Mushroom Monday: The Brown Roll-rim (Paxillus involutus) and Its Vital Tree-Fungal Partnership

Mushroom Monday: The Brown Roll-rim (Paxillus involutus) and Its Vital Tree-Fungal Partnership

Introduction

Exploring forest fungi in Texas introduces us to a variety of mushrooms that form essential partnerships with trees. Though the Brown Roll-rim mushroom (Paxillus involutus) isn’t commonly found in Houston’s forests, understanding its role in broader ecosystems helps us appreciate similar tree-fungal connections occurring locally. These mycorrhizal relationships are vital for trees and fungi, where fungi support trees in accessing nutrients and water, while trees provide essential sugars for the fungi’s survival. This fascinating fungus, found throughout other parts of North America, demonstrates how mushrooms and trees collaborate to maintain healthy forest environments.

Understanding Mushroom Structure

The Brown Roll-rim mushroom is known for its distinctive cap that curves inward at the edges. The cap spans between 1.5 to 4.7 inches and starts off convex, gradually forming a funnel-like depression with age. Young specimens have a suede-like texture, which smooths out as the mushroom matures, while the brown coloration remains consistent.

Identifying Features

Under the cap, you’ll find a network of gills that extend down the stem—a feature known as ‘decurrent.’ These gills start off pale brownish-yellow but turn brown when touched or damaged. The stem, typically 0.8 to 3.1 inches tall and 0.4 to 1.2 inches thick, mirrors the cap’s brown tones.

Habitat and Growing Conditions

Paxillus involutus prefers acidic soils and thrives in woodland areas with both hardwood and coniferous trees. It can also adapt to human-altered landscapes, frequently appearing along trails, in parks, or in newly landscaped areas. While it typically fruits in late summer and fall, it can emerge earlier in the damp conditions of the Pacific Northwest.

Similar Species

Several mushrooms might resemble Paxillus involutus, including the Velvet Roll-rim (Tapinella atrotomentosa) and Paxillus filamentosus. The Velvet Roll-rim grows on wood and has a velvety stem, while Paxillus filamentosus is smaller and specifically associated with alder trees. Key identification features of the Brown Roll-rim include its brown-staining gills, rolled-in cap edge, and its preference for soil over wood.

Tree-Fungal Partnerships

Underground, Paxillus involutus forms an intricate mycorrhizal network with tree species such as birch (Betula), pine (Pinus), and spruce (Picea). The fungus envelops tree roots in a protective sleeve, extending its hyphal threads into the soil to access water and nutrients. In return, the tree shares sugars produced through photosynthesis, forming a vital underground exchange that supports both species.

Forest Distribution and Growth

While absent from Houston’s urban forest, Paxillus involutus is widespread across North America, Europe, and Asia. Besides its role as a mycorrhizal partner, it also acts as a decomposer, breaking down organic matter and enriching forest soils. This dual function highlights the essential ecological roles fungi play in maintaining healthy forest ecosystems.

Important Safety Information

Despite its humble appearance, the Brown Roll-rim mushroom is toxic and can cause severe reactions in humans. Always exercise caution and avoid handling wild mushrooms without expert guidance. It’s best to appreciate this fascinating fungus from a distance.

Professional Tree Care

Mushrooms found near your trees can offer valuable insights into the soil’s biological health. Fungi like Paxillus involutus play an important role in your trees’ well-being by forming mycorrhizal relationships that promote nutrient exchange. A certified arborist can evaluate these relationships, helping your trees to thrive and addressing potential issues in your landscape.

If you liked learning about Brown Roll-rim (Paxillus involutus) mushrooms, check out our other Mushroom Monday posts on our TrueTreeTalk blog.  Follow us on Facebook to keep up with these and other posts.  Check out our list of blog posts grouped by topic for more to read. 

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