Landscapes Forest and Global Change - ESA - Escola Superior ...

N. Zurbriggen et al. 2010. Modeling feedbacks between avalanches and forests under a changing environment
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snow profile, temperature). The distinct weather conditions within a few days of the event,
which in reality strongly influence avalanche release probability, were not included due to the
larger time scale of TreeMig (1 year steps). To be compatible with TreeMig, the avalanche
module also uses one year steps. A random component was added to the probabilistic release
module instead of explicit weather related variables, to simulate short-term (within-year)
weather variability. The historical avalanche and forest data used as input was stratified into
coniferous and broadleaf forest, and the GLM is run separately for each forest type.
The selection criteria to include variables in the GLM were (a) significant improvement of
variance explained, (b) sensitivity of at least some of the variables to climatic or land use
change, and (c) the possibility to calculate or estimate the variables from TreeMig output. To
implement the equation resulting from the GLM in the avalanche module, the values for slope
angle were taken from the Swiss digital elevation model (DEM) at 25m resolution (Swiss
Federal Office of Topography), and maximum gap size and other forest-related variables were
calculated allometrically from TreeMig output. For example, crown projection could be
calculated from TreeMig output using the percent cover equations established by Lischke and
Zierl (2002).
2.2 Mortality and regeneration modeling
The increased mortality of trees standing in an avalanche path will be modeled based on a metamodel
of RAMMS, which is currently in development. In TreeMig, different model versions
will be compared, using either full mortality of all trees in the avalanche path, or partial
mortality based on tree size and species. For preliminary versions of the avalanche subroutine in
the forest-landscape model, mortality will be set to one, and adapted later to include the
RAMMS meta-model output.
To simulate regeneration after avalanche disturbances, the dispersal, germination, establishment,
and growth subroutines of TreeMig will be used (Lischke et al. 2006). Here, growth is modeled
in height class increments, with a maximum possible growth rate per species, modified by
environmental factors such as light, temperature, precipitation, or local disturbances. As trees
are in competition for light, an increased light availability due to avalanches changes the growth
potential of surviving seedlings or freshly germinated seeds.
TreeMig is set up so that primary succession in the destroyed cells is given by potentially
remaining seeds or seedlings, dispersal distance from mature forest, and germination and
growth response to environmental conditions. Seedlings that survive the avalanches will have
higher growth rates due to the lower competition and higher light levels, making seedling shade
tolerance and the shading function highly critical. To increase the accuracy of the seedling
growth, we will partition the lowest height class (previously including all individuals 0-1.37m
height) into 4 smaller classes, and improve the shading subroutine. Furthermore, the previously
strict allometry between size and growth, both simulated in units of height classes, will be
improved by allowing more variability between size and growth. This is especially important at
treelines, where individuals are often found to be relatively old, but short, with a relatively high
stem diameter.
2.3 Merging of model components
The four components of avalanche release and flow inside and outside of forested areas will be
implemented in the forest landscape model TreeMig, and analyzed for sensitivity, scaling
effects, and model uncertainties. To implement the avalanche module in TreeMig, both the
avalanche submodel and the growth and regeneration subroutines require careful calibration.
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.