Ectomycorrhizal colonization reduces ultra-fine root abundance

Ectomycorrhizal fungi (ECM) form mutualistic associations with tree roots, enhancing nutrient and water uptake and improving tree resistance to drought stress. ECM associations generally improve plant performance but act as strong carbon sinks, altering carbon allocation within the root system and modifying its architecture. A high degree of ECM colonization has been associated with increased root branching and the formation of short, swollen root tips. However, the role of ECM in shaping root system functional architectural traits remains unclear.

For instance, root diameter is a key trait differentiating fundamentally different functions, with fine roots mainly serving resource uptake and coarse roots mainly serving axial transport. Yet, studies observed ECM colonization leading to both stimulation and suppression of fine‑root production. These contrasting findings might derive from the widespread use of the <2 mm diameter threshold to define fine roots, a broad range lumping together root structures that differ both anatomically and functionally.

In this study, we distinguish two functionally different components of the fine-root system: ultrafine, absorptive feeder roots (diameter <0.5 mm) and thicker, transport/structural fine roots (0.5–2 mm). We then determine whether the degree of ECM colonization is associated with relative changes in ultrafine (<0.5 mm) root abundance.

We collected root samples from oak (Quercus petraea), beech (Fagus sylvatica), and larch (Larix decidua) saplings planted in 2024 at three sites in Northern Luxembourg that differ in land‑use history. At planting, half of the saplings received a commercial mycorrhizal inoculant. We stained root system subsamples with lactophenol cotton blue to facilitate ECM detection and counted the colonized root tips under a digital microscope. All root samples were later imaged using a flatbed scanner, and images were analyzed with WinRhizo software to quantify root length distribution across seven diameter classes (from <0.1 mm to 0.6-2 mm, in 0.1 mm increments). We then assessed relationships between ECM colonization and the proportion of ultrafine roots (< 0.5 mm) for each root sample.

The analysis revealed an overall negative correlation between ECM colonization and ultrafine root proportion. However, when examining sites separately, this trend was not observed at the site with former pasture land use. There, inoculated saplings showed both relatively high ECM colonization and high ultrafine root proportion. As the other two sites were former spruce stands (typically nutrient-poor), we argue that the greater nutrient availability at the ex-pasture site promoted the production of ultrafine roots, whereas high ECM colonization may reflect the legacy of the inoculation treatment rather than indicating substantial hyphal activity. Ongoing soil nutrient analyses will help confirm this interpretation.

Overall, the results suggest that, in nutrient poor soils, root systems may adjust their ultrafine feeder root proportion according to the degree of mycorrhization. We argue that this adjustment may potentially allow plants to maximize the benefits of the ECM association by reducing investment in short-lived feeder roots, whose function can be replaced by ECM.