arid, and dry sub-humid environments (drylands) receiving average annual rainfall between 850 and 200 mm. An analysis of the impacts of shrub encroachment on ecosystem structure and function has already been reported using a large subset of these data. This updated data set can provide an opportunity to test further hypotheses about the effects of encroachment on plant and animal communities and on soil processes related to ecosystem functioning.

Salt marsh‐mangrove ecotones: using structural gradients to investigate the effects of woody plant encroachment on plant–soil interactions and ecosystem carbon pools

“…; Eldridge et al . ). In arid and semi‐arid terrestrial grasslands (i.e., where mean annual precipitation ≤850 mm), woody plant encroachment can result in lower soil pH, higher soil carbon and nitrogen pools, higher potential nitrogen mineralization, and higher levels of exchangeable calcium (Eldridge et al .…”

“…In arid and semi‐arid terrestrial grasslands (i.e., where mean annual precipitation ≤850 mm), woody plant encroachment can result in lower soil pH, higher soil carbon and nitrogen pools, higher potential nitrogen mineralization, and higher levels of exchangeable calcium (Eldridge et al . ). In mesic terrestrial grasslands where above‐ground net primary productivity is already relatively high due in part to high freshwater availability, woody plant encroachment can result in a decrease (Jackson et al .…”

“…; Throop & Archer ; Eldridge et al . ). However, the effects of woody plant encroachment on ecosystem structure and function are often location dependent and greatly influenced by rainfall‐plant trait interactions (Knapp et al .…”

Spatial heterogeneity of fine root biomass and soil carbon in a California oak savanna illuminates plant functional strategy across periods of high and low resource supply

“…Woody elements on the landscape generally concentrate nutrients and carbon beneath their canopies by depositing plant biomass in sub-canopy soils and translocating nutrients from surrounding treeless areas via roots (Vinton and Burke, 1995;Jobbagy and Jackson, 2000;Hibbard et al, 2001;Archer et al, 2004). However, they have also been found to reduce soil carbon stocks compared with surrounding vegetation at some sites (Belsky, 1994;Eldridge et al, 2012;O'Donnell and Caylor, 2012). Whereas most research on this topic addresses the phenomenon of woody encroachment into formerly treeless areas, these insights also serve for regions where woody landscape elements are at risk.…”

Trees and shrubs as invasive alien species – 2013 update of the global database

“…Our criteria for inclusion of species remain unchanged: the alien species should be not only naturalized (consistently reproducing), but invasive (spreading) in one or more of the 15 recognized geographical regions (for definitions see Pyšek et al ., ). Because we deal explicitly with non‐native species, cases of encroachment of native woody species (Eldridge et al ., ) are not included in our database.…”