Boreal Forest Floor Research Articles

Response of Boreal Plant Communities and Forest Floor Carbon Fluxes to Experimental Nutrient Additions

Response of Boreal Plant Communities and Forest Floor Carbon Fluxes to Experimental Nutrient Additions

Tree species replacement from birch to spruce affects eukaryome in boreal forest soil

Large-scale replacements of native birch with spruce have been carried out in Western Norway for economic reasons. This tree species shift potentially affects biotic components such as the eucaryome, consisting of microscopic animals (Metazoa), protists and fungi, which are key players in the functioning of forest ecosystem. The impact on the belowground eukaryome and its interactions with vegetation and soil properties is not well assessed. We examined the impact of replacing native birch with Norway spruce plantations on the eukaryome of the boreal forest floor in Western Norway using 18S rDNA metabarcoding. The tree species shift from birch to spruce had significant impacts on the eukaryome at both taxonomic (Metazoa) and functional categories (phagotrophs, phototrophs, parasites and osmotrophs). The distinct differences in eukaryome communities were related to changes in understorey vegetation biomass and soil chemistry following the tree species shift. This had a negative effect on eukaryome richness, particularly affecting phagotrophs and parasites, while the opposite was observed for osmotroph richness. Our results indicated that the spruce plantations altered the eukaryome communities and their food-web patterns compared to what was found in the native birch forest soil. This information should be taken into consideration in forest management planning.

Unraveling global and diazotrophic bacteriomes of boreal forest floor feather mosses and their environmental drivers at the ecosystem and at the plant scale in North America.

Feather mosses are abundant cryptogams of the boreal forest floor and shelter a broad diversity of bacteria who have important ecological functions (e.g., decomposition, nutrient cycling). In particular, nitrogen (N2-) fixation performed by feather moss-associated diazotrophs constitutes an important entry of nitrogen in the boreal forest ecosystem. However, the composition of the feather moss bacteriome and its environmental drivers are still unclear. Using cDNA amplicon sequencing of the 16S rRNA and nifH genes and cyanobacterial biomass quantification, we explored the active global and diazotrophic bacterial communities of two dominant feather moss species (i) at the ecosystem scale, along a 500-km climatic and nutrient deposition gradient in the North American boreal forest, and (ii) at the plant scale, along the moss shoot senescence gradient. We found that cyanobacteria were major actors of the feather moss bacteriome, accounting for 33% of global bacterial communities and 65% of diazotrophic communities, and that several cyanobacterial and methanotrophic genera were contributing to N2-fixation. Moreover, we showed that bacteria were occupying ecological niches along the moss shoot, with phototrophs being dominant in the apical part and methanotrophs being dominant in the basal part. Finally, climate (temperature, precipitation), environmental variables (moss species, month, tree density) and nutrients (nitrogen, phosphorus, molybdenum, vanadium, iron) strongly shaped global and diazotrophic bacteriomes. In summary, this work presents evidence that the feather moss bacteriome plays crucial roles in supporting moss growth, health, and decomposition, as well as in the boreal forest carbon and nitrogen cycles. This study also highlights the substantial effects of climate and nutrients on the feather moss bacteriome, suggesting the importance of understanding the impacts of global change on moss-associated bacterial growth and activity.

Bryosphere Loss Impairs Litter Decomposition Consistently Across Moss Species, Litter Types, and Micro-Arthropod Abundance

The bryosphere (that is, ground mosses and their associated biota) is a key driver of nutrient and carbon dynamics in many terrestrial ecosystems, in part because it regulates litter decomposition. However, we have a poor understanding of how litter decomposition responds to changes in the bryosphere, including changes in bryosphere cover, moss species, and bryosphere-associated biota. Specifically, the contribution of micro-arthropods to litter decomposition in the bryosphere is unclear. Here, we used a 16-month litterbag field experiment in two boreal forests to investigate bryosphere effects on litter decomposition rates among two moss species (Pleurozium schreberi and Hylocomium splendens), and two litter types (higher-quality Betula pendula litter and lower-quality P. schreberi litter). Additionally, we counted all micro-arthropods in the litterbags and identified them to functional groups. We found that bryosphere removal reduced litter decomposition rates by 28% and micro-arthropod abundance by 29% and led to a colder micro-climate. Litter decomposition rates and micro-arthropod abundance were uncorrelated overall, but were positively correlated in B. pendula litterbags. Bryosphere effects on litter decomposition rates were consistent across moss species, litter types, and micro-arthropod abundances and community compositions. These findings suggest that micro-arthropods play a minor role in litter decomposition in the boreal forest floor, suggesting that other factors (for example, micro-climate, nutrient availability) likely drive the positive effect of the bryosphere on decomposition rates. Our results point to a substantial and consistent impairment of litter decomposition in response to loss of moss cover, which could have important implications for nutrient and carbon cycling in moss-dominated ecosystems.

Microbial Substrate Utilization and Vegetation Shifts in Boreal Forest Floors of Western Canada

Boreal forest soils are highly susceptible to global warming, and in the next few decades, are expected to face large increases in temperature and transformative vegetation shifts. The entire boreal biome will migrate northward, and within the main boreal forest of Western Canada, deciduous trees will replace conifers. The main objective of our research was to assess how these vegetation shifts will affect functioning of soil microbial communities and ultimately the overall persistence of boreal soil carbon. In this study, aspen and spruce forest floors from the boreal mixedwood forest of Alberta were incubated in the laboratory for 67 days without (control) and with the addition of three distinct 13C labeled substrates (glucose, aspen leaves, and aspen roots). Our first objective was to compare aspen and spruce substrate utilization efficiency (SUE) in the case of a labile C source (13C-glucose). For our second objective, addition of aspen litter to spruce forest floor mimicked future vegetation shifts, and we tested how this would alter substrate use efficiency in the spruce forest floor compared to the aspen. Tracking of carbon utilization by microbial communities was accomplished using 13C-PLFA analysis, and 13C-CO2 measurements allowed quantification of the relative contribution of each added substrate to microbial respiration. Following glucose addition, the aspen community showed a greater 13C-PLFA enrichment than the spruce throughout the 67-day incubation. The spruce community respired a greater amount of 13C glucose, and it also had a much lower glucose utilization efficiency compared to the aspen. Following addition of aspen litter, in particular aspen leaves, the aspen community originally showed greater total 13C-PLFA enrichment, although gram positive phospholipid fatty acids (PLFAs) were significantly more enriched in the spruce community. While the spruce community respired a greater amount of the added 13C-leaves, both forest floor types showed comparable substrate utilization efficiencies by Day 67. These results indicate that a shift from spruce to aspen may lead to a greater loss of the aspen litter through microbial respiration, but that incorporation into microbial biomass and eventually into the more persistent soil carbon pool may not be affected.

Age-related response of forest floor biogenic volatile organic compound fluxes to boreal forest succession after wildfires

The amplification of global warming in the Northern regions results in a higher probability of wildfires in boreal forests. On the forest floor, wildfires have long-term effects on vegetation composition as well as soil and its microbial communities. A large variety of biogenic volatile organic compounds (BVOCs) such as isoprene, monoterpenes, sesquiterpenes have been observed to be emitted from soil and understory vegetation of boreal forest floor. Ultimately, the fire-induced changes in the forest floor affect its BVOC fluxes, and the recovery of the forest floor determines the quantity and quality of BVOC fluxes. However, the effects of wildfires on forest floor BVOC fluxes are rarely studied. Here we conducted a study of the impacts of post-fire succession on forest floor BVOC fluxes along a 158-year fire chronosequence in boreal Scots pine stands near the northern timberline in north-eastern Finland throughout a growing season. We determined the forest floor BVOC fluxes and investigated how the environmental and ground vegetation characteristics, soil respiration rates, and soil microbial and fungal biomass are associated with the BVOC fluxes during the post-fire succession. The forest floor was a source of diverse BVOCs. Monoterpenes (MTs) were the largest group of emitted BVOCs. We observed forest age-related differences in the forest floor BVOC fluxes along the fire chronosequence. The forest floor BVOC fluxes decreased with the reduction in ground vegetation coverage resulted from wildfire, and the decreased fluxes were also connected to a decrease in microbial activity as a result of the loss of plant roots and soil organic matter. The increase in BVOC fluxes was associated with the recovery of aboveground plant coverage and soils. Our results suggested taking into consideration the implications of BVOC flux variations on the atmospheric chemistry and climate feedbacks.

Rapid Changes in Ground Vegetation of Mature Boreal Forests—An Analysis of Swedish National Forest Inventory Data

The boreal forest floor vegetation is critical for ecosystem functioning and an important part of forest biodiversity. Given the ongoing global change, knowledge on broad-scale changes in the composition and abundance of different plant species and species groups is hence important for both forest conservation and management. Here, we analyse permanent plot data from the National Forest Inventory (NFI) on changes in the vegetation over a 10-year period in four regions of Sweden. To limit the direct and relatively well-known effects of forest management and associated succession, we only included mature forest stands not influenced by forestry during the 10 years between inventories, and focused on vegetation change mainly related to other factors. Results show strong decrease among many species and species groups. This includes dominant species such as Vaccinimum myrtillus and Deschampsia flexuosa as well as several forest herbs. The only species increasing are some mosses in the southern regions. Our data do not allow for a causal interpretation of the observed patterns. However, the changes probably result from latent succession in combination with climate change and nitrogen deposition, and with time lags complicating the interpretation of their relative importance. Regardless of the cause, the observed changes are on a magnitude that suggest impacts on ecosystem functioning and hence highlight the need for more experimental work.

Topography-based statistical modelling reveals high spatial variability and seasonal emission patches in forest floor methane flux

Abstract. Boreal forest soils are globally an important sink for methane (CH4), while these soils are also capable of emitting CH4 under favourable conditions. Soil wetness is a well-known driver of CH4 flux, and the wetness can be estimated with several terrain indices developed for the purpose. The aim of this study was to quantify the spatial variability of the forest floor CH4 flux with a topography-based upscaling method connecting the flux with its driving factors. We conducted spatially extensive forest floor CH4 flux and soil moisture measurements, complemented by ground vegetation classification, in a boreal pine forest. We then modelled the soil moisture with a random forest model using digital-elevation-model-derived topographic indices, based on which we upscaled the forest floor CH4 flux. The modelling was performed for two seasons: May–July and August–October. Additionally, we evaluated the number of flux measurement points needed to get an accurate estimate of the flux at the whole study site merely by averaging. Our results demonstrate high spatial heterogeneity in the forest floor CH4 flux resulting from the soil moisture variability as well as from the related ground vegetation. The mean measured CH4 flux at the sample points was −5.07 µmol m−2 h−1 in May–July and −8.67 µmol m−2 h−1 in August–October, while the modelled flux for the whole area was −7.42 and −9.91 µmol m−2 h−1 for the two seasons, respectively. The spatial variability in the soil moisture and consequently in the CH4 flux was higher in the early summer (modelled range from −12.3 to 6.19 µmol m−2 h−1) compared to the autumn period (range from −14.6 to −2.12 µmol m−2 h−1), and overall the CH4 uptake rate was higher in autumn compared to early summer. In the early summer there were patches emitting high amounts of CH4; however, these wet patches got drier and smaller in size towards the autumn, changing their dynamics to CH4 uptake. The mean values of the measured and modelled CH4 fluxes for the sample point locations were similar, indicating that the model was able to reproduce the results. For the whole site, upscaling predicted stronger CH4 uptake compared to simply averaging over the sample points. The results highlight the small-scale spatial variability of the boreal forest floor CH4 flux and the importance of soil chamber placement in order to obtain spatially representative CH4 flux results. To predict the CH4 fluxes over large areas more reliably, the locations of the sample points should be selected based on the spatial variability of the driving parameters, in addition to linking the measured fluxes with the parameters.

Rhizosphere response to predicted vegetation shifts in boreal forest floors

The boreal forest is the single largest terrestrial store of carbon on Earth, and approximately 25% of these carbon stocks are in the forest floor. Climate change is expected to alter boreal vegetation, and aspen-dominated stands will replace conifers in Western Canada. We investigated how these vegetation shifts could affect the composition and function of soil microbial communities, using forest floor samples from the Ecosystem Management Emulating Natural Disturbance (EMEND) project in northwestern Alberta, Canada. Soil microbial communities were surveyed in rhizosphere and bulk forest floor of 17-year-old spruce clear-cuts where aspen was naturally regenerating, mature stands of aspen and spruce, and 17-year-old clear-cuts of aspen. Phospholipid fatty acid (PLFA) analysis was used to characterize microbial community composition and multiple substrate induced respiration (MSIR) to quantify microbial community functional capacity. Carbon source utilization by microorganisms was investigated through natural abundance isotope analysis of individual PLFAs. Rhizosphere samples had a significantly higher proportion of fungi and a higher gram-negative to gram-positive bacteria ratio compared to bulk soil. Fungi and gram-negative bacteria biomarkers in the rhizosphere showed 13C depletion compared to bulk forest floor, indicating that they had assimilated more newly-photosynthesized carbon than bulk forest floor microbes. Aspen trees exerted a greater influence over their rhizospheres than spruce trees in terms of microbial community composition and functional capacity, and aspen rhizospheres showed the highest basal respiration. In less than two decades, aspen regeneration in former spruce stands shifted the composition of microbial communities towards that of native aspen stands, with the rhizosphere microbiome responding more quickly than bulk forest floor. This study suggests that predicted vegetation shifts in the boreal have the potential to cause more immediate and profound changes in the rhizosphere, and emphasizes the need to include the rhizosphere in future studies.

Wildfire effects on BVOC emissions from boreal forest floor on permafrost soil in Siberia

One of the effects of climate change on boreal forest will be more frequent forest wildfires and permafrost thawing. These will increase the availability of soil organic matter (SOM) for microorganisms, change the ground vegetation composition and ultimately affect the emissions of biogenic volatile organic compounds (BVOCs), which impact atmospheric chemistry and climate. BVOC emissions from boreal forest floor have been little characterized in southern boreal region, and even less so in permafrost soil, which underlies most of the northern boreal region. Here, we report the long-term effects of wildfire on forest floor BVOC emission rates along a wildfire chronosequence in a Larix gmelinii forest in central Siberia. We determined forest floor BVOC emissions from forests exposed to wildfire 1, 23 and > 100 years ago. We studied how forest wildfires and the subsequent succession of ground vegetation, as well as changes in the availability of SOM along with the deepened and recovered active layer, influence BVOC emission rates. The forest floor acted as source of a large number of BVOCs in all forest age classes. Monoterpenes were the most abundant BVOC group in all age classes. The total BVOC emission rates measured from the 23- and >100-year-old areas were ca. 2.6 times higher than the emissions from the 1-year-old area. Lower emissions were related to a decrease in plant coverage and microbial decomposition of SOM after wildfire. Our results showed that forest wildfires play an important indirect role in regulating the amount and composition of BVOC emissions from post-fire originated boreal forest floor. This could have a substantial effect on BVOC emissions if the frequency of forest wildfires increases in the future as a result of climate warming.

Stand type affects fluxes of volatile organic compounds from the forest floor in hemiboreal and boreal climates

AimsThe forest floor is a significant contributor to the stand-scale fluxes of biogenic volatile organic compounds. In this study, the effect of tree species (Scots pine vs. Norway spruce) on forest floor fluxes of volatile organic compounds (VOC) was compared in boreal and hemiboreal climates.MethodsMonoterpenoid and sesquiterpene flux rates were determined during 2017–2018 using dynamic (steady-state flow-through) chambers placed on permanent soil collars on boreal and hemiboreal forest floors, where the canopy was formed by Scots pine (Pinus sylvestris), Norway spruce (Picea abies) or dominated by Scots pine with small coverage of Norway spruce and birches (Betula pendula and Betula pubescens).ResultsThe total monoterpenoid fluxes were higher from the Scots pine forest floor (boreal 23 μg m−2 h−1) and from the mixed forest floor (hemiboreal 32 μg m−2 h−1) compared to the Norway spruce forest floor in both boreal (12 μg m−2 h−1) and hemiboreal (9 μg m−2 h−1) climates. Due to higher litterfall production, the forest floor seems to be a greater source of monoterpenoids and sesquiterpenes in the hemiboreal mixed stand dominated by Scots pine compared to the boreal Scots pine stand, although the difference was not statistically significant. Forest floor VOC fluxes followed a similar seasonal dynamic in different forest stands, with the highest flux rates in spring and summer. Significant VOC sources in the boreal forest floor were synthesis and release from vegetation and living roots together with litter decomposition of fungi and other microbes, where VOCs are released from needle storage pools. Ground vegetation in the hemiboreal forest is scanty under the dense tree canopy, meaning soil processes, such as litter decomposition, microbial metabolism and root release, were likely the dominating VOC sources. VOC fluxes from the hemiboreal forest floor were reduced by increases in soil moisture.ConclusionsThis study indicates that if the warming climate changes tree species’ abundance and stand biomass, by increasing tree growth and coverage of broadleaf species, it may affect VOC fluxes from the forest floor and impact the total VOC emissions from northern soils.

Boreal forest soil is a significant and diverse source of volatile organic compounds

AimsVegetation emissions of volatile organic compounds (VOCs) are intensively studied world-wide, because oxidation products of VOCs contribute to atmospheric processes. The overall aim of this study was to identify and quantify the VOCs that originate from boreal podzolized forest soil at different depths, in addition to studying the association of VOC concentrations with VOC and CO2 fluxes from the boreal forest floor.MethodsThis is the first published study that measures belowground VOC concentrations at different depths in a podzol soil combined with simultaneous flux measurements from the boreal forest floor. The VOC concentrations were determined by sampling VOCs from air inside soil layers using the gas collectors and adsorbent tubes. Forest floor VOC fluxes were determined using a dynamic enclosure technique. All the VOC samples were analysed using a thermal desorption-gas chromatograph-mass spectrometer.ResultsMore than 50 VOCs, dominated by monoterpenes and sesquiterpenes, were detected in the air space in the soil during two measurement campaigns. The O-horizon was a significant monoterpene source, because it contained fresh isoprenoid-rich litter. Belowground monoterpene concentrations were largely decoupled from forest floor monoterpene fluxes; thus, it seems that production processes and storages of VOCs partly differ from those VOCs that are simultaneously released from the forest floor. Both fluxes and concentrations of the monoterpenes and sesquiterpenes correlated with the CO2 fluxes in autumn, indicating that VOC release was driven by microbial activity.ConclusionsThis is the first study where below-ground VOC concentrations were quantified in situ, and for this reason, this study provides valuable insights to the VOC sources present in soils.

Boreal Forest Floor Greenhouse Gas Emissions Across a Pleurozium schreberi-Dominated, Wildfire-Disturbed Chronosequence

The boreal forest is a globally critical biome for carbon cycling. Its forests are shaped by wildfire events that affect ecosystem properties and climate feedbacks including greenhouse gas (GHG) emissions. Improved understanding of boreal forest floor processes is needed to predict the impacts of anticipated increases in fire frequency, severity, and extent. In this study, we examined relationships between time since last wildfire (TSF), forest floor soil properties, and GHG emissions (CO2, CH4, N2O) along a Pleurozium schreberi-dominated chronosequence in mid- to late succession located in northern Sweden. Over three growing seasons in 2012–2014, GHG flux measurements were made in situ and samples were collected for laboratory analyses. We predicted that P. schreberi-covered forest floor GHG fluxes would be related to distinct trends in the soil properties and microbial community along the wildfire chronosequence. Although we found no overall effect of TSF on GHG emissions, there was evidence that soil C/N, one of the few properties to show a trend with time, was inversely linked to ecosystem respiration. We also found that local microclimatic conditions and site-dependent properties were better predictors of GHG fluxes than TSF. This shows that site-dependent co-variables (that is, forest floor climate and plant-soil properties) need to be considered as well as TSF to predict GHG emissions as wildfires become more frequent, extensive and severe.

Interannual and Seasonal Dynamics of Volatile Organic Compound Fluxes From the Boreal Forest Floor.

In the northern hemisphere, boreal forests are a major source of biogenic volatile organic compounds (BVOCs), which drive atmospheric processes and lead to cloud formation and changes in the Earth’s radiation budget. Although forest vegetation is known to be a significant source of BVOCs, the role of soil and the forest floor, and especially interannual variations in fluxes, remains largely unknown due to a lack of long-term measurements. Our aim was to determine the interannual, seasonal and diurnal dynamics of boreal forest floor volatile organic compound (VOC) fluxes and to estimate how much they contribute to ecosystem VOC fluxes. We present here an 8-year data set of forest floor VOC fluxes, measured with three automated chambers connected to the quadrupole proton transfer reaction mass spectrometer (quadrupole PTR-MS). The exceptionally long data set shows that forest floor fluxes were dominated by monoterpenes and methanol, with relatively comparable emission rates between the years. Weekly mean monoterpene fluxes from the forest floor were highest in spring and in autumn (maximum 59 and 86 μg m-2 h-1, respectively), whereas the oxygenated VOC fluxes such as methanol had highest weekly mean fluxes in spring and summer (maximum 24 and 79 μg m-2 h-1, respectively). Although the chamber locations differed from each other in emission rates, the inter-annual dynamics were very similar and systematic. Accounting for this chamber location dependent variability, temperature and relative humidity, a mixed effects linear model was able to explain 79–88% of monoterpene, methanol, acetone, and acetaldehyde fluxes from the boreal forest floor. The boreal forest floor was a significant contributor in the forest stand fluxes, but its importance varies between seasons, being most important in autumn. The forest floor emitted 2–93% of monoterpene fluxes in spring and autumn and 1–72% of methanol fluxes in spring and early summer. The forest floor covered only a few percent of the forest stand fluxes in summer.

Contrasting effects of reindeer grazing on CO2, CH4, and N2O fluxes originating from the northern boreal forest floor

AbstractReindeer (Rangifer tarandus L.) is considered to be an important mammalian herbivore, strongly influencing Arctic lichen‐dominated ecosystems. There is no wide knowledge about the effect of reindeer on greenhouse gas (GHG) fluxes in northern boreal forests. Ground vegetation plays an important role in absorbing nitrogen (N) and carbon dioxide (CO2) from the atmosphere. Lately, it has also been found to be a significant source of nitrous oxide (N2O) and a small source of methane (CH4). We investigated the influence of reindeer grazing on field layer GHG (CO2, CH4, and N2O) fluxes, ground vegetation coverage and biomass, and soil physical properties (temperature and moisture) in a northern boreal forest. At our study site, the reindeer‐induced replacement of lichen by mosses had contrasting effects on the GHG fluxes originating from the field layer. Field layer CO2 efflux was significantly higher in grazed areas. The field layer was a CH4 sink in all areas, but grazed areas absorbed more CH4 compared to non‐grazed areas. Although total N2O fluxes remained around 0 in grazed areas, a small N2O sink occurred in non‐grazed areas with lower moss biomass. Our results indicated that grazing by reindeer in northern boreal forests affects GHG fluxes from the forest field layer both positively and negatively, and these emissions largely depend on grazing‐induced changes in vegetation composition.

Contribution of understorey vegetation and soil processes to boreal forest isoprenoid exchange

Abstract. Boreal forest floor emits biogenic volatile organic compounds (BVOCs) from the understorey vegetation and the heterogeneous soil matrix, where the interactions of soil organisms and soil chemistry are complex. Earlier studies have focused on determining the net exchange of VOCs from the forest floor. This study goes one step further, with the aim of separately determining whether the photosynthesized carbon allocation to soil affects the isoprenoid production by different soil organisms, i.e., decomposers, mycorrhizal fungi, and roots. In each treatment, photosynthesized carbon allocation through roots for decomposers and mycorrhizal fungi was controlled by either preventing root ingrowth (50 µm mesh size) or the ingrowth of roots and fungi (1 µm mesh) into the soil volume, which is called the trenching approach. Isoprenoid fluxes were measured using dynamic (steady-state flow-through) chambers from the different treatments. This study aimed to analyze how important the understorey vegetation is as a VOC sink. Finally, a statistical model was constructed based on prevailing temperature, seasonality, trenching treatments, understory vegetation cover, above canopy photosynthetically active radiation (PAR), soil water content, and soil temperature to estimate isoprenoid fluxes. The final model included parameters with a statistically significant effect on the isoprenoid fluxes. The results show that the boreal forest floor emits monoterpenes, sesquiterpenes, and isoprene. Monoterpenes were the most common group of emitted isoprenoids, and the average flux from the non-trenched forest floor was 23 µg m−2 h−1. The results also show that different biological factors, including litterfall, carbon availability, biological activity in the soil, and physico-chemical processes, such as volatilization and absorption to the surfaces, are important at various times of the year. This study also discovered that understorey vegetation is a strong sink of monoterpenes. The statistical model, based on prevailing temperature, seasonality, vegetation effect, and the interaction of these parameters, explained 43 % of the monoterpene fluxes, and 34–46 % of individual α-pinene, camphene, β-pinene, and Δ3-carene fluxes.

The contribution of ericoid plants to soil nitrogen chemistry and organic matter decomposition in boreal forest soil

Nitrogen (N) availability restricts plant carbon assimilation from the atmosphere in N-limited boreal ecosystems. Boreal forest floor is covered with ericoid plants and in soil, ectomycorrhizal and ericoid mycorrhizal plant roots and fungal hyphae are intermixed. How different mycorrhizal plants affect N mobilization from soil organic matter (SOM) has yet to be elucidated. Here we compared the effects of ericoid plants (Calluna vulgaris, Vaccinium myrtillus, Vaccinium vitis-idaea), forming ericoid mycorrhiza (ERM), on soil N chemistry, plant N uptake and SOM decomposition and compared them with ectomycorrhizal (ECM) Pinus sylvestris in a microcosm experiment. Pine and ericoid plants affected soil N similarly, accessing both degradable and recalcitrant N pools. Both ericoid plants and pine took up N from organic source. Ericoid plant roots contained more phenolic compounds and condensed tannins than pine roots which together with lower pH under ericoids suggests substantial changes in soil chemistry and biology. This study underlines the plant stimulation of SOM degradation with special emphasis on ericoid plants. We point out that both ericoid and ectomycorrhizal plants and their interactions should be of interest when N uptake and SOM decomposition in boreal forests are studied.

Litter heterogeneity modulates fungal activity, C mineralization and N retention in the boreal forest floor

The implications of forest floor heterogeneity for fungal activity, nutrient retention and carbon sequestration within the forest floor remain poorly documented. This information would be particularly relevant to situations where large changes in fresh litter occur such as following stand replacing disturbances. Numerous laboratory studies have documented mycelial translocation of simple forms of C and nutrients between contrasting substrates, allowing fungi to overcome local deficiencies. In slightly more complex but less controlled conditions, we assessed how factors contributing to forest floor heterogeneity—decay state, litter origin and tree species—individually affect fungal activity during decomposition. We also assessed how the juxtaposition of litters of contrasting nutrient and C status (decay state) alter fungal activity within individual substrates. We expected fungal biomass to be reallocated to C-rich litters while lignocellulolytic activity would increase in all juxtaposed litters. A microcosm experiment was conducted in which wood and leaf litters of one softwood species (Pinus banksiana) and one hardwood species (Populus tremuloïdes) were incubated alone or in combination with litters of contrasting decay states. Litter mass loss, change in N content, C mineralization, fungal biomass, specific respiration rate and lignocellulolytic activity were measured after 15 and 30 weeks. The decay state of litter had the most pronounced and consistent effects on fungal activity, with higher fungal biomass and lignocellulolytic enzyme activity in well decomposed litters and higher mass loss, C mineralization and specific respiration rate in fresh litters. In juxtaposed litters, fungal biomass was initially reallocated to fresh litters when incubated with well decomposed litters. Cellulolytic activity also increased by 30% in juxtaposed fresh litters while Mn-peroxidase activity increased by 42% in both fresh and well decomposed litters. With the longer incubation period, C mineralization and specific respiration of fungal biomass increased in juxtaposed well decomposed wood, indicating an increase in overflow metabolism presumably in response to an increase in labile C. Fresh litters that were juxtaposed increased their N content while that of single litters decreased. A better understanding of fine scale mechanisms affecting litter decomposition could improve our ability to forecast ecosystem response to disturbance.

Application of body size spectra to nematode trait-index analyses

Current trait-based indices of soil fauna for assessing soil quality rely on extensive knowledge of life history traits for hyper-diverse groups. We look at the effects of clear-cut forest harvesting and subsequent wood ash application on the free-living nematode community using trait-based indices and body size spectra. We test the hypothesis that body size is a unifying trait and that the body size spectra can be used to demonstrate changes in soil quality while overcoming taxonomic impediments. Undisturbed boreal forest floor soils were compared with clear-cut sites amended with zero, one-half calcium equivalent wood ash, equivalent calcium reintroduction, and twice calcium reintroduction. Total abundance, trait-based metrics for the Maturity Index, and two metrics of body size spectra were calculated. Nematode abundance was not a good indicator of soil disturbance, while traditional trait-based approaches (the summed Maturity Index) demonstrated predicted changes in community structure. Analysis of two types of body size spectra elucidated how the community had changed under disturbance, and revealed that smaller bodied organisms were lost under ash amended treatments compared to both clear-cut and undisturbed forest. Using the body size spectra for nematodes is a simple addition to current trait-based approaches that can supplement soil assessments for disturbance and quality without requiring taxonomic expertise.

Interactions with successional stage and nutrient status determines the life-form-specific effects of increased soil temperature on boreal forest floor vegetation.

The boreal forest is one of the largest terrestrial biomes and plays a key role for the global carbon balance and climate. The forest floor vegetation has a strong influence on the carbon and nitrogen cycles of the forests and is sensitive to changes in temperature conditions and nutrient availability. Additionally, the effects of climate warming on forest floor vegetation have been suggested to be moderated by the tree layer. Data on the effects of soil warming on forest floor vegetation from the boreal forest are, however, very scarce. We studied the effects on the forest floor vegetation in a long-term (18 years) soil warming and fertilization experiment in a Norway spruce stand in northern Sweden. During the first 9 years, warming favored early successional species such as grasses and forbs at the expense of dwarf shrubs and bryophytes in unfertilized stands, while the effects were smaller after fertilization. Hence, warming led to significant changes in species composition and an increase in species richness in the open canopy nutrient limited forest. After another 9 years of warming and increasing tree canopy closure, most of the initial effects had ceased, indicating an interaction between forest succession and warming. The only remaining effect of warming was on the abundance of bryophytes, which contrary to the initial phase was strongly favored by warming. We propose that the suggested moderating effects of the tree layer are specific to plant life-form and conclude that the successional phase of the forest may have a considerable impact on the effects of climate change on forest floor vegetation and its feedback effects on the carbon and nitrogen cycles, and thus on the climate.