Do the energy fluxes and surface conductance of boreal coniferous forests in Europe scale with leaf area?
Earth observing systems are now routinely used to infer leaf area index (LAI) given its significance in spatial aggregation of land surface fluxes. Whether LAI is an appropriate scaling parameter for daytime growing season energy budget, surface conductance (Gs), water- and light-use efficiency and surface-atmosphere coupling of European boreal coniferous forests was explored using eddy-covariance (EC) energy and CO2fluxes. The observed scaling relations were then explained using a biophysical multilayer soil-vegetation-atmosphere transfer model as well as by a bulk Gsrepresentation. The LAI variations significantly alter radiation regime, within-canopy microclimate, sink/source distributions of CO2, H2O and heat, and forest floor fluxes. The contribution of forest floor to ecosystem-scale energy exchange is shown to decrease asymptotically with increased LAI, as expected. Compared with other energy budget components, dry-canopy evapotranspiration (ET) was reasonably 'conservative' over the studied LAI range 0.5-7.0 m2m-2. Both ET and Gsexperienced a minimum in the LAI range 1-2 m2m-2caused by opposing nonproportional response of stomatally controlled transpiration and 'free' forest floor evaporation to changes in canopy density. The young forests had strongest coupling with the atmosphere while stomatal control of energy partitioning was strongest in relatively sparse (LAI ~2 m2m-2) pine stands growing on mineral soils. The data analysis and model results suggest that LAI may be an effective scaling parameter for net radiation and its partitioning but only in sparse stands (LAI <3 m2m-2). This finding emphasizes the significance of stand-replacing disturbances on the controls of surface energy exchange. In denser forests, any LAI dependency varies with physiological traits such as light-saturated water-use efficiency. The results suggest that incorporating species traits and site conditions are necessary when LAI is used in upscaling energy exchanges of boreal coniferous forests.
Nature Resources Institute Finland, Environmental Impacts of Production, Jokiniemenkuja 1, Vantaa, Finland.
Samuli
S
Launiainen
Nicholas School of the Environment, Duke University, PO Box 90328, Duke University, Durham, NC 27708-0328, USA.
Gabriel G
GG
Katul
Department of Physics, University of Helsinki, PO Box 64, 00140 University of Helsinki, Finland.
Pasi
P
Kolari
Department of Earth and Ecosystem Sciences, Lund University, Sölvegatan 12, Lund, 223 62, Sweden.
Anders
A
Lindroth
Finnish Meteorological Institute, P.O. Box 503, FI-00101, Helsinki, Finland.
Annalea
A
Lohila
Finnish Meteorological Institute, P.O. Box 503, FI-00101, Helsinki, Finland.
Mika
M
Aurela
A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Leninsky pr. 33, Moscow, 119071, Russia.
Andrej
A
Varlagin
Department of Ecology, Swedish University of Agricultural Sciences, PO Box 7044, Uppsala, 750 07, Sweden.
Achim
A
Grelle
Department of Physics, University of Helsinki, PO Box 64, 00140 University of Helsinki, Finland.|Department of Forest Sciences, University of Helsinki, PO Box 27, 00140, Helsinki, Finland.
Timo
T
Vesala
D002245:Carbon Dioxide / Q000032:analysis; D005060:Europe; D018515:Plant Leaves / Q000502:physiology*; D018526:Plant Transpiration; D066188:Taiga; D014197:Trees / Q000502:physiology*
D002245:Carbon Dioxide
boreal forest; ecosystem modeling; eddy-covariance; energy budget; evapotranspiration; latent and sensible heat flux; leaf area index; light-use efficiency; remote sensing; water-use efficiency