Mulch Makes Room for Mushrooms

©Peter Werner
Original publication: Mycena News, March 2005

When we think of habitats where one can find lots of mushrooms and a great diversity of species, the first thing that comes to mind is some kind of woodland. In particular, woodlands dominated by ectomycorrhizal host trees, such as oaks, pines, spruces, and Douglas-fir. These are the places we turn to when we wish (to at least attempt) to stuff our baskets full of porcini, chanterelles, matsis, velosi, and so on. However, a unique and diverse mycota can also be found in a decidedly more mundane and often overlooked habitat – urban and suburban woodchip beds.

Over the last several decades, the practice of woodchip mulching has become very common throughout the world. Deep mulching around established plants is a very effective means of controlling weeds without the use of herbicides and woodchips produced as a byproduct of arboriculture are an extremely inexpensive and readily available source for this mulch.

Anybody who has looked for mushrooms in urban parks will note that such woodchip beds support an abundance and variety of macrofungi. Formal mycological investigation of what kind of mushrooms are found in this habitat has not been carried out until very recently, however. Peter Shaw has published several articles since 2001 based upon close observation and survey of the fungi of woodchip beds around Surrey, UK for the better part of the last decade.

Looking over Shaw’s species lists, I was immediately struck by how similar the British woodchip mycota was to that of the SF Bay Area. Psilocybe cyanescens and Stropharia aurantiaca (= Hypholoma aurantiaca) are reported to be the most widespread species; these are, of course, also common species here in the SF Bay Area, with S. aurantiaca being perhaps our single most common and abundant woodchip species.

Other species that turn up on Shaw’s lists read like a ‘who’s who’ of local woodchip species – Agrocybe praecox, Volvariella gloiocephela, Macrocystidia cucumis, and Hygrophoropsis aurantiaca. Stropharia percevalli, a less commonly encountered woodchip species in the UK, looks an awful lot like the ‘freeway stropharia’ that has become so abundant here in the last couple of years. (The ‘freeway stropharia’ usually is called ‘Stropharia riparia’, though it is distinctly larger and otherwise different from the true Stropharia riparia of montane and Northwest forest habitats.) Clathrus archeri, the octopus stinkhorn, sometimes fruits in abundance in woodchip areas around introduced bamboo in Kew Gardens, much the same way we see occasional large fruitings of this species in similar habitats in Santa Cruz.

There are a couple of notable traits shared by many species of woodchip mushrooms. The first is that many are observed very little, if at all, in non-anthropogenic (“natural”) environments and their geographic origins are often unclear. Psilocybe cyanescens is occasionally found growing on rotting wood and small branches within woodlands, but this is very unusual. As far as I know, Stropharia aurantiaca has never been reported from undisturbed woodlands at all. Psilocybe cyanescens is thought to have originated in the Pacific Northwest, where it is most abundant. However, it was first collected in Kew Botanical Gardens in 1910, over 30 years before the first Northwest collection was made in Seattle.

The geographic origin of Stropharia aurantiaca are even more unclear, having first been collected in Kew Gardens in 1887, but turning up in Australia very soon after. Interestingly, both the 1979 edition of Mushrooms Demystified and the 1983 edition of Mushrooms and Other Fungi of Great Britain and Europe describe S. aurantiaca as “rare”. Twenty-odd years later, it is the single most common mushroom to be found growing on woodchips, both in California and in Europe. This is a good example of another phenomena we see with many woodchip fungi -– rare species becoming more common over time.

Other examples include the above-mentioned ‘freeway stropharia’ that seems to be on the increase in both European and California parklands, and Psilocybe azurescens, a species first collected in the Northwest in 1979, and that was reported for the first time in Europe in 2000 and recently is turning up in California as well. (Of course, hallucinogenic Psilocybe species can spread particularly fast, as they are able to encourage human symbionts to disperse them.) The Psilocybe subaeruginascens group are found in woodchip beds in Asia (particularly Japan), but in the last year a mushroom belonging to this group has been collected in Golden Gate Park.

These patterns of the emergence and spread of “new” species and the fact that many of them are rarely seen outside of woodchip beds suggest that woodchip beds represent a unique and emerging niche that is actively being colonized. Wood-rotting fungi are extremely common in woodlands, yet the common woodland wood-rotters tend not to be common woodchip fungi and vice-versa. Shaw suggests several reasons why woodchips are a unique habitat: 1) the depth, extent, and abundance of surface area are far greater than those of logs and course woody debris, which means nutrients are far more available; 2) woodchip beds often undergo regular disturbance and replenishment with fresh chips (woodchip beds that aren’t replenished will dramatically decrease in mushroom abundance and diversity, but species diversity actually increases in established woodchip beds that are regularly replenished); and 3) woodchips often come from many different sources, typically being a mixture of wood from several species of trees and shrubs, and often chips coming from a number of different locations.

All of this creates selective pressures that are very different from those found in ‘natural’ wood environments, where lack of nutrients and resulting environmental stress are more important and competition pressure from other species is somewhat less. Species common in woodchips might have been very rare and restricted to marginal niches in such an environment. In rich environments, this situation is reversed, and these ‘weedy’ species that are able to rapidly colonize, reproduce, and compete with other species are favored.

Species lists for northern California fungi come from our larger forays, particularly the ones we have for the Fungus Fair. These forays are typically in wildlands – west Marin, Salt Point, the East Bay Hills, etc. These are the areas that need to emphasized, but sometimes urban environments are overlooked. (The last Fungus Fair received exactly one collection from San Francisco!) There are clearly some interesting fungal dynamics afoot in our cities, suburbs, and parklands and we should definitely be paying attention to them.