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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Furthermore, to be able to model avalanches in sufficient detail, the cell size will be reduced
from 100m side length to 25m. To ensure applicability of the forest-landscape model with
reduced cell size, we will compare model runs with both cell side lengths.
The feedback effects will emerge once the avalanche release probability, flow dynamics,
destruction, and regeneration subroutines are set up. These will have to be fine-tuned for current
climatic and land-use conditions to make sure that there are no runaway effects, which could
develop due to the positive feedback cycle between forests and avalanches. Keeping the
environmental conditions constant, we will make sure there is a balance between avalanches and
forest dynamics, by tuning the avalanche and forest parameters involved. Once the feedbacks
are established, we will compare the output to current treeline positions and patterns, to estimate
the precision of the model.
In a sensitivity analysis, environmental conditions are again kept constant while disturbance
intensity (frequency and magnitude) are varied, and the influence on output variables such as
total biomass per area, location and pattern of treelines, and species composition will be
analyzed. The merged model TreeMig-Av will then be applied to the study area of the Davos
municipality, in the eastern Swiss Alps. The model will be validated against historical avalanche
frequency data and compared to the output of other models such as those used for avalanche risk
mapping. IPCC AR4 climatic change scenarios and different land use scenarios will then be
used to simulate avalanche release and forest cover scenarios for the next centuries, and to
analyze how the feedback effects develop under the changing environmental conditions.
3. Preliminary and expected results
In the GLM, the final variables used for the estimation of avalanche release inside forests are
similar between the two forest types: While the coniferous forest model equation uses slope
angle, crown projection, and maximum gap size, the broadleaf forest equation uses slope angle,
crown projection, and proportion of coniferous trees instead of the gap size.
A sensitivity analysis in the current version of TreeMig, with single cell disturbances, showed
that it is sensitive to changes in disturbance frequency and magnitude. Changes in these
variables led to changes not only in total biomass, but also species composition and structural
diversity in the affected cells. Furthermore, the single cell disturbance regime was compared to
spatially connected disturbances on a transect, which confirmed the expected difference in
regeneration patterns after the two disturbance types. The main feature of the spatially
connected disturbance type is the potentially larger distance of a recently disturbed cell to the
nearest vegetated cell, from where seeds may disperse into the disturbed cell. Regeneration
dynamics therefore strongly depend on accurate simulation of dispersal kernels and size of the
disturbed area.
The Intermediate Disturbance Hypothesis (IDH) was shown to apply to TreeMig's disturbance
rates at the single cell level, and for connected disturbances along the one-dimensional transect,
and is expected to also apply to spatially connected disturbances in two-dimensional space.
Highest species diversity was found at intermediate disturbance frequency and magnitude. The
exact location of the peak diversity along the two axes, however, was influenced by altitude, i.e.
by limitations given by climatic variables. For example, where growth and survival is limited by
climate, a relatively high disturbance frequency could lead to the disappearance of the forest
altogether. Here, the peak of diversity would shift to lower values of disturbance frequency. The
IDH applies to species composition but not necessarily to forest structure (size distribution), as
avalanches can increase the number of smaller trees relative to the larger trees. Applicability of
IDH to structural diversity will further be studied using the adapted version of TreeMig.
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.