Purple Moor Grass prevents Oaks from getting help finding nutrients

Conservation of species is more than just about the species you’re conserving. The species you’re saving has to interact with the rest of its environment, and understanding these interactions is essential for successful forest management. A recent study conducted by Marine Fernandez and colleagues examined the effects of purple moor grass (Molinia caerulea) on the root system size and ectomycorrhizal colonisation of oak seedlings, as well as the consequences on nitrogen content in oak and soil. This study’s results, published in AoB PLANTS, provide important insights into the complexity of interspecific interactions and the role of allelopathy in these interactions.

Fernandez and colleagues were interested in how oak seedlings interacted with ectomycorrhizae. These are fungi that associate with the roots of a plant, allowing them to form a symbiotic relationship with their host. The fungus provides the plant with nutrients and water, while the plant provides the fungus with carbohydrates. This process is important for the growth and health of the plant, as it helps the plant access nutrients and water that it would not be able to access on its own. It allows a plant to massively increase the reach of its roots, so it is an important process.

The experiment found that when mixed-grown with moor grass, both the numbers of oak fine lateral roots and ectomycorrhizal root tips were markedly reduced, leading to a 2.4-fold decrease of oak ectomycorrhization rate. Molecular typing of ectomycorrhizal root tips identified six different taxa in sole-grown versus five in mixed-grown treatment. The presence of moor grass had a negative effect on the ectomycorrhizal fungal community, likely due to allelochemical compounds, chemical compounds that can affect the growth of other species.

Oak seedlings had a lower nitrogen content in their shoots when mixed-grown with moor grass, suggesting a conservative strategy for nitrogen allocation. The moor grass had higher biomass when mixed-grown with oak, indicating an antagonistic interaction between the two species.

The team found that the oaks also had fewer lateral roots, despite having the same access to nitrogen. Root foraging ability associated with root system architecture, particularly branching, plays a critical role in nutrient uptake, particularly of nitrogen and phosphorus. The botanists comment that shoot and root extracts of some species, generally grass and crops, can inhibit lateral root formation and length in neighbours. Allelochemical exudation by roots may also inhibit both endomycorrhizae, fungi that colonise roots to partner with plants, and ectomycorrhizae, fungi that form a sheath around roots.

Fernandez and colleagues note that there are some limitations to their study. In their paper, they write:

Our experiment needs to be replicated in situ, as growth in pots can modify root architecture compared with naturally regenerated oak seedlings (Tsakaldimi et al. 2009), and impacts on ectomycorrhizal rate could be altered due to reduced space in the pot. The role of soil bacteria modulating N provision in the rhizosphere could be studied to gain a better understanding of the mechanisms by which lateral root formation and ectomycorrhization are inhibited. Then, in order to better characterise the interactions and question the ecological theories on plant competition, it would be interesting to measure the amount of nutrients (N, C, P, K) in the soil throughout the experiment.

Fernandez et al. 2023.

The findings of this study suggest that forest management should consider the balance between thinning intensity and the consequences on the interactions between understory species and seedlings. Thinning boosts the growth of the understorey, which can aid the renewal of the soil, but comes with the cost of promoting the growth of moor grass at the expense of the oak seedlings.

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Fernandez, M., Malagoli, P., Vincenot, L., Vernay, A., Améglio, T. and Balandier, P. (2022) “Molinia caerulea alters forest Quercus petraea seedling growth through reduced mycorrhisation,” AoB PLANTS, 15(2), p. lac043. Available at: https://doi.org/10.1093/aobpla/plac043.