Relevance of secondary tropical forest for landscape restoration
Introduction
Since the 1920 s, most deforestation has taken place in tropical countries (FAO, 2001, FAO, 2012b, Hansen et al., 2013, FAO, 2018). Although the rate of net tropical forest loss dropped after 1990, between 2015 and 2020 the total area of tropical forest disappeared at a huge average rate of 9.28 million ha per year! (Keenan et al., 2015, FAO, 2020). Old-growth forests now only account for 25–30% of all existing tropical forests (Mercer, 2015, FAO, 2020) and most tropical forests have already been substantially altered (Laurance et al., 2014, FAO, 2018, Vancutsem et al., 2021)
Loss of forested areas during the decades of deforestation across the tropics is mainly the result of agricultural development including shifting agriculture and commodity-driven deforestation, which has led to the clearance of millions of hectares of primary forest (Curtis et al., 2018). Unsustainable logging practices in tropical regions have also been an indirect cause of deforestation, by facilitating access to and colonization of the forest (Lambin et al., 2001, Noriko et al., 2012, Finegan, 2016, FAO, 2018), and by encouraging the conversion of degraded forests to more profitable land uses: agriculture, pasture, industrial plantations, and mining (Lambin et al., 2001, Noriko et al., 2012, Zimmerman and Kormos, 2012, Finegan, 2016).
The loss of soil fertility in deforested areas has led to the abandonment of marginal agricultural lands, where forests were able to regenerate. Forests that regenerate in previously deforested areas are usually called secondary forests (SFs). In this paper, we define SFs as forests that regenerate naturally in abandoned deforested areas. This definition follows that of Brown and Lugo (Brown and Lugo, 1990) and is widely used in the literature (Poorter et al., 2016, Rozendaal et al., 2019). Even if today there is no precise estimation of the extent of tropical SFs, it is acknowledged that SFs are now the dominant forest ecosystems in many tropical regions and landscapes (Chazdon, 2003, Lugo and Helmer, 2004). Brown and Lugo (2009) estimated that at the time, SFs in the tropics, accounted for about 40% of the total tropical forest area, and that their rates of formation were about 9 million ha yr−1. Across Southeast Asia, Mukul et al. (2016) estimated that SFs accounted for 63% of the remaining forest cover.
In the current global context of climate change and biodiversity loss, forest ecosystems will play an increasingly important role in providing key environmental services. The potential of SFs to provide such services is widely characterized and evaluated in the literature (Feldpausch et al., 2004, Arroyo-Mora et al., 2005, Chazdon et al., 2009, Poorter et al., 2016, FAO, 2018, Bastin et al., 2019, Rozendaal et al., 2019). SFs regulate the water cycle, provide critically important habitats that preserve biodiversity, and enable carbon storage in above and belowground biomass thereby improving soil fertility, even if biodiversity and associated ecosystem services in SFs are lower than in primary forest ecosystems (Benayas et al., 2009, Gibson et al., 2011). However, the wood production potential of SFs has rarely been studied, or even taken into account. This is vital, as the current global context is also characterized by increasing global demand for (tropical) wood products (Malhi et al., 2014, FAO, 2015, FAO, 2018), meaning logging pressure on the remaining tropical primary forests will not let up. SF expansion and decline in old growth forest highly justifies considering SFs as one of the sustainable future sources of wood, in a way that avoids further decline of natural forests and biodiversity loss while enabling sustainable climate mitigation (Lewis et al., 2019).
In this position paper based on relevant literature and expert knowledge, we argue that in many situations, tropical SFs should be actively managed as sustainable and complementary alternatives to selective logging of old growth forests for future wood production. Currently, many tropical SFs are vulnerable forest ecosystems: many are in transition and not currently under sustainable forest management (Blaser et al., 2011, Guimarães Vieira et al., 2014, Finegan, 2016). Without appropriate management, and because of uncontrolled use along with other human (agriculture) and non-human disturbances (fire), they may become even more degraded and again be converted into pastures or agricultural fields, resulting in further degradation (Chazdon et al., 2007, Marin-Spiotta et al., 2008, Hirota et al., 2011, Jakovac et al., 2015, Reid et al., 2018). Producing wood in SFs systems may increase their value and reduce their vulnerability. The sustainability of wood production in tropical SFs would be based on combining wood production with the preservation of ecosystem services already provided by these forest ecosystems, with a positive effect on their long-term maintenance in pressured landscapes. Moreover, considering SFs as a potential source of wood is fully consistent with the need to reduce logging pressure on tropical old growth forests.
The paper is organized in three sections aimed at answering three questions. Why should we enhance wood production in tropical SFs? What are the limitations and obstacles to wood production in tropical SFs? How could future research in forest ecology and management enhance active restoration of wood production in SFs? The paper also provides practical guidance and suggests future research priorities to support the sustainable implementation of wood production in tropical SFs.
Section snippets
We need to reduce logging pressure on the remaining tropical primary forests
At global scale, reducing logging pressure on remaining tropical primary forests may be the most important reason to enhance wood production in tropical SFs. Indeed, non-sustainable logging operations in tropical primary production forests have increased due to the increasing demand for tropical timber (Malhi et al., 2014) leading to forest degradation (Calaboni et al., 2018, Torrella et al., 2018). Using current rotation times, at best, only about 50% of the volume of timber extracted from
What are the limits and obstacles to wood production in tropical Sfs?
Considering SFs as potential systems for wood production involves both limits and obstacles. In this section, we describe the limits and obstacles to more general SF dynamics, and discuss how to overcome them and reverse the trend, by providing practical guidance and identifying priorities to support the sustainable implementation of active restoration in tropical SFs through wood production.
How could future research in forest ecology and management enhance active restoration for wood production in SFs?
SFs may have serious biophysical limitations to the development of ecologically and economically viable forestry. Depending on the previous land use and its intensification, natural regeneration dynamics in SFs may be hindered by biophysical factors (Guariguata and Ostertag, 2001). Biophysical limitations include environmental factors such as soil degradation, local climate variability, seed availability and seed sources (that allow secondary dynamics and succession) within the forest itself,
Conclusion
Tropical forests are currently disappearing at an alarming rate, and the increasing demand for wood in the near future will increase pressure on the remaining tropical natural forests. Secondary forests (SFs) that grow in former deforested areas today account for a considerable proportion of the natural tropical forests that remain. These SFs are vulnerable ecosystems located in highly pressured landscapes. They are also exposed to catastrophic events including fire. Without appropriate
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the CGIAR Research Program on Forests, Trees and Agroforestry (FTA). We would like to thank all the financial partners who supported this research through their contributions to the CGIAR Fund. To view a list of donors to the Fund, visit: http://www.cgiar.org/about- us / our-funders / http://www.cgiar.org/about-%20us/our-funders/
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