On the Ecology of Mountainous Forests in a Changing Climate: A ...
On the Ecology of Mountainous Forests in a Changing Climate: A ...
On the Ecology of Mountainous Forests in a Changing Climate: A ...
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Introduction 5<br />
<strong>in</strong> a chang<strong>in</strong>g climate thus would require to formulate time-variant transition probabilities.<br />
However, such a formulation would not be causal and does not appear trustworthy<br />
enough for a study <strong>of</strong> climatic change. O<strong>the</strong>r landscape models concentrate on primary<br />
productivity (e.g. Kauppi & Posch 1985, 1988). A prom<strong>in</strong>ent model that <strong>in</strong>cludes primary<br />
productivity and <strong>the</strong> hydrological cycle is FOREST-BGC (Runn<strong>in</strong>g et al. 1987,<br />
Runn<strong>in</strong>g & Coughlan 1988, Runn<strong>in</strong>g & Gower 1991). Runn<strong>in</strong>g & Nemani (1991) used<br />
this model for assess<strong>in</strong>g <strong>the</strong> possible impact <strong>of</strong> climatic change on forest productivity and<br />
hydrology.<br />
A major drawback <strong>of</strong> models on <strong>the</strong> landscape scale is that none <strong>of</strong> <strong>the</strong>m was designed to<br />
predict <strong>the</strong> structure <strong>of</strong> <strong>the</strong> landscape (e.g. species composition or vegetation types) and<br />
its productivity simultaneously. However, both features are <strong>of</strong> <strong>in</strong>terest <strong>in</strong> <strong>the</strong> present<br />
study, and landscape models <strong>the</strong>refore are <strong>of</strong> limited value.<br />
Ecosystem models: A large effort for build<strong>in</strong>g ecosystem models was <strong>in</strong>itiated by <strong>the</strong><br />
International Biological Programme (Reichle 1981). Models on this scale typically<br />
assume ei<strong>the</strong>r that a forest consists <strong>of</strong> a s<strong>in</strong>gle species (Swartzman 1979, McMurtrie &<br />
Wolf 1983) or that its composition does not change with time (Shugart et al. 1974,<br />
Soll<strong>in</strong>s et al. 1976, Dixon et al. 1978a,b, Aber et al. 1991). The temporal resolution <strong>of</strong><br />
<strong>the</strong>se models is on <strong>the</strong> scale <strong>of</strong> hours to weeks, and <strong>the</strong> compartments ignore any differences<br />
between <strong>in</strong>dividuals, species, and <strong>of</strong>ten even trophic levels. They take <strong>the</strong> forest as<br />
a functional entity with superorganism-like behaviour (cf. Huston et al. 1988). This<br />
makes it difficult to apply such models to study <strong>the</strong> transient behaviour <strong>in</strong> function <strong>of</strong> climatic<br />
variables (Davis 1990). However, <strong>the</strong>y can be quite useful to assess productivity,<br />
assimilate allocation, transport mechanisms, and energy flow through ecosystems.<br />
Soll<strong>in</strong>s et al. (1981) noted that a major problem with models formulated on <strong>the</strong> ecosystem<br />
scale is <strong>the</strong> lack <strong>of</strong> sufficient validation data, such as gross ecosystem respiration or <strong>the</strong><br />
effects <strong>of</strong> defoliat<strong>in</strong>g <strong>in</strong>sects on net primary productivity. Moreover, <strong>the</strong> scope <strong>of</strong> <strong>the</strong>se<br />
models was to <strong>in</strong>crease <strong>the</strong> understand<strong>in</strong>g <strong>of</strong> forests as <strong>the</strong>y are today. This justifies <strong>the</strong>ir<br />
basic assumptions but renders <strong>the</strong>m <strong>in</strong>appropriate for studies <strong>of</strong> climatic change.<br />
Models us<strong>in</strong>g populations and functional groups: Models at this scale were used<br />
to simulate <strong>the</strong> management <strong>of</strong> s<strong>in</strong>gle-species stands (e.g. Kimm<strong>in</strong>s et al. 1981). O<strong>the</strong>r<br />
applications <strong>in</strong>cluded studies <strong>of</strong> <strong>the</strong> <strong>in</strong>teractions between a few populations or functional<br />
groups <strong>of</strong> organisms, most <strong>of</strong>ten plants (Malanson 1984, Moore & Noble 1990, Osho<br />
1991). These models typically were built for management purposes, thus ignor<strong>in</strong>g many