The high hydraulic conductivity of three wooded tropical peat swamps in northeast Peru : measurements and implications for hydrological function

This research was supported by a NERC PhD studentship to the lead author, as well as by a Dudley Stamp Memorial Award from the Royal Geographical Society.The form and functioning of peatlands depend strongly on their hydrological status, but there are few data available on the hydraulic properties of tropical peatlands. In particular, the saturated hydraulic conductivity (K) has not previously been measured in neotropical peatlands. Piezometer slug tests were used to measure K at two depths (50 and 90 cm) in three contrasting forested peatlands in the Peruvian Amazon: Quistococha, San Jorge and Buena Vista. Measured K at 50 cm depth varies between 0.00032 and 0.11 cm s−1, and at 90 cm, it varies between 0.00027 and 0.057 cm s−1. Measurements of K taken from different areas of Quistococha showed that spatial heterogeneity accounts for ~20% of the within-site variance and that depth is a good predictor of K. However, K did not vary significantly with depth at Buena Vista and San Jorge. Statistical analysis showed that ~18% of the variance in the K data can be explained by between-site differences. Simulations using a simple hydrological model suggest that the relatively high K values could lead to lowering of the water table by >10 cm within ~48 m of the peatland edge for domed peatlands, if subjected to a drought lasting 30 days. However, under current climatic conditions, even with high K, peatlands would be unable to shed the large amount of water entering the system via rainfall through subsurface flow alone. We conclude that most of the water leaves these peatlands via overland flow and/or evapotranspiration.PostprintPeer reviewe

High permeability explains the vulnerability of the carbon store in drained tropical peatlands

Tropical peatlands are an important global carbon (C) store but are threatened by drainage for palm oil and wood pulp production. The store's stability depends on the dynamics of the peatland water table, which in turn depend on peat permeability. We found that an example of the most abundant type of tropical peatland—ombrotrophic domes—has an unexpectedly high permeability similar to that of gravel. Using computer simulations of a natural peat dome (NPD) and a ditch-drained peat dome (DPD) we explored how such high permeability affects water tables and peat decay. High permeability has little effect on NPD water tables because of low hydraulic gradients from the center to the margin of the peatland. In contrast, DPD water tables are consistently deep, leaving the upper meter of peat exposed to rapid decay. Our results reveal why ditch drainage precipitates a rapid destabilization of the tropical peatland C store

First estimates of fine root production in tropical peat swamp and terra firme forests of the central Congo Basin

Tropical peatlands are carbon-dense ecosystems because they accumulate partially-decomposed plant material. A substantial fraction of this organic matter may derive from fine root production (FRP). However, few FRP estimates exist for tropical peatlands, with none from the world's largest peatland complex in the central Congo Basin. Here we report on FRP using repeat photographs of roots from in situ transparent tubes (minirhizotrons), measured to 1 m depth over three one-month periods (spanning dry to wet seasons), in a palm-dominated peat swamp forest, a hardwood-dominated peat swamp forest, and a terra firme forest. We find FRP of 2.6 ± 0.3 Mg C ha-1 yr-1, 1.9 ± 0.5 Mg C ha-1 yr-1, and 1.7 ± 0.1 Mg C ha-1 yr-1 in the three ecosystem types respectively (mean ± standard error; no significant ecosystem type differences). These estimates fall within the published FRP range worldwide. Furthermore, our hardwood peat swamp estimate is similar to the only other FRP study in tropical peatlands, also hardwood-dominated, from Micronesia. We also found that FRP decreased with depth and was the highest during the dry season. Overall, we show that minirhizotrons can be used as a low-disturbance method to estimate FRP in tropical forests and peatlands

Constraining recent ice flow history at Korff Ice Rise, West Antarctica, using radar and seismic measurements of ice fabric

The crystal orientation fabric of ice reflects its flow history, information which is required to better constrain projections of future ice sheet behavior. Here we present a novel combination of polarimetric phase‐sensitive radar and seismic anisotropy measurements to provide independent and consistent constraints on ice fabric at Korff Ice Rise, within the Weddell Sea sector of West Antarctica. The nature and depth distribution of fabric in the ice column is constrained using the azimuthal variation in (1) the received power anomaly and phase difference of polarimetric vertical radar soundings and (2) seismic velocities and shear wave splitting measurements. Radar and seismic observations are modeled separately to determine the nature and strength of fabric within the ice column. Both methods indicate ice fabric above 200‐m depth which is consistent with present‐day ice‐divide flow. However, both measurements also indicate an oblique girdle fabric below 230‐m depth within the ice column, inconsistent with steady state divide flow. Our interpretation is that this deeper fabric is a remnant fabric from a previous episode of flow, which is currently being overwritten by ongoing fabric development associated with the present‐day flow regime. The preexisting fabric is consistent with ice flow from the south prior to ice‐divide formation, in agreement with models of Holocene ice sheet evolution. These findings apply new constraints to the flow history at Korff Ice Rise prior to divide formation and demonstrate the capacity of radar and seismic measurements to map fabric and thus constrain past ice flow

Ebullition of methane from peatlands: Does peat act as a signal shredder?

Bubbling (ebullition) of greenhouse gases, particularly methane, from peatlands has been attributed to environmental forcings, such as changes in atmospheric pressure. However, observations from peat soils suggest that ebullition and environmental forcing may not always be correlated and that interactions between bubbles and the peat structure may be the cause of such decoupling. To investigate this possibility, we used a simple computer model (Model of Ebullition and Gas storAge) to simulate methane ebullition from a model peat. We found that lower porosity peat can store methane bubbles for lengthy periods of time, effectively buffering or moderating ebullition so that it no longer reflects bubble production signals. Our results suggest that peat structure may act as a “signal shredder” and needs to be taken into account when measuring and modeling ebullition

Prospectus, October 1, 1986

https://spark.parkland.edu/prospectus_1986/1023/thumbnail.jp

The effect of sampling effort on estimates of methane ebullition from peat

We investigated the effect of sample size and sampling duration on methane bubble flux (ebullition) estimates from peat using a computer model. A field scale (10 m), seasonal (> 100 days) simulation of ebullition from a two-dimensional structurally-varying peat profile was modelled at fine spatial resolution (1 mm × 1 mm). The spatial and temporal scale of this simulation was possible because of the computational efficiency of the reduced complexity approach that was implemented, and patterns of simulated ebullition were consistent with those found in the field and laboratory. The simulated ebullition from the peat profile suggested that decreases in peat porosity – which cause increases in gas storage – produce ebullition that becomes increasingly patchy in space and erratic in time. By applying different amounts of spatial and temporal sampling effort it was possible to determine the uncertainty in ebullition estimates from the peatland. The results suggest that traditional methods to measure ebullition can equally overestimate and underestimate flux by 20% and large ebullition events can lead to large overestimations of flux when sampling effort is low. Our findings support those of field studies, and we recommend that ebullition should be measured frequently (hourly to daily) and at many locations (n > 14)

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

This research was funded by the Gordon and Betty Moore Foundation, through grant #5349 ‘Monitoring protected areas in Peru to increase forest resilience to climate change’, and NERC standard grant ‘Carbon Storage in Amazonian Peatlands: Distribution and Dynamics’(NE/R000751/1).The peat-forming wetland forests of Amazonia are characterised by high below-carbon 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 (ie 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.Publisher PDFPeer reviewe