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.
Research reports have suggested that the soil seed banks can have seedling recruitment favoured by fire, especially in grassy plant communities prone to this environmental factor. We examine effects of fire on the soil seed bank of grasslands in the Neotropical wetland Pantanal (hyperseasonal savanna). Our hypothesis was of negative effect, given the contrasting traits associated to seed-seedling tolerance to fire or flooding in addition to the strength of flood as the main ecological driver in floodable areas. Soil samples were collected before and immediately after a prescribed burn in floodable savanna grasslands, at different depths (2 cm layers down to the 10 cm depth), plus litter. The soil samples were taken to a greenhouse for censuses of seedling emergence, to assess the species composition of the seed bank. We recorded a density of 7404 seeds/m 2 and 49 morphospecies, mostly aquatic plants (63%). Among the analysed ecological parameters, the species richness and composition did not change significantly between the pre and postfire conditions; yet the abundance was significantly lower only in the first 2 cm layer of the burned soil. Both abundance and richness decrease with soil depth. This apparent fire tolerance of the soil seed bank of floodable savanna grasslands is here proposed as a clue to understand fire as another ecological driver, as well as flood, but further long-term studies are needed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.