Fine roots are the principal organs for absorption of water and nutrients in soil, and their growth patterns control forest production and sustainability. In most boreal and temperate forest trees, the fine root nutrient acquisition is often mediated by ectomycorrhizae and soil and rhizosphere bacteria. Despite our growing understanding of the importance of fine roots and their associated mycorrhiza and bacterial communities in the rhizosphere for carbon and nutrient cycling in forests, studies of the functioning and adaptability of “the root-mycorrhiza-bacteria continuum” to a range of environmental conditions are still in their infancy.

In this paper we analysed adaptive foraging mechanisms of ectomycorrhizal and fine roots of Picea abies, Pinus sylvestris and Betula pendula along a gradient from temperate to subarctic boreal forest (38 sites between latitudes 48°N and 69°N) in Europe. The study was carried out in cooperation with colleagues and research groups from Finland, Germany, United Kingdom and Lithuania.

Root tips with their symbiotic fungi and associated bacterial communities are metabolically most active, making many of their traits, both quantitative and qualitative characteristics, good indicators of root system adaptability. We used variables describing tree resource uptake structures and processes (absorptive fine root biomass and morphology, N concentration in absorptive roots, extramatrical mycelium (EMM) biomass, community structure of root-associated EcM fungi, soil and rhizosphere bacteria) to analyse relationships between root system functional traits and climate, soil and stand characteristics. As a result, we constructed a conceptual, multidimensional framework applicable to the description and analysis of resource capture strategies employed by the root-mycorrhiza-bacteria communities in forest soils.

Figure. A conceptual scheme of fine root foraging strategy related to latitudinal climate and soil carbon : nitrogen (C : N) gradient from boreal to temperate forests. The soil C : N ratio increases from left to right, from N-rich temperate forests to N-poor northern boreal forests. Foraging strategies are based on the adaptation of biomass allocation to absorptive fine roots associated with fine root turnover rate, fine root morphology and changes in root-associated ectomycorrhizal (EcM) fungi and rhizosphere bacterial communities. The EXTENSIVE strategy refers to investment in larger absorptive fine root biomass per forest stand basal area (BA; kg m ^-2), whereas the INTENSIVE strategy denotes the tendency to establish smaller absorptive root biomass, associated with functional changes in root morphology and a larger reliance on EcM and bacterial communities in the rhizosphere. (Figure 7 in the paper)

Our concept of fine root foraging strategies puts forward the notion that quantitative differences in absorptive fine root biomass per stand basal area are concurrent with changes in root morphology. At the same time, a foraging strategy involves qualitative shifts in multitrophic interactions in the rhizosphere involving host trees, ectomycorrhizal fungi and associated bacteria.

We integrated the root-mycorrhiza-bacteria continuum into the concept of adaptive fine root foraging and showed that the variety of alternatives within the root-mycorrhiza-bacteria continuum enables adaptive root foraging in both northern subarctic boreal and southern temperate forests. We provided strong empirical evidences that bi- and trilateral shifts in the root-mycorrhiza-bacteria continuum are part of changing biomass and nutrient cycling fluxes along climatic and environmental gradients,  driving fine root foraging efficiency and affecting essentially the potential of forest trees adaptability in global change.

Citation: Ostonen, I., Truu, M., Helmisaari, H. S., Lukac, M., Borken, W., Vanguelova, E., … & Truu, J. (2017). Adaptive root foraging strategies along a boreal–temperate forest gradient. New Phytologist, DOI: 10.1111/nph.14643 (link to full text)

Fine roots that are not so fine, but at the same time completely fine (pic from here)

Summary

The tree root–mycorhizosphere plays a key role in resource uptake, but also in the adaptation of forests to changing environments.

The adaptive foraging mechanisms of ectomycorrhizal (EcM) and fine roots of Picea abies, Pinus sylvestris and Betula pendula were evaluated along a gradient from temperate to subarctic boreal forest (38 sites between latitudes 48°N and 69°N) in Europe. Variables describing tree resource uptake structures and processes (absorptive fine root biomass and morphology, nitrogen (N) concentration in absorptive roots, extramatrical mycelium (EMM) biomass, community structure of root-associated EcM fungi, soil and rhizosphere bacteria) were used to analyse relationships between root system functional traits and climate, soil and stand characteristics.

Absorptive fine root biomass per stand basal area increased significantly from temperate to boreal forests, coinciding with longer and thinner root tips with higher tissue density, smaller EMM biomass per root length and a shift in soil microbial community structure. The soil carbon (C) : N ratio was found to explain most of the variability in absorptive fine root and EMM biomass, root tissue density, N concentration and rhizosphere bacterial community structure.

We suggest a concept of absorptive fine root foraging strategies involving both qualitative and quantitative changes in the root–mycorrhiza–bacteria continuum along climate and soil C : N gradients.