Restoration plantings of non-pioneer tree species in open fields, young secondary forests, and rubber plantations in Bahia, Brazil

Highlights

• Survival of non-pioneer tree species was similar across all treatments.

• Growth of non-pioneer tree species was, in general, higher in open fields.

• Planting pioneer species in the mixture did not improve the growth of non-pioneers.

• Trees become functionally active for wildlife earlier when planted in open fields.

Abstract

The Atlantic forest of Brazil is a biodiversity hotspot that retains less than 12% of its original area. In this biome, non-pioneer tree species with limited dispersal are the most impacted by recent habitat loss and fragmentation. As attempts to establish non-pioneer tree species in pastures and agricultural fields in the Atlantic forest have failed, restoration strategies that consider planting these species in other habitats, such as the understory of secondary forests and tree plantations may be more successful. To test this, five native non-pioneer tree species (Sloanea obtusifolia, Garcinia macrophylla, Copaifera lucens, Symphonia globulifera, and Pouteria reticulata) were planted in complete randomized blocks in April 2009, with four site replications and four treatments (open fields, open fields with pioneers, young secondary forests, and rubber plantations). Survival, height, and dbh (diameter at the breast height) were measured every year for six years. We also measured canopy cover and soil physical and chemical parameters. Six years after planting, we found similar survival of non-pioneer tree species in open fields, young secondary forests and within rubber plantations. However, height and dbh growth were, in general, significantly higher in open fields. Planting pioneer species in mixture with non-pioneers did not improve the growth of non-pioneers. We also found differences among species, with Copaifera lucens showing higher survival and growth than the other species. Even though enriched young secondary forests and rubber plantations could be important contributors to biodiversity conservation, higher growth rates of non-pioneer species in open fields have the benefit of accelerating the recovery of wildlife activity in restoration plantings.

Introduction

Land clearance by humans has reduced forest cover in the Brazilian Atlantic forest to 12% of its original extent (Ribeiro et al. 2009). Recent estimations predict that a third of tree species in the biome may go extinct because of fragmentation and degradation (Canale et al., 2012, Silva and Tabarelli, 2000). Non-pioneer tree species include a wide array of long-lived canopy tree species that attain dominance of a forest later in successional development (Swaine and Whitmore, 1988). Non-pioneer tree species have limited dispersal and as a large group of species, they are generally, compared to pioneers, the most affected by the current habitat loss and fragmentation (Tabarelli and Peres, 2002, Metzger, 2000, Tilman et al., 1994) and, consequently, conservation programs have focused their efforts on these species, e.g. the IUCN Red List. The main strategy to conserve non-pioneer tree species has been the creation of protected areas that retain the last remnants of old-growth forests. However, these relict forests may not be sufficient to sustain viable populations of non-pioneer tree species, because the old-growth remnants in the Atlantic forest are too small and frequently lack the full complement of mammals and birds that act as specialized dispersers (Galetti et al., 2013, Silva and Tabarelli, 2000).

In Brazil, an increase in the exigencies of environmental laws has led to a growth in the demand for restoration projects, some of which are based on mixed plantations of native tree species. These plantations are usually established in open and isolated areas where secondary forest succession is unlikely to happen and have included a high diversity of non-pioneer and pioneer native tree species (Rodrigues et al. 2009). The use of mixed tree plantations of native species has been employed as a restoration strategy in the Atlantic forest since the 1970′s. Several plantation designs were developed over the past 50 years, including the random distribution of native species plantings in open fields (Nogueira 1977), the distribution of native species plantings in open fields based on the composition of adjacent forests (Joly et al. 2000), and the most commonly adopted strategy of combining native pioneer and non-pioneer species together in open field plantings (Kageyama and Castro 1989). This last design assumes that the shade of pioneer species provides adequate conditions for non-pioneer species to grow. However, studies have reported extremely low survival rates of non-pioneer species in these plantations. Souza and Batista (2004) reported that out of 42 tree species planted only four pioneer species shared overstory dominance (Cecropia pachystachya, Croton floribundus, Genipa americana, and Peltophorum dubium) in a 10 year-old plantation. In another area reforested with 39 species, the same authors found that only two pioneer species (C. floribundus and C. pachystachya) comprised more than 50% of the total tree density after 9.4 years. Moreover, Brancalion et al. (2018) reported that in addition to the low survival of non-pioneer species, species with large-sized seeds (diameter >12 mm) have been rarely used in Atlantic forest restoration projects due to problems associated with their collection, storage and germination. However, if conservation and restoration efforts are to advance, finding ways to incorporate non-pioneer species that would be considered relatively rare and dependent upon large mammals or birds for dispersal needs to be considered.

The poor performance of non-pioneer species when mixed with pioneers in open areas reinforces the idea that restoration techniques must be based on a thorough understanding of the species ability to establish and grow under different environmental conditions (Sayer et al. 2004). As some attempts to establish non-pioneer tree species in open full sun areas in the Atlantic forest have failed (Brancalion et al., 2018, Souza and Batista, 2004), restoration projects need to consider other strategies in order to increase the probability of successfully restoring populations of these species. There have been several successful experiences of enrichment planting of non-pioneer species in the understory of natural forests in the tropics (Atondo-Bueno et al., 2018, Keefe et al., 2009, Piotto, 2007, Montagnini et al., 2006, Montagnini et al., 1997, Schulze et al., 1994, Ramos and del Amo, 1992). However, few studies have used the understory of tree plantations to grow non-pioneer tree species (Rappaport and Montagnini, 2014, Ashton et al., 1997). Ashton et al. (1997) demonstrated that pine plantations can facilitate the establishment of non-pioneer tree species in the mixed dipterocarp forests of south-east Asia, while Rappaport and Montagnini (2014) showed that rubber plantations were suitable for enrichment planting of non-pioneer trees. Therefore, similar to natural forests, tree plantations may ameliorate soil and microclimatic conditions, which can favor the establishment of non-pioneer tree species (Kaewkrom et al., 2005, Cusack and Montagnini, 2004, Keenan et al., 1997, Parrotta, 1992).

In this study, we compare the performance of non-pioneer tree species growing in open areas and within the understory of young secondary forests and rubber plantations for six years. We also examine whether mixing pioneer tree species with non-pioneer tree species in open areas improves the survival and growth of the latter. We hypothesized that after six years: (i) non-pioneer tree species growth and survival are better beneath partial shade (young secondary forests and rubber plantations) than in open areas, because microclimatic conditions of the understory will promote the establishment of the more shade-tolerant non-pioneer tree species; and (ii) in open areas, non-pioneer tree species growth and survival are better in mixed stands of pioneer and non-pioneer tree species than in stands of only non-pioneer tree species, because pioneer tree species can potentially provide partial shade to non-pioneer species while simultaneously shading out sun-loving grasses, thereby diminishing competition for soil resources.