Can mulch be effective in controlling exotic grasses and promoting natural regeneration in ecological restoration?

Bruno Santos Francisco brunofrancisco@estudante.ufscar.br

Universidade Federal de São Carlos, Brasil

Felipe Bueno Dutra

Universidade Federal de São Carlos, Brasil

Emerson Viveiros

AES Brasil, Brasil

Lausanne Soraya de Almeida

Universidade Federal de São Carlos, Brasil

Matheus Fontes Souza

Universidade Federal de São Carlos, Brasil

Paulo Cesar Souza Filho

Ceiba Consultoria Ambiental, Brasil

José Mauro Santana da Silva

Universidade Federal de São Carlos, Brasil

Universidade Federal de São Carlos, Brasil

Fatima Conceição Márquez Piña-Rodrigues

Universidade Federal de São Carlos, Brasil

Universidade Federal de São Carlos, Brasil

The need to restore degraded ecosystems has generated international targets to restore 350 million hectares of native forest by 2030 (Dave et al., 2019), supporting the United Nations - UN resolution that considers the period 2021-2030 the decade of ecosystem restoration (United Nations Organization [UNO], 2019). Ecological restoration is defined as the assisted recovery of an ecosystem that has been degraded or destroyed (Society for Ecological Restoration International [SER], 2004). It is a practice that aims to return to the recovered environment the ecological processes linked to resilience, resistance, and stability (Clewell & Aronson, 2007), which will ensure the continuity of ecological processes.

In Brazil, the goal of restoring 12 million hectares by 2030 was established (Brasil, 2015); however, the areas intended for forest restoration are highly uncharacterized from the point of view of native species composition, represented mostly by degraded pastures (Rodrigues, Brancalion, & Isernnhagen, 2009). Many of these areas to be recovered have coverage of exotic and invasive plants that hinder the establishment of native species, becoming a major cause of failure of ecological restoration (Cornish & Burgin, 2005).

Regardless of the occurrence and interference of exotic plants, measures should be adopted for suppressing the growth and or reducing the number of these plants in the area to be restored until the coexistence with native species (Gazziero, Vargas, & Roman, 2004). The control can be done by applying preventive, biological, cultural, physical, and chemical methods, with the integration between different methods, considered as an effective way to control exotic plants (Silva, Ferreira, Ferreira, & Santos, 2009; Brighenti & Oliveira, 2011).

The physical method uses an agent capable of affecting the survival of the exotic plant community by preventing light radiation from reaching the soil surface (Silva et al., 2009). Mulching acts via shading and soil smothering due to the dead plant biomass from management practices, such as weeding and desiccation by herbicide (Almeida, 1988; Chen et al., 2014; Kader, Senge, Mojid, & Nakamura, 2017). The term mulch is defined as the use of any material, other than the soil itself, that will perform the function of permanent or semi-permanent protective cover over the soil surface (Jordán, Zavala, & Muñoz-Rojas, 2011).

The use of mulching in agriculture according to McCalla and Army (1961) has been known since 1802 as the practice of spreading plant remains such as dry leaves and straw on the soil to prevent erosion and water loss (Jordán et al., 2011), as well as to reduce the weed competition in agriculture with unwanted plants (Chopra & Koul, 2020). Although it is an old practice, it is considered essential for soil conservation (Prosdocimi, Tradolli, & Cerdá, 2016; Kader et al., 2019; Li, Li, & Pan, 2020).

Mulch management is employed in agriculture associated with conservationist practices such as no-till farming (Kader et al., 2017). It aims to inhibit the growth of C4-plants that are highly dependent on light, thus decreasing photosynthetic activity and primary production (Almeida, 1988). Mulching also has an effect on the seeds of some weeds (Mayer & Poljakoff-Mayer, 1989) by decreasing germination rate, loss of vigor, increased seedling mortality, leaf chlorosis, reduced tillering, and root stunting (Sarrantonio & Gallandt, 2003; Gallandt, 2006), as well as potential allelopathic processes from mineralization of plant residue (Theisen, Vidal, & Fleck, 2000).

In addition to weed competition control, mulching has a positive impact on increasing the height of agricultural and forest plants (Chaudhry, Malik, & Sidhu, 2004; Dostálek, Weber, Matula, & Frantík, 2007) and on protecting soil against erosion from post-fire factors in forest areas (Díaz-Raviña et al., 2021). Mulching also improves soil quality (Chen et al., 2014) and promotes seedling establishment in restoration (Benigno, Dixon, & Stevens, 2013; Mollard, Naeth, & Cohen‐Fernandez, 2014; Breton, Crosaz, & Rey, 2016). In addition, it reduces the emergence of invasive plants such as Urochloa decumbens (Stapf) R.D. Webster (Silva & Vieira, 2017) and contributes to moisture retention and control of temperature fluctuation (Resende et al., 2005) and reduction of soil erosion by rainfall (Montenegro, Abrantes, Lima, Singh, & Santos, 2013;Li et al., 2020).

Despite the already known benefits in agriculture, this practice has been more adopted in the restoration of mining areas (Silva & Corrêa, 2008; Salomão, Júnior Brienza, & Rosa, 2014) or degraded soils (Schirone, Salis, & Vessella, 2011), and in studies on direct seeding (Silva & Vieira, 2017). However, there is still little research focused on the impact of mulching, particularly with regard to active restoration. Given the need to promote conservationist methods to control weed competition and verify its effects on native species, the objective of the study was to analyze the use of mulching and its impact on the community of regenerating plants and its contribution to the control of exotic grasses in an area under ecological restoration.

A total of 42 seedlings (8,400 seedlings ha-1) of eight species from seven botanical families were sampled, leaving only a single unidentified individual. The Jaccard index was 0.33, indicating that the similarity between treatments was low. The most abundant species in the treatment with mulch were Schinus terebinthifolia Raddi (14 individuals; 33%) followed by Solanum mauritanum Scop. (11 individuals; 26.2%) and Platypodium elegans (3 ind; 7.1%). The most abundant species in the control treatment with exposed soil (SM) were P. elegans and S. terebinthifolia, both with three individuals (Table 1). There was a significant difference in abundance between treatments by Mann-Whitney test (p <0.0001) (Figure 2).

Table 1. Floristic list of the species sampled in the regenerating seedling community in the ecological restoration area, in the municipality of Itapira, state of São Paulo. CM = number of seedlings sampled in the treatment with mulch; SM = number of seedlings sampled in the control treatment with exposed soil, Sucessional Class = S. Class, Dispersion Syndrome = D. Synd., ANE = anemochoric, ZOO = zoochoric, and AUT = autochoric. NID = Not identified.

Figure 2. Box-plot of the median and quartiles of the abundance of individuals of the seedling community sampled in the 100 plots of each treatment in the ecological restoration area, in the municipality of Itapira, state of São Paulo. Legend: MU = number of seedlings found in the treatment with mulch, CO = number of seedlings found in the control treatment with exposed soil. Mann-Whitney test (p <0.0001).

According to successional class, 40 seedlings (96% of the individuals) were classified as pioneer, one seedling was classified as non-pioneer, and one seedling was not identified (Table 1). In the treatment with mulch, 32 seedlings (94%) were classified as pioneer, one seedling (3%) was classified as non-pioneer, and one seedling (3%) was not identified. In the exposed soil treatment, we observed only pioneer species (nine individuals).

The most abundant dispersal syndrome was zoochoric with 31 seedlings (73% of the individuals), followed by anemochoric, with nine seedlings (21%), and autochoric with one seedling (2%) and one unidentified individual (2%). In the treatment with mulching (MU), zoochoric (76% of seedlings) and anemochoric (18% of seedlings) dispersions predominated, and one seedling (3%) was not identified (Figure 3). In the control treatment with exposed soil (SM) zoocoria (63% of the seedlings) was also dominant, but less abundant than in the area with mulching, and the same was observed for anemochoric (37% of the seedlings). Among the zoochoric species, ornithocory was dominant with 18 individuals (42.3%) represented by the species S. terebinthifolia and E. uniflora.

Figure 3. Number of individuals per dispersal syndrome in the seedling community sampled in the ecological restoration area, in the municipality of Itapira, state of São Paulo. Legend: MU= treatment with mulch, CO = treatment with exposed soil. ANE - anemochoric; AUT - autochoric; ZOO - zoochoric; NID - not identified.

We observed the presence of exotic grass cover in 80 plots (40%) and 120 plots (60%) free of exotic grasses. In the treatment with mulch the representative presence of exotic grasses occurred in only 13 plots (13%) and in 87 plots (87%) the presence of exotic grasses was not recorded. In the control treatment with exposed soil, we found 67 plots (67%) with exotic grasses and 33 plots (33%) without exotic grasses (Figure 4). There was a significant difference in the coverage of exotic grasses between treatments by the Mann-Whitney test (p <0.0001) (Figure 5). The chi-square test evidenced the difference between the frequency of seedlings in the different mulching height classes (Figure 4) with a value of p = 0.009.

Figure 4. Histogram of the presence and absence of seedlings in different height classes of the mulch in the evaluated plots in the ecological restoration area, in the municipality of Itapira, state of São Paulo. Legend: C1 = class 1, from 5 to 21 cm; C2 = class 2, from 21 to 37 cm; C3 = class, 3 from 37 to 53 cm; C4 = class 4, from 53 to 69 cm; C5 = class 5, from 69 to 85 cm; and C6 = class 6, from 85 to 101 cm.

Figure 5. Median graph and quartiles of the coverage of exotic grasses sampled in the plots assessed in the ecological restoration area, in the municipality of Itapira, state of São Paulo. Legend: MU = coverage of exotic grasses in percentage found in the treatment with mulch, CO = coverage of exotic grasses in percentage found in the control treatment with exposed soil. Mann-Whitney test (p <0.0001).

In this study, the use of the mulching technique influenced the regeneration of native species, because the treatment with mulching (MU) showed higher abundance (80.9%) and species richness (87.5%) compared to the one with exposed soil. Areas in restoration process with strong presence of exotic grasses present negative impacts on the establishment of herbs, shrubs, and trees (Ferreira, Walter, & Vieira, 2015). Moreover, weed competition can interfere with the composition of naturally regenerating communities (Damasceno et al., 2018), and with the growth and survival of native species seedlings, as observed by Passaretti et al. (2020). We should point out that the greater presence of zoochoric species sampled indicate the possibility of mulch on the ground favoring germination niches for species disseminated by fauna, which was evident with the higher proportion of seedlings with zoochoric dispersal syndromes, mostly ornithochoric. These species tend to have small colored seeds, with more rounded shapes (Gagetti, Piratelli, & Piña-Rodrigues, 2016), which have germination favored by the mulch (Silva & Vieira, 2017). Despite the lower incidence of light on the ground, the desiccated mulch may have provided the maintenance of humidity and ideal temperature for germination of these species, which require temperatures below 30°C (Wielewicki, Leonhardt, Schlindwein, & Medeiros, 2006).

There was low similarity in floristic composition between both treatments, noting the negative effect of the presence of exotic grasses on the composition of the regenerating seedling community. The mulching technique provided higher species richness, a result that was also observed by Doležal et al. (2011), who verified the effect of mulching on the functional diversity of plant species in Prairie regions of Central Europe. Thakur, Singh, Jawandha, and Kaur (2012) observed similar results where mulch was efficient in controlling weeds in an agricultural production area. The use of U. decumbens (Stapf) R. D.Webster's own mulch can keep the temperature, as well as the moisture in the soil (Resende et al., 2005; Montenegro et al., 2013) in ideal conditions for native plants caused by increased microbial activity and decomposition rates of organic matter in the system, adding nutrients to the soil (Athy, Keiffer, & Stevens, 2006). The increment of these conditions and resources in the restored area can promote an increase in the survival and establishment of regenerating native plants (Benigno et al., 2013; Mollard et al., 2014; Breton et al., 2016).

We observed that the mulch decreased the emergence of exotic grasses (13%) in relation to the control treatment with exposed soil (67%). There was an 80.6% reduction in the presence of exotic grasses in the treatment with mulch in relation to the control (CO), bringing evidence as to the efficiency of this simple technique for the control of invasive exotic grasses. This is an important fact, because exotic grasses present competitive superiority over native grasses in relation to resource retention (Levine et al., 2003;Tecco, Di, Cabido, & Urcelay, 2010) and in the efficiency of nutrient use even in areas with low availability of these resources (Funk, 2013). Furthermore, grasses of the species U. decumbens have allelopathic effects, inhibiting the germination of native species (Barbosa, Pivello, & Meirelles, 2008). The reduction in exotic grasses may have occurred because of the decrease in light entry caused by the mulch barrier, a condition that, for the seeds of exotic grasses, affects their germination rate, vigor loss, and increased seedling mortality, resulting in leaf chlorosis, reduced tillering, and root stunting (Sarrantonio & Gallandt, 2003; Gallandt, 2006.).

Another factor besides the control of weed competition was the height of the mulch on the ground. Our results showed that there is an ideal height of desiccated mulch in the range of 21 to 37 cm, in which we observed a higher frequency of regenerating seedlings. Therefore, it is not enough to propose the practice of mulching with the grass itself after its desiccation; it is also important to control the height of the layer.

The practice of maintaining the mulch to provide nutritional, moisture, and light conditions, and controlling its height should be added to the post-planting management to reduce the risk of invasion of exotic grasses and ensure the development of seedlings arising from natural regeneration (Kader et al., 2019). Our results reinforce the potential of using mulch as a conservationist technique to control exotic grasses that can be complementary to the application of chemical herbicides, provided that the conditions of layer height and initial soil management are maintained.

When compared to other exotic grass control methods, such as herbicide use, manual mowing (Thaxton, Cordell, Cabin, & Sandquist, 2012), and topsoil removal (Buisson et al., 2008), mulching can be considered an adaptive management practice that contributes to reducing inputs and controlling competition while providing crucial conditions for the establishment and survival of naturally regenerating seedlings.

In this study, we verified that the mulching technique with desiccated U. decumbens itself favored the natural regeneration of tree species and contributed to the control of exotic grasses. Furthermore, we observed that there is an ideal height of the desiccated mulch and should be between 21 and 37 cm high. We suggest that adaptive management practices such as the mulching technique can be recommended in the preparation stages in ecological restoration areas, as they favored seedling establishment and contributed an 80% rate of exotic grasses control.

The present authors would like to express their thanks to the company AES Brasil and CEIBA consultoria ambiental for providing all human and material resources for the field experiment. This study was partly financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES).