The encroachment of woody vegetation in open-canopy ecosystems, i.e. grasslands and savannas, has been observed in many parts of the world (Archer, 1990; Bond and Midgley, 2000; Bond et al., 2003; Duarte et al., 2006). This process can be defined as increasing density, cover and biomass of trees and/or shrubs species in open-canopy ecosystems (Van Auken, 2000). The causes are mainly related to changes in climatic variables (i.e. warmer and more humid conditions), land use modifications and decreased frequency or cessation of disturbance regimes, resulting herbaceous species being replaced by tussock grasses, shrubs and/or trees (Hobbs and Mooney, 1986; Overbeck et al., 2005).

Woody encroachment often presents conservation concerns (Van Auken, 2000; Overbeck et al., 2007), as it modifies functions and processes of open-canopy ecosystems (Eldridge et al., 2011), such as total primary productivity, decomposition rates, nutrient availability and soil carbon dynamics (Lett and Knapp, 2003). As a result, these modifications could facilitate the growth of other woody species, for example some trees acting as nurse plants (e.g.Duarte et al., 2006), creating a positive feedback that might result in an irreversible woody encroachment process (Callaway and Davis, 1998; Barnes and Archer, 1999). Consequently, community and environmental changes can modify resource availability below some species requirements and thus, result in certain species being suppressed (Hobbs and Mooney, 1986). In this way, herbaceous plant cover and productivity may be reduced, which often results in significant decreases in grassland species richness and diversity (Hobbs and Mooney, 1986).

In the southernmost state of Brazil, Rio Grande do Sul, highland grasslands occur in the Mata Atlântica biome, within shrubby and forest physiognomies belonging to Araucaria pine tree and seasonal forests. Fire and cattle grazing are thought to be responsible for the maintenance of these forest-grassland mosaics under a climate more suitable for forest development (Pillar and Quadros, 1997). Vegetation composition and structure have changed over the last millennia, and it is known that woody species have increased in density and cover, mainly due to climate change and lack of management regimes (i.e. fire and grazing suppression) (Behling and Pillar, 2007; Overbeck et al., 2007; Andrade et al., 2015, 2016). Baccharis uncinella D.C. (Asteraceae) is one of these woody species, with spontaneous occurrences found in southern and southeastern regions of Brazil. In Rio Grande do Sul state, it is considered an endemic shrub of the “Campos de Cima da Serra” region. This region is characterized as a high altitude humid grassland region in forest-grassland mosaics (Andrade et al., 2016). Although B. uncinella encroachment is a recognized process in Brazilian grasslands-forest mosaics (Duarte et al., 2006), there is a lack of information about its effects on grassland vegetation. B. uncinella expansion could have important implications for forest-grassland mosaics conservation, as it may represent the initial stage of grassland conversion to forest. Currently, there is a debate about the best approach to conserve forest-grasslands mosaics in southern Brazil (see Pillar and Vélez-Martin, 2010; Luza et al., 2014; Carlucci et al., 2016; Overbeck et al., 2016). By gaining an understanding of the impacts B. uncinella could have on grassland ecosystems, conservationists would acquire important information about the encroachment process.

The aim of this study was to evaluate the effect of B. uncinella performance in a forest-grassland mosaic in southern Brazil. Management practices, i.e. fire and cattle grazing, were excluded from the study area since 1994. Specifically, we evaluated the influence of B. uncinella on grassland plant composition and richness, and whether this influence affects certain life forms (i.e. shrubs, grasses and forbs). Previous studies conducted in this region have shown changes in grassland species composition and richness when disturbance practices are excluded, mainly by increasing tussock grasses and shrubs cover (Oliveira and Pillar, 2004; Overbeck et al., 2005; Andrade et al., 2016). Thus, we expect the encroachment of B. uncinella to affect grassland community vegetation, promoting not only differences in species composition but also reducing the richness of typical grassland plants.

We recorded 46 plant species in total, belonging to 15 families (see Appendix S1). The most common families were Asteraceae (28%), Poaceae (20%) and Melastomataceae (15%). Grass species had the highest percentage of vegetation cover in both situations, with (67%) and without (64%) B. uncinella. Among the 46 recorded species, 34 of them (12 exclusive) were recorded in plots with B. uncinella; 29% of them grasses, 44% shrubs, 24% forbs and 3% trees. Trachypogon montufarii (Kunth) Nees (10%) and Baccharis trimera (Less.) DC (9%) were the most frequently occurring species in plots with B. uncinella. Moreover, 37 species (9 exclusive) were recorded in plots without B. uncinella, where 27% of them were grasses, 35% shrubs and 38% forbs. Axonopus siccus (Nees) Kuhlmand and Schizachyrium tenerum Nees, both caespitose grasses, were the most frequently occurring species (10%) when B. uncinella was absent.

Despite the exploratory analysis (PCoA) showing a tendency of distinct composition between communities with and without B. uncinella (Fig. 1), we did not find significant differences in species composition between both groups (p=0.47).

Fig. 1.

Diagram of the distribution of the plots with and without Baccharis uncinella, according with the PCoA of the species cover matrix.

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Species richness in grassland communities showed a negative significant relationship with B. uncinella performance (R2=0.36; p=0.04; Fig. 2A). Regarding plant life forms, forb was the only group that showed a significantly negative relationship with B. uncinella performance (R2=0.43; p=0.017; Fig. 2D). Shrub and grass species also showed a negative trend, but these relationships were not significant (Fig. 2B and C). Moreover, vegetation height (R2=0.004; p=0.85), total vegetation cover (R2=0.125; p=0.25) and standing dead biomass (R2=0.139; p=0.23) did not show significant relationships with B. uncinella performance.

Fig. 2.

Relationship between community species richness and B. uncinella performance per plot (10×10m; n=12). (A) Relationship between total species richness and B. uncinella performance. (B) Relationship between shrub richness and B. uncinella performance. (C) Relationship between grass richness and B. uncinella performance. (D) Relationship between forbs richness and B. uncinella performance.

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Shrub encroachment in southern Brazilian grasslands as a result of management suppression (i.e. cattle grazing and fire) deserves particular attention in the Mata Atlântica biome, where current temperature and precipitation conditions favor forest expansion, reducing native grassland vegetation (Oliveira and Pillar, 2004; Pillar and Vélez-Martin, 2010; Andrade et al., 2016; Overbeck et al., 2016). Our study contributes to understanding one of the main concerns about Brazilian forest-grassland mosaics conservation, an important and uniquely biodiverse ecosystem that is currently threatened by global climate change and inadequate local management practices (Oliveira and Pillar, 2004). We showed that B. uncinella encroachment had a significant effect on grassland community, reducing native plant richness, and particularly the number of forbs species. However, our results suggested that the presence of B. uncinella does not affect the richness of other life forms.

The encroachment of shrubs in natural grasslands could alter resource availability (e.g. light interception and soil moisture), affecting the ability of some species to establish and grow (Lett and Knapp, 2003). Our results suggest that some forbs species were mainly observed growing in plots without B. uncinella, such as Galium hirtum Lam., Lucilia sp. Cass, Polygala brasiliensis L. and Sysirinchium sp. L. Therefore, the presence of B. uncinella would lead to a reduction of certain species, such as forbs, which exhibit morphological, life history and ecophysiological traits that contrast sharply with those of the dominant C4 tussock grasses. These traits may limit certain plants ability to obtain resources, mainly light, beneath a B. uncinella canopy. The shrubs canopy would decrease light interception, which could mainly affect forbs species due to their shorter height and different specific leaf area without conferring any advantage to caespitose grasses. Previous studies which evaluated the effect of shrub encroachment in open-canopy ecosystems, i.e. grassland with no or few trees, reported significant community changes. Such changes were mainly related to a reduction in herbaceous species due to alterations in resource availability (Hobbs and Mooney, 1986; Lett and Knapp, 2003).

In Brazil, conservation efforts are primarily focused on forest ecosystems with grasslands receiving less attention (Overbeck et al., 2007). Nevertheless, woody encroachment in open ecosystems is well documented for many parts of the world (Archer, 1990; Bond and Midgley, 2000; Van Auken, 2000; Bond et al., 2003; Duarte et al., 2006). As other studies in southern Brazil have suggested (e.g.Oliveira and Pillar, 2004; Overbeck et al., 2005; Blanco et al., 2014; Andrade et al., 2016), we observed that the exclusion of disturbances regimes in forest-grassland mosaics would initially result in grassland species loss, mainly due to the encroachment of woody species, such as B. uncinella. The presence of such woody species may lead to competitive exclusion of subordinate plants (e.g. forbs). Therefore, studies with management practices that reconcile the maintenance of forest-grassland mosaics as a whole are urgently needed. For example, grassland patches can be conserved with adequate fire and grazing regimes to prevent woody expansion and corresponding reduction in grassland species richness. However, these management practices should be avoided inside woody patches to maintain tree recruitment. Longer-term studies are required to evaluate the response of each vegetation type to different management practices, and therefore, to decide which strategies are more appropriate for forest-grassland mosaics conservation.

In our study site, vegetation composition was homogeneous, a few native caespitose grasses dominated the community (e.g. A. lateralis and A. siccus), and populations of B. uncinella seem to be expanding into the open-canopy ecosystem. The capacity of B. uncinella to occupy adjacent open ecosystems and to reduce its richness indicates that grassland vegetation is being suppressed in the absence of management practices. However, our study is a first step to understand more about the effects behind B. uncinella expansion. Longer-term studies on the causes and mechanisms of this encroachment process are needed; as such encroachment has important implications for Brazilian forest-grassland mosaics conservation.

The authors declare no conflicts of interest.