Shrub encroachment influences herbaceous communities in flooded grasslands of a neotropical savanna wetland

Abstract

Question The encroachment of woody species has been globally reported over much of arid and semi‐arid biomes, and has been associated with a decrease in cover and number of herbaceous species. How does the encroachment of a woody shrub affect herbaceous community structure and species composition in grasslands of a wetland ecosystem? Location Seasonally flooded grasslands in a Neotropical Hyperseasonal Savanna, the Pantanal wetland, Brazil. Methods We investigated the effect of the encroaching plant Combretum laxum on a herbaceous community from seasonally flooded grasslands in the Pantanal wetland using 29 vegetation samples representing encroachment at different spatio‐temporal stages. The point quadrat method was used to acquire vegetation data, as plant cover, species richness (S) and Shannon‐Wiener diversity index (H’). We evaluated the existence of stages of encroachment related to differences in vegetation structure and species composition using non‐metric multidimensional scaling ordination and analysis of similarity. The response of the herbaceous community to shrub advance was analysed using linear and quadratic polynomial regression models. Model fitness was tested using the ACI. Results The advance of the woody encroaching plant C. laxum over the seasonally flooded grassland occurs in three stages: grassland stage, shrub islands stage and shrubland stage. The initial advance of C. laxum over the grassland, represented by the shrub islands stage was correlated to an increase in species richness and a decrease in important native forage grasses. Critical changes in the herbaceous community were observed when the encroaching plant covered >30% of the periodically flooded grassland, when the richness of herbaceous species dropped from 22 to four. Conclusions Woody encroachment causes impoverishment and simplification of the herbaceous community. The shift from a grass‐ to a shrub‐dominated state is related to the reduction in important grassland‐obligate species, forage resources for herbivore livestock and wild animals, affecting the ecological dynamics and the economy of rangelands. Conservation of these grassland ecosystems depends on rangeland management practices guided by scientific knowledge on the causes and consequences of plant community changes.

A multifaceted view on the impacts of shrub encroachment

“…Increasing evidence suggests that the positive effects of encroachment might far outweigh the perceived negative effects. More recent global-scale field studies (Soliveres et al 2014) and regional syntheses (Eldridge & Soliveres 2015) indicate that plant diversity and multiple ecosystem functions are maximized under moderate levels of woody plant cover (the same response reported by Barbosa da Silva et al [2016] for species diversity). Furthermore, the effects of shrub encroachment on ecosystem structure and functioning, either positive or negative, are strongly dependent on the functional traits of both the encroaching shrubs and the grasses being replaced (Maestre et al 2009), as well as on environmental conditions (e.g.…”

“…While the results and conclusions of Barbosa da Silva et al. (2016) and Pittarello et al (2016) are entirely valid, both studies focus on a particular functional group (grasses) and ecosystem service (provision of forage).…”

“…The two articles included in this issue of Applied Vegetation Science evaluate the consequences of encroachment for grassland vegetation and possible management actions to reverse its negative effects on grasses. Barbosa da Silva et al (2016) show that shrub encroachment simplified a herbaceous community in seasonally flooded grasslands from the Pantanal ecosystem in Brazil, and Pittarello et al (2016) illustrate how two management actions, the arrangement of temporary night camp areas and the strategic placement of mineral mix supplements for cattle, can be used to restore the cover, diversity and biomass of grasses in encroached grasslands from the Italian Alps. Both articles provide useful insights into the (deleterious) effects of shrub encroachment on grass diversity and abundance in these ecosystems and how it can be managed to maximize the provision of forage for livestock.…”

Soil surface complexity has a larger effect on food exploitation by ants than a change from grassland to shrubland

“…Alterations to vegetation structure lead to changes in landscapeand habitat-level complexity and above-ground productivity for grass-or shrub-dependent organisms. These changes in ecosystem structure can alter the abiotic and biotic environments of the original grassland by increasing leaf litter beneath shrub canopies (Maestre et al, 2009;Tighe et al, 2009) and reducing the abundance and richness of perennial grasses (Barbosa da Silva et al, 2016). These changes in ecosystem structure can alter the abiotic and biotic environments of the original grassland by increasing leaf litter beneath shrub canopies (Maestre et al, 2009;Tighe et al, 2009) and reducing the abundance and richness of perennial grasses (Barbosa da Silva et al, 2016).…”

“…Above-ground productivity may be more spatially variable in shrublands than in the original grassland from which they are derived (Huenneke et al, 2002) because shrub encroachment is often associated with large patches of bare soil, particularly where sites are overgrazed (Eldridge & Soliveres, 2015;. These changes in ecosystem structure can alter the abiotic and biotic environments of the original grassland by increasing leaf litter beneath shrub canopies (Maestre et al, 2009;Tighe et al, 2009) and reducing the abundance and richness of perennial grasses (Barbosa da Silva et al, 2016). Shrub encroachment can alter the abiotic environment by increasing surface temperatures in the interspaces, and change water infiltration and the availability of carbon and nitrogen (Schlesinger et al, 1990;Eldridge et al, 2011).…”