Abstract

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 (G(s)), water- and light-use efficiency and surface-atmosphere coupling of European boreal coniferous forests was explored using eddy-covariance (EC) energy and CO2 fluxes. The observed scaling relations were then explained using a biophysical multilayer soil-vegetation-atmosphere transfer model as well as by a bulk G(s) representation. 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 0.5-7.0m(2) m(-2). Both ET and G(s) experienced a minimum in the LAI range 1-2m(2) m(-2) caused 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 similar to 2m(2) m(-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 <3m(2) m(-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.

Keywords

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

Published in

Global Change Biology
2016, Volume: 22, number: 12, pages: 4096-4113

SLU Authors

• Grelle, Achim

  • Department of Ecology, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Forest Science
Ecology


Publication identifier

DOI: https://doi.org/10.1111/gcb.13497

Permanent link to this page (URI)

https://res.slu.se/id/publ/79899