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.

The international Long-Term Ecological Research Network (ILTER) encompasses hundreds of long-term research/monitoring sites located in a wide array of ecosystems that can help us understand environmental change across the globe. We evaluated long-term trends (1990-2015) for bulk deposition, throughfall and runoff water chemistry and fluxes, and climatic variables in 25 forested catchments in Europe belonging to the UNECE International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems (ICP IM). Many of the IM sites form part of the monitoring infrastructures of this larger ILTER network. Trends were evaluated for monthly concentrations of non-marine (anthropogenic fraction, denoted as x) sulphate (xSO) and base cations x(Ca+Mg), hydrogen ion (H), inorganic N (NO and NH) and ANC (Acid Neutralising Capacity) and their respective fluxes into and out of the catchments and for monthly precipitation, runoff and air temperature. A significant decrease of xSO deposition resulted in decreases in concentrations and fluxes of xSO in runoff, being significant at 90% and 60% of the sites, respectively. Bulk deposition of NO and NH decreased significantly at 60-80% (concentrations) and 40-60% (fluxes) of the sites. Concentrations and fluxes of NO in runoff decreased at 73% and 63% of the sites, respectively, and NO concentrations decreased significantly at 50% of the sites. Thus, the LTER/ICP IM network confirms the positive effects of the emission reductions in Europe. Air temperature increased significantly at 61% of the sites, while trends for precipitation and runoff were rarely significant. The site-specific variation of xSO concentrations in runoff was most strongly explained by deposition. Climatic variables and deposition explained the variation of inorganic N concentrations in runoff at single sites poorly, and as yet there are no clear signs of a consistent deposition-driven or climate-driven increase in inorganic N exports in the catchments.

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