Northern peatlands have acted as persistent sinks of CO2 throughout the Holocene largely owing to their ability to maintain shallow water table depths that limit decomposition rates and supports the growth of keystone vegetation including Sphagnum mosses. There is concern, however, that the future success and ecosystem function of these northern peat deposits may be at risk to climate change, where temperatures and evaporation rates are predicted to increase substantially in the next century. While numerous studies have examined the hydrology and carbon dynamics in large expansive peatland systems where a water table (WT) is ever-present, relatively little research has been done on small scale peat-accumulating systems where their vulnerability remains unknown. One region where a broad spectrum in the scale of peat accumulation is present is in the bedrock depressions of Canadian Shield rock barrens, which are of special importance as many peat deposits here provide habitat to species at risk including the Blanding’s Turtle and the Massassauga Rattlesnake. This study examines the controls that govern water storage dynamics and moss water availability in 18 different peat-accumulating depressions that vary in size, catchment area, and sediment composition.
The magnitude of WT variability was often several times greater in shallower bedrock depressions (<50 cm deep) as compared to deeper ‘bogs’ (>60 cm deep). The magnitude of depression WT variability appeared to be closely linked to the WT depth (WTD), the relative proportions of different sediment types within the depression, and the depth dependant specific yield (Sy) of each sediment type. Sites which contained large fractions of Polytrichum moss or mineral soil – which were more common in shallow depressions ¬¬– had the greatest WT variability due to the lower porosity and Sy of this sediment as compared to Sphagnum peat. Sphagnum dominated ‘vernal pools’ (30-50 cm deep) had a WT variability two to three times greater than Sphagnum dominated bogs at WTDs > 20-25 cm, which may be related to exceptionally high ash concentrations near the base of vernal pools which reduced peat porosity and Sy as compared to more organic-rich peat. As compared to bogs, pits (<15 cm deep) and vernal pools had greater rates of WT decline during drying intervals, deeper average WTDs when a WT was present, and extended periods of WT absence during the summer months. As such, moss growing in pits and vernal pools generally had lower near-surface water availability as compared to bogs, though the importance of depression depth in determining the timing of moss stress is also dependant on the hydrophysical properties (Kunsat and moisture retention) of the moss species in question. WT dynamics and moss water availability were generally weakly correlated to depression catchment size, although during wetter periods of the year the rate of WT recession was moderated in pits and vernal pools which had an upslope depression that could provide sustained water inputs for multiple days after rainfall. The results of this study suggest that depression depth may be a first order control in determining peatland vulnerability to future regime shifts induced by external forcings or disturbances. Furthermore, this study suggests that systematic differences may exist between the hydrophysical properties of peat in shallow vs. large bedrock depressions, potentially resulting from contrasts in fire frequency/severity, and/or the degree of humification/compression among geological settings. / Thesis / Master of Science (MSc) / Canada is home to one of the largest reservoirs of organic carbon stored on land in the world, in unique ecosystems called peatlands. Peatlands are formed in wetland environments where a thick layer of organic matter has accumulated over time due to the average rate of vegetation growth on the surface of peatlands exceeding the rate of decomposition of the underlying organic matter. This net accumulation of organic matter over time has caused peatlands to act as a long term sink of carbon dioxide, which is a greenhouse gas that is a primary driver of global warming. The ability of peatlands to have slow decomposition rates and support the growth of key peatland vegetation, most notably various species of ‘peat moss’, is highly dependent upon their ability to keep their water table (i.e. the surface below which pore spaces in the organic matter are saturated with water) close to their growing surface. There is concern, however, that a warmer and dryer climate in the future could cause deeper water table positions in peatlands, thereby increasing decomposition rates, decreasing the growth rate of peat moss, and potentially turning peatlands into a net source of carbon dioxide.
Most peatland studies to date, however, have focused on water storage/movement and carbon exchange in large, deep peatland systems, whereas relatively little research has been conducted on smaller peatlands. As such, the vulnerability of these smaller peatlands to future climate change remains uncertain. One region where peatlands exist over a wide range of different sizes and landscape positions is in bedrock depressions of the Canadian Shield, which are of special interest as they also provide habitat for species at risk including the Blanding’s Turtle and the Massassauga Rattlesnake. This study looked at how the water table positions and water availability to different species of peat moss compared over the growing season between 18 peatlands of different sizes and landscape position (i.e. peatlands with a relatively ‘small’ and ‘large’ area upslope of them).
This study finds that deeper peatlands (with organic matter layers > 60 cm deep) usually had a shallower water table over the summer months than shallower peatlands (< 50 cm deep), primarily due to differences in the properties of the organic matter underlying their growing surfaces. Furthermore, each of the 12 studied peatlands < 50 cm deep lost their water table for a considerable amount of time during the summer (when their water table position dropped below the underlying bedrock of the depression), whereas each of the six peatlands > 60 cm deep had a water table present for the entire growing season. Surprisingly, a peatland’s position on the landscape seemed to have a relatively minor effect on determining the depth/presence of its water table. As deeper peatlands usually had a water table that was closer to the growing surface and was always present, more moisture was available to the peat moss growing at their surface than for peat moss in shallower depressions, though this moisture availability also depended upon the growth form of the different species of peat moss (some species of peat moss were better at accessing subsurface water than others). Through its impact on water table positions and moisture availability for peat moss, peatland depth is likely a primary control governing peatland vulnerability climate change, with shallower peatlands being more vulnerable to warmer and dryer conditions in the future.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/20536 |
| Date | January 2016 |
| Creators | Didemus, Benjamin |
| Contributors | Waddington, James Michael, Geography and Earth Sciences |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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