From Sink to Source: Peatland Community Structure and Its Influence on Drought Response in Peatland CO2 Exchange

Canadian Peatland

Peatlands contain vast stores of organic carbon; however, these stores are sensitive to changes in temperature and moisture. Periods of drought can cause peatlands to temporarily act as sources of carbon rather than carbon sinks. As rising temperatures and the potential for more intense droughts in many areas are predicted under climate change projections, we could see an outflow of greenhouse gas emissions from peatlands instead of carbon sequestration. A previous article posted a few months ago brings a local perspective to this issue. This article looked at flux measurements taken during a drought at a bog in Polly’s Cove, Nova Scotia. It was noted that after our hot, dry summer last year, there was widespread drought stress in plant communities and severe drought conditions unlike any seen in recent years, with bogs and other similar systems experiencing drying. With all this in mind, the work of Kuiper et al. (2014) is becoming increasingly relevant as they examined how the presence of different plant functional types can affect the drought response of peatland carbon fluxes.

Plant Functional Types in the PeatlandsSphagnum.magellanicum.2

The study by Kuiper et al. examined three major plant functional types in their experimental peatland. Sphagnum mosses are the dominant species group in bogs, acting as ecosystem engineers, with interesting properties such as their ability to inhibit microbial activity, thereby controlling the decomposition and efflux of carbon from stored peat. They also looked at two dominant vascular plant species: ericoid dwarf-shrubs and graminoids.  Kuiper et al. set out to determine how exactly each of these plant functional types (PFT) affects the overall stability of the ecosystem in the face of disturbances such as drought, and how that affects the uptake of carbon by peatland systems; more specifically the resistance during, and the recovery after, a period of drought.

Kuiper et al. (2014)

Samples were taken in November 2009 from Tofte mose, Lille Vildmose Natural Park, Denmark and removed to a growth room in Utrecht University, The Netherlands. 32 intact peat cores were collected, half representative of each of two different microhabitats; wet lawns and relatively dry hummocks. Each core contained equal amounts of the three dominant PFTs. Keeping conditions similar to the natural environment of their respective microhabitats, the cores were allowed to acclimate before measurements started. To examine how the presence of ericoids and graminoids affect the resistance and recovery of the CO2 uptake, the cores were divided up amongst four different sets of treatments: a control, a set each with only one of the vascular plants removed and one with both removed to leave only Sphagnum. The experiment consisted of three different periods: pre-drought, drought (where the cores were allowed to dry) and then the recovery period after re-wetting. Measurements were taken using closed flux chamber systems. They monitored two different elements for carbon exchange; the net ecosystem CO2 exchange (NEE), which was measured regularly during the procedure, and the total ecosystem respiration (Re), which was measured before and after the initial removal of the  vascular PFTs and then throughout the course of the drought and recovery by darkening the chambers.

Their Findings

Removing vascular PFTs resulted in a lower initial NEE before the drought period. For lawns, graminoid removal or the removal of both reduced NEE, while for hummocks it was only the ericoid removal. As NEE was a way of measuring net ecosystem productivity, this means that without the vascular plants present and the loss of that biomass, the peatland system was less productive even before the drought.

20170221073643

Resistance to drought: both NEE and Re decreased immediately after the beginning of the drought period in all treatments. The rate at which they decreased, and therefore their resistance to drought, differed between microhabitats; the lawns saw a faster decrease in NEE than the hummocks, while the opposite was true of ecosystem respiration. Cores from the lawn microhabitats switched to acting as CO2 sources faster than those from the hummocks, and the removal of graminoids accelerated this process. This lower resistance in the lawn cores also resulted in a lower level of NEE and therefore productivity after the drought event was over.

Resilience to drought, while they did find there was an initial burst of CO2 emissions immediately after rewetting, uptake began again shortly after. The removal of the vascular PFTs didn’t have much of an overall effect on the resilience of the peat, but again, cores from the hummocks were able to recover faster.  

Their findings show that, while vascular plants certainly contribute to the overall ecosystem productivity of the peatland, they do not play as large a role in combating the effects of drought, particularly in terms of recovery. They do help in a sense, however, with the resistance of the peat to drought, as they contribute to higher initial levels of ecosystem productivity, it will take longer for the peatland to switch from being a CO2 sink to a CO2 source. Interestingly, they also found that even ten weeks after rewetting, the CO2 uptake was still only 45% of what it was before the drought. This demonstrates the importance of protecting undisturbed peatlands, as these results show how even a single drought event can significantly impact the ability of these regions to sequester carbon.  Kuiper et al. did acknowledge that as a laboratory based plant-removal experiment, there are certain inherent limitations to the applicability of the data in natural systems. In-situ measurements, similar to those taken during the drought at Polly’s Cove bog, can help corroborate, and extend the results from Kuiper et al. to improve our understanding of these natural systems.

Peatland_Pool_-_geograph.org.uk_-_496291 (1)