The mushroom season has shifted by nearly two weeks

Fungal phenology — when fruiting bodies appear — was a blank for a long time. The mycelium is invisible and the fruiting body is ephemeral. Only dated herbarium collections gave material on which anything could be counted.

In 2008 Håvard Kauserud and colleagues analysed roughly 34,500 dated herbarium records from Norway covering 1940 to 2006. The average delay in autumn fruiting after 1980 was 12.9 days. The change was not evenly spread: early-fruiting species were delayed more than late ones, so the season as a whole was compressed. There is also a geographic gradient — fruiting is earlier in the northern and more continental parts of the country than in the southern and oceanic parts.

Independent confirmation came from Switzerland. Ulf Büntgen and colleagues compiled 115,417 weekly fruit-body counts from permanent inventory plots between 1975 and 2006. After 1991 autumn fruiting was delayed by an average of 12 days, the annual number of fruit bodies rose from 1,801 to 5,414, and mean species richness doubled from 10 to 20. The authors stress that this contradicts earlier reports of declining harvests.

Two things deserve stating plainly. First, both datasets also measure collector effort: a herbarium grows where somebody collects, an inventory where somebody walks. The authors correct for this statistically but cannot remove it entirely. Second, delayed fruiting is not the same as more mushrooms — the timing changes, not the promise of a basket.

For conservation the significance runs against intuition. If the season shifts and compresses, monitoring dates fixed decades ago may fall beside the fruiting peak, and year-on-year comparisons made "in the same week" stop measuring the same thing.

Nearly two weeks—but for a specific metric

Kauserud and colleagues analysed about 34,500 dated Norwegian herbarium records from 1940–2006. For 1980–2006, their model estimated a 12.9 ± 1.2-day delay in mean fruiting date per 20 years. Early-fruiting species shifted more strongly, so the beginning moved later while the end changed little. An independent Swiss programme counted 115,417 fruit bodies weekly. After 1991, autumn fruiting was 12 days later on average, while annual fruit-body counts rose from 1,801 to 5,414 and mean richness from 10 to 20. Both studies measure visible fruiting phenology, not underground mycelial biomass. Herbarium collections and permanent plots carry different biases, so the results should not be merged into one universal European trend.