Use of Eucalytpus mixed with native species for initial reforestation in the Atlantic Forest

Introduction

Assuming that the increase in the CO₂ concentration in the atmosphere is the main cause of accelerating climate change, one of the first steps needed to mitigate climate change is to reduce the CO₂ concentration (Yamasaki, 2003Yamasaki A, 2003. An Overview of CO₂ Mitigation Options for Global Warming-Emphasizing CO₂ Sequestration Options. J Chem Eng Jpn 36(4): 361-375.
). Planting trees is often advocated by governments and non-governmental organizations as one of the simplest approaches for mitigating the increasing CO₂ concentration, as well as for generating strong public support (Kirschbaum et al., 2024Kirschbaum MUF, Cowie AL, Penuelas J, Smith P, Cotrufo MF, Conant RT, Sage RF, Brandão M, Co, Luo Y, Way DA, Robinson SA, 2024. Is tree planting an effective strategy for climate change mitigation? Sci Total Environ 909: July 2023.
). Thus, Brazil is committed to restoring approximately 21 million degraded hectares in permanent preservation areas (PPAs) and legal reserves on rural properties (Soares-Filho et al., 2014Soares-Filho B, Rajão R, Macedo M, Carneiro A, Costa W, Coe M, Rodrigues H, Alencar A, 2014. Cracking Brazil’s Forest Code. Science 344: 363-364.
; Brancalion et al., 2016Brancalion PHS, Garcia LC, Loyola R, Rodrigues RR, Pillar VD, Lewinsohn TM, 2016 A critical analysis of the Native Vegetation Protection Law of Brazil (2012): updates and ongoing initiatives. Braz J Nat Conserv 14: 1-16. https://doi.org/10.1016/j.ncon.2016.03.004
).

Nevertheless, in order to achieve the proposed bold goals, it is necessary for the projects to be economically attractive since these kinds of projects have high costs (Brancalion et al., 2019Brancalion PHS, Meli P, Tymus JRC, Lenti FEB, Benini RM, Paula A, Silva M, Isernhagen I, Holl KD, 2019. What makes ecosystem restoration expensive? A systematic cost assessment of projects in Brazil. Biol Conserv 240: 108274. https://doi.org/10.1016/j.biocon.2019.108274
). The Brazilian Forest Code under law 12,651/2012 (Brazil, 2012Brazil, 2012. Law No. 12,651 of May 25, 2012. Establishes rules for native vegetation protection, Permanent Preservation Areas (APPs), and Legal Reserves. Official Gazette of the Federative Republic of Brazil, May 28, 2012.
) requires that 20% of the rural property area in the Atlantic Forest in Brazil be covered by native vegetation as a Legal Reserve (LR). However, Brazilian legislation allows rural landowners to exploit their Legal Reserve areas for economic gain provided that a sustainable management plan is prepared. A maximum of 50% of the planting can be performed with exotic species, and such plans must be approved by the governing body. Based on these requirements, different systems can be used to reduce implementation costs, such as the system presented by Amazonas et al. (2018Amazonas NT, Forrester DI, Oliveira RS, Brancalion PHS, 2018. Combining Eucalyptus wood production with the recovery of native tree diversity in mixed plantings: Implications for water use and availability. For Ecol Manage 418: 34-40. https://doi.org/10.1016/j.foreco.2017.12.006
, 2021)Amazonas NT, Forrester DI, Silva CC, Almeida DRA, Oliveira RS, Rodrigues RR, Brancalion PHS, 2021. Light and nutrient-related relationships in mixed plantations of Eucalyptus and a high diversity of native tree species. New For 52(5): 807-828. https://doi.org/10.1007/s11056-020-09826-x
. In these systems, Eucalyptus spp. are planted with a density of up to 50% coupled with a high diversity of native tree species, representing a compromise between production and restoration (Amazonas et al., 2018Amazonas NT, Forrester DI, Oliveira RS, Brancalion PHS, 2018. Combining Eucalyptus wood production with the recovery of native tree diversity in mixed plantings: Implications for water use and availability. For Ecol Manage 418: 34-40. https://doi.org/10.1016/j.foreco.2017.12.006
).

The use of Eucalyptus spp. is strategic because it reduces implantation costs, as the cost of Eucalyptus spp. seedlings is low and these plants have high plasticity, resistance to drought, high silviculture technology, and a well-established economic market (Silva, 2017Silva CC, 2017. Impacto ecológico e silvicultural do uso e colheita de eucalipto consorciado com espécies arbóreas nativas para a restauração da Mata Atlântica. USP / Escola Sup Agric Luiz de Queiroz.
). The use of Eucalyptus spp. trees may also favour understorey regeneration, as observed in other studies (Brockerhoff et al., 2013Brockerhoff EG, Jactel H, Parrotta JA, Ferraz SFB, 2013. Role of eucalypt and other planted forests in biodiversity conservation and the provision of biodiversity-related ecosystem services. For Ecol Manage 301: 43-50. https://doi.org/10.1016/j.foreco.2012.09.018
; Pryde et al., 2015Pryde EC, Holland GJ, Watson SJ, Turton SM, Nimmo DG, 2015. Conservation of tropical forest tree species in a native timber plantation landscape. For Ecol Manage 339: 96-104. https://doi.org/10.1016/j.foreco.2014.11.028
; Wu et al., 2015Wu J, Fan H, Liu W, Huang G, Tang J, Zeng R, Huang J, Liu Z, 2015. Should Exotic Eucalyptus be Planted in Subtropical China? Insights from Understory Plant Diversity in Two Contrasting Eucalyptus Chronosequences. Environ Manage, August. https://doi.org/10.1007/s00267-015-0578-x
). However, the use of Eucalyptus spp. may also have some noteworthy disadvantages, such as the possibility of competition with native species and the potential to deplete soil nutrients in the long term. Therefore, it is important to carefully evaluate each project and understand the growth dynamics of species belonging to different ecological groups intercropped with Eucalyptus spp. and their influence on the recovery of degraded areas.

Thus, the objective of this study was to evaluate the growth of native and exotic species, particularly the Eucalyptus genus, for restoring an Atlantic Forest region in Brazil, as well as the carbon stock potential in the aboveground (trunk/canopy) and belowground (root) biomass of the trees. Although Amazon et al. (2018)Amazonas NT, Forrester DI, Oliveira RS, Brancalion PHS, 2018. Combining Eucalyptus wood production with the recovery of native tree diversity in mixed plantings: Implications for water use and availability. For Ecol Manage 418: 34-40. https://doi.org/10.1016/j.foreco.2017.12.006
found a significant effect on the introduction of the Eucalyptus genus in restoration studies, our focus is to quantify its influence on the growth of native species at different planting densities up to 50% of the planting density, as permitted by Law 12,651/2012 (Brazil, 2012Brazil, 2012. Law No. 12,651 of May 25, 2012. Establishes rules for native vegetation protection, Permanent Preservation Areas (APPs), and Legal Reserves. Official Gazette of the Federative Republic of Brazil, May 28, 2012.
). We hypothesise that: (i) the growth and survival of native species will be minimally affected by incorporating different Eucalyptus urophylla x Eucalyptus grandis hybrid densities in the area; (ii) E. urophylla x E. grandis hybrid trees will perform better in treatments with a greater proportion of native species. Despite being a fast-growing species and a strong competitor, Eucalyptus hybrids may exhibit enhanced growth in such conditions due to reduced intraspecific competition.

Material and methods

The experiment was implemented at Fazenda da Lagoa, which is part of the Federal University of Lavras (UFLA) in the municipality of Santo Antônio do Amparo, Minas Gerais (20° 56’40”S, 44°55′8″W), at an elevation of approximately 1,000 m. According to the Koppen climate classification system, the region has a Cwa climate type, which is characterised by tropical and subtropical rainy mesothermic conditions with dry winters and rainy summers (Alvares et al., 2013Alvares CA, Stape JL, Sentelhas PC, Gonçalves JLM, Sparovek G, 2013. Koppen’s climate classification map for Brazil. Meteorol Z 22(6): 711-728. https://doi.org/10.1127/0941-2948/2013/0507
). The average annual temperature is 19.8°C, and the average annual rainfall is between 1400 and 1700 mm. The soil was characterised as a Dystrophic Oxisol (LVd in the Brazilian soil classification system). The region is composed of a mosaic of Brazilian savanna (Cerrado) and Atlantic Forest phytophysiognomies and is in an agricultural zone. Previous land uses consisted of coffee plantations and pasture.

An E. urophylla x E. grandis hybrid and 18 native species were used for the present study (Table 1). The seeds of the native species were collected in the municipalities of Lavras, MG, and Mariana, MG, and the seedlings were germinated in the Forestry Nursery of the Department of Forest Sciences (UFLA) in the municipality of Lavras, MG, from June 2019 to February 2020. The climatic conditions during germination included an average temperature of 28.3°C and average monthly rainfall ranging from 8.6 to 190.2 mm. Tubes measuring 120 cm³ were used for germination. The E. urophylla x E. grandis hybrid seedlings were produced in 55 cm³ tubes and purchased from a commercial nursery located in the city of Lavras.

After sowing and germination, the native plants were allowed to germinate for 60 days in a shade house (with 50% irradiance and irrigation supplied by microaspersion three times a day for five minutes each at a flow rate of 140 L h^-1). The plants were subsequently transferred to full sun for hardening (irrigation four times a day for five minutes each, with a flow rate of 95 L h^-1) until being transferred to the field.

Leaf-cutting ants and weeds were periodically manually controlled in the planting area. Soil correction was performed by applying 2 Mg ha^-1 of calcitic limestone to supply Ca and Mg. The soil was prepared by harrowing the entire area. Planting was performed in March 2020, with 150 g of formulated NPK 06-30-06 fertilizer per hole. Next, topdressing fertilization was performed in two doses, with 100 g of formulated NPK 20-00-20 being applied each time. Boric acid was applied at a concentration of 10 g per plant in May 2021, which is the beginning of the dry season in the region.

The experiment consisted of a completely randomised block design with four replicates. A 3 × 19 factorial scheme was used, in which the first factor corresponded to the percentage of Eucalyptus plants relative to native species and the second factor was the species that composed the plantation. Two control treatments were also implemented for comparison: one with 100% native species and one with 100% Eucalyptus (Table 1). The plots consisted of nine rows with nine plants each, totalling 81 trees per plot at a spacing of 3 × 2 m (1,667 seedlings per hectare). Additionally, one row of native species was planted between the plot rows to serve as a border area. The arrangement of species in each treatment can be found in Fig. S1, in the supplementary materials.

The planting models were characterised by the proportions of native (pioneer and non-pioneer) and exotic (Eucalyptus) species. Control 1 (T1) had 100% native species; control 2 (T2) had 100% Eucalyptus; model 1 (M1) had 25% Eucalyptus per ha; model 2 (M2) had 33% Eucalyptus per ha; and model 3 (M3) had 50% Eucalyptus per ha.

The percentage of dead plants per species, height (H) and diameter at breast height (DBH) at 42 months after planting were evaluated. The diameters for trees with up to three branches at a height of five centimetres from the soil were converted into fused diameters, as described by Scolforo and Thiersch (2004)Scolforo JR, Thiersch CR, 2004. Biometria florestal medição, volumetria e gravimetria. UFLA/FAEPE.
. Then, the stem cross-sectional area (SA) of each tree was determined from the diameter based on equation (i).

In which: SA is the cross-sectional area (cm² tree¹) and DBH is the diameter at breast height (cm).

In turn, the model described by Binkley et al. (2020)Binkley D, Campoe OC, Alcarde C, Lorenzato R, Stape JL, 2020. Variation in whole-rotation yield among Eucalyptus genotypes in response to water and heat stresses: The TECHS project. For Ecol Manage 462: 117953. https://doi.org/10.1016/j.foreco.2020.117953
(equation (ii)) was used to obtain the trunk biomass of the Eucalyptus trees. This equation was developed for the clone investigated in this study, E. urophylla x E. grandis, across climatic gradients in Brazil.

In which: y is the biomass in kg tree^-1, DBH is the diameter at breast height (cm), and H is the height (m).

The stem biomass of the native species was calculated from equation (iii), which was obtained by (Luz, 2024Luz MS, 2024. Práticas silviculturais intensivas influenciam positivamente no estoque de carbono de florestas de restauração [Thesis]. Univ Fed Lavras (UFLA).
) for Atlantic Forest species across climatic gradients in Brazil

In which: B is the biomass (kg tree^-1), DBH is the diameter at breast height (cm), H is the height (m), and ρ is the density (g cm^-³).

The total biomass (B) was obtained using the biomass expansion factor BEF and Root-to-Shoot Ratio (R) values provided by the Intergovernmental Panel on Climate Change (IPCC). The BEF and R values for Eucalyptus were 1.20 and 0.35, respectively, while the BEF and R values for native species were 3.16 and 0.37, respectively. Total carbon was calculated by multiplying the total biomass by 0.47 (IPCC, 2006IPCC, 2006. Guidelines for National Greenhouse Gas Inventories. http://www.ipcc.ch.
).

The natural regeneration of the plantation was also evaluated at 42 months by counting each species and determining their occurrence frequencies. Three subplots were established in the upper left corner, centre and lower right corner of each plot. A quadrant of 1 m² was used, and all individual trees greater than 0.5 m in height were counted.

Next, we assessed the normality of the data using the Shapiro-Wilk test at a 5% significance level to evaluate the overall and individual growth potentials of the species in the planting models. An analysis of variance (ANOVA) was conducted following the normality test. When significant differences were detected, the means of the planting models were compared using the Tukey’s test at a 5% significance level, while the means of the species were compared using the Scott-Knott test (5%). Furthermore, the native species and Eucalyptus species groups were analysed separately using ANOVA to better understand the growth behaviours of the different measured variables. The means were again compared using the Tukey’s test when significant differences were identified (5%).

Results

Influence of Eucalyptus on the growth of native species

 ⌅

The treatments under study exhibited a wide range of values for the analysed growth parameters (Fig. 1). We observed that the highest absolute values among the species groups were for Eucalyptus for all variables. The highest values (except for the mortality rate) were observed in treatment M2, specifically: 19.66 m for height, 21.8 cm for diameter at breast height (DBH), 0.03 m² tree⁻¹ for stem cross-sectional area (SA), 216.71 Mg tree⁻¹ for biomass (B), and 101.85 Mg tree⁻¹ for carbon (C). The treatment with the highest average mortality rate was M3, at 20.7%.

Figure 1.  Means and range values found for height (H), diameter at breast height (DBH), stem cross-sectional area (SA), total biomass (B) and total carbon (C), and percentage of dead plants (M) for Eucalyptus and native species in an Atlantic Forest restoration in Brazil at 42 months after planting.

The interaction between species and planting model had no effect on the height, DBH and plant mortality variables (Fig. 2). The species which showed the greatest increases in height was Eucalyptus (18.4 m), followed by Solanum granulosoleprosum (6.6 m); on the other hand, the lowest height was observed for Psidium guajava, with a value of only 2.3 m. The species with the greatest DBH was Eucalyptus (15.9 cm), followed by Enterolobium contortisiliquum (8.6 cm) and S. granulosoleprosum (8.5 cm); no significant differences were observed between the means of the latter species. The species with the highest plant mortality percentages were: Apuleia leiocarpa (41.7%), Citharexylum myrianthum (45.1%) and Guazuma ulmifolia (49.6%).

Figure 2.  Means found for height (H), diameter at breast height (DBH) and percentage of dead plants (M) for Eucalyptus and native species in an Atlantic Forest restoration in Brazil at 42 months after planting. The means followed by the same letter in the column do not differ from each other according to the Scott-Knott test at the 5% probability level. Averages followed by an asterisk (*) indicate values less than 0.009.

Significant interactions between the planting models and the studied species were found for the SA, B and C variables (Fig. 3). Regardless of the variable analysed, the greatest values were found for Eucalyptus. The highest values for the native species were generally found in models M2 and M3. S. granulosoleprosum and E. contortisiliquum outperformed all the other native species and had similar growth rates (Fig. 3), with the planting models having different effects on the variables. The M1 model showed the best performance for E. contortisiliquum, with SA of 0.83 m² ha^-1 and B and C of 5.06 and 2.63 Mg ha^-1 respectively. Similar behaviour was observed for E.contortisiliquum. In contrast, the smallest increases for all variables were observed in Sapindus saponaria and P. guajava.

Figure 3.  Means of the interactions between species and planting models for stem cross-sectional area per tree (SA), total biomass (B) and total carbon (C) in an Atlantic Forest restoration in Brazil at 42 months after planting. Capital letters represent the species and lowercase letters represent the planting models. Means followed by the same letter in the same column did not differ from each other according to the Scott-Knott test at the 5% error probability level. Averages followed by an asterisk (*) indicate values less than 0.009.

Growth of species groups

 ⌅

A significant effect on DBH, AS, B and C was observed for the native plants, with the M3 treatment showing worse growth, with averages of 2.8 cm, 1.2 m² ha^-1 7.3 Mg ha^-1 and 3.4 Mg ha^-1, respectively (Table 2). In addition, the mean mortality of the seedlings differed, with the highest mortality level occurring in M3 (32.0%). Moreover, DBH, SA, B and C differed significantly between the planting models for Eucalyptus, while the values in the M1 and M2 models did not differ from each other for DBH (16.45 cm), and M2 and M3 for SA, B and C, with averages 11.8 m² ha^-1 and 62.2 and 29.2 Mg ha^-1, respectively.

Table 2.  Means of height (H), diameter at breast height (DBH), stem cross-sectional area (SA), total biomass (B), total carbon (C) and percentage of dead plants (M) for the native species and for Eucalyptus as a function of the models of Eucalyptus proportions in an Atlantic Forest restoration in Brazil 42 months after planting.

Native
MP	H	DBH	SA	B	C	M
	(m)	(cm)	(m-2 ha-1)	(Mg ha-1)	(Mg ha-1)	(%)
T1	3.4 ±0.4a	3.8 ±0.4ab	2.95 0.5a	16.9 ±1.6a	7.9 ±0.7a	19.1 ±0.05a
M1	3.7 ±0.2a	4.6 ±0.8a	3.2 ±0.8a	20.1 ±4.0a	9.4 ±2.1a	16.1 ±0.02a
M2	3.8 ±0.4a	4.0 ±0.4ab	2.09 ±0.4a	13.9 ±3.6a	6.5 ±1.7a	19.6 ±0.06a
M3	3.0 ±0.3a	2.8 ±0.5b	1.26 ±0.1b	7.3 ±1.5b	3.4 ±0.7b	32.0 ±0.09b
Eucalyptus
MP	H	DBH	SA	B	C	M
	(m)	(cm)	(m-2 ha-1)	(Mg ha-1)	(Mg ha-1)	(%)
T2	15.7 ±0.9a	11.4 ±2.8c	19.7 ±1.9a	97.1 ±10.9a	45.6 ±5.1a	10.2 ±0.03a
M1	16.8 ±0.5a	16.3 ±2.1a	7.18 ±7.1c	38.1 ±6.3c	17.9 ±2.9c	9.7 ±0.06a
M2	17.8 ± 0.8a	16.6 ±3.0a	11.5 ±11.5b	61.4 ±4.1b	28.9 ±2.0b	3.7 ±0.03a
M3	16.5 ±0.8a	13.6b ±2.6	12.1 ±1.1b	62.7 ±6.9b	29.4 ±3.2b	9.0 ±0.04a

MP is the planting model; T1 is 100% native species; T2 is 100% Eucalyptus; M1 is 25% Eucalyptus; M2 is 33% Eucalyptus; and M3 is 50% Eucalyptus. Means followed by the same letter in the same column did not differ from each other according to the Tukey’s test at the 5% probability level.

Next, the predominant species regarding the natural regeneration process for all planting models was Vernonia polysphaera, which is considered an aggressive fast-growing species (Fig. 4). In addition, Schinus terebinthifolius and Baccharis dracunculifolia were found in T1, P. guajava was found in M1, B. dracunculifolia was found in M2, Albizia polycephala was found in M3, and S. terebinthifolius and Solanum paniculatum were found in T2.

Figure 4.  Frequencies of naturally regenerating species in the plantation for each planting model: T1, 100% native; M1, 25% Eucalyptus; M2, 33% Eucalyptus; M3, 50% Eucalyptus; and T2, 100% Eucalyptus, in an Atlantic Forest restoration in Brazil at 42 months after planting. E.1: Albizia. polycephala; E.2: Baccharis dracunculifolia; E.3: Schinos terebinthifolius; E.4: Vernonia polysphaera; E.5: Psidium guajava; E.6: Solanum. paniculatum.

Discussion

The use of Eucalyptus to regenerate the legal reserve area significantly interfered with the growth of S. granulosoleprosum and E. contortisiliquum, while the models which favoured the growth of native species were M1 and M2 (Fig. 3). This contradicts the first hypothesis because planting Eucalyptus may interfere with the growth of some native species, especially fast-growing species such as S. granulosoleprosum and E. contortisiliquum. It was also observed that a Eucalyptus density of 50% increased the mortality of native species. Similar results were reported by Amazonas et al. (2018)Amazonas NT, Forrester DI, Oliveira RS, Brancalion PHS, 2018. Combining Eucalyptus wood production with the recovery of native tree diversity in mixed plantings: Implications for water use and availability. For Ecol Manage 418: 34-40. https://doi.org/10.1016/j.foreco.2017.12.006
, in which Eucalyptus negatively affected the growth of pioneer species (i.e. those with faster growth). Because it is a species which has a rapid initial onset, Eucalyptus aggressively competes for available resources such as water, light and nutrients.

When the cutting cycle of a Eucalyptus tree is complete, the supply of available resources will increase and favour the growth of other native species (Brancalion et al., 2019Brancalion PHS, Meli P, Tymus JRC, Lenti FEB, Benini RM, Paula A, Silva M, Isernhagen I, Holl KD, 2019. What makes ecosystem restoration expensive? A systematic cost assessment of projects in Brazil. Biol Conserv 240: 108274. https://doi.org/10.1016/j.biocon.2019.108274
). Since species with secondary growth grow slower than pioneer species, they require shade to germinate and become established in an area, so the rapid growth of Eucalyptus may aid in developing these species. Thus, Eucalyptus planting densities of up to 33% are recommended for regenerating LR areas so as not to compromise plant quality and to prevent seedlings of native species from being suppressed by Eucalyptus.

Among the different models, the native plants only showed significant differences for DBH, SA, B and C, while the M1 and M2 models showed similar growth patterns. These results once again highlight the potential of using Eucalyptus at the age evaluated with a planting density of 33% for restoring legal reserve areas.

The species which generally showed the greatest growth were classified as pioneers. Increases in the SA, B, and C of Eucalyptus were greater in the M1 and M2 models, which led us to accept the second hypothesis. Eucalyptus is an aggressive competitor and can benefit from the conditions in mixed plantations; it can particularly take advantage of light resources because their canopy is taller than those of other species. Eucalyptus stands out in B and C production, which supports its use in the commercialisation of by products such as sawn wood (at more advanced ages of approximately 12-20 years, as there will be no thinning) and energy, ensuring a reduction in the cost of implementing a restoration project.

Other pioneer species with exceptional performances are S. granulosoleprosum and E. contortisiliquum, which presented greater growth and B stocks than any of the other native species. In addition to the financial gain from commercialising Eucalyptus wood, native species can be exploited in the long term for commercialising economically valuable timber, provided that the limitations of Law N^o. 12.651 are respected (Brazil, 2012Brazil, 2012. Law No. 12,651 of May 25, 2012. Establishes rules for native vegetation protection, Permanent Preservation Areas (APPs), and Legal Reserves. Official Gazette of the Federative Republic of Brazil, May 28, 2012.
), and this approach may make projects eligible for the carbon credit market.

Furthermore, the rapid growth of Eucalyptus is a very important feature for recovering degraded areas and may be an interesting strategy for creating rapid soil cover. In addition, leaf fall increases soil organic matter, favouring nutrient cycling (Carvalho et al., 2017Carvalho DC, Pereira MG, Toledo LO, Simon CA, Silva Rodrigues J, Fernandes JCF, Neto ECS, 2017. Ciclagem de nutrientes de um plantio de eucalipto em regeneração de espécies nativas no sub-bosque. Floresta 47(1): 17-27. https://doi.org/10.5380/rf.v47i1.43652
). Oliveira et al. (2014)Oliveira EB, Santos LMF, Gobor D, Moris AC, Tina V, 2014. Produtividade de plantações de Eucalyptus intercaladas com espécies nativas em áreas de pastagens degradadas no noroeste do estado do Paraná. Encontro Bras Silvic 337-340.
implemented a mix of native and exotic species (E. grandis and C. citriodora) in different municipalities in the state of Paraná, Brazil, and obtained satisfactory wood production values which could finance the associated forest restoration project. This result confirms that Eucalyptus has great potential to considerably increase and improve the incomes of rural landowners or dampen investments in restoration projects.

This study showed that it is feasible to establish highly diverse mixed plantations with Eucalyptus and native tree species. A Eucalyptus planting density of up to 33% is suitable for restoring legal reserves, as the results showed that this density did not negatively affect the growth of native species up to the evaluated age. Moreover, Eucalyptus seedlings are less expensive than those of native species, which further reduces the costs of implementing restoration projects.

The species with the highest incidence regarding natural regeneration was V. polysphaera, which is considered an aggressive fast-growing species (Dutra et al., 2003Dutra S, Filho APSS, Mascarenhas REB, 2003. Controle Integrado das Espécies Invasoras Assa-peixe e Casadinha em Pastagens Cultivadas de Paragominas, Nordeste Paraense. Bol Pesq Desenv 13: 1-33.
) and was especially abundant in T1. This treatment resulted in smaller individuals (Fig. 1) and less soil cover, allowing greater light entry, which benefitted propagation of species that thrive in early successional stages.

Schinus terebinthifolius, P. guajava and A. polycephala were observed in the other planting models. In addition, the reduced number of species found in the area may be related to the fact that weed control was performed up to approximately two years after planting. Despite weed control, Brancalion et al. (2019)Brancalion PHS, Meli P, Tymus JRC, Lenti FEB, Benini RM, Paula A, Silva M, Isernhagen I, Holl KD, 2019. What makes ecosystem restoration expensive? A systematic cost assessment of projects in Brazil. Biol Conserv 240: 108274. https://doi.org/10.1016/j.biocon.2019.108274
observed that areas with seed rain and adequate conditions for germination in restored plantations subjected to intensive silviculture with plants up to two years of age tended to have greater soil cover, leaf area index values, and diversity of regenerants than plantations without intensive silviculture.

Viani et al. (2010)Viani AGR, Durigan G, Melo ACG, 2010. A regeneração natural sob plantações florestais: desertos verdes ou redutos de biodiversidade? Cien Florest 20(3): 533-552.
indicated that the richness, density and structure of natural regeneration are influenced by several factors, such as the characteristics of an area and planting age. Thus, it remains necessary to track the development of planted areas over time to understand the quality of the natural regeneration in the area. According to Brancalion et al. (2020)Brancalion PHS, Amazonas NT, Chazdon RL, van Melis J, Rodrigues RR, Silva CC, Sorrini TB, Holl KD, 2020. Exotic eucalypts: From demonized trees to allies of tropical forest restoration? J Appl Ecol 57(1): 55-66. https://doi.org/10.1111/1365-2664.13513
, the introduction of Eucalyptus to mixed planting areas with native species favoured natural regeneration throughout the understorey after five years and considerably reduced the costs associated with planting in the restoration area. The authors also observed that Eucalyptus did not regenerate through seed germination in the area, meaning it did not become an invasive species, allowing the area to be recomposed through germination and growth of native species.

The present study only considered growth variables and an initial evaluation of natural regeneration in a transition area between the Cerrado and Atlantic Forest biomes. The species used in this study are also native to the Cerrado, making the results particularly relevant to restoration efforts in both biomes. For future studies, it is important to evaluate additional indicators of restoration success, such as the presence of seed dispersers, seed rain, and the diversity and density of regenerating native species. Furthermore, this project can be adapted to Cerrado-specific restoration efforts, given its relevance to both ecosystems. Additionally, long-term monitoring of plant growth is necessary, as this evaluation was conducted 42 months after planting.

Conclusion

This study demonstrates that incorporating up to 33% Eucalyptus in mixed-species plantations does not significantly affect the growth or survival of native species within the first 42 months. Specifically, proportions of 25% and 33% Eucalyptus increased the diameter at breast height (DBH) and stem cross-sectional area (SA), respectively, while no significant effects were observed on plant height, mortality, biomass accumulation, or carbon storage. These findings support our hypothesis that native species growth would be minimally impacted by the inclusion of Eucalyptus, although its influence should continue to be monitored over time.

The results also confirm that Eucalyptus benefits from reduced intraspecific competition in mixed plantings, aligning with our second hypothesis that it would perform better in treatments with a higher proportion of native species. However, longer term studies are needed to fully understand the dynamics of Eucalyptus-native species interactions, particularly in terms of biodiversity, and the economic potential for carbon credits

In conclusion, our findings suggest that integrating Eucalyptus into reforestation projects can contribute to both ecological restoration and economic objectives, providing a scalable model for Brazil’s reforestation goals, when managed appropriately.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Competing interests

The authors have declared that no competing interests exist.

Authors’ contributions

Bruna S. Crivilin: Conceptualization, Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Fernanda L. Cunha: Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Josiana J. N. Basílio: Writing - original draft, Writing - review & editing. Lucas A. de Melo: Conceptualization, Formal analysis, Methodology, Writing - review & editing. Soraya A. Botelho: Formal analysis, Methodology, Writing - review & editing.

Funding

References