Tuesday, 8 December 2009

Dry autumn leaves a legacy

While the wet weather of recent weeks may be sufficient to cloud the memory, clearing it of recollections of dry autumn days, a visible legacy of autumn’s ‘drought’ has been the poor crop of mushrooms and toadstalls. My weekly walks through mixed and broad-leaf woodland, across fields and heath, have turned up little in the way of fungal fruiting bodies. I would imagine that my experience has been repeated in those who take a more detailed interest in fungi than I achieve with my casual eye.

The building blocks of fungi are the fungal hyphae, thread-like tubes that occur in vast numbers in damp leaf-litter; scoop up a handful of this leaf litter from a damp deciduous woodland and you will soon reveal a cobweb-like mat of fungal strands. These absorb nutrients, either from the soil or from the substance in which they are growing (some fungi grow in and digest wood, while others tackle hair, feathers or the hard exoskeletons of insects). For the greater part of the year hyphae are hidden from view, held within the substrate, but come the autumn they may form fruiting bodies, the familiar mushrooms and toadstalls being one such type. The fruiting bodies allow reproduction and the formation of new colonies, as spores are released to drift on the wind.

Fruiting bodies are produced once the mat of fungal hyphae has reached a certain age or size. Development is then triggered by environmental conditions and research has helped to pinpoint what is involved in this process. In addition to nutrient availability, both soil temperature and humidity are important, with dry ground inhibiting production of the complex fruiting bodies. As anyone who has collected or watched a Shaggy Ink Cap will testify, many of the fruiting bodies are very short-lived (perhaps lasting only for a few brief hours or a couple of days). The tough, woody, bracket fungi, however, are much more robust, with some lasting for 20 years or more and adding a new spore-producing layer each autumn.

The process by which the body develops from the thread like hyphae is fascinating, with a quick progression from a knot of hyphae through to the ‘button’ stage (which is effectively a fully-formed but miniature version of the final body). The next stage is one of expansion, the use of hydraulic pressure increasing the size of the body and forcing it up through the soil’s surface. Such is the strength of the hydraulic pressure that fruiting bodies have been recorded lifting paving slabs and other obstructions.

It is easy to overlook the network of hyphae that drives the production of fruiting bodies but the lack of fungi this autumn is a gentle reminder of their function.

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