Saint-Nicolas, France
Saint-Nicolas, France

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Observations performed over many years, concerning aspens of a subalpine forest growing in Haute-Maurienne (Savoie), provide clear evidence of a "climacic" balance between a population of a Mycetophilid fly, Sciophila bonnevalensis n. sp. and the fungus Phellinus tremulae, a strict parasite of aspen trees above 1800 m. We have recently outlined the disastrous effects of the very low temperatures that occurred during the 2004-2005 winter, attaining -28°C. Very few Sciophila larvae survived to the following summer. At the same time, the protective webs of others were partly destroyed, being reduced to white dots dispersed on the carpophores of Phellinus. Consequently, we thought that an almost irreversible situation had been attained. However, despite a second hard winter (-23°C), the population of Sciophila was again observed to be slowly growing during July 2006. Moreover, the 2006-2007 winter (-17.5°C) was not strongly destructive and consequently we observed a relatively strong increase of the Sciophila population in July 2007: protective webs were eight times as numerous as in July 2005. Nevertheless, this increase in larval activity may have been slightly slowed in 2007 as a consequence of a brief period of frost and snow at the beginning of July. This was strong enough to induce death of Yponomeuta padella larvae everywhere, the nym-phosis of which is known to locally hinder Sciophila larval growth. We believe that our climatic and biological observations, patiently built up during many years, could provide a starting point for a more complete study (we give also new results for 2008 and some details for 2011). Such a study concerns only a local geographical and ecological subject, but it could be compared with well-known strong global disturbances observed by eminent specialists. In fact, in these limited, subalpine mountain conditions, it could be possible to take into account some parameters that are relatively easy to evaluate on yearly basis. Thus, we observed that local warming strongly favours the growth of aspen, which can climb along rocky slopes and among frontal moraines of the Arc glacier. After some years, this can consequently enhance Phellinus growth, as described in this work. On the other hand, we still lack knowledge about the effects of wind and snow: are these determinant or minor factors? As detailed here, it is clear that very low winter temperatures affect Phellinus growth and induce Sciophila larval mortality. Such parameters can be easily studied, evaluated, and, in combination, could constitute an appropriate model to investigate local climatic disturbances and perhaps even serve as a reference for comparisons with similar studies undertaken elsewhere, in other biotopes.

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