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The Growth of Fungal Hyphae in Stones: Importance for Tree Nutrition


Healthy forest growth in the Black Forest, Germany, contradicts our expectations for what is an acidic soil environment, characterized by poor base saturation and low neutral cation availability in the fine soil. This leads to the assumption that a further source of plant-available nutrients exists.
By splitting gneiss fragments along inherent weathering planes, it can be observed that fungal rhizomorphs grow into the vermiculite-rich weathering material of these internal micro fissures. Thus, it is assumed that the weathering of coarse soil particles which are not considered in conventional soil analysis actually contributes significantly to tree nutrition through the penetration and nutrient extraction of fungi.
In order to investigate this hypothesis, undisturbed samples were taken from two forest stands in the Black Forest, Conventwald and Haslach, the latter exhibiting much better fine soil nutrient availability in terms of cation exchange capacity and base saturation. Polished sections from the different soil horizons were photographed at random points magnified to 500~times. Using GIS and spatial statistics, the digitalized photographs were analysed and used to describe the distribution of soil hyphae with respect to their micropedological environment. By this means, evidence was sought of soil microstructure influencing the growth of fungal hyphae.
As expected, hyphal density was found to increase significantly as the soil depth decreases. Results for the 1.20m deep B/C~horizon show an average density of 4 hyphae/mm², although the heterogenity of hyphal distribution in this depth is so high that the standard deviation of the different observation areas exceeds the average. The closer analysis of point patterns within these observation windows also indicates high variability in the spatial distribution of hyphae. Nonetheless, mean curves of empty space measurements and pair correlation functions show a uniform slight clustering of hyphae for A/B, B and B/C horizons as well as for both forest stands.
Hyphae show a clear preference for growing in fine soil. However, where sand grain sized particles constitute a greater proportion of soil volume, hyphae are also frequent in these coarse soil fragments. It was demonstrated that fungal hyphae tend to avoid coarse particles bigger than 2 mm, although it was also shown that they can penetrate these particles and thus reach the important nutrient pool within this fraction.

Duration: 01.09.02 – 31.05.05