Our results showed that active restoration triggered a healthy succession on sites formerly dominated by African grasses, with weak or nor natural regeneration. Forest structure in active restoration sites developed in a similar rate as sites naturally regenerating, therefore subject to passive restoration. Stem density and rarefied richness were higher in active restoration sites, due to seed and seedling addition and, especially, to the stimulus of new stems and species recruitment. Species composition in active restoration sites included a shared dominance of short life-spam, pioneer trees, typical of less disturbed natural regeneration sites, in opposition to the Vismia-dominated natural regeneration sites (Mesquita et al. 2015).
Active restoration is a matter of eliminating regeneration filters
The success of the active restoration was explained by preparing a loose soil, free from exotic grasses, with small furrows and mounds, associated with a high availability of seeds of pioneer trees. That resulted in high stem density and basal area of pioneer trees, as seen in the older sites with no direct seeding intervention. Planted species (direct seeding and seedlings) contributed to 50 and 25% of the species richness in younger sites and older, respectively, and only 48% and 28% of the basal area. In an experiment designed for decoupling the effects of soil preparation and seed and seedling addition, stem density was similar in plots with only soil preparation and those with soil preparation and seeding or seedling planting (Rezende and Vieira 2019). These result show that the barrier to natural regeneration in this landscape is not seed availability, but the conditions to germinate and establish.
For the restoration project studied here, direct seeding is a safety strategy for sites that do not have enough seed density from seed rain or seed bank. However, in landscapes with even lower forest cover and lower abundance of pioneer trees in the agricultural matrix, direct seeding might be necessary. Direct seeding of highly abundant pioneer trees is a relatively cheap strategy (Rezende and Vieira 2019), but one cannot decide on doing it after seeing the early results of the restoration. It needs to be applied during the first months after soil harrowing, which is the window of opportunity for germination and establishment.
Forest landscape cover affect the effect size of the active restoration. Active restoration is more necessary to guarantee restoration success in landscapes with low forest cover (Crouzeilles and Curran 2016). An effective active forest restoration method should introduce pioneer trees that contribute to fast structuring a canopy, and introduce late successional species, that will guarantee a long-term forest (Rodrigues et al. 2009). However, in the studied landscape planted species promoted higher species richness in the restored sites only in the first few years. Species colonization was happening in natural regeneration sites to a point of no difference in species richness. We suggest that with the forest cover in the studied landscape of Amazonia (37%; Rocha et al. 2016), restoration intervention should focus in structure a tree canopy in order to facilitate natural colonization. In the southern border of Amazonia, in a landscape with 18% of forest cover on average, non-planted species colonize sites in an average rate of 0.4 species/year along 10 years after direct-seeding restoration (considering a plot size of 500m2 for adults, 100m2 for saplings and 25m2 for seedlings; Freitas et al. 2019). Thus, even within highly deforested landscapes, active interventions will be more efficient if designed for triggering natural regeneration, than planting an assembled forest.
Differently from the southern Amazonia, other restored tropical forests are sustained by the planted seedlings (Rodrigues et al. 2019; César et al. 2018; Shoo et al. 2015; Sansevero et al. 2011). In the Atlantic forest, 97% of the above ground biomass in 7–20 year-old active restoration sites were from planted trees (César et al. 2018), evidencing that natural regeneration was scarce. Yet, in older (up to 53 years) active restoration sites in Atlantic forest, there is an increase of species- and life-form diversity, becoming similar in structure and species composition to old growth forests (Suganuma and Durigan 2015).
Active restoration × naturally regenerating forests in Amazonia
In our study, pastures were mechanized and used for cattle ranching during 9.3 years on average (Rocha et al. 2016). Forest structure and richness in these pastures before active restoration were lower than the heavy-use pastures (> 7 years used as pasture, 1–2 fires and mechanized for grass seeding) described by Uhl et al. (1988). Before active restoration interventions, those sites were dominated by a healthy U. brizantha pasture, cultivated through intense soil management, and removal of trees resprouting and recruitment by harrowing and herbicide application (Rezende and Vieira 2019). Probably as a result of intensive pasture management all seed bank and sprouts were eliminated, which may explain why not even the most important tree-weed in Amazonian pastures, Vismia spp. could vigorously resprout.
The active restoration sites of up to 7.5 years old were dominated by different species when compared with the, formerly more resilient, natural regeneration sites. Natural regeneration sites were dominated by Vismia species responsible for 58 and 45% of basal area in younger and older sites. After active restoration, sites with a stable U. brizantha cover, were dominated by Senna alata, Cecropia spp., Solanum spp, Muntingia calabura and Trema michranta in the first three years and then by Cecropia purpurascens. The dominance of Cecropia spp. makes actively restored sites more similar to highly resilient sites of Amazon forests than to the Vismia-dominated sites. In other parts of Amazonia, sites that have low resilience because they were subject to decades of exotic grasses cultivation and frequent fires to prevent forest recovery, tend to be dominated by Vismia spp. Sites that were subjected to milder use and thus have higher resilience tend to be dominated by Cecropia and to be more biodiverse (Jakovac et al. 2016; Mesquita et al. 2015).
The cause for the impoverished regenerating community at Vismia dominated sites is attributed to the degradation of soil quality, loss of sprouting ability of most species, and non-facilitative life-history traits of Vismia spp. (Jakovac et al. 2016; Williamson et al. 2012). In this project (see also Rezende and Vieira 2019), we were able to verify that preparing soil and eliminating U. brizantha cover triggered an early community dominated by Cecropia. Thus, our study adds that the exotic grass cover is a strong filter that affects compositional trajectory. Removing the grass cover is a low-cost intervention that allows for a healthier succession. However, further studies are needed to assess if the early community facilitated by the intervention advances in a similar way to the highly resilient Cecropia-dominated sites in Central Amazonia.
Active restoration sites were remarkably different from natural regeneration sites in respect to species composition. Natural regeneration sites were dominated by Vismia spp., species that maintain dense stem populations in heavily used pastures by resprouting (Mesquita et al. 2015). Self-thinning in Vismia-dominated secondary forests is slower than in more resilient forests, because Vismia species have a long life-cycle and because they maintain the recruitment of new individuals by sprouting (Norden et al. 2011). This resulted in less emergent trees and less species recruitment, compared to active restoration sites.
In active restoration sites, initial recruitment originated from seed rain of short life-cycle trees, such as Solanum spp. and T. michrantha, and from the directly seeded S. alata. In high stem density sites, these species died after ca. 3 years, opening space for new recruitment. Solanum spp. is a genus of shrubs and small trees that colonize active pastures (Uhl et al. 1988) and, in our study, it colonized sites with prepared soil. Solanum spp. attract high densities of bats that bring Cecropia seeds to the site (Silveira et al. 2011). Consequently, Cecropia spp. emerged. This extremely fast compositional change promoted higher size-structure heterogeneity, increasing density of recruits and reducing intermediary size-classes. In addition, planted seedlings developed large trees (> 25.1 cm DBH) in active restoration sites.
Active restoration or Natural regeneration?
This study contributes to the understanding of how restoration methods can shape early trajectories of forest recovery. Our study does not allow to recommend which one is more cost-effective, since active restoration was applied only at less-resilient sites and natural regeneration was applied at more-resilient sites. Given the low levels of funding for restoration, a simple recommendation would be to use natural regeneration where the natural regeneration potential is high enough to trigger and maintain a successional trajectory (Chazdon and Uriarte 2016; Holl and Aide 2011). However, different compositional trajectories (Jakovac et al. 2016; Mesquita et al. 2015; Uhl et al. 1988) and even more contrasting rates of structural development have been observed in natural regeneration (Heinrich et al. 2021), depending on the land use history. Variations are also observed in actively restored sites, due to different intervention methods (Gardon et al. 2020; Giudice Badari et al. 2020; Freitas et al. 2019; Shoo et al. 2015). Thus, a relevant question is if we can significantly improve secondary forests quality, for biodiversity and carbon increment, with cost-effective interventions. We suggest that simple and cheap interventions directed at improving the odds of natural regeneration can be effective for non-resilient sites located in resilient landscapes.
Passive restoration might not be an option in some regions in southern Amazonia. In the very state of Rondônia, there are municipalities with 15% of forest cover, and landscapes with less than 5% of forest cover if we consider landscape sizes of 25 km2 (analyzed from Project MapBiomas Collection [4.1] www.mapbiomas.org.br), which is landscape size in which forest cover affects restoration success worldwide (Crouzeilles and Curran 2016). Southern Amazonia has the largest area demanding for restoration in order to comply with Brazilian environmental laws (Rajão et al. 2020); sites will probably need active restoration or assisted natural regeneration.