On the Ecology of Mountainous Forests in a Changing Climate: A ...

56 Chapter 3
ature of the coldest month (January). In several models (Pastor & Post 1985, Solomon
1986, Prentice et al. 1992) such correlations were used to estimate the winter minimum
temperature from the actual mean January temperature. However, the month with the
lowest long-term mean temperature is not necessarily the month with the lowest actual
mean temperature. Therefore the FORCLIM model uses the minimum of the actual mean
temperature of the winter months December, January, and February as a proxy for the
winter minimum temperature (Fig. 3.4).
Degree-days (uDD)
The concept of degree-days, i.e. a linear dependency of the growth rate on temperature
above a threshold temperature, was used in most forest gap models developed to date
(Shugart 1984). The tree species native to the European Alps have rather similar threshold
temperatures of net photosynthesis (Lyr et al. 1992); it is therefore justified to use a
general threshold temperature, which is independent of the single tree species. By doing
so, the annual sum of degree-days becomes an abiotic index of the forest environment
(Fig. 3.4).
Evapotranspiration (uAET) and drought stress (uDrStr)
There are many models available to calculate evapotranspiration and the water balance
(e.g. Penman 1948, Thornthwaite & Mather 1957; see review in Mintz & Serafini 1992).
The more accurate methods require many weather variables with a high temporal resolution.
The model by Thornthwaite & Mather (1957), although an entirely empirical approach,
is especially useful because it is based on monthly mean temperatures (T m,y,l )
and monthly precipitation sums (P m,y,l ) only, and it provides a reasonable estimate of potential
and actual evapotranspiration (PET and AET, respectively). Correspondingly, it
was used in many empirical and modelling studies (e.g. Müller 1982, Meentemeyer et al.
1985, Mintz & Serafini 1992) as well as in most forest gap models (Shugart 1984). In
FORCLIM, this approach is used as well.
In the Thornthwaite & Mather model, actual evapotranspiration is assumed to be independent
of the vegetation cover and is based on an average leaf area index. Since canopy
openings caused by the death of single trees are relatively small (