Work package no 12: Integrative modelling activities

A hierarchy of models scaling up from the individual leaf or branch to the stand and the landscape (catchment) will be used to quantify controls on CO₂ and water fluxes within the Alpine ecosystems and landscapes (watersheds). A hierarchy of models is required in order to bring together information on species-specific response to climate conditions, large differences in vegetation canopy structure due to natural variation in vegetation distribution and to anthropogenic influences), and elevation and slope effects on climate which affect phenology and vegetation composition.

	Leaf and Branch Gas Exchange: Dependent on the determined light climate, temperature, relative humidity, carbon dioxide concentration, and wind speed at a particular point in the canopy, the momentary steady-state gas exchange response of leaves will be calculated. At the level of individual leaves, species-specific stomatal control and "responsiveness" to changes in radiation, temperature, and water stress will be included in the models.
	Canopy Gas Exchange: The model STANDFLUX will provide a framework for integrating three-dimensional aspects of forest stand structure and light interception, one-dimensional aspects (with depth) of stand microclimate, and the gas exchange behaviour of plant organs (needles, branches with needles, and potentially respiring branches) distributed throughout the stand. GASFLUX is an alternative model that may be applied to vegetation of relatively homogeneous structure, such as that in meadows and grasslands at high elevation.
	Landscape Gas Exchange: To model the partitioning of water flux between transpiration and discharge at the landscape scale, initial conditions will be defined within a GIS based on remote sensing, maps, or field studies. From this GIS, homogeneous "source areas" with similar vegetation, soils, and topography may be recognized that exhibit essentially the same potential response. From the coded "source area" information derive spatial "correlative models" will be derived, which provide boundary conditions affecting the simulation of vegetation or soil processes. Simultaneously, techniques will be developed to derive parameter values for spatial hydrological models, such as the "topographic index" of TOPMODEL which expresses hydrologic similarity of source areas dependent on their upslope contributing area and local topographic gradient. Spatially explicit landscape assessments of the effects of vegetation change on water balance, watershed discharge, carbon dioxide exchange, and dry deposition will be achieved by coupling 1) the spatial climate models, 2) the semi-distributed hydrological TOPMODEL and 3) the ecosystem BIG-LEAF stand process simulator.

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