Why one year is full of mushrooms and another almost empty

Sometimes last year your baskets wouldn't close, and this year the same spots are almost empty. Spanish researchers spent years weighing mushrooms on the same forest plots and tried to work out why. We'll go through what they found and where their conclusions stop, so that you don't expect more from them than they can give.

The short answer: rain, above all

In all three studies we'll look at, swings in yield are linked first and foremost to rainfall. But that is a conclusion about averages across survey plots. It doesn't tell you what will happen in a particular wood on a particular weekend. ¹²³

How different can years be? Many times over

The clearest picture comes from the work of Alday and colleagues (Scientific Reports). For eight years running, from 2007 to 2014, they surveyed 19 permanent plots in Mediterranean areas of Spain, at altitudes of 684 to 1,615 metres above sea level. They measured fruit body biomass, that is, the weight of the mushrooms themselves, in kilograms per hectare. ¹

Up to 2009 the yield was rising, by 2011 it had collapsed, and in 2014 it reached its maximum for all eight years. The '±' sign shows the margin of error: these are averages across plots, not the weight picked in one place. Almost all of this weight, 94% of the total biomass, came from species that the authors classify as mycorrhizal, so their curve across the years almost matches the overall one. ¹

Which matters more: the year or the place? The year, but not by much

Since yield changes so much over time, the question arises: perhaps it is all down to where the plot is? Alday and colleagues split the variation in biomass into parts. The largest share, 42%, comes from differences between years, 34% from differences between plots, and 17% from differences between localities. So place trails behind year by only a little. ¹

Which rain matters most? Late summer and early autumn

It was rainfall in late summer and early autumn that swung yields most in these observations. The authors conclude that in Mediterranean areas rainfall is a limiting resource: fruiting is held back precisely by a lack of it. Bear in mind how this conclusion was reached. Nobody changed the rain on purpose; it is a link found in observations. And there is one more caveat from the authors themselves: they show no long-term trend. Eight years give a picture of fluctuations, not a trend. ¹

Does forest density matter? Yes, but medium is best

If the difference between years is mostly down to weather, where does the difference between places come from? That is addressed by the work of Sánchez-González and colleagues (Forest Ecosystems). It contains no new observations. The authors built yield models from survey plot data in northern Spain: 90 plots of 100 m² in pure stands of P. sylvestris and P. pinaster. ²

The authors assume that differences between years come mainly from weather, and differences between places from soil, management, stand composition and structure, and site characteristics. According to their results, yield is affected by weather, chiefly rainfall, and by the sum of trunk cross-sectional areas. Put simply, this is the sum of the cross-sectional areas of all the trunks on a hectare, a rough measure of how densely the forest stands. Yield depends on it along a single-humped curve, and the highest predicted yield falls at 30–40 m² per hectare. ²

It's easy to trip up here. These 30–40 m² per hectare are the peak of the model's curve, not a measurement taken in any particular forest, and not a recipe. What's more, the authors themselves write that existing models rely on fairly local data and that tools for large areas are still lacking. ²

What counts for more in pine stands: rain or warmth? Rain

The third study is no longer about past yields but about scenarios. Herrero and colleagues (Forests) built a model for stands of Maritime pine and used it to calculate scenarios. The model included three things: the ratio of the sum of trunk cross-sectional areas to stand density as an indicator of forest management, rainfall, and mean temperatures in September and November. ³

The main point for us: in these stands, rainfall affected total mushroom productivity more strongly than temperature did. But this is the result of a model for one tree species, and the scenarios in it are calculations, not something anyone saw in a forest. ³

What can be concluded from these studies, and what cannot

The studies are built differently: one is observations, another is models based on plot data, the third is a calculation of scenarios. And they agree on one thing: it all comes down to rain. Here is where the boundaries lie. ¹²³

• Follows: according to multi-year surveys in Spanish forests, yield swings many times over from one year to the next, and these swings are linked most strongly to rainfall; in Alday and colleagues, to rainfall in late summer and early autumn.
• Follows: place matters too. The difference between plots explains only a slightly smaller share of the variation than the difference between years.
• Does not follow: that rain this week guarantees mushrooms in your wood on Saturday. All the conclusions concern averages across survey plots.
• Does not follow: that a result from pine stands in one area is a rule for all forests, or that 30–40 m² per hectare is practical advice. It is a model's prediction based on local data.
• Does not follow: that eight years of observations show a long-term trend. The authors say outright that they do not.

And one last thing: the MicoRadar forecast map is also a model calculation, not an observation of yield, so the same caveats apply to it.

The text was checked against the sources on 3 October 2026.