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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The forest model FORCLIM 51<br />
Disturbances extr<strong>in</strong>sic to <strong>the</strong> forest patch, such as forest fires and w<strong>in</strong>dthrow, provide a<br />
third source <strong>of</strong> mortality, which is episodic (Shugart 1984). This mortality is <strong>in</strong>cluded <strong>in</strong><br />
<strong>the</strong> FORCLIM model us<strong>in</strong>g a simple approach: All <strong>the</strong> trees currently present on <strong>the</strong> patch<br />
are killed if such a disturbance occurs. O<strong>the</strong>r sources <strong>of</strong> tree mortality, such as forest<br />
management practices like th<strong>in</strong>n<strong>in</strong>g and logg<strong>in</strong>g, are disregarded <strong>in</strong> FORCLIM.<br />
3.2.2 Soil submodel: Turnover <strong>of</strong> soil organic matter<br />
Nitrogen is one <strong>of</strong> <strong>the</strong> major plant nutrients, and its availability limits plant growth <strong>in</strong><br />
many terrestrial ecosystems (Kimm<strong>in</strong>s 1987). The nitrogen cycle <strong>in</strong> forests is <strong>in</strong>timately<br />
coupled with <strong>the</strong> carbon cycle (Shaver et al. 1992): The amount <strong>of</strong> organic matter returned<br />
to <strong>the</strong> soil depends on primary productivity, which is limited by nitrogen availability<br />
(War<strong>in</strong>g & Schles<strong>in</strong>ger 1985, Lyr et al. 1992). In turn, nitrogen availability is largely<br />
determ<strong>in</strong>ed by nitrogen m<strong>in</strong>eralization, <strong>the</strong> conversion <strong>of</strong> organic nitrogen to ammonium<br />
with concomitant release <strong>of</strong> CO 2 (Alexander 1977); nitrogen m<strong>in</strong>eralization itself depends<br />
on climate and on <strong>the</strong> type <strong>of</strong> carbon compounds with which <strong>the</strong> nitrogen is associated<br />
(Mellilo et al. 1982, McClaugherty et al. 1985). Thus <strong>in</strong> an analysis <strong>of</strong> <strong>the</strong> turnover <strong>of</strong><br />
soil organic matter at <strong>the</strong> ecosystem scale, <strong>the</strong> coupl<strong>in</strong>gs between <strong>the</strong> carbon and nitrogen<br />
cycles should be considered explicitly (Pastor & Post 1985).<br />
The basic paradigm for most decomposition models developed to date was formulated by<br />
Jenny et al. (1949); Olson (1963) formalized it <strong>in</strong> a simple exponential-decay model.<br />
However, <strong>the</strong> parameters <strong>of</strong> this model are specific for each soil, depend<strong>in</strong>g on climate<br />
and <strong>the</strong> type <strong>of</strong> litter returned to <strong>the</strong> soil. Thus, it was a logical step to relate decay rates to<br />
environmental parameters such as temperature and precipitation (or a comb<strong>in</strong>ation <strong>of</strong><br />
<strong>the</strong>se), and to simple chemical <strong>in</strong>dices <strong>of</strong> substrate quality (e.g. Meentemeyer 1978,<br />
Melillo et al. 1982). Several models <strong>of</strong> <strong>the</strong> carbon cycle were constructed for forests<br />
(Aber & Melillo 1982, We<strong>in</strong>ste<strong>in</strong> et al. 1982, Pastor & Post 1985, Aber et al. 1991), and<br />
grasslands (Jenk<strong>in</strong>son & Rayner 1977, Parton et al. 1987, Verberne et al. 1990) (cf. <strong>the</strong><br />
review <strong>in</strong> Ågren et al. 1991). However, most <strong>of</strong> <strong>the</strong>se models do not treat explicitly <strong>the</strong><br />
effects <strong>of</strong> climatic parameters and substrate chemistry on decomposition rates. The LINK-<br />
AGES model by Pastor & Post (1985) fulfils both requirements. Moreover, this model<br />
was used successfully <strong>in</strong> many subsequent studies (Pastor & Post 1986, 1988, Shugart<br />
& Urban 1989, Mart<strong>in</strong> 1992, Pastor & Naiman 1992). Therefore, <strong>the</strong> FORCLIM submodel<br />
for soil organic matter turnover was derived from LINKAGES (Fig. 3.3).