Introduction
Tropical forest restoration is a central strategy in the global efforts
to mitigate climate change (Bernal, Murray, & Pearson, 2018; Griscom et
al., 2017; Houghton, Unruh, & Lefebvre, 1993; Pugh et al., 2019;
Silver, Ostertag, & Lugo, 2000). Regenerating forests in the lowland
tropics have high rates of carbon accumulation in aboveground biomass.
On average, regenerating forests recover over 100 Mg of biomass per
hectare within 20 years (L. Poorter et al., 2008) and reach biomass
stocks comparable to old-growth forests within 66 to 80 years (Martin,
Newton, & Bullock, 2013). Protecting young regenerating forests in
Neotropics over the next 40 years could accumulate an amount of carbon
sufficient to offset emissions from fossil fuel burning and industrial
processes across all Latin American and Caribbean countries over the
1993-2014 period (Robin L. Chazdon et al., 2016).
The Bonn challenge, launched in 2011, is a global commitment of
countries, NGOs, and private companies that aims to restore 350 M ha of
degraded forests by 2030 (R. L. Chazdon et al., 2017; Grassi et al.,
2017; K. D. Holl, 2017). The success of large-scale restoration programs
will rely crucially on their economic viability over both short- and
long- time scales, as well as the benefits they may provide to human
wellbeing in relation to alternative land uses (Pedro H. S. Brancalion
et al., 2019). Restoring large areas of tropical forests requires
knowledge about which restoration approach provides the best returns on
investment for accumulating carbon and other expected benefits, such as
reducing extinction risk, improving water supplies, and increasing food
security (Pedro H.S. Brancalion et al., 2018; Crouzeilles et al., 2017;
Luke P. Shoo et al., 2017).
Natural regeneration is widely recognized as a core restoration strategy
to mitigate climate change due to its potential to accumulate biomass at
large spatial scales and at lower costs compared to tree plantations (R.
L. Chazdon & Guariguata, 2016; Evans et al., 2015; Gilroy et al., 2014;
Lewis, Wheeler, Mitchard, & Koch, 2019). This potential is already well
demonstrated worldwide through several examples of forest transitions,
where the abandonment of marginal agricultural lands led to the
widespread expansion of forest cover through natural regeneration (Aide
et al., 2013; Nanni et al., 2019; Rudel et al., 2005). As a spontaneous,
uncontrolled reassembling process, natural regeneration trajectories can
vary remarkably even among stands with similar local biophysical
conditions and disturbance regime (Arroyo-Rodríguez et al., 2016;
Mesquita, Massoca, Jakovac, Bentos, & Williamson, 2015; Norden et al.,
2015). This variation has to be accounted for in restoration planning
(Pedro H. S. Brancalion, Schweizer, et al., 2016; Uriarte & Chazdon,
2016).
In contrast with natural regeneration, tree plantings permit tighter
control on the characteristics of the restored forest stand. Tree
planting has often been established by restoration practitioners when
ecosystem resilience is expected to be insufficient to support effective
natural regeneration (K. D. Holl & Aide, 2011), or where rapid recovery
of forest structure is required for legal compliance (Pedro H. S.
Brancalion, Schweizer, et al., 2016; Chaves, Durigan, Brancalion, &
Aronson, 2015; Rodrigues et al., 2011). Intensive management practices,
such as planting trees at regular spacing, fertilizing soil, and
controlling weeds and leaf-cutter ants, can promote rapid accumulation
of above-ground biomass in tree stands (Rubilar et al., 2018; Wheeler et
al., 2016). However, plantation management is costly, and restoration
projects do not provide sufficient revenues to offset high
implementation and maintenance costs (P.H.S.; Brancalion et al., 2019).
In addition, most low-resilience sites where tree planting is required
to support restoration have been historically used for intensive
agricultural production and therefore have high land opportunity costs
(Laurance, Sayer, & Cassman, 2014) and altered biophysical conditions
that reduce the potential for natural regeneration (R.L. Chazdon, 2014).
Comparing restored forests established by tree planting and natural
regeneration (i.e. second-growth forests) within the same geographic
region is key to guide the selection of appropriate restoration
approaches for different socioecological contexts and expected benefits
(Luke P. Shoo et al., 2017).
As
stated above, tree planting and natural regeneration are usually found
in contrasting biophysical conditions within landscapes. Consequently,in situ comparisons cannot be used to develop general guidelines
about the relative costs and benefits of restoration practice (Reid,
Fagan, & Zahawi, 2018), to provide a basis for guiding decisions
regarding how to restore a given piece of land. Rather, such
comparisons have the potential to highlight the actual efficiencies of
“real-world” restoration practices, as they are determined by
environmental, social and economic constraints. Such comparisons can
help to guide decisions regarding how to invest limited resources for
implementing forest restoration approaches across a range of
socio-ecological conditions.
At the global scale, natural regeneration was shown to be more effective
(Crouzeilles et al., 2017) or present similar outcomes (Meli et al.,
2017) than tree plantations for recovering forest biodiversity and
structure. At the local scale, however, L. P. Shoo, Freebody, Kanowski,
and Catterall (2016) found that restoration plantations 1-25 yr old in
the Australian wet tropics had a faster recovery of wood volume than
natural regeneration sites, and César et al. (2017) found similar
results in the Atlantic Forest of Brazil for 7-15 yr old forests
undergoing restoration. These previous works focused mainly on
above-ground carbon, with no assessment of the cost-effectiveness of
restoration approaches. Cost-effectiveness assessments, rather than the
evaluation of costs and effectiveness independently, are essential for
guiding decision-making in restoration (Birch et al., 2010; Molin,
Chazdon, Ferraz, & Brancalion, 2018; Strassburg et al., 2019).
Moreover, the relative effect of tree planting and natural regeneration
for soil carbon sequestration remain unknown.
Here, we assessed the carbon accumulation (above-ground and soil),
implementation and land opportunity costs of forests established by
natural regeneration (i.e, passive restoration) and high-diversity
native tree plantations (i.e, active restoration). Our study was based
on chronosequences (10-60 yr) in human-modified landscapes of Brazil’s
Atlantic Forest. Brazil’s Atlantic Forest is a highly degraded
biodiversity hotspot with low remnant native forest cover (only 12-28%
of the original Atlantic Forest remains today, depending on remote
sensing resolution; Rezende et al., 2018; Ribeiro, Metzger, Martensen,
Ponzoni, & Hirota, 2009). Deforestation and fragmentation resulted
mainly from the expansion of profitable agricultural and forestry
commodities such as sugarcane, cattle ranching, and eucalypt plantations
(Joly, Metzger, & Tabarelli, 2014), making cost-effectiveness a
fundamental topic for the upscaling of restoration in this biome (Pedro
Henrique Santin Brancalion, Viani, Strassburg, & Rodrigues, 2012). This
work aimed to compare current management practices in order to guide the
ambitious restoration programs recently established for the Atlantic
Forest (Molin et al., 2018; Soares et al., 2014; Strassburg et al.,
2019). Restoration practitioners and organizations worldwide have to
make hard decisions on where and how to invest in restoration to enhance
carbon accumulation while promoting biodiversity and other ecosystem
services (Luke P. Shoo et al., 2017). The solutions identified for
carbon farming through restoration with native trees in the Atlantic
Forest of Brazil may be readily transferrable to other tropical forest
regions with comparable economic and environmental contexts, and
hopefully will contribute to more effective use of the limited resources
currently available for forest restoration.