Red oak: a North American newcomer. Do mushrooms grow under it?

Yes, mushrooms do grow under red oak. It forms mycorrhiza with our native European fungi, and researchers have found porcini, chanterelles, brittlegills and amanitas in its plantations. But there are usually fewer mushrooms under it than under native pedunculate oak: one Polish study counted roughly half as many species of mycorrhizal fungi under red oak. Its leaves also settle in a dense layer that's slow to rot, and everything living under that carpet, from grass to moss, grows more poorly.

Red oak (Quercus rubra) is a North American tree that has been planted in Ukraine for more than two hundred years. Foresters like it because it grows fast, while botanists consider it invasive. Below we look at how to recognize it, why it stirs up so much argument, and what a forager can expect.

How it got into our forests

Red oak is native to the eastern United States and south-eastern Canada. It was brought to Europe as a park tree back in the late 17th century, and the first trees in Ukraine were planted in 1809 in a manor park. Large-scale forest planting began in 1959. According to the forest inventory as of January 2020, stands where it's the main species cover more than 66 thousand hectares in Ukraine, mostly in the west of the country. It's the most widespread of all introduced forest trees in Ukraine.

The reason is simple: on moist sod-podzolic soils it grows one and a half to two times faster than pedunculate oak, and it isn't fussy about soil fertility. It was often planted alongside pine. Here's just such a mixed stand of pine and red oak at the end of May: light-green, sharp-lobed oak leaves on the right, pines on the left, and a grassy clearing below.

In just this kind of mixed forest of pine, red oak and hornbeam, that same May, we found an inkstain bolete, a bolete that turns black where it's cut. So there are mushrooms here, especially where red oak isn't growing on its own.

How to recognize red oak

The leaves are the easiest giveaway. They're large, 12–20 cm long, with 7–11 lobes. Each lobe is pointed and ends in several fine bristle-like tips. The notches between the lobes are shallow, usually reaching less than halfway to the midrib. The leaf stalk is long, 2.5–5 cm, and often reddish.

Our pedunculate oak is the opposite in every way: its lobes are rounded, with no bristles at all, and the leaf stalk is so short that the leaf seems to sit right on the twig. The photo shows a wet red oak leaf on bright-green moss in November. Look at the tips of the lobes: they're pointed, with fine little “needles”.

There are other clues too. The bark is gray, with broad smooth stripes that seem to shine between shallow cracks. The acorns are large and flattened, with a shallow, saucer-like cup covering only a quarter or a third of the nut. They take two years to ripen, rather than one as in pedunculate oak, whose acorns also hang on long stalks.

A red autumn unlike ours

In autumn, red oak leaves turn red or brown. Polish ecologists assessing the risks posed by this tree even noted that its red autumn clashes with the familiar “golden autumn” of our forests. Our native maples, lindens and birches turn yellow in autumn. Here are yellow maples in mid-October, an autumn scene we all know well.

In November, red oak leaves fall and settle on the ground in a brown layer. Dry, they rustle underfoot; wet, they darken and shine.

Why grass and moss grow poorer under it

Red oak leaves are slow to rot. Czech researcher Dominika Dobrylovská explained why: they absorb water poorly, they're dense, and they contain a lot of carbon relative to nitrogen. Ukrainian botanists from the Institute of Botany of the National Academy of Sciences of Ukraine confirm this: leaf litter in red oak stands decomposes more slowly than in Ukraine's natural broadleaf forests, and the litter layer is thicker. They even warn that in hot weather this litter raises the risk of fire.

That said, not every measurement shows a big difference. In a Lithuanian experiment, red oak leaves lost about as much mass over two years as pedunculate oak leaves do in other studies. And in Polish forests the litter under red oak was thicker, but not by enough for the difference to be statistically significant.

The dense carpet of leaves is a physical barrier to seeds and seedlings, and little light gets through the thick crown. As a result, there's almost no grass in stands dominated by red oak: according to Ukrainian data, other plants cover no more than 5% of the ground there, and under dense three-year-old self-sown saplings there was no herb layer at all. Polish botanists describe grasses, mosses and ground lichens gradually disappearing from such plots. The last to hold out are mosses that form tall cushions and mosses at the base of tree trunks.

Here's what it looks like in November: red oak leaves cover the moss, with only the odd green patch showing through.

And here are cushions of moss on a stone, photographed in the second half of November. Dense, tall cushions like these are exactly the kind of moss that survives longest under a carpet of leaves, although these particular ones weren't growing under an oak.

Is red oak invasive?

Opinions differ here, and it's only fair to give you both sides.

• European Union. Red oak is not on the EU-wide list of invasive alien species.
• Poland. The official 2018 risk assessment using the Harmonia+PL method classed it as a moderately invasive alien species. Polish botanists have shown that it reduces the number and abundance of native plants and suppresses the regeneration of native trees.
• Lithuania. Researchers found that on poorer soils it spreads more readily than pedunculate oak, and that the soil under it holds a third fewer microscopic fungi than soil under pedunculate oak.
• Ukraine. Botanists consider it an invasive species. In April 2023 Ukraine's Ministry of Environmental Protection added red oak to a list of 13 invasive trees banned from use in reforestation, but in October of the same year the order was revoked.

Foresters counter that in Ukraine red oak's invasiveness is confirmed mainly on rich, moist soils, where it competes with pedunculate oak. In poor pine forests on sand, their data show it poses no threat to the local flora. Its acorns are heavy and only animals carry them, so it spreads slowly beyond the plantations.

Which mushrooms grow under red oak

Like all oaks, red oak forms ectomycorrhiza: its roots live in partnership with fungi. At home it has its own partners, and in Europe it finds new ones among the local fungi. Here's what research from Poland has shown.

• Fewer species of fungal partners. Researchers from the Institute of Dendrology of the Polish Academy of Sciences compared 35-year-old trees planted side by side in identical conditions. Pedunculate oak had about 120% more mycorrhizal fungus species in total than red oak, in other words more than double. All the fungi found under red oak were local, common species.
• Who dominates on the roots. Another study compared red oak with North American hickories growing nearby. The most frequent fungi on red oak roots were inconspicuous tomentelloid fungi, which form thin films on rotting wood and soil, along with members of the brittlegill family, that is, brittlegills and milkcaps. Earthballs (Scleroderma, false puffballs) and webcaps were abundant only on red oak roots, not on the hickories. Here, too, all the fungi turned out to be local, European species.
• Which fruiting bodies were found. In two red oak stands aged 52 and 61 years in an arboretum in central Poland, 40 species of fungi were recorded in 2007–2010, 20 of them mycorrhizal. They included porcini, the chanterelle, the bare-toothed russula, the green brittlegill, the ochre brittlegill and the poisonous sickener, the blusher, the inedible false death cap, the tawny grisette, the poisonous brown rollrim, the amethyst deceiver, and webcaps. For comparison, other authors counted 67 species of fungi in a natural linden-hornbeam forest.

An important caveat: the plots were small, 0.04 hectares each, with pine, beech, hornbeam and pedunculate oak growing nearby. A mushroom that came up in such a stand could have been partnered with any of these trees. So the list shows that these fungi do occur in red oak stands, but it doesn't prove they're all its partners.

As for the summer bolete, we found no reliable data on it under red oak specifically. It lives with our native oaks, beech and hornbeam, so look for it where those grow.

What this means for foragers

A pure, dense red oak forest with a thick layer of leaves isn't the best place for a mushroom walk: there's almost no grass, the litter is thick, and the tree has fewer fungal partners. But mixed forests where red oak grows alongside pine, pedunculate oak, hornbeam or birch can be very good for mushrooms, because there the fungi have partners to live with. If you're after oak-loving mushrooms in particular, our article which mushrooms grow in an oak forest explains where to go.

Our November photos show one more thing: under dense leaves a mushroom is easy to miss. If you head into this kind of forest in late autumn, gently push the leaves aside near the trunks and along the edges of the moss. For what you can find at this time of year, see our article which mushrooms to pick in November.