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 53<br />
decay <strong>of</strong> each year's litter is tracked through time, thus mimick<strong>in</strong>g many litterbag studies.<br />
When <strong>the</strong> critical nitrogen concentration (Alexander 1977) is reached, <strong>the</strong> litter is transferred<br />
to a common “humus” compartment, and nitrogen m<strong>in</strong>eralization starts. The<br />
amount <strong>of</strong> nitrogen available for plant growth (uAvN) is calculated as <strong>the</strong> difference between<br />
<strong>the</strong> nitrogen m<strong>in</strong>eralized from <strong>the</strong> humus pool and <strong>the</strong> immobilization demand <strong>of</strong><br />
<strong>the</strong> litter cohorts (gImmob, Fig. 3.3).<br />
Pastor et al. (1984) and Pastor & Post (1985) found good correlations between litter<br />
decay rates, actual evapotranspiration (uAET), litter lign<strong>in</strong> content (gLign), and <strong>the</strong><br />
nitrogen to mass ratio <strong>of</strong> <strong>the</strong> litter (gNMR); <strong>the</strong>se <strong>in</strong>dices are used to formulate litter decay<br />
rates <strong>in</strong> LINKAGES and FORCLIM (Fig. 3.3). The leach<strong>in</strong>g <strong>of</strong> nitrate from nitrogen-rich<br />
litter is taken <strong>in</strong>to account (Cole & Rapp 1981) as well as a constant atmospheric deposition<br />
rate <strong>of</strong> soluble nitrogen compounds. The more recalcitrant litter types (twigs and<br />
wood) are assumed to decay at a constant rate. The hypo<strong>the</strong>sized effects <strong>of</strong> canopy open<strong>in</strong>gs<br />
on litter and humus decay rates as <strong>in</strong>corporated <strong>in</strong> LINKAGES were not <strong>in</strong>cluded <strong>in</strong><br />
<strong>the</strong> FORCLIM model (Fig. 3.3).<br />
Most <strong>of</strong> <strong>the</strong> litterbag studies to date were conducted under boreal conditions and/or <strong>in</strong><br />
America; only few data are available for central European conditions and species (e.g.<br />
Berg & Staaf 1981, Ellenberg 1986, Lüscher 1991). Thus it was necessary to collapse<br />
<strong>the</strong> 17 litter types dist<strong>in</strong>guished <strong>in</strong> LINKAGES to three types <strong>of</strong> leaf litter (fast, medium,<br />
and slow decay), twig litter, root litter, and stemwood litter.<br />
At least two weaknesses rema<strong>in</strong> <strong>in</strong> <strong>the</strong> LINKAGES as well as <strong>the</strong> FORCLIM-S model: First,<br />
<strong>the</strong>re is no compartment with a very slow turnover rate <strong>of</strong> soil organic matter. It is well<br />
known that part <strong>of</strong> <strong>the</strong> organic matter is “protected” or “stabilized” and very recalcitrant to<br />
decay (e.g. Parton et al. 1987, Verberne et al. 1990); nei<strong>the</strong>r <strong>the</strong> LINKAGES nor <strong>the</strong><br />
FORCLIM model simulate <strong>the</strong>se processes. Second, Pastor & Post (1985) had to use <strong>the</strong><br />
N:C ratio <strong>of</strong> <strong>the</strong> litter to formulate <strong>the</strong> m<strong>in</strong>eralization rate <strong>of</strong> <strong>the</strong> humus compartment;<br />
while this approach was phenomenologically correct (Pastor et al. 1984), it represents an<br />
empirical, not a causal relationship. Lüscher (1991) <strong>in</strong>vestigated <strong>the</strong> dynamics <strong>of</strong> <strong>the</strong><br />
forest floor and <strong>the</strong> humus <strong>of</strong> many soils <strong>in</strong> Switzerland; however, this data base does not<br />
allow to reformulate <strong>the</strong> rate <strong>of</strong> nitrogen m<strong>in</strong>eralization from <strong>the</strong> humus as a function <strong>of</strong><br />
abiotic variables on a more mechanistic basis. Thus, <strong>the</strong> data from Pastor et al. (1984)<br />
had to be used aga<strong>in</strong> <strong>in</strong> FORCLIM (Fig. 3.3).