Canadian Forest Service Publications

The ecohydrology of forested peatlands: simulating the effects of tree shading on moss evaporation and species composition. 2013. Kettridge, N.; Thompson, D.K.; Bombonato, L.; Turetsky, M. R.; Benscoter, B. W.; Waddington, J. M. Journal of Geophysical Research: Biogeosciences 118(2):422–435.

Year: 2013

Issued by: Northern Forestry Centre

Catalog ID: 35504

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1002/jgrg.20043

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Forested peatlands represent an important global carbon pool, storing 48.0 Pg of carbon within continental western Canada alone. Peatland hydrology regulates the carbon dynamics and future stability of this carbon store and provides a critical control on regional water dynamics. Drying associated with land-use change and climate change has the potential to increase tree growth, modifying the density, size, and spatial arrangement of trees. This can reduce peatland evaporation and offset the associated increase in transpiration. To determine the magnitude of this negative ecohydrological feedback, we simulated spatial variations in radiation, turbulent energy fluxes, and temperatures in peatlands with real and idealized tree densities and distributions. For a random tree distribution, an increase in tree density from 0 to 4 trees per m2 reduced available energy at the peat surface, decreasing average evaporation by 25%. At higher tree densities, feather moss species covered a larger fraction of the ground because of lower light availability. In combination with the lower energy availability, this change in moss composition reduced evaporation by ~70%. The reduction in evaporation was greater (83%) when the effects of increased canopy cover on peatland aerodynamic properties were incorporated. Additionally, we found that evaporation was dependent on the spatial arrangement of trees, with evaporation being higher when trees were clustered. Overall, our model showed that the trade-off between reduced evaporation and increased transpiration with increasing tree densities reduced landscape variation in evapotranspiration, with simulated evapotranspiration remaining approximately constant across a broad range of peatland ecosystems despite varying canopy densities.

Plain Language Summary

We used a combination of fieldwork and modelling to describe the interaction between feather mosses and peat-forming sphagnum mosses in forested bogs near Slave Lake, Alberta. We already knew that feather mosses can out-compete sphagnum mosses only in shadier areas with more trees. Working in the field, we found that feather mosses evaporate water more slowly than sphagnum mosses under the same weather conditions. We then used a physical model of competition between the two types of moss to examine the effect of increasing tree density on water losses in the whole ecosystem, both trees and mosses. We found that increasing the number of trees in a certain area increased the water lost directly by trees. Initially, this loss of water was compensated by increased shading of the moss and increased ground coverage by feather mosses. Beyond a certain level of tree density, however, the total amount of water lost by the ecosystem increased, causing the water table to decline, thus reducing carbon capture, and increasing fire risk.