Discussion
Native tree plantations were shown to be more effective in accumulating
above-ground and soil carbon stocks than natural regeneration during the
first 50 years, but their higher implementation (tree plantations:
US$2,788 versus natural regeneration: US$1,250) and land
opportunity (tree plantations: US$324 per year versus natural
regeneration: US$106 per year; Molin et al. 2018) costs make them less
cost-effective for carbon farming. It is important to note that our work
assessed high-diversity plantations of native tree species rather than
monoculture tree plantations, to which natural regeneration has often
been compared (Lewis et al., 2019). Several independent factors have
contributed to this outcome. Exploring them is key to understanding how
the cost-effectiveness of different restoration approaches may vary
within human-modified tropical landscapes.
Most of the plantations included in the present study were found in
riparian buffers − a privileged environmental condition for biomass
accumulation due to the reduced water deficit and higher soil fertility
− and in landholdings dominated by intensive agriculture, where soils
usually display high nutrient and clay content. The studied tree
plantations may therefore have benefited from higher nutrient and water
availability than second-growth forests, which usually regenerate on
slopes and sandy soils distant from watercourses. Water deficit and low
soil fertility are known to limit tropical forest successional
development (Jakovac, Pena-Claros, Kuyper, & Bongers, 2015; Martins,
Marques, dos Santos, & Marques, 2015; Lourens Poorter et al., 2016;
Zermeno-Hernandez, Mendez-Toribio, Siebe, Benitez-Malvido, &
Martinez-Ramos, 2015), and may have reduced the biomass accumulation
rates of second-growth forests in our study. Water limitation, in
particular, may have played a critical role for the differential
performance of restoration approaches, since the study region has a
seasonal climate with annual water deficits of 20 mm or more depending
on geographical features (Alvares, Stape, Sentelhas, Gonçalves, &
Sparovek, 2013). In addition, tree plantations were fertilized, weeded,
and planted at a regular spacing, which allow an efficient occupation of
the deforested area by trees and enhance their growth, resulting in a
higher accumulation of biomass per area (P.H.S.; Brancalion et al.,
2019; Ferez et al., 2015). Our observation that tree plantations
initially accumulate more carbon than second-growth forest corroborates
previous results obtained in southern Costa Rica (Karen D. Holl &
Zahawi, 2014) and Queensland, Australia (L. P. Shoo et al., 2016), but
contradict the findings of Lewis et al. (2019) based on commercial
forestry plantations.
The predominance of soil properties over land use effects on soil carbon
stocks in the tropics has been reported by Powers, Corre, Twine, and
Veldkamp (2011) and is confirmed in our study landscape. Greater biomass
productivity and carbon inputs are expected to increase soil carbon
stocks (Jandl et al., 2007; Karlen & Cambardella, 1996), even though
changes in soil microbial communities may affect this causal dependency
(Fontaine, Bardoux, Abbadie, & Mariotti, 2004). As a result, intensive
forest management, afforestation, and reforestation are commonly
associated with increased soil carbon stocks (Don, Schumacher, &
Freibauer, 2011; Guo & Gifford, 2002). We do not report such an
association between above-ground biomass and soil carbon stocks, as
plantations displayed soil C stocks comparable to naturally regenerated
forests, and the temporal accumulation of above-ground biomass in
restored forests was not accompanied by a similar increase in soil
carbon stocks (Fig. 1 and 2). Variations in soil properties among study
plots may have obscured the relationship between soil carbon stocks and
restored forest age at landscape level, as also reported by Mora et al.
(2018) and Martin et al. (2013). Strikingly, none of the restoration
management types recovered soil carbon stocks comparable to reference
forests. Our results thus corroborate a global meta-analysis in tropical
regions that found that second-growth forests stored 9% less soil
carbon than primary forests (Don et al., 2011). The differences between
forest types was in our case remarkably higher, with reference forests
having approximately 50% higher soil carbon stocks at similar soil clay
content. Taken as a whole, our results show that both above-ground
biomass and soil carbon were enhanced in plantations compared to second
growth forests, but that 60 years of stand development was not
sufficient for the restored areas to recover stocks comparable to
reference forests.
The inclusion of restoration costs in the analysis reversed the priority
order of restoration approaches for carbon farming, and natural
regeneration emerged as the most cost-effective solution. In other
words, the cost reduction allowed by natural regeneration compared to
planting more than compensated for the slower rate of accumulation of
biomass, even in the unfavorable environmental conditions for tree
growth in which second-growth forests regenerated in the study region.
It is also important to note that we included in our analysis the direct
financial costs of passive restoration (i.e., natural regeneration;
Zahawi, Reid, & Holl, 2014) and considered both implementation and land
opportunity costs. Natural regeneration may show even higher
cost-effectiveness in regions where these costs are reduced, such as in
landscapes not dominated by agriculture, where land rental prices are
usually low. In addition, natural regeneration can be assisted (Shono,
Cadaweng, & Durst, 2007), potentially at much lower costs than tree
planting over the whole area. Assisted regeneration has the potential to
enhance the growth performance of spontaneously regenerating trees, thus
enhancing the biomass accumulation potential with lower implementation
costs. We note that our results may be affected by a site selection
bias, possibly resulting in an over-estimation of natural regeneration
success. As was highlighted recently (Reid et al., 2018), natural
regeneration is commonly conducted at sites closed to secondary forests
remnants, which could bias conclusions of restoration practice
comparisons. Additional studies would be needed to adequately represent
the Atlantic Forest, a 1.3 million km² ecosystem with several
biogeographical zones and socioeconomic conditions that would affect the
comparisons made in this study.
Carbon market currently values a carbon credit approximately 5 US$
(Hamrick & Goldstein, 2016) for 1 ton of CO2 (273 kgC),
i.e. 54.6 kgC.US$-1. Here, we report average total
cost-effectiveness values for above-ground carbon of 9.1
kgC.US$-1 and 15.1 kgC.US$-1, for
plantations and naturally regenerated forests, respectively. The market
price of 5 US$ thus underestimates by more than a factor of 3 the
actual price of carbon accumulation in restored forests. This clearly
demonstrates that the revenues potentially obtained by trading carbon
credits do not adequately cover the basic costs of both active and
passive restoration. Overall, our results suggest that carbon markets as
they are today offer a very low potential to up-scale restoration
efforts in the Atlantic Forest. Other complementary revenue sources like
those resulting from timber and non-timber forest products’ exploitation
and payments for watershed services have been proposed to make tropical
forest restoration financially viable (P.H.S.; Brancalion et al., 2017),
which could be bundled with carbon farming for more favorable financial
results. Notwithstanding, carbon farming will continue to be one of the
major demands of environmental organizations, private companies, and
governments supporting forest restoration in tropical regions, and
finding the most cost-effective restoration approaches for this
objective remains as a critical research challenge.