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Ecological restoration in pasture areas with direct seeding

The use of direct seeding in projects for the recovery of degraded areas has stood out in some regions of the country because of its effectiveness, lower operating costs, and ease of implementation. Thus, the present study aimed to evaluate the potential of direct seeding the haul in ecological restoration of a deactivated pasture dominated by Urochloa brizantha, here characterized by an invasive weed plant . The experiment consisted of eight treatments resulting from different combinations of seeds (tree and herbaceous cover crops), the use of treatments to break the dormancy of tree species seeds (with and without), and the use of different types of substrates (clay and sawdust). The tested treatments did not affect seedling emergence, survival, or initial growth. The most established species in the study area were Piptadenia gonoacantha , Mabea fistulifera , Dalbergia nigra , and Senegalia polyphylla , which could potentially compose the list of species to be used in forest restoration projects through the use of direct sowing techniques. Plant survival at the end of the evaluation period every three months until the 14 months of experiment implementation corresponded to a density equivalent to 4300 plants per hectare, this result compared to other techniques, shows muvuca as a seeding technique viable direct for the area under study. However, further studies using higher seed densities of cover species are necessary to control the invasive grass Urochloa brizantha effectively


INTRODUCTION
In Brazil, agricultural activity has converted complex ecosystems into cultivated areas, areas that, in large part, have been degraded as a result of the adoption of inappropriate practices. According to data from Embrapa (2019), the country has approximately 200 million hectares of pasture, and more than half of this land extension has some level of degradation, and most of the land is already at an advanced stage of degradation. Currently, Minas Gerais is the state with the most deforestation of the Atlantic Forest in Brazil. In this state, livestock occupy large tracts of land without proper management (ROCHA JÚNIOR et al., 2017).
Although there is a great loss of forest area with the advancement of agriculture, the greatest threat to ecological restoration has been biological invasion, mainly by African grasses, and the changes brought about in the fire regime . Invasion by exotic species represents a problem worldwide; however, some territories are more susceptible to invasion than others (MARCHANTE, 2011).
The Urochloa genus, which includes some of the most aggressive invasive grasses, has favorable characteristics that contribute to its invasion in Brazilian biomes, among which the following stand out: high photosynthetic efficiency and use of nutrients, high rates of growth, regrowth, and regeneration, high reproductive efficiency because of the fast reproductive cycle, and intense production of seeds with high viability (DAMASCENO et al., 2018).
The species Urochloa brizantha, which is widespread in the study region, alters the intrinsic and extrinsic characteristics of the combustible material, retaining moisture, and increasing the vertical distribution of the combustible material, which are characteristics that alter the fire regime in the areas where this species occurs (GORGONE-BARBOSA et al., 2015). Additionally, its high biomass acts as a barrier for the arrival of propagules and light in the soil, which consequently prevents or reduces the establishment of native species from natural regeneration or causes high mortality of seedlings of tree species used in planting for forest restoration in these areas.
Faced with the challenges of restoring pasture areas, it is necessary to use effective and low-cost techniques, as well as to know when and where to use them.
Among the techniques that show promise in ecological restoration, direct sowing stands out (PALMA; LAURANCE, 2015), which is a cheap and versatile procedure. In addition to reducing costs, it eliminates the entire seedling production phase in a nursery and can be carried out in areas with difficult access and significant declivity.
There is also the possibility of mechanization for use in large areas (CAMPOS-FILHO et al., 2013), as long as they present a flat relief.
One way to use direct seeding is muvuca, which consists of mixing different species of native seeds together with some substrate (earth, sand and sawdust, for example), avoids the separation between pioneers and other non-pioneer species (SILVA et al., 2015). To accelerate the process of soil covering and to create the microclimatic conditions necessary for the development of native species, seeds of non-invasive annual exotic species, such as pork beans, pigeon peas, and/or crotalaria, are added to the mixture (RAUPP et al., 2020).
Although the muvuca technique has been standing out in some regions of the country, it is still a little known technique, despite its high potential. In the literature, studies using this restoration technique are scarce, especially in the biome under study.
Therefore, the goal of the present study was to evaluate the potential of direct sowing through direct seeding in the ecological restoration of a pasture of Urochloa brizantha. To improve the technique in the region, the influence of different substrates associated with seeds was tested along with the overcoming of dormancy in seed germination and initial seedling development.

Characterization of the study area
The experiment was implemented in an area of deactivated pasture located at Fazenda Pitangueiras, belonging to the Anglo American mining company. The farm is  (AYOADE, 1996). The soils occurring at the site are the Cambisols, occurring in relief, varying from wavy to mountainous.

Implementation and conduct of the experiment
The experiment was conducted during the rainy season in December 2017.
The experiment was a randomized block with eight treatments and four replications each. Four blocks were implanted along a gentle slope, each containing nine plots of 4 m × 4 m (16 m 2 ) with 1 m spacing between them.
About three days before sowing, the soil was prepared by plowing and harrowing the area. Subsequently, direct broadcast sowing was carried out manually of the mix of seeds of tree species belonging to different ecological groups (pioneers, early secondary and late secondary) occurring in forest remnants in the study region, together with cover herbaceous seeds or green adubation. The seed density used in the composition of the mix was six seeds/species/m 2 . For treatments that presented dormancy break (T1, T3, T5, and T7), this was done only for the species that presented this requirement ( Table 1). The exotic species Tamarindus indica was added to the seed mixture to act as a bait plant to avoid damage caused by the attack of leafcutting ants on the other species used. According to the Instituto Socioambiental (ISA) the species is consumed by ants, which is preferred over seedlings of other species. Therefore, tamarind seeds are used in the seed mixture to compose the mixture that will serve as bait to attract ants and, thus, interrupt the consumption of the seedlings of the species of interest.
The seeds were collected from matrices located on the premises of the Anglo American mining company in the year before the implementation of the experiment, except for the species Cordia trichotoma, Handroanthus ochraceus, and Machaerium nyctitans, which were collected about six months before the experiment. Seeds were beneficiaries and stored later. The seeds of the following species: Cassia ferruginea, Anadenanthera colubrina, Piptadenia gonoacantha, Mabea fistulifera, Senegalia polyphylla, and Tamarindus indica remained stored in a cold chamber at the Center for Propagation of Forest Species (CIPEF) at UFVJM, at a temperature of 6 °C and relative humidity of 40%.  (2020) The variables evaluated were emergence, survival, establishment, growth of tree species, and soil cover by Urochloa brizantha and herbaceous species. In the first three months, the evaluation was carried out every two weeks, and after that, it was conducted monthly. The seedlings were marked with a numbered plate; thus, enabling their identification and counting for emergence and survival. The evaluation of the increase in height (cm) and diameter (mm) of the seedlings began 184 days after the implementation of the experiment (End of rainy season), using a measuring tape and a digital caliper, respectively. The growth evaluation was carried out every three months until 14 months after the implementation of the experiment.
The Braun-Blanquet scale was used to assess the Urochloa brizantha coverage in the plot and the coverage of herbaceous species, which is a subjective measure that visually estimates the coverage in percentage values (BRAUN-BLANQUET, 1979).
Values from 1 to 5 were assigned to cover species in the area, where 1: individuals covering less than 5% of the plot, 2: individuals covering 5%-25% of the plot, 3: individuals covering 25%-50% of the plot, 4: individuals covering 50%-75% of the plot, and 5: individuals covering >75% of the plot.

Data analysis
Data were tested for normality and homogeneity and, when necessary, transformed according to Box and Cox, using the MASS package (VENABLES; RIPLEY, 2002), and then submitted to analysis of variance (ANOVA), with trough of the package ExpDes.pt . The significance of the data was detected (p < 0.05), and the Tukey's test was performed to compare the means. The data were analyzed with the aid of the statistical program R version 3.5.0 (2018).

Emergency
Of the 21 tree species sown in the area, only seven emerged. These include The ANOVA of the emergency data did not result in a statistically significant difference between the treatments tested (p > 0.05). However, the descriptive analysis of the data showed that the treatments in which dormancy breaking techniques were not applied obtained slightly higher averages than the treatments in which the techniques were applied. As for the tree/herbaceous association, the treatments with this association showed higher averages in comparison with treatments using only tree species. The substrate factor of the seed mixture resulted in the highest averages for the treatments in which sawdust was used.
The species Tamarindus indica had the greatest emergence, followed by Piptadenia gonoacantha. The species with the least emergence was Anadenanthera colubrina.

Plant survival
The overall average survival of the plants in the treatments was 82% and 54% at 7 and 14 months, respectively. The ANOVA of the data showed statistically different averages at 5% significance for the treatments tested at 7 months ( Figure 1A), which were not significant at 14 months ( Figure 1B). In where: *Bars followed by the same letters do not differ based on Tukey's test at 5% significance.
Tamarindus indica had a mortality rate higher than 90%, due to herbivory of leaf-cutting ants. It is noteworthy that this was the only species to be attacked by ants during the experiment. This fact converges with the observation of the local population regarding the attraction of the species to ants, which motivated its choice to be part of the seed mix used for the restoration of the area. Survival rates ranged from 24% to 64% for Cassia ferruginea and Anadenanthera colubrina, respectively. Piptadenia gonoacantha had the highest number of plants per plot, followed by Mabea fistulifera (Table 2).

Establishment of plants
The

Growth and increment of established plants
At 14 months after the implementation of the experiment, the plants showed an average growth of 1.5 mm in diameter and 11.5 cm in height. The ANOVA did not result in a significant difference between the treatments tested for height, diameter, and increment (p > 0.05). The descriptive analysis of the data showed the highest averages for the treatments in which sawdust was used as a substrate for seed release, especially for the averages of the height variable (Table 3). The species with the highest growth and overall average increase was Mabea fistulifera, and the species with the lowest growth was Cassia ferruginea (Figure 3). In where: *Bars followed by the same letters do not differ based on Tukey's test at 5% significance.

Covering of Urochloa brizantha and herbaceous species used for sowing
Three months after the implementation of the experiment, the area had approximately 50% of its coverage composed of Urochloa brizantha, which persisted until the end of the evaluations of the experimental area. In the same period of three months, the herbaceous species sown in the plots reached a maximum coverage of 25%, and at six months, the total area already had more than 75% of grass cover. Among the cover species, Canavalia ensiformes was the one that stood

DISCUSSION
Only seven of the 21 species used for sowing emerged in the study area.
However, the estimated population density of individuals in treatments T8 (1.93 plants m -2 ) and T6 (1.87 plants m -2 ) showed the highest plant averages established at the end of 14 months. The seeds that did not germinate were precisely the seeds stored under controlled conditions of temperature and humidity, which may have affected their viability. Six of the seven germinated species were stored in a cold chamber at the CIPEF at UFVJM.
The ANOVA of the data carried out at 7 months showed different averages at 5% significance, however, for the seedling emergency data at the end of 14 months, there were no significant differences between the treatments tested. Some authors have obtained better conditions for establishing tree species in response to the decrease in competition between native and invasive exotic species resulting from the consortium between tree species and herbaceous cover species (NUNES et al., 2015;. Beltrame and Rodrigues (2008) obtained better results with respect to survival and increase in the basal area of native species in consortium with Cajanus cajan; however, it was observed that the positive results of this consortium depended on the planting density of Cajanus cajan and the length of the intercropping. As in this study, the coverage of herbaceous species did not exceed 25% of the plots, and the absence of influence of this treatment may be related to the low sowing density of these species in the plots.
Results similar to those found in the present study were obtained by Pietro-Sousa and Silva (2014), who studied the sowing of seed mixture for ecological restoration of degraded permanent preservation areas. These authors did not find any influence of Canavalia ensiformis as a cover plant on the emergence and survival of native species in the evaluated plots. Alves (2016), when testing different densities of Stylosanthes capitata and Stylosanthes macrocephala, found no differences in the density of native planted species.
Another treatment evaluated in the present study was of breaking the dormancy of the seeds of the species for which this was needed. It was observed that of the nine species that received dormancy-overcoming treatments, only Cassia ferruginea and Tamarindus  The descriptive analysis of the data, in general, revealed higher averages for the treatments in which sawdust was used as an aggregating material for the release of the seeds. In the mixture, the seed mixture must be homogenized with some aggregating material to avoid stratification by size and uneven distribution of the seeds. Sawdust may have contributed to a better spreading of the seeds in the area because of its lower density, thus reducing the initial competition and increasing the emergence, survival, and growth of the seedlings in comparison with the treatments in which clay was used.
According to the analysis of the species, Tamarindus indica was the one that had the greatest emergency. However, it is an exotic species used by the Instituto Socioambiental (ISA) to minimize attacks by leaf-cutting ants on newly germinated seedlings in the field.  Cava et al. (2016), comparing different techniques for restoring vegetation in abandoned pasture areas, found that direct seeding promotes a density of species higher than the treatments of assisted natural regeneration, passive restoration and planting of seedlings.
The average growth of species in height was 11.5 cm and 1.45 mm in diameter.
Similar results were obtained by Souza (2013), evaluating the direct seeding of native forest species, such as alternative ecological restoration. Resende and Pinto (2013) evaluating seed mixture in a degraded area in southern Minas Gerais obtained similar averages in some of the species used, as well as Soares (2007) evaluating the effect of rhizobia inoculation in native species by direct seeding. These values are considered satisfactory for cerrado areas (SILVA et al., 2015), however, the area in question is an area of Atlantic forest.
The species Mabea fistulifera showed the highest growth values, reaching 22 cm in height and 2 mm in diameter. Gomes Júnior (2011) studying the growth of the species on different substrates and levels of shading concluded that the initial growth of Mabea fistulifera seedlings is favored by the level of shading. What may have given prominence to this species in this study, since the plots showed a considerable percentage of coverage of the invasive grass Urochloa brizantha throughout the growth evaluation. The strong presence of this grass may have interfered with the growth of the other species, keeping low values of height and mainly diameter. This is due to the competitive superiority of grasses over native ones, explained by their greater ability to retain resources (TECCO et al., 2010), and in the efficiency of the use of nutrients even in areas with low availability of these resources (FUNK, 2013).

CONCLUSIONS
The treatments to overcome dormancy, the association of tree and herbaceous species, and substrate of mixture did not affect emergence, survival, and initial growth of seedlings.
The species with the greatest establishment in the study area were Piptadenia gonoacantha, Mabea fistulifera, Dalbergia nigra, and Senegalia polyphylla. These species are recommended for use in ecological restoration projects through the use of seed mixture.
The seed mixture proved to be a viable technique for the recovery of the study area, presenting several surviving individuals in the evaluated period.
The growth of the species was slow because they suffered interference from competitiveness with the exotic grass Urochloa brizantha, which remained in the area until the end of the evaluation, and its control was not effective. Therefore, it is necessary to carry out further studies by applying a higher density of seeds of the cover species used in the area.