The response of forest ecosystems to increased atmospheric CO₂is constrained by nutrient availability. It is thus crucial to account for nutrient limitation when studying the forest response to ...climate change. The objectives of this study were to describe the nutritional status of the main European tree species, to identify growth‐limiting nutrients and to assess changes in tree nutrition during the past two decades. We analysed the foliar nutrition data collected during 1992–2009 on the intensive forest monitoring plots of the ICP Forests programme. Of the 22 significant temporal trends that were observed in foliar nutrient concentrations, 20 were decreasing and two were increasing. Some of these trends were alarming, among which the foliar P concentration in F. sylvatica, Q. Petraea and P. sylvestris that significantly deteriorated during 1992–2009. In Q. Petraea and P. sylvestris, the decrease in foliar P concentration was more pronounced on plots with low foliar P status, meaning that trees with latent P deficiency could become deficient in the near future. Increased tree productivity, possibly resulting from high N deposition and from the global increase in atmospheric CO₂, has led to higher nutrient demand by trees. As the soil nutrient supply was not always sufficient to meet the demands of faster growing trees, this could partly explain the deterioration of tree mineral nutrition. The results suggest that when evaluating forest carbon storage capacity and when planning to reduce CO₂emissions by increasing use of wood biomass for bioenergy, it is crucial that nutrient limitations for forest growth are considered.
Explaining the large-scale diversity of soil organisms that drive biogeochemical processes-and their responses to environmental change-is critical. However, identifying consistent drivers of ...belowground diversity and abundance for some soil organisms at large spatial scales remains problematic. Here we investigate a major guild, the ectomycorrhizal fungi, across European forests at a spatial scale and resolution that is-to our knowledge-unprecedented, to explore key biotic and abiotic predictors of ectomycorrhizal diversity and to identify dominant responses and thresholds for change across complex environmental gradients. We show the effect of 38 host, environment, climate and geographical variables on ectomycorrhizal diversity, and define thresholds of community change for key variables. We quantify host specificity and reveal plasticity in functional traits involved in soil foraging across gradients. We conclude that environmental and host factors explain most of the variation in ectomycorrhizal diversity, that the environmental thresholds used as major ecosystem assessment tools need adjustment and that the importance of belowground specificity and plasticity has previously been underappreciated.
During the last decade it has been increasingly acknowledged that carbon (C) contained in root exudates can accelerate decomposition of soil organic matter (SOM), a phenomenon known as rhizosphere ...priming effect (RPE). However, the controlling factors and the role of different soil microorganisms in RPE are not yet well understood. There are some indications that the response of the soil microbial decomposers to labile C input in the rhizosphere depends on microbial demand of nutrients for growth and maintenance, especially that of C and nitrogen (N). To test this hypothesis, we assessed SOM decomposition induced by 13C-glucose additions during one week in forest soils with different C:N ratios (11.5–22.2). We estimated SOM respiration, the potential activity (concentration) of a range of extracellular enzymes, and incorporation of 13C and deuterium (D) in microbial phospholipid fatty acids (PLFAs).
Glucose additions induced positive priming (a 12–52% increase in SOM respiration) in all soil types, but there was no linear relationship between priming and the soil C:N ratio. Instead, priming of SOM respiration was positively linked to the C:N imbalance, where a higher C:N imbalance implies stronger microbial N limitation. The total oxidative enzyme activity and the ratio between the activities of C and N acquiring enzymes were lower in soil with higher C:N ratios, but these findings could not be quantitatively linked to the observed priming rates. It appears as if glucose addition resulted in priming by stimulating the activity rather than the concentration of oxidative enzymes. Microbial incorporation of D and 13C into in PLFAs demonstrated that glucose additions stimulated both fungal and bacterial growth. The increased growth was mainly supported by glucose assimilation in fungi, while the increase in bacterial growth partly was a result of increased availability of C or N released from SOM. Taken together, the findings suggest that the soil C:N ratio is a poor predictor of priming and that priming is more dependent on the C:N imbalance, which reflects both microbial nutrient demand and nutrient provision.
•Soil C:N ratio alone was a poor predictor for the magnitude of priming effect (PE).•PE in soils with a varying C:N ratios was positively linked to the C:N imbalance.•Glucose addition increased activity rather than concentration of oxidative enzymes.•Both bacteria and fungi were taking up and growing on the added glucose.•CUE of glucose and ratio of fungal/bacterial growth correlated positively with PE.
•Stand density and age were strong predictors of European forest growth.•We found direct and indirect impacts of N deposition on forest growth.•Forest growth showed a positive, but in some cases ...quadratic response to N deposition.•Beech forest growth was significantly reduced at high N deposition levels.•Climate predictors were species-specifically related to European forest growth.•There was limited evidence of an ozone impact on European forest growth.
Changing environmental conditions may substantially interact with site quality and forest stand characteristics, and impact forest growth and carbon sequestration. Understanding the impact of the various drivers of forest growth is therefore critical to predict how forest ecosystems can respond to climate change. We conducted a continental-scale analysis of recent (1995–2010) forest volume increment data (ΔVol, m3 ha−1 yr−1), obtained from ca. 100,000 coniferous and broadleaved trees in 442 even-aged, single-species stands across 23 European countries. We used multivariate statistical approaches, such as mixed effects models and structural equation modelling to investigate how European forest growth respond to changes in 11 predictors, including stand characteristics, climate conditions, air and site quality, as well as their interactions. We found that, despite the large environmental gradients encompassed by the forests examined, stand density and age were key drivers of forest growth. We further detected a positive, in some cases non-linear effect of N deposition, most pronounced for beech forests, with a tipping point at ca. 30 kg N ha−1 yr−1. With the exception of a consistent temperature signal on Norway spruce, climate-related predictors and ground-level ozone showed much less generalized relationships with ΔVol. Our results show that, together with the driving forces exerted by stand density and age, N deposition is at least as important as climate to modulate forest growth at continental scale in Europe, with a potential negative effect at sites with high N deposition.
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.
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•Soil organic carbon (SOC) stock changes of upland forest sites were studied in 1995–2016.•Mean annual SOC stock increase was 36 g m−2 yr−1 in the organic layer and mineral soil ...(0–40 cm).•An opposite trend was observed on harvested plots or plots where part of the standing tree biomass was lost due to disturbances.•Total forest vegetation biomass correlated positively and soil C/N ratio negatively with the annual SOC stock increase.•Top mineral soil layer played an important role in annual SOC stock increase.
This study’s aim was to evaluate the soil organic carbon (SOC) stock changes of the upland forest sites (ICP Forests, Level II network) in relation to factors affecting the stocks in the long run, such as soil and stand properties. The SOC stock changes were based on repeated soil surveys carried out in 1995, 2006 and 2016 on ten plots on managed boreal forest ecosystems, and they represent typical mature forest stands in Finland. In addition, two plots in old-growth forests and two with a short monitoring history were studied. SOC stocks (organic layer, mineral soil 0–40 cm depth) were significantly higher in the spruce stands than in pine stands due to the higher site fertility of the spruce stands, and the biomass in the litter layer was positively correlated with the SOC stocks. The mean annual increase in the SOC stock was 36 g m−2 yr−1 for ten of the plots, which were monitored for the period between 1995 and 2016. An opposite trend was observed on two of the monitored plots, which were on harvested stands or plots where part of the standing tree biomass was lost due to disturbances before 2016. Total forest vegetation biomass was positively correlated, and the soil C/N ratio negatively, with the SOC stock’s annual increase. The top mineral soil layer (0–20 cm) played an important role in the SOC stock’s average annual increase.
•Biodiversity of the understorey vegetation of boreal forests was studied.•Increasing stand density weakened the positive effect of broadleaved trees on plant species number.•Increase of broadleaved ...correlated positively with nutrient level of the soil organic layer.•Response of bilberry cover to broadleaved depended on the site type and zone.
Understorey vegetation constitutes a remarkable part of the total biodiversity of boreal forests. Mixing broadleaved trees in coniferous forests has been shown to enhance vascular plant abundance and species richness, while high stand density has a counteracting effect. However, more information is needed on how environmental factors modify these biodiversity responses.
We investigated how the proportion of broadleaved trees of total basal area, tree species richness, stand density, and shrub cover were reflected in the number of vascular plant species and percentage cover of herbs and bilberry (Vaccinium myrtillus). We also studied the response of site fertility indicators, i.e. C:N ratio and extractable calcium concentration in the soil organic layer, to broadleaved species in mixed-forests.
The study was based on the data of understorey vegetation, soil, and stand properties in forests on mineral soil, inventoried on a systematic sample plot network in Finland in 2006. We used a subset of 307 plots in the hemi-southern and middle boreal zones and divided the plots either to xeric or mesic types. We used generalized additive models in analysing the data.
Birches (Betula sp.) were the main broadleaved species. Vascular species richness and herb cover generally increased with increasing proportions of broadleaved trees and tree species number both on mesic and xeric sites but decreased with increasing stand density on mesic sites. An increase in proportion of broadleaved trees and stand density resulted in decreased bilberry cover in the southern boreal zone, probably due to stand shading, but in increased cover in the middle boreal zone. Vegetation responses to increasing shrub cover often showed unimodal shape. As increase of proportion of broadleaved trees correlated positively with calcium and negatively with C:N of the organic layer, we suggest that nutrient input from easily decomposable birch leaf litter promotes species richness and herb cover via improved nutrient availability in the soil.
Our results indicate that many stand characteristics determining understorey biodiversity can be directed by forest management regimes. Generally, slight increase of broadleaved trees benefitted the understory plant richness, given that stand density is not too high. Because site type and zone controlled many responses, we suggest that it would be beneficial to have habitat and zone-specific instructions in forest management practices that conserve understorey vegetation and its multi-purpose ecosystem services.
•Study on change in the cover % of boreal forest understorey in Finland in 1985–2006.•Species-specific responses to cuttings differed within plant functional groups.•Light requirements directed the ...response of understorey plants to cuttings.•The plant species showed contrasting responses to clear-cuttings and thinnings.•Light requirements also accounted for the recovery rate after cutting disturbance.
We analysed the driving forces of temporal change in the cover of boreal forest plant species in Finland over 21years (1985–2006). Our study focused on the effects of forest cuttings of different intensity, i.e. regeneration cuttings (RC) and intermediate cuttings (IC), on 11 common understory species (including vascular plants, mosses and lichens) and their recovery after cuttings. General linear models (LM) were applied in modelling the change in the cover of plant species. The plant species displayed contrasting responses to RC and IC in accordance with their light demand, which also accounted for their recovery rate after cutting disturbance. After RC of old forest, shade and semi-shade tolerant species (herb Oxalis acetosella, moss Hylocomium splendens, dwarf shrub Vaccinium myrtillus) decreased strongly in cover (>70%) and recovered relatively slowly, but increased (40–100%) after IC. Species adapted to semi-light conditions (mosses Dicranum polysetum and Pleurozium schreberi, dwarf shrub Vaccinium vitis-idaea) decreased rather strongly (20–60%) after RC, but recovered relatively fast, reaching their old-forest cover level at a stand age of ca. 30years. These species also increased after IC. In contrast, light-demanding species (dwarf shrubs Empetrum nigrum and Calluna vulgaris, grass Deschampsia flexuosa) increased after RC and decreased after IC. Cladina lichens decreased across the country. In general, early successional light-demanding species increased and late successional shade-tolerant species decreased after RC. IC favoured all but the most light-demanding species. Species-specific responses to cuttings showed considerable variation within plant functional groups (e.g. dwarf shrubs, herbs, mosses). Cuttings determined the principal change in vegetation, while only slight signals of the effect of climate warming were found in few species. The legacy of forest management deserves special attention when e.g. tracking the effect of global change on vegetation.
Fine root acclimation to different environmental conditions is crucial for growth and sustainability of forest trees. Relatively small changes in fine root standing biomass (FRB), morphology or ...mycorrhizal symbiosis may result in a large change in forest carbon, nutrient and water cycles. We elucidated the changes in fine root traits and associated ectomycorrhizal (EcM) fungi in 12 Norway spruce stands across a climatic and N deposition gradient from subarctic‐boreal to temperate regions in Europe (68°N–48°N). We analysed the standing FRB and the ectomycorrhizal root tip biomass (EcMB, g m−2) simultaneously with measurements of the EcM root morphological traits (e.g. mean root length, root tissue density (RTD), N% in EcM roots) and frequency of dominating EcM fungi in different stands in relation to climate, soil and site characteristics. Latitude and N deposition explained the greatest proportion of variation in fine root traits. EcMB per stand basal area (BA) increased exponentially with latitude: by about 12.7 kg m−2 with an increase of 10° latitude from southern Germany to Estonia and southern Finland and by about 44.7 kg m−2 with next latitudinal 10° from southern to northern Finland. Boreal Norway spruce forests had 4.5 to 11 times more EcM root tips per stand BA, and the tips were 2.1 times longer, with 1.5 times higher RTD and about 1/3 lower N concentration. There was 19% higher proportion of root tips colonized by long‐distance exploration type forming EcM fungi in the southern forests indicating importance of EcM symbiont foraging strategy in fine root nutrient acquisition. In the boreal zone, we predict ca. 50% decrease in EcMB per stand BA with an increase of 2 °C annual mean temperature. Different fine root foraging strategies in boreal and temperate forests highlight the importance of complex studies on respective regulatory mechanisms in changing climate.
•Belowground litter was 23–37% of total litter C inputs in boreal pine forests.•Belowground litter C inputs did not significantly vary between site types.•EcM fungi and understory roots constituted ...8–13% and 18–41% of belowground litter.•Understory root litter equaled to that of pine in mesic site type.
Forest ecosystem productivity is strongly linked to site nutrient availability, which is influenced by litter inputs and their decomposition rates. Fine roots and mycelia are key contributors in belowground soil carbon (C) accumulation, but studies have seldom reported how belowground litter C input is related to site types in boreal forests. In this study, three mature and one young Pinus sylvestris forests along a site type gradient in southern Finland were chosen for measurements of fine root biomass, fine root longevity, below- and aboveground growth phenology and annual litter input from tree and understorey vegetation. Site types were distinguished by understorey vegetation composition, which indicated the site fertility. Fine root biomass per tree stand basal area decreased significantly from nutrient-poor to nutrient-rich sites, the nutrient-poor sites with longer fine root longevity resulted in an equal belowground litter input with the nutrient-rich site. Above- and belowground annual litter inputs were 131–236 and 70–91 g m−2 year−1, respectively. Aboveground litter increased with site fertility, resulting into belowground litter having a decreasing trend from 37% to 23% of total litter inputs with increasing site fertility. Ectomycorrhizal mycelia and understory production contributed 8–13% and 18–41% of belowground production, respectively. Contribution of understorey vegetation to the belowground litter C input was lower than that of trees at xeric and sub-xeric sites but equaled to that of trees at the mesic site. Our study showed distinct dimensions of below- and aboveground litter inputs influenced by site types. Moreover, we emphasize that the belowground C inputs from ectomycorrhizal mycelia and the understorey in addition to those of trees should always be considered in C balances and C reporting in boreal conifers.
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