Woody Plant Encroachment: Causes and Consequences

Abstract

Woody vegetation in grasslands and savannas has increased worldwide over the past 100-200 years. This phenomenon of "woody plant encroachment" (WPE) has been documented to occur at different times but at comparable rates in rangelands of the Americas, Australia, and southern Africa. The objectives of this chapter are to review (1) the process of WPE and its causes, (2) consequences for ecosystem function and the provision of services, and (3) the effectiveness of management interventions aimed at reducing woody cover. Explanations for WPE require consideration of multiple interacting drivers and constraints and their variation through time at a given site. Mean annual precipitation sets an upper limit to woody plant cover, but local patterns of disturbance (fire, browsing) and soil properties (texture, depth) prevent the realization of this potential. In the absence of these constraints, seasonality, interannual variation, and intensity of precipitation events determine the rate and extent of woody plant expansion. Although probably not a triggering factor, rising atmospheric CO 2 levels may have favored C 3 woody plant growth. WPE coincided with the global intensification of livestock grazing that by reducing fine fuels, hence fire frequency and intensity, facilitated WPE. From a conservation perspective, WPE threatens the maintenance of grassland and savanna

Winter survival of North American grassland birds is driven by weather and grassland condition in the Chihuahuan Desert

“…Our results demonstrate that the presence of shrubs also can lower winter survival, and suggest that shrub encroachment into wintering habitat may be a driver of population declines in these migratory species, given the widespread nature of this phenomenon in their wintering areas (Archer et al. ). As such, shrub removal could increase survival of wintering sparrows in the Chihuahuan Desert by reducing availability of perches, nest sites, and larders for Loggerhead Shrikes and perches for raptors, thus reducing predation risk.…”

“…Shrub encroachment, possibly due to grazing mismanagement and increased atmospheric CO 2 (Van Auken ), is a widespread and primary driver of grassland loss in the Chihuahuan Desert (Archer et al. ). Shrub removal has been a frequent focus in grassland restoration (Rango et al.…”

“…These habitat relationships would have implications for grassland conservation and management, especially in light of well‐documented and widespread overgrazing that has resulted in woody‐plant encroachment (Van Auken , Archer et al. ) and loss of perennial grass cover (Estrada‐Berg et al. , Manzano et al.…”

Plant community response to Artemisia rothrockii (sagebrush) encroachment and removal along an arid elevational gradient

“…Woody species, predominantly shrubs, are particularly responsive to changing climate (Walker et al., ) and their range expansions (Kopp & Cleland, ; Sturm, Racine, & Tape, ) could have significant impacts on ecosystems into which they encroach (Knapp et al., ). In grasslands and other herbaceous‐dominated communities, the arrival of shrubs may alter light, water and nutrient availability, but outcomes will vary from community to community (Archer et al., ; Eldridge et al., ). Thus, experiments testing community response to shrub encroachment can help shape our understanding of how ecosystems will function in the face of climate‐induced plant species range alterations.…”

“…Arid grasslands are susceptible to ecosystem degradation in response to shrub encroachment, with declines in grass species cover and abundance a common result (Archer et al., ; Eldridge et al., ). However, shrubs can promote establishment of target species during restoration by acting as “nurse plants” (Gomez‐Aparicio, ).…”