Intensive field sampling increases the known extent of carbon-rich Amazonian peatland pole forests

Abstract

Peatland pole forest is the most carbon-dense ecosystem in Amazonia, but its spatial distribution and species composition are poorly known. To address this knowledge gap, we quantified variation in the floristic composition, peat thickness, and the amount of carbon stored above and below ground of 102 forest plots and 53 transects in northern Peruvian Amazonia. This large dataset includes 571 ground reference points of peat thickness measurements across six ecosystem types. These field data were also used to generate a new land-cover classification based on multiple satellite products using a random forest classification. Peatland pole forests are floristically distinctive and dominated by thin-stemmed woody species such as Pachira nitida (Malvaceae), Platycarpum loretense (Rubiaceae), and Hevea guianensis (Euphorbiaceae). In contrast, palm swamps and open peatlands are dominated by Mauritia flexuosa (Arecaceae). Peatland pole forests have high peat thickness (274 ± 22 cm, mean ± 95% CI, n = 184) similar to open peatlands (282 ± 46 cm, n = 46), but greater than palm swamps (161 ± 17 cm, n = 220) and seasonally-flooded forest, terra firme, and white-sand forest where peat is rare or absent. As a result, peatland pole forest has exceptional carbon density (1,133 ± 93 Mg C ha−1). The new sites expand the known distribution of peatland pole forest by 61% within the Pastaza-Marañón Foreland basin, mainly alongside the Tigre river, to cover a total of 7540 km2 in northern Peruvian Amazonia. However, only 15% of the pole forest area is within a protected area, whilst an additional 26% lies within indigenous territories. The current low levels of protection and forest degradation but high threat from road paving projects makes the Tigre river basin a priority for conservation. The long-term conservation of peatland pole forests has the potential to make a large contribution towards international commitments to mitigate climate change.

The presence of peat and variation in tree species composition are under different hydrological controls in Amazonian wetland forests

“…Our results indicate that peatland pole forest also has a distinctive ecohydrological regime, with a low pH, a water table permanently close to the surface and a range in water table depth of less than 0.70 m. This ecohydrological regime would not be found if this forest type was derived from a palm swamp through selective harvesting of female M. flexuosa palms for their fruit, as palm swamps are associated with a greater range of water table depths and higher levels of flooding than peatland pole forests (Figure 2). A similar sex ratio of adult male:female individuals of M. flexuosa in palm swamps and peatland pole forests in the Tigre river basin also supports the argument that these two forest types are different natural formations (Honorio Coronado et al, 2021). Peatland pole forests therefore appear to be analogous to the small stature forests growing on domed ombrotropic peatlands in Southeast Asia (Draper et al, 2018; Lähteenoja et al, 2009b), rather than a degraded version of other forest types.…”

“…This wetland type is most common in the Ucayali and Marañón river basins (Draper et al, 2014). Peatland pole forest, locally known as ‘varillal hidromórfico’, is dominated by a few species of thin‐stemmed trees (Draper et al, 2018; Honorio Coronado et al, 2021). These forests occur typically on deep peat deposits and are rarely flooded (del Aguila‐Pasquel, 2017; Draper et al, 2014).…”

“…This wetland type is most commonly found between the Tigre and Marañón rivers (Draper et al, 2014; Honorio Coronado et al, 2021). Seasonally flooded forests are more diverse than palm swamps and pole forests (Draper et al, 2018), and occur on areas that flood for two to nine months during the year (Kvist & Nebel, 2001) and only occasionally have peat (Draper et al, 2014; Honorio Coronado et al, 2021; Hastie et al, 2022). Seasonally flooded forests occur alongside the major rivers of all basins but are particularly extensive alongside the Ucayali river (Draper et al, 2014).…”

“…Here, a range of ecosystem types, including palm swamps, peatland pole forests, open herbaceous swamps and seasonally flooded forests are associated with the presence of peat (Draper et al, 2014; Lähteenoja et al, 2012; Lähteenoja et al, 2009a; Ruokolainen et al, 2001). These ecosystems store large amounts of carbon below ground (Draper et al, 2014; Honorio Coronado et al, 2021; Hastie et al, 2022) and provide natural resources to local communities, supporting livelihoods and the regional economy (Padoch, 1988; Kvist & Nebel, 2001; Schulz et al, 2019). However, climatic changes related to the increased variability of the hydrological cycle leading to more locally severe drought and flooding events, as well as drainage associated with increasing demand for agricultural land, may lead to dramatic changes in the hydrological regime of these ecosystems (Baker et al, 2019; Gloor et al, 2013; Roucoux et al, 2017; Zulkafli et al, 2016).…”

“…Observations of the distribution of the different ecosystem types (palm swamps, peatland pole forests, open herbaceous swamps, and seasonally flooded forests; see Materials and Methods) and peat deposits in the PMFB suggest that the controls on peat formation do not correspond with hydrological controls on tree species composition. Peat formation can be associated with very different species composition above‐ground: for example, herbaceous swamps that look like open savannas with scattered stems of the palm Mauritia flexuosa , and peatland pole forests that have a closed canopy and are dominated by thin‐stemmed woody species, contain different species but both consistently have thick peat deposits (Honorio Coronado et al, 2021). Sites within the same forest type with similar species composition also vary in whether they are associated with the presence of peat.…”