Summary: Patterns of plant biomass allocation and functional adjustments along climatic gradients are poorly understood, particularly belowground. Generally, low temperatures suppress nutrient release and uptake, and forests under such conditions have a greater proportion of their biomass in roots. However, it is not clear whether ‘more roots’ means better capacity to acquire soil resources. Herein we quantified patterns of fine‐root anatomy and their biomass distribution across Scots pine (Pinus sylvestris) populations both along a 2000‐km latitudinal gradient and within a common garden experiment with a similar range of populations. We found that with decreasing mean temperature, a greater percentage of Scots pine root biomass was allocated to roots with higher potential absorptive capacity. Similar results were seen in the common experimental site, where cold‐adapted populations produced roots with greater absorptive capacity than populations originating from warmer climates. These results demonstrate that plants growing in or originated from colder climates have more acquisitive roots, a trait that is likely adaptive in the face of the low resource availability typical of cold soils.

Ectomycorrhizal (ECM) fungi can influence the establishment and performance of host species by increasing nutrient and water absorption. Therefore, understanding the response of ECM fungi to expected changes in the global climate is crucial for predicting potential changes in the composition and productivity of forests. While anthropogenic activity has, and will continue to, cause global temperature increases, few studies have investigated how increases in temperature will affect the community composition of ectomycorrhizal fungi. The effects of global warming are expected to be particularly strong at biome boundaries and in the northern latitudes. In the present study, we analyzed the effects of experimental manipulations of temperature and canopy structure (open vs. closed) on ectomycorrhizal fungi identified from roots of host seedlings through 454 pyrosequencing. The ecotonal boundary site selected for the study was between the southern boreal and temperate forests in northern Minnesota, USA, which is the southern limit range for Picea glauca and Betula papyrifera and the northern one for Pinus strobus and Quercus rubra. Manipulations that increased air and soil temperature by 1.7 and 3.4 °C above ambient temperatures, respectively, did not change ECM richness but did alter the composition of the ECM community in a manner dependent on host and canopy structure. The prediction that colonization of boreal tree species with ECM symbionts characteristic of temperate species would occur was not substantiated. Overall, only a small proportion of the ECM community appears to be strongly sensitive to warming.

Fine roots play a significant role in plant and ecosystem respiration (RS); therefore, understanding factors controlling that process is important both to advancing understanding and potentially in modelling carbon (C) budgets. However, very little is known about the extent to which ectomycorrhizal (ECM) identity may influence RS or the underlying chemistry that may determine those rates. In order to test these relationships, we examined RS, measured as O₂ consumption, of first-order ECM root tips of Pinus sylvestris L. and Quercus robur L. saplings in relation to their ECM fungal symbionts and associated nitrogen (N), C and non-structural carbohydrate concentrations. Roots of P. sylvestris were colonized by Rhizopogon roseolus, Tuber sp. 1 and an unknown species of Pezizales. Fungal species colonizing Q. robur roots were Hebeloma sp., Tuber sp. 2 and one unidentified ECM fungus described as Tuber-like based on ECM morphology. ECM RS rates for different host species were significantly different and more than 97% of the variation in RS within a host species was explained by ECM root tip N concentrations. This may indicate that some of the variability in fine root RS–N relationships observed between and within different host species or their functional groups may be related to intraspecific host species differences in root tip N concentration among ECM fungal associates.

Understory shrubs are frequently attacked by insect herbivores. However, very little is known regarding possible interactions between light condition, defoliation (D) and fine root vitality (% live roots) and metabolic activity, and whether different plant strategies (compensation, trade-off and equilibrium) to defoliation depend on individual species light requirements. To explore the response of roots to such conditions, an experiment was established in which we experimentally removed 50% of leaves in 1-year-old seedlings of Sambucus nigra, Cornus sanguinea, Prunus serotina, Frangula alnus and Corylus avellana grown in 15% and full sunlight. On average, defoliation leads to a 15% reduction in fine root (<2 mm) vitality (% live roots). However, a statistically significant reduction in root vitality after defoliation was detected only in those species that are less herbivorized in nature (48% in S. nigra and 5% in C. sanguinea). On average, shade conditions (L) resulted in 18% decline in root vitality, and the effects of defoliation were also 22% higher than for plants grown in full light. Root vitality in both treatments (D and L) was significantly correlated with their dry mass, concentration of total phenol (TPh) and carbon to nitrogen ratio, and negatively correlated with nitrogen, soluble carbohydrates, starch and total non-structural carbohydrates (TNC). To a large extent, root vitality and chemistry varied by species. Higher root vitality was related to higher concentrations of phenolics, more than to N and TNC concentrations. Concentrations of phenolics also differed significantly between defoliated plants and controls. However, in defoliated plants, an increase in TPh was observed only in two species, which belong to two different groups in light requirements and susceptibility to insect grazing (C. sanguinea and P. serotina). This study indicated that higher vitality of roots occurred in species that are characterized by higher insect defoliation under natural conditions. It is likely that higher root vitality of these species was related to their high level of TPh and tannins. This was especially noticeable for the reduced light treatment, which represents natural conditions under which insect defoliation is highest. Our results suggest that varied strategies of resource allocation were used by the different species in response to variations in light and defoliation.