Using Nonvascular Water Storage to Characterize Controls of Rainfall and Condensation on Understory Evapotranspiration in a Mature Jack Pine Stand
Date
2025-06-13
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-1987-3314
Type
Thesis
Degree Level
Masters
Abstract
This study examined the contribution of a moss and lichen understory to the total ecosystem evapotranspiration (ET) of a mature jack pine forest in the boreal plains ecozone of central Saskatchewan, Canada. Using laboratory-calibrated electrical impedance sensors to estimate the water-storage of three nonvascular forest floor cover types—lichen-dominant, moss-dominant, and mixed lichen and moss—this study made observations in a boreal stand from June through October, 2023, to observe the understory response to rainfall and condensation as well as a relationship to below-canopy ET determined using eddy covariance. Additionally, the controls on the contribution of the lichen and moss mat understory exerted by its water storage were characterized, demonstrating the importance of nonvascular processes in boreal hydrology. The hydrology of nonvascular plants is a poorly understood topic. Despite their importance and contribution to ET, lichens and mosses are often poorly represented in models or not included at all. Fire prediction and hydrologic modelling would greatly benefit from measurements of such a key water storage, especially in the use of those measurements to examine fluxes of water between the atmosphere, understory vegetation, and soil.
Observations using impedance sensors showed wetting responses to rainfall and condensation events as well as drying during periods of high ET. This sensitivity allowed for comparisons of response to different types of events. The lichen and moss understory showed similar wetting behaviour during rainfall and condensation, though the latter served mostly to keep the understory wet by regular dampening from morning dew and the former served to more heavily saturate it. Days without condensation exhibited consequential drops in below-canopy ET while longer-duration rainfall events resulted in higher contribution of the understory to total ecosystem ET for longer after rainfall. Moss-dominant understory ground cover showed the highest water storage and lichen-dominant the least.
Below-canopy ET contributed 51% of ET observed above the canopy. Understory contribution was highest 2-8 hours after last rain at 62%. Early understory ET was suppressed by rainfall interception of the lichen and moss mat and decreased thereafter with time after rainfall to a minimum of 29% after one week. Though understory ET followed a diurnal cycle, it showed no notable relationship to shallow soil water. Contrastingly, above-canopy ET was independent of time after rainfall and more dependent on soil moisture and vapour pressure deficit. This was consistent with the ability of the overstory to uptake soil water through roots while the understory depended on atmospheric water directly in the form of rainfall and condensation. The understory ET responded more quickly than that above due to a greater nonvascular capacity to transpire in the presence of canopy interception storage. Due in part to its high interception capacity, the lichen and moss understory also showed wet conditions for longer than the overstory or leaf wetness measurements made below the canopy. Long-duration rainfall events resulted in higher saturation of the understory which allowed it to remain wet for longer, a phenomenon exacerbated by regular condensation which the nonvascular lichens and mosses could absorb. This maintenance of understory wetness by frequent wetting events was mirrored in energy partitioning and understory ET alike. Latent heat fluxes dominated during wet conditions and lasted longer after rainfall below the canopy than above. Similarly, contribution to total ecosystem ET stayed higher for longer after long-duration wetting and was reduced after an absence of morning condensation.
The lichen and moss understory influenced ET through water storage mechanisms which behave differently from a vascular canopy. Nonvascular ET was shown to be dependent on differing water sources, to have different evaporation and transpiration controls, and exhibit different partitioning of latent and sensible heat fluxes. This study affirms the need to account for nonvascular vegetation and its distinct properties in our understanding of boreal hydrology.
Description
Keywords
Nonvascular, Evapotranspiration, Eddy Covariance, Boreal, Forest, Hydrology, Moss, Lichen, Rainfall, Condensation, Understory, Interception, BERMS
Citation
Degree
Master of Science (M.Sc.)
Department
Civil and Geological Engineering
Program
Civil Engineering