By Dr. Ricardo Viani
Ricardo Viani is a professor at the Universidade Federal de São Carlos, Brazil, where he coordinates the LASPEF (viani@ufscar.br)
The Atlantic Forest in Brazil is a highly diverse tropical forest, listed as a global hotspot for biodiversity conservation. It is also the home of the two biggest Brazilian cities, São Paulo and Rio de Janeiro, and it is where most Brazilians live; around 150 million people inhabit and depend on the Atlantic Forest for the provision of ecosystem services.
Originally covering about 1.2 million square kilometers (297 million acres), the Atlantic Forest now covers less than 20% of its original area because of centuries of degradation. It is no wonder that the Atlantic Forest stands out globally as a region where forest restoration is urgently needed and is being taken seriously. In recent decades, many large-scale Atlantic Forest restoration programs have been implemented and, in 2009, the Pacto (Atlantic Forest Restoration Pact), a coalition of hundreds of institutions working for Atlantic Forest restoration, was launched. For its work, the Pacto was recognized as a World Restoration Flagship by the United Nations Decade on Ecosystem Restoration, highlighting it as a global example of an ongoing, large-scale, and long-term ecosystem restoration effort.
Landscapes where forest restoration plantings are usually done in the Atlantic Forest in Brazil. A sugarcane plantation (left, photo: Paulo Molin) and a pasture matrix (right, photo: Ricardo Viani), with variable levels of scattered remaining forest.
After decades of Atlantic Forest restoration efforts, it is time to evaluate what we have done so far, not only to assess the outcomes but also to inform other large-scale restoration initiatives worldwide. Thus, we recently investigated which tree species were included in 1,073 forest restoration plantings implemented from 2002 to 2018 in the Brazilian Atlantic Forest by restoration programs carried out by the NGO SOS Mata Atlântica.
Overall, 423 tree species were included in the evaluated plantings, which represent less than 8% of the Atlantic Forest tree flora. In comparison with remaining forest patches, restoration plantings skewed towards nitrogen-fixing, non-animal-dispersed, and pioneer species. Plantings had poorly included endangered and endemic species as well as species that were previously indicated as priority for restoration based on their ecological interactions, carbon storage, and conservation values.
However, the more striking result is that restoration initiatives are planting the same set of limited tree species across the whole Atlantic Forest. Although the Atlantic Forest has three types of forests (the Araucaria Forest, the Rainforest, and the Seasonal Forest), each one with their own singular floristics, restoration plantings are overall more similar in species composition to each other than to the remnants of the type of forest of the region where they exist. In other words, we plant the same species without considering spatial variation in local and regional floras.
This pattern raises an uncomfortable question. Is large-scale restoration via tree planting contributing to biotic homogenization, that is, the tendency for distinct places to become more similar?
Why should we pay attention to the trees we plant for tropical forest restoration?
One may say that our role in tropical forest restoration is to trigger ecological succession, and it is not relevant if we plant just a few tree species across a wide, diverse region. Actually, some argue that we should plant the same species everywhere, by selecting and planting the best species to accelerate early forest development . This is one explanation for the overrepresentation of pioneer trees in restoration plantings, and a reason to increase the representation of generalist animal-dispersed trees, which attract native fauna to restoration sites and favor natural regeneration under planted trees. Another argument may be that evaluating what we plant for forest restoration is not so important because non-planted species will naturally colonize planting sites later on and change their floristic composition. However, this statement and the previous one are acceptable only if regional tree species can colonize restoration sites in fragmented landscapes – a questionable premise in typical landscapes of southeastern Brazil.
The few studies available in rich tropical forests show that many non-planted trees species colonize restoration plantings, which is good news. However, some functional groups, such as slow-growing tree species dispersed by gravity or by larger animals tend to be bad colonizers, especially in fragmented or defaunated landscapes. In other words, if they are not planted, many tree species will not reach restoration sites on their own, and we should pay more attention to their inclusion in restoration plantings.
Finally, we need also to consider that restoration is a strategy to promote in situ tree species conservation. The Atlantic Forest has more than 4,000 native tree species, almost half of them endemic, and dozens threatened by extinction. However, few endemic and threatened tree species are included in restoration plantings and some of the ones that are included are only used infrequently and in low abundance. Probably, some of them are also rare, absent in fragmented landscapes, and not good colonizers, which increases the importance of their planned and careful inclusion in restoration efforts.
The way forward
Restoration initiatives have done great work for the Atlantic Forest in recent decades, but there are always points that could be improved, such as the representation of regional flora and of some groups of species currently underrepresented in plantings (e.g., animal-dispersed, endangered, high value species for conservation, etc.). Solving this issue involves much more than just recommending their inclusion in restoration initiatives. Many of these species are rare, lots of them do not have seeds or seedlings available for restoration and, many, if not most, are slow-growing trees. Knowing how restoration practitioners perceive the importance of species diversity, selection, and composition for restoration plantings and potential trade-offs in prioritizing diversity representation versus fast early recovery may give insights on strategies to overcome this challenge.
In addition, as many underrepresented species in tropical forest restoration plantings are slow-growing, it is worth studying ways of introducing these species in restoration plantings. For instance, should we plant slow-growing trees under full-sun or under the canopy of previously planted pioneer trees? How can we balance the composition of restoration plantings to include slow-growing species without losing the benefits of rapid forest development promoted by fast-growing species? All these questions are still to be answered and part of my ongoing research project Optimizing high-diversity restoration: perceptions and approaches to add tree diversity in tropical forest restoration plantings.
We are on our way, but there is still a long path to go before restoration will truly represent the diversity of complex and unique biomes.
For more information, read our paper recent paper in Forest Ecology and Management or contact Dr. Viani (viani@ufscar.br).
Natural History of Ecological Restoration 