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

Abstract

The peat-forming wetland forests of Amazonia are characterized by high belowcarbon stocks and supply fruit, fibres and timber to local communities. Predicting the future of these ecosystem services requires understanding how hydrological conditions are related to tree species composition and the presence, or absence, of peat.Here, we use continuous measurements of water table depth over 2.5 years and manual measurements of pore-water pH and electrical conductivity to understand the ecohydrological controls of these variables across the large peatland complex in northern Peruvian Amazonia. Measurements were taken in permanent forest plots in four palm swamps, four seasonally flooded forests and four peatland pole forests. All trees ≥10 cm diameter were also measured and identified in the plots to assess floristic composition. Peat occurs in eight of these twelve sites; three seasonally flooded forests and one palm swamp are not associated with peat. Variation in tree species composition among forest types was linked to high flood levels (maximum flooding height) and pH: seasonally flooded forests experience high flood levels (up to 3.66 m from the ground surface) and have high pH values (6-7), palm swamps have intermediate flood levels (up to 1.34 m) and peatland pole forests experience shallow flooding (up to 0.28 m) and have low pH (4). In contrast, the presence of peat was linked to variation in maximum water table depth (i.e. the depth to which the water table drops below the ground surface). Surface peat is found in all forest types where maximum water table depth does not fall >0.55 m below the ground surface at any time.Peat formation and variation in tree species composition therefore have different ecohydrological controls. Predicted increases in the frequency and strength of flooding events may alter patterns of tree species composition, whereas increases in drought severity and declines in minimum river levels may pose a greater risk to the belowground carbon stores of these peatland ecosystems.

Influence of flooding variability on the development of an Amazonian peatland

“…Over 60% of grains were larger than 40 µm, which indicates the presence of Mauritia flexuosa . As this taxon is not tolerant of prolonged or frequent flooding above 1.2 m deep (Junk et al, 2015; Flores Llampazo et al, 2022), its presence may be used to infer a reduction in flooding depth and frequency. The prevalence of Poaceae undiff.…”

“…So‐called ‘open peatlands’ ( pantano abierto de turbera , or pantano herbaceo‐arbustivo ), which are named as such due to their significantly more open canopies compared with peatland palm swamps and pole forests, occupy a comparatively smaller area of 4888 km 2 (Draper et al, 2014; Draper, 2015; Hastie et al, 2022) and have an AGC of 41.26 ± 22.71 Mg C ha −1 , a BGC of 15.59 ± 8.72 Mg C ha −1 and SOC of 628.27 ± 131.85 Mg C ha −1 (Honorio Coronado et al, 2021). More rarely, peat has been found in seasonally flooded forests, specifically in the roughly 2% that maintain a consistently high water table throughout the year (Lähteenoja and Page, 2011; Flores Llampazo et al, 2022; Hastie et al, 2022).…”

“…Peat thickness at the site, as determined by augering, ranged from 30 cm at the edge to 580 cm in the centre. The peatland is permanently waterlogged, the water table being at or near the surface year‐round, enabling peat to accumulate (Flores Llampazo et al, 2022). At the time of sampling (July 2019), during the end of the wet season, standing water height measured 10–20 cm above the peat surface.…”

“…The Pastaza–Marañon Foreland Basin (PMFB), situated in the Loreto Department in northeast Peru, hosts the largest peatland area in Amazonia, most of which is hydrologically intact (Schulman et al, 1999; Freitas Alvarado et al, 2006; Lähteenoja et al, 2009a, b; Draper et al, 2014; Hastie et al, 2022). Within this region, four ecosystem types associated with peat have been identified, namely peatland palm swamps, peatland pole forests, open peatlands and occasionally seasonally flooded forests (Lähteenoja and Page, 2011; Flores Llampazo et al, 2022). Although these ecosystem types are floristically distinct in the PMFB (Draper et al, 2018; Honorio Coronado et al, 2021; Hastie et al, 2022), recent fieldwork and palynological investigations have revealed a range of intermediate ecosystems, suggesting greater complexity of peatland ecosystems and environments than had previously been appreciated.…”

“…At both sites, the shift towards a palm swamp, dominated by the Mauritia flexuosa palm, was attributed to changes in the flooding regime. This involved a shift from deep seasonal flooding (3–5 m), unsuitable for Mauritia flexuosa due to its intolerance to flooding above 1.2 m, to permanently waterlogged conditions with less frequent and/or shallower seasonal floods tolerated by Mauritia flexuosa (Junk et al, 2015; Flores Llampazo et al, 2022). Based on what is known about peatland terrestrialization and the possible patterns of ecological succession from lake to bog after foundational work by Walker (1970), the records at Quistococha and San Jorge match the ‘expected’ pathways of succession from open water to closed forest.…”