Altitudinal changes in humus form dynamics in a spruce forest at the montane level

Abstract

An altitudinal transect was studied at the montane level in a spruce forest, in order to describe changes in humus form dynamics. Whatever the elevation, the periodicity of humus form changes is copied on forest dynamics. Spruce regeneration is restricted both in space and time to favourable micro-site conditions accompanying tree fall gaps. One of these conditions is a shift from moder to mull humus form. As a result, this forest ecosystem shows a space-time mosaic pattern. The recovery of mull humus form takes more time as elevation increases and thus the ratio mull:moder progressively decreases. Simultaneously, burrowing earthworms become scarce. Nevertheless mull humus form does not change basically at increased elevation. On the other hand, moder has little organic matter at the lower montane level. At the mid montane level, accumulation of holorganic faeces into OH horizon is maximum. A mor-moder (few faunal traces) is observed during tree growth phase at the upper montane level. Lack of nutrient availability was hypothesized to explain the observed shift from mull to moder or mor-moder during the phase of intense tree growth. An increased independancy between the building of a moder humus profile and the actual accumulation of organic matter was the main change occurring with elevationl The transition to the subalpine level may thus be defined as the highest elevation up to which the forest ecosystem is able to reverse the mor-building process produced by tree growth. The discoupling between primary producers, plant debris accumulation and decomposers observed in the subalpine spruce forest leads to a breaking point in the carrying capability of humus for spruce seedlings. At high elevation spruce seedlings are mainly observed on rotten wood.The influence of humus form dynamics upon forest sustainability was discussed. The lack of humus biological activity at higher elevation was found to be a reason for the weakness of the forest ecosystem faced to the bilberry heath. Forest sustainability depends both on mull and moder. It was observed that moder failed to recover after mull along the forest cycle and turned to mor in harsh climate conditions. Consequences of management practices on the issue of competition between forest and heath were also discussed.

Humus forms in terrestrial ecosystems: a framework to biodiversity

“…Such a system, by its high biodiversity, generates the conditions of its own stability, according to the bootstrapping effect described by Perry et al (1989). Nevertheless it should be stressed that initial conditions for its build-up are a wealth of nutrients and a mild climate, as shown by Figure 1, and that severe limits appear when an important function such as burying of organic matter is ensured by only one and the same organism, for instance the earthworm Lumbricus terrestris (Bernier, 1996).…”

“…It is harder to understand how moder ecosystems are stabilized, and more knowledge about negative feed-back loops prevailing in moder is needed before approaching the reasons why this intermediary humus form is so frequent in deciduous and coniferous ecosystems, at least within the temperate zone. The existence of cyclic processes in moder ecosystems, with mull prevailing during the regeneration phase (Bernier, 1996), might suggest a solution to this enigma. Moder being associated with forest ecosytems (deciduous as well as coniferous), the existence of moder could well be explained by tree physiology.…”

Structure of diagnostics horizons and humus classification

“…Moder e Mor avrebbero dunque OH di spessore maggiore dell'OF, mentre l'inverso accadrebbe negli Amphi, fatto questo che, se confermato da ulteriori e specifici studi, permette rebbe di assegnare ai Moder un funzionamento diffe rente da quello degli Amphi. Questo fatto viene per altro confermato da Zampedri (in litteris) che, come già Bernier (1996) e avevano descritto, riesce a distingue i Moder tipici da quelli in evoluzione dinamica verso i Mull.…”

Changes in the composition of humus profiles near the trunk base of an oak tree (Quercus petraea (Mattus.) Liebl.)

“…An alternative method is the analysis of biological traits by morphological assessment, using structural components of humus profiles (plant debris, animal faeces, roots, mineral particles) and the succession of horizons created by their accumulation as parameters describing the activity of soil organisms. A morphological method using the observation of small volumes of litter and soil has been devised, allowing qualitative [31,32,33,34,35,36], thereafter quantitative analysis [7,6,3,4] of the transformation of organic and mineral matter by fauna and microbes and the development of the root system, i.e. the development of humus profiles [37].…”