Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis

Abstract

Encroachment of woody plants into grasslands has generated considerable interest among ecologists. Syntheses of encroachment effects on ecosystem processes have been limited in extent and confined largely to pastoral land uses or particular geographical regions. We used univariate analyses, meta-analysis and structural equation modelling to test the propositions that (1) shrub encroachment does not necessarily lead to declines in ecosystem functions and (2) shrub traits influence the functional outcome of encroachment. Analyses of 43 ecosystem attributes from 244 case studies worldwide showed that some attributes consistently increased with encroachment (e.g. soil C, N), and others declined (e.g. grass cover, pH), but most exhibited variable responses. Traits of shrubs were associated with significant, though weak, structural and functional outcomes of encroachment. Our review revealed that encroachment had mixed effects on ecosystem structure and functioning at global scales, and that shrub traits influence the functional outcome of encroachment. Thus, a simple designation of encroachment as a process leading to functionally, structurally or contextually degraded ecosystems is not supported by a critical analysis of existing literature. Our results highlight that the commonly established link between shrub encroachment and degradation is not universal.

It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands

“…The phenomenon of shrub encroachment (i.e. the expansion of woody vegetation into former grasslands [99]), a key land-cover change affecting drylands worldwide, has multiple effects on ecosystem structure and functioning [100]. Although this phenomenon has not been found to significantly affect ecosystem attributes such as biodiversity at the global scale [100], it may either enhance or reduce plant species richness at local and regional scales [97,101].…”

Shrub encroachment of Iberian dehesas: implications on total forage productivity

“…For both the Retama and Cistus shrubs, plots with a certain level of shrub cover were more productive than shrub-free plots, although browsing by livestock could be limited at high shrub cover. It is recommended that good management practices of Iberian dehesas and similar extensive silvopastoral systems should maintain the shrub layer (at optimal densities) to take advantage of the many positive roles of shrubs for ecosystem functioning (Eldridge et al 2011) without compromising the overall productivity of the system.…”

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

“…and creosote shrubland (Larrea tridentata) encroaching across the Rio Grande basin and piñon-juniper woodland (Pinus edulis-Juniperus monosperma) encroaching downslope in the Los Piños mountains. These vegetation changes are characteristic of those occurring across large extents of the semiarid basin and range landscape of the southwestern U.S. [Eldridge et al, 2011;Van Auken, 2009]. The criteria used in the selection of the sites were (a) each site must be characteristic of an end-member vegetation state over the two monitored ecotones; (b) sites must be of comparable area, dimensions, and slope to allow comparison between monitored rainfallrunoff relationships; (c) sites must be located on a planar interfluve slope; and (d) at the two ecotones, sites selected as being representative of end-member vegetation states must be as close as possible, thus minimizing variation in geology, soil characteristics, and meteorological variables.…”

“…Many drylands globally, including large areas of the southwestern United States (southwestern U.S.), have recently experienced, or are currently undergoing, extensive environmental change, driven by the encroachment of woody vegetation into grasslands [King et al, 2012;Zafar et al, 2005]. In the U.S., lands undergoing encroachment are estimated to cover 330 × 10 6 ha [Eldridge et al, 2011;Knapp et al, 2008]. Observations from the grass-woody vegetation transitions occurring in drylands across the southwestern U.S. suggest that the competitive ability of grass species is being reduced, allowing woody species to encroach and that ecosystems are adjusting to a change in current or historic controls on vegetation patterns [Archer, 1995].…”