Tree logs for grassland restoration? Lessons from an unintentional experiment

Introduction

In temperate grassland restoration, hay transfer, direct seeding and topsoil application have been used to increase seed recruitment (Kiehl et al. 2010; Klimkowska et al. 2010; Rydgren et al. 2010). Restoration of tropical and subtropical grasslands and savannas also is limited by seed availability (Buisson et al. 2019), making passive restoration difficult (e.g. Koch et al. 2016; Torchelsen et al. 2019). Degradation in these ecosystems due to human action, such as afforestation or agricultural activities, among others, also may affect soil features (e.g. physical and chemical characteristics; soil seed bank). This makes active restoration necessary as the natural community has low resilience to severe degradation (Buisson et al. 2019; Nerlekar & Veldman 2020). Removal of vegetation that leads to the loss of belowground structure makes re-establishment of native species especially difficult (Buisson et al. 2019).

For subtropical grasslands in Brazil, studies that tested techniques of seed introduction still are scarce (e.g. Thomas et al. 2019a, 2019b; Porto et al. 2022) and few generalizations can be made about effectiveness of different seed introduction techniques. At the same time, seeds of native grassland species are not available on the market (Rolim et al. 2022), making restoration projects challenging. Hence, it is important to investigate techniques that modify the environment in a way that seed arrival and plant establishment from seeds in degraded areas is enhanced.

In forest restoration, perches are successfully used to increase seed rain at restoration sites (Guidetti et al. 2016). In grasslands, seed dispersal by animals also may play a role in restoration (see, e.g. Minervini Silva & Overbeck 2021 for the role of endozoochory), but perches should be less successful as few grassland plant species are dispersed by birds. The most common species in subtropical grassland belong to the Poaceae and Asteraceae (Andrade et al. 2019), plant families that contain many species that exhibit anatomical adaptations in their diaspores that facilitate dispersion by wind or surface dispersal (e.g. pappus in Asteraceae; silky hairs on dispersal units, lightweight seeds, awns, dispersal as “tumbleweeds”; Cheplick 1998, Cavanagh et al. 2020). This seed rain can be intercepted by physical barriers (PBs), increasing seed establishment, which may promote vegetation development at degraded sites (Fick et al. 2016). In addition, PBs can also increment plant establishment by changes of site conditions, creating safe site for germination, due to shading, accumulation of organic material and retention of sediment and litter (Fick et al. 2016).

Here we took advantage of an “unplanned experiment” to analyze the potential of PBs, created by wooden logs, to trap seeds and facilitate the establishment of grassland species. At a site originally covered by natural grassland where the topsoil had been removed, wooden logs had been placed to prevent further degradation due to human action in the area. Seven years later, we observed some recovery of the native species. We compared plots closer to and distant to the logs in terms of species richness and diversity to verify if logs served as seed traps. We identified species that exhibit attributes in their diaspores that facilitate dispersal (well-dispersed species [WDS]) in our sample and in the regional species pool and compared the percentage of WDS with these attributes in the degraded area with those that could potentially colonize.

Methods

Study Area and Data Collection

The degraded grassland site is located in Saint'Hilaire Municipal Natural Park (−30.102319°S, −51.088843°W), in Viamão and Porto Alegre municipalities in Rio Grande do Sul State, Brazil (Fig. 1). The vegetation in Saint'Hilaire Park consists of a mosaic of grassland and forest patches. Climate is Cfa according to the updated Köppen system, that is, subtropical without a marked dry season and with hot summers (Peel et al. 2007). At the study site (total area 1700 m²) vegetation and topsoil had been removed to perform aeromodelling activities (flying of airplane models). To prevent this activity in the protected area, wooden (eucalypt) logs were placed at the site in 2012 (Fig. 1D). To evaluate the effect of the logs in enhancing vegetation development, we evaluated, in December 2019 (7 years after placement of logs), vegetation cover in 10 transects, established randomly perpendicular to pairs of logs.

Along each transect, the vegetation was surveyed in square plots of 0.25 m². The number of plots varied according to the distance between logs, totaling 125 plots. All plant species were identified within plots and their ground cover was estimated using a decimal scale (Londo 1976). The species nomenclature followed the Tropicos website (Tropicos 2020) for exotic species and the Flora do Brasil (Flora do Brasil 2020) for native species.

Results

A total of 74 species from 15 families were found in plots at the degraded site (Table S2). Nine individuals were not identified to the species level. Poaceae was the richest family with 26 species, followed by Asteraceae (22) and Rubiaceae (6). The species with highest mean cover (≥5%) were Aristida filifolia (Arechav.) Herter, Andropogon lateralis Nees, Eryngium horridum Malme, Galianthe fastigiata Griseb., and Chamaecrista repens (Vogel) H.S.Irwin & Barneby (Table 1).

Species richness was highest closest to the PBs and generally decreased with distance from the log (Fig. 2A). For estimated Shannon diversity, the plot closest to the PB (0–0.5 m) presented the highest value compared to the other distances (Fig. 2B). Simpson's diversity showed higher values for those plots far from the logs (Fig. 2C). Furthermore, total ground cover was also negatively affected by distance from logs (Fig. S1).

Our characterization of the species pool resulted in 318 species (Table S2). The richest families were Asteraceae with 90 species, followed by Poaceae (63) and Fabaceae (26). One third of these species (33%) had diaspores with attributes that facilitate dispersal (WDS). At the degraded site, 43.2% had these attributes, and these species also showed the highest cover values (Table 1). Logs trapped more WDS than expected, based on the regional species pool (Fig. 3, χ² = 88.59, p = 0.18) or the reference grassland (χ² = 5.86, p = 0.05).

Discussion

Our data indicated that the presence of PBs enhanced the establishment of native plant species in a degraded area with bare soil. The most species-rich families were Asteraceae (22 species) and Poaceae (26 species), and the established community included WDS such as Aristida and Andropogon species. Rubiaceae (six species) was the third family in the degraded area, including several Richardia species (R. brasiliensis Gomes, R. grandiflora (Cham. & Schltdl.) Steud. and R. humistrata (Cham. & Schltdl.) Steud.). Species of this genus can be considered ruderal and are commonly found in anthropic environments and areas modified by humans (Lorenzi 1991; Cruz & Martins 2015). The presence of ruderal species in degraded grassland areas after highly impacting activities is common (Le Stradic et al. 2018; de Souza Vieira & Overbeck 2020), especially Richardia species (Vieira et al. 2022).

Fick et al. (2016) demonstrated the effectiveness of barrier structures to mitigate desertification processes and enhance plant recruitment in a degraded semiarid grassland. In their study, Fick et al. (2016) combined barriers with seed addition and soil disturbances, but the increased survival of grass seedlings was likely improved due to the barriers, which suggests that these structures may have functioned as artificial “nurse plants.” Importantly, barriers can have effects beyond enhancing seed arrival: the high cover of moss (mean value of 8.16% in all plots) in the plots closest to the logs in our study indicates that the logs also lead to a modification of local microclimate, maintaining higher levels of soil moisture. Furthermore, moss may also serve as seed traps, as shown in peat bogs and peatlands where this group of plants is recommended for restoration (Groeneveld et al. 2007). Planting nurse plants has been applied and evaluated in different open ecosystems (see Castellanos et al. 1994 and Rubio-Casal et al. 2001: saltmarshes; Hulvey et al. 2017: dryland in general, “restoration island”; Franks 2003: “coastal dunes”; Holl et al. 2021: planting patches, “applied nucleation”) and may lead to the establishment of “islands of fertility” (Gornish et al. 2021) where vegetation development is facilitated. However, the use of nucleation-based techniques in the restoration of grasslands is still little explored (Shaw et al. 2020).

The spatial patterns of vegetation at the degraded site demonstrates that the PBs provided a greater richness of individuals close to them with a gradual decrease as the distance increased. The Simpson index, which gives more weight to abundant species, indicate that the plots closest to the PBs showed a less even community composition in their abundances. Species of the genus Aristida had the highest cover values and occurred mainly closer to the logs. Aristida spp. form large tussocks which contributes to their dominance, reflected by the Simpson index. However, when we measured plant diversity using Shannon diversity, plots closest to the PBs appeared as more diverse, due to higher weight of species richness.

Despite the effectiveness of PBs to trap seeds, 7 years after they were installed (2013–2022, personal observation), a large portion of the soil remained exposed, especially further in all away from the logs, even though the study site is immersed in conserved natural grasslands. Apparently, the proximity of grasslands did not supply enough propagules. Alternatively, the site conditions were too severe (e.g. Fick et al. 2016) to promote fast establishment and vegetation cover, that is, there could be microsite limitation, wherein recruitment of individuals remains limited in spite of seed availability, often requiring disturbances, for example in soil surfaces to break physical crusts and facilitate seed burial (Fick et al. 2016). While the use of PBs appears to be promising, apparently it is not sufficient in the studied system and the creation of adequate conditions for plant establishment is also important. Nonetheless, the efficiency of PBs to increase seed rain in grassland restoration should be studied to disentangle the mechanism(s) at play (seed trap vs. change in microsite conditions). Needless to say, such barriers do not need to be heavy tree logs, but could be made of light and portable materials and thus could be a low-cost and easy to implement technique. Such experiments should also include the evaluation of factors such as the importance of wind direction and ideal height of barriers. Overall, improved knowledge on dispersal and recruitment processes in grasslands is important to find ways to increase seed rain in grassland restoration and thus facilitate restoration (Arruda et al. 2018; Török et al. 2021), even if seed introduction may remain necessary in many cases.