Supporting Information Appendix A2. The applicability of soil heat transfer equation for
modeling of heat transfer in sub-arctic bryophytes.
According to Koorevaar, Menelik & Dirksen (1983) in soils being warmed by the sun at day
and releasing energy at night the decrease of soil temperature can be calculated as:
A  Ao * exp( c * s / d ) ,
(1)
where A is the temperature amplitude at depth s, A0 is the temperature amplitude at the soil
surface, d is soil thermal diffusivity and c is a constant depending on fluctuation frequency,
equaling 0.36 for a 24 hour cycle.
Equation 1 describes an idealized situation, when the air temperature (i.e. temperature on the
bare soil in our situation) only fluctuates around a constant average, without a consistent
decrease or increase trend. We considered this idealization to be applicable for the data of the
growing season, based on the weather records of Abisko research station collected in 19132008 (Zhenlin, Hanna & Callaghan 2011): mean temperature in June, July, August, first half
of September were 8.27, 11.69, 10.05, 7.20 ºC, respectively.
Further on Equation 1 does not account for latent heat fluxes. However, although the
bryophyte mats are recognized as a major contributor to the total evapotranspiration in tundra
(Stuart, Oberbauer & Miller 1982; McFadden, Eugster & Chapin 2003; Heijmans, Arp &
Chapin 2004), due to cool and wet subarctic weather and poor soil conditions restraining
photosynthesis, the net evapotranspiration values reported for polar regions are low: typically
less than 0.5-1.5 mm/day, hardly reaching 2 mm/day at the warmest mid-summer days
(McFadden, Eugster & Chapin 2003; Heijmans, Arp & Chapin 2004; Fitzjarrald & Moore
1992; Harding & Lloyd 1998; Soegaard et al. 2001). Indeed, numerous studies (cf. reviews
Sellers et al. 1997; Wang & Dickinson 2012) point out that heat fluxes in tundra and boreal
areas are dominated by sensible heat with latent heat constituting a small fraction of the total
heat flux. Blok et al. (2011) directly showed in similar climatic conditions that the
temperature under the surface of bryophyte mats and bare soil was not noticeably reduced by
evapotranspiration. Moreover, in our own field experiment we also detected absence of soil
temperature reduction effect of bryophytes observed over the whole vegetation season (see
the Results for details), which as well suggests that latent heat fluxes in bryophyte mats are
very small. Therefore, we considered ignorance of these fluxes in bryophyte mats to be an
acceptable assumption in our model.
References
Blok , D., Heijmans, M.M.P.D., Schaepman-Strub, G., van Ruijven, J., Parmentier, F.J.W.,
Maximov, T.C. & Berendse, F. (2011) The cooling capacity of mosses: controls on
water and energy fluxes in a siberian tundra site. Ecosystems, 14, 1055–1065.
Fitzjarrald, D.R. & Moore, K.E. (1992) Turbulent transports over tundra. Journal of
Geophysical Research-Atmospheres, 97, 16717-16729.
Harding, R.J. & Lloyd, C.R. (1998) Fluxes of water and energy from three high latitude
tundra sites in Svalbard. Nordic Hydrology, 29, 267-284.
Heijmans, M., Arp, W.J. & Chapin, F.S. (2004) Controls on moss evaporation in a boreal
black spruce forest. Global Biogeochemical Cycles, 18.
Koorevaar, P., Menelik, G. & Dirksen, C. (1983) Elements of soil physics. Elsvier,
Amsterdam.
McFadden, J.P., Eugster, W. & Chapin, F.S. (2003) A regional study of the controls on water
vapor and CO2 exchange in arctic tundra. Ecology, 84, 2762-2776.
Sellers, P.J., Hall, F.G., Kelly, R.D., Black, A., Baldocchi, D., Berry, J., Ryan, M., Ranson,
K.J., Crill, P.M., Lettenmaier, D.P., Margolis, H., Cihlar, J., Newcomer, J.,
Fitzjarrald, D., Jarvis, P.G., Gower, S.T., Halliwell, D., Williams, D., Goodison, B.,
Wickland, D.E. & Guertin, F.E. (1997) BOREAS in 1997: Experiment overview,
scientific results, and future directions. Journal of Geophysical ResearchAtmospheres, 102, 28731-28769.
Soegaard, H., Hasholt, B., Friborg, T. & Nordstroem, C. (2001) Surface energy- and water
balance in a high-arctic environment in NE Greenland. Theoretical and Applied
Climatology, 70, 35-51.
Stuart, L., Oberbauer, S. & Miller, P.C. (1982) Evapotranspiration measurements in
eriophorum-vaginatum tussok tundra in Alaska. Holarctic Ecology, 5, 145-149.
Wang, K.C. & Dickinson, R.E. (2012) A review of global terrestrial evapotranspiration:
observation, modeling, climatology, and climatic variability. Reviews of Geophysics,
50.
Zhenlin, Y., Hanna, E. & Callaghan, T.V. (2011) Modelling surface-air-temperature variation
over complex terrain around Abisko, Swedish Lapland: uncertainties of measurements
and models at different scales. Geografiska Annaler Series a-Physical Geography,
93A, 89-112.