Shrub Invasion of Grassland: Recruitment Is Continuous and Not Regulated by Herbaceous Biomass or Density

Brown, Joel R.; Archer, Stephen D.

doi:10.2307/176918

Cited by 89 publications

(109 citation statements)

References 40 publications

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“…Disturbance may be a necessary precursor of the exclusion of large herbivores to cause substantial increases in herbaceous cover (Brown and Archer 1999). Our findings of limited effects of protection from grazing on plant community composition cannot be taken as evidence that large effects of grazing on species diversity did not occur before exclosures were established.…”

Section: Discussionmentioning

confidence: 60%

Large herbivores influence the composition and diversity of shrub-steppe communities in the Rocky Mountains, USA

Manier

Hobbs

2005

Oecologia

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It is widely believed that wild and domestic herbivores have modified the structure and composition of arid and semi-arid plant communities of western North America, but these beliefs have rarely been tested in long-term, well-replicated studies. We examined the effects of removing large herbivores from semi-arid shrublands for 40-50 years using 17 fenced exclosures in western Colorado, USA. Shrub cover was greater (F=5.87, P=0.0020) and cover (F=3.01, P=0.0601) and frequency (F=3.89, P=0.0211) of forbs was less inside the exclosures (protected) relative to grazed plots. However, we found no significant effects (minimum P=0.18) of protection from grazing on cover or frequency of grasses, biotic crusts, or bare soil. Although mean species richness and diversity were similar between treatments, protected areas had much higher dominance by fewer species, primarily sagebrush. Exclusion of herbivores changed the relationship between species richness and evenness. Consistent with theoretical expectations, species evenness was positively correlated with richness in protected plots (r2=0.54). However, contrary to theory, evenness and richness were inversely related in grazed plots (r2adjacent=0.72, r2distant=0.84). We suggest that these differences resulted because grazing acts as a stressor promoting facilitative relationships between plant species that might compete for resources in the absence of grazing. We conclude that exclusion of grazing in the sites we studied caused minor changes in cover and diversity of herbaceous plants, but caused a clear increase in the cover of shrubs. Importantly, the exclusion of ungulates changed the relationship between evenness and richness.

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Section: Discussionmentioning

confidence: 60%

Large herbivores influence the composition and diversity of shrub-steppe communities in the Rocky Mountains, USA

Manier

Hobbs

2005

Oecologia

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“…In areas without significant exotic annual grasses and fire suppression, sagebrush recovery after burning may be predictable (Harniss and Murray 1973;Wambolt et al 2001). Such an increase in sagebrush cover into early successional grass and forb dominated stands provides direct connections to work on ecosystem consequences of woody plant invasions into grasslands (Brown and Archer 1999). Sagebrush stand productivity may decline over time due to dominance by senescent shrubs with a concurrent loss of herbaceous production; this represents a stable state that resists changes in grazing practices (Laycock 1991).…”

Section: Introductionmentioning

confidence: 99%

Spatial patterns in leaf area and plant functional type cover across chronosequences of sagebrush ecosystems

Ewers

Pendall

2007

Plant Ecol

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Since most studies of ecosystem dynamics after disturbance require longer durations of study than the life span of most research careers, many studies rely on chronosequence approaches to substitute space for time. We tested the chronosequence approach for assessing the change in plant functional type cover and leaf area index (L) using three replicated mountain big sagebrush (Artemesia tridentata var. vaseyana (Rydb.) Boivin) dominated ecosystems in southern Wyoming. We further tested our broader inferences of mountain big sagebrush ecosystem chronosequences by assessing whether dynamics in spatial patterning of plant functional type cover and leaf area index would compromise the chronosequence approach. We hypothesized that (1) L and total cover increase with age at similar rates across replicated chronosequences, (2) spatial autocorrelation is greatest with shrub cover, and (3) spatial autocorrelation increases with age. We failed to reject all three hypotheses. Our analyses showed that mean shrub cover, total cover, and L all increased linearly with time since disturbance across all three replicated chronosequences. While neither graminoid nor forb cover was correlated with time since disturbance, graminoid cover did show an inverse relationship with shrub cover and L. Semivariogram analysis showed that spatial patterning increased with shrub cover and time since disturbance. Thus, while we cannot yet provide a process to fit the spatial patterns, the chronosequence approach for sagebrush ecosystems recovering from disturbance has survived a rigorous test because the mean changes in shrub cover, total cover, and L were replicable across three different sites.

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“…There are several examples of fire suppression promoting woody encroachments in grasslands (mesic grasslands of North America: reviewed in Briggs et al 2005; South Africa: Cowling et al 1986). Grazers rarely deter woody invaders in grasslands; in fact, they promote invasion by dispersing seeds, and creating safe sites for the establishment of invaders by reducing competition and disturbing soil (Brown and Archer 1999;Brown and Carter 1998;Drewa et al 2001). Diversity and density of the herbaceous resident community, at best, has a minor role in resisting woody invasions due to niche separation between the two growth forms (Bellingham and Coomes 2003;Brown and Archer 1989;Sankaran et al 2004).…”

Section: Introductionmentioning

confidence: 99%

Exotic shrub invasion in a montane grassland: the role of fire as a potential restoration tool

Srinivasan

2011

Biol Invasions

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In recent years, invasion of native grasslands by exotic woody plants has been recognized as a global problem with multiple adverse ecological and socio-economic consequences. Reasons for such expansions are numerous, including fire suppression. An important example of this problem is the native montane grassland in the Nilgiris of the Western Ghats in India, a biodiversity hotspot threatened by invasion of multiple woody species. In this study, the impacts of the highly invasive, nitrogen fixing exotic shrub Cytisus scoparius (Scotch broom) on the grassland community and ecosystem function have been quantified and the role of fire as a potential management tool evaluated. I established paired plots in uninvaded and broom-invaded grasslands that were either unburned or burned by an unplanned wildfire event. Invasion negatively impacted the grassland community structure and composition, favoring shade tolerant and weedy native plants, but did not greatly alter ecosystem function. Burning broom patches to eliminate the stands resulted in lower soil moisture and nitrogen levels 18 months after the fire. Yet, there were no notable fire effects on the grassland communities or ecosystem properties. Taken together, the results suggest that fire might be an effective tool for broom control. At the end of the study period burned-broom communities did not become more similar to uninvaded-grasslands; presumably the recovery process may be slow without additional management intervention.

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Shrub Invasion of Grassland: Recruitment Is Continuous and Not Regulated by Herbaceous Biomass or Density

Cited by 89 publications

References 40 publications

Large herbivores influence the composition and diversity of shrub-steppe communities in the Rocky Mountains, USA

Large herbivores influence the composition and diversity of shrub-steppe communities in the Rocky Mountains, USA

Spatial patterns in leaf area and plant functional type cover across chronosequences of sagebrush ecosystems

Exotic shrub invasion in a montane grassland: the role of fire as a potential restoration tool

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