Master of Science (MSc)
Geography & Environmental Studies
Faculty of Arts
Forested peatlands in the Western Boreal Plain (WBP) represent hydrologically sensitive ecosystems that often support an open-crown forest of Picea mariana and/or Larix laricina. These systems store globally significant soil carbon, containing one-fourth to one-third of the world’s soil organic carbon pool (Turunen et al., 2002),serving a critical role in regulating atmospheric CO2. Recent studies indicate that the hydrological conditions are the critical determinant of a peatland’s carbon budget (Price et al., 2005; Aurela et al., 2007).To understand current hydrological conditions, it is essential to accurately estimate the rate of ET, due to its dominance within a peatland’s water balance (Price and Maloney, 1994; Fraser et al., 2001; Lafleur, 2008). The mechanism by which peatlands retain and exchange water with the atmosphere is important to maintain the stability of these systems. However, this stability is threatened by the impacts of both warmer and drier conditions associated with climate change, and altered hydroclimatic cycles as a result of landscape disturbance. Increasing drought (frequency and severity) has the potential to increase tree growth, modifying density, size and spatial arrangement of the trees (Kettridge et al., 2013). This expansion impedes incoming solar radiation from reaching the peat surface, potentially limiting surface evapotranspiration (ET), which at present, represents the main flux water loss from these systems. A reduction in surface ET(ETsurf)could further produce a reduction in total fen ET, despite predicted increases in canopy transpiration (T) attributed to the higher stem density.
This research partitions ET between the canopy and understory between two typical fens, under current climate conditions, within the oil sands region of Fort McMurray, Alberta. The effects of climate, tree canopy and surface vegetation on the energy balance and ET processes were analyzed using a micrometeorology (MET) and eddy-covariance (EC) data in two typical Western Boreal Plain (WBP) fens during the growing 2013 season. Flux data were partitioned through the application of the stem heat balance (SHB) method and dynamic closed chambers. The two fens are distinguished as a poor fen with an open canopy composed of Picea mariana, and a rich fen, with a dense Larix laricina canopy. Additionally, the two fens are distinguished by differences in localized climate, with the poor fen subjected to significantly cooler air (Ta) and soil (Tg) temperatures.
The energy balance of both fens was regulated by the latent heat flux (QE). The seasonal pattern of ET was closely linked with growing season net radiation (Q*),vapour pressure deficit (VPD), Ta and precipitation (P) events, averaging 2.3 mm d-1 and 3.5 mm d-1 between the poor, open canopy fen and rich, dense canopy fen, respectively. A strong, positive linear correlation was exhibited between the control parameters Q*, Ta, and daily transpiration (T).Seasonal mean T rates varied over the four month growing season at the Picea mariana poor fen, averaging 0.3 mm d-1, while T rates at the Larix laricina rich fen supplied a higher contribution to the fen’s total ET flux, averaging 2.7 mm d-1. Both ET and T reached maxima in conditions of high Q*, Ta, and moderate to high VPD, that coincided with lower relative humidity (RH) and moderate windspeed (u). Neither ET nor T demonstrated a direct relationship with volumetric moisture content (VMC), due to the consistently high water table, generally at or above the peat surface, maintained at both sites.
The poor fen’s discontinuous Picea mariana canopy permitted a larger degree of incoming radiation to reach the underlying peat surface, while the rich fen’s higher tree density composed of the Larix laricina, limited incoming radiation due to shading. Subsequently, surface vegetation of the former was dominated by Sphagnum moss, while the latter was composed of a variety of feather moss and the brown moss, Tomenthypnum nitens. The poor fen’s open canopy and dominant Sphagnum moss resulted in the dominance of the ETsurf, with a mean of 0.8 mm d-1, contributing approximately > 80% to the daily ET budget. Conversely, the rich fen’s dense canopydiminished the impact of ETsurf to 0.5 mm d-1, contributing < 20% to the total ET flux. Increased tree density from a Picea mariana open-canopy, to a Larix laricina dense canopy,reduced average PAR reaching the underlying surface to < 500 μmolm-2 s-1 and < 300 μmol m-2 s-1. Although the presence of an overstory did not produce a microclimate that was statistically different between open and covered plot conditions, it did generally support cooler, wet conditions that inhibited ETsurf.
Gabrielli, Elise C., "Partitioning Evapotranspiration in Forested Peatlands within the Western Boreal Plain, Fort McMurray, Alberta, Canada" (2016). Theses and Dissertations (Comprehensive). 1820.