Assessing the growth and climate sensitivity of secondary forests in highly deforested Amazonian landscapes

Abstract

Tropical forests hold 30% of Earth’s terrestrial carbon and at least 60% of its terrestrial biodiversity, but forest loss and degradation are jeopardizing these ecosystems. Although the regrowth of secondary forests has the potential to offset some of the losses of carbon and biodiversity, it remains unclear if secondary regeneration will be affected by climate changes such as higher temperatures and more frequent extreme droughts. We used a data set of 10 repeated forest inventories spanning two decades (1999–2017) to investigate carbon and tree species recovery and how climate and landscape context influence carbon dynamics in an older secondary forest located in one of the oldest post‐Columbian agricultural frontiers in the Brazilian Amazon. Carbon accumulation averaged 1.08 Mg·ha−1·yr−1, and species richness was effectively constant over the studied period. Moreover, we provide evidence that secondary forests are vulnerable to drought stress: Carbon balance and growth rates were lower in drier periods. This contrasts with drought responses in primary forests, where changes in carbon dynamics are driven by increased stem mortality. These results highlight an important climate change–vegetation feedback, whereby the increasing dry‐season lengths being observed across parts of Amazonia may reduce the effectiveness of secondary forests in sequestering carbon and mitigating climate change. In addition, the current rate of forest regrowth in this region was low compared with previous pan‐tropical and Amazonian assessments—our secondary forests reached just 41.1% of the average carbon and 56% of the tree diversity in the nearest primary forests—suggesting that these areas are unlikely to return to their original levels on politically meaningful time scales.

Benchmark maps of 33 years of secondary forest age for Brazil

“…For this estimate, we considered a linear net carbon uptake rate of 3.05 ± 0.19 Mg C ha −1 yr −1 (mean for the neotropical secondary forests) 9 during the first 20-years of secondary forest succession, followed by a subsequent stabilization of the process, with a null growth 9,31,32 . Despite not considered in our estimates, it is important to highlight that carbon uptake rates vary among tropical secondary forests depending on climatic and environmental conditions 33 .…”

Large carbon sink potential of Amazonian Secondary Forests to mitigate climate change

“…Even in the regions of no disturbance and favourable environmental conditions, where SF AGC recovered to old-growth forest levels up to 4 times more rapidly, we estimated the minimum time taken to reach old-growth forest AGC to be ~100 years ( Supplementary Tables 8 and 9). SF will therefore never replace old-growth forests on policyrelevant timescales, stressing the continued need to conserve existing old-growth forests (Supplementary Table 9) 43 .…”

Secondary forests offset less than 10% of deforestation‐mediated carbon emissions in the Brazilian Amazon

“…Variation in local climate is known to influence carbon sequestration in secondary forest (Elias et al., 2019). However, accounting for it involves a number of spatial and temporal issues.…”

“…With the promotion of secondary forest growth being suggested as an important climate change mitigation strategy (Griscom et al., 2017; Pan et al., 2011; Rogelj et al., 2018), the need to improve our understanding grows more pressing. Second, the trajectory and rate of secondary forest growth are influenced by numerous climatic, landscape and local factors, which contribute to a 10‐fold difference in estimates of carbon sequestration rates across the tropics (Elias et al., 2019). Carbon accumulation in secondary forests is strongly linked to climatic conditions, with longer, more intense dry seasons, and lower annual rainfall known to slow accumulation (Poorter et al., 2016).…”