Effects of ectomycorrhizal species identity and colonisation extent on growth and nitrogen uptake in Pinus sylvestris seedlings

Sammanfattning

Boreal forests are commonly nitrogen-limited, and Scots pine (Pinus sylvestris) therefore forms symbiotic associations with ectomycorrhizal fungi (EMF) to increase nutrient acquisition in exchange for photosynthates. This association is particularly important for young seedlings due to their small root systems, and most P. sylvestris seedlings in boreal forests are therefore highly colonised by EMF. The importance of this symbiosis for seedling growth and nitrogen uptake is usually assessed by comparing mycorrhizal and non-mycorrhizal seedlings, with less attention given to the variation in colonisation extent. This study therefore examined whether colonisation extent can serve as a proxy for symbiotic function and predict growth and nitrogen uptake. We grew P. sylvestris seedlings under controlled greenhouse conditions and inoculated them with varying amounts of two EMF species, Laccaria bicolor and Suillus variegatus, to test whether inoculum dose affected colonisation. A stable isotope tracer (15N) was applied to quantify 15N uptake, and relationships between colonisation, growth, nitrogen content and 15N recovery were assessed. Both EMF successfully colonised seedlings, with colonisation increasing over time and reaching high levels after 14 weeks, independent of inoculum dose. Despite successful colonisation, mycorrhizal seedlings did not show increased growth, nitrogen content, or ¹⁵N uptake compared with non-mycorrhizal controls, and in some cases showed reduced biomass and tracer recovery. Under these experimental conditions, mycorrhizal association therefore did not confer a growth or nitrogen benefit compared with non-mycorrhizal controls. Fungal identity strongly influenced outcomes. Seedlings inoculated with S. variegatus had higher biomass, nitrogen content, and ¹⁵N uptake than those inoculated with L. bicolor, despite lower levels of colonisation. Colonisation extent therefore did not predict plant performance under these experimental conditions, suggesting ectomycorrhizal effects were more strongly influenced by fungal identity.