Grazing abandonment in Iceland: unearthing the consequences for root biomass and community-level root functional traits

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Abstract Background and aims In northern ecosystems, plants allocate biomass predominantly to roots, which are central to the transfer of recent photosynthate from plants to soil and thus soil carbon accumulation. Grazing typically modifies above-ground vegetation composition and structure. Yet, little is known about how root systems adjust structurally and functionally in the long-term after grazing abandonment in northern ecosystems. In Iceland, grassland and heathland have historically been maintained by traditional livestock grazing, although grazing practices are increasingly being abandoned. The aim of this study was to assess how long-term grazing abandonment affects roots in Iceland. Methods We analysed community-level root distribution and root functional traits and their association with soil carbon in 0 – 60 cm soil from 34 sites of adjacent grazed and long-term ungrazed (> 20 years) land. Results We found that fine root biomass was strongly reduced in long-term grazing abandoned grassland, but not in heathland. Fine root biomass in the top 60 cm of soil was on average 16.1 Mg ha −1 in grazed land and 12.7 Mg ha −1 in ungrazed land, with 71% of all roots distributed in the top 10 cm. Despite these differences in biomass, community-level root functional traits were largely unrelated to grazing abandonment, indicating that grazing was not a main driver of root foraging and resource acquisition across our sites. Conclusion Our findings suggest that, especially in grassland, grazing abandonment limits the below-ground allocation of photosynthates and thus transfer into soil but has no impact on root functional traits related to root economics space.

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