Climate, Trees & Macrofungi

A synopsis of a short article I wrote for the German mycological quarterly Der Tintling (1-2010, p. 5-6) on the assumed correlation between the fruiting of ectomycorrhizal macrofungi and climate change follows.

In 2009, in Germany and The Netherlands an exuberant fruiting of the Chanterelle (Cantharellus cibarius) and of boletes such as Boletus edulis occured, which was attributed to climate change as the dominant causing factor by a German author (D. Richter, Klima und Pilze. Der Tintling, 3-2009, p. 31-42).

At the end of the 20th century, during a period of 15 years, in Dutch decideous and coniferous forests research was carried out, in which the presence of the Chanterelle as a symbiotic partner of oaks, pines and spruce was monitored and documented, because C. cibarius was said to be undergoing a dramatic decline in fruiting, assumed to be caused by too much "harvesting" for consumption, setting of the soil (lack of oxygen) because of too many people going "off track" searching for a meal and/or acid rain.

The research however showed, that the ectomycorrhizal Chanterelle is part of the succession in the life cycle of the tree species it associates with, which disappears once the trees is old enough to enter the next phase and "selects" other symbiotic partners to live with. Another factor turned out to be the acidity of the soil, with C. cibarius vanishing when the soil became more acid then pH 4. In this case, acid tolerant generalistic ectomycorrhizal species, such as Boletus badius, Scleroderma citrinum and Paxillus involutus, substituted for the loss, the last two species also being capable of temporarely surviving as a saprotrophic recycler on dead wood as long as there is no living tree partner at hand.

And then there was the change of wood-fired ovens to electric ovens by the bakers, who no longer needed the wood from the annual cut thickets of Quercus rubra, which caused the trees to "overgrow" the phase its roots are colonized by the mycelium of the Chanterelle, which as a consequence of succession, brought the fruiting to a hold.

The fruiting of the mycelium of the Chanterelle is predominantly determined by the availability of the energy reserves stored in the roots and the supply of sugars to the ectomycorrhizae. In the preceding article on "Climate and Fungi" was stated, that the fruiting of the Chanterelle was more depending on rain, then on temperature.

The tree partners, however, are the ones, which react stronger to rain then to temperature. In particular for "deep rooting" oaks, the amount of rain and snow fall in winter, which determines the water reserve in the soil in spring, the tree can rely on for developing foliage and shoots and for blossoming and fruiting, are the most important. Dendrochronological research has shown, that the growing of year rings is determined by the energy reserves before winter stored in the preceding year and/or by the reserves, which the tree has built up by the end of the following autumn, i.e. as long as the tree had the time to photosynthesis and assimilate.

Because of this, it is to be understood, that the fruiting of C. cibarius, as was the case in 2007, together with the development of leaves and buds, could take place at the end of May, because the "wealthy" oaks could afford the spending of energy to the mycelium after a late started and early ended rainy and mild winter period. So in this case, it would be a serious mistake to interpret the (much too) early fruiting of the Chanterelle as panic reproduction of a mycelium in distress.

Beeches, for their water supply highly dependend on rain because of their superficial root system, on a hot summer's day can evaporate 300 to 400 liters of water with their leaves. Most of the water reserve in the soil has to be brought to the tree roots by the hyphae of the mycelia of the ectomycorrhizal symbionts and then transported by the tree to the foliage of the crown. If, like in 2009, from the end of July until the beginning of August, a longer period of extreme heat and drought occurs, the tree has to immediately stop the evaporation to keep its roots from drying out completely and the ectomycorrhizae with it. Unlike its normal defoliage in autumn, the tree then sheds its leaves without withdrawing the chlorophyl and because of this looses 6 to 8 weeks of energy production and the reuse of otherwise stored chlorophyl next spring.

Instead of partially transporting and storing its energy reserves in the roots, the beech now only stores the sugars and starch in its trunk and radial rays, resulting in stagnation of the fruiting of ectomycorrhizal symbionts such as Chanterelles and boletes, which normally takes place in this period of the year, i.e. in autumn. In 2009, from the end of October until the middle of November, the months of extreme drought were followed by a short, relatively warm and wet period. Preparing for the winter period, the trees then transported and stored sugars to/in the roots. And then, in the German Eifel and the eastern parts of The Netherlands, another abnormal phenomenon was documented : the mycelia of ectomycorrhizal macrofungi, such as C. cibarius and Boletus edulis, "quick" started fruiting with extreme quantities (panic reproduction) to limit the damage otherwise done to their reproductive needs and with it took so much of the stored energy of the trees, that they were left with insufficient reserves to fully develop leaves and fruit next spring.

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fascinating.