Modelling interacting effects of invasive earthworms and wildfire on forest floor carbon storage in the boreal forest

In forest ecosystems, earthworms and wildfire are two ecological agents that cause carbon (C) stored in the forest floor to be transferred to the atmosphere as greenhouse gases, either through heterotrophic respiration (earthworms) or through periodical combustion (wildfire). For centuries, wildfire has been an important ecological driver in the boreal forests of Canada where most fire emissions to the atmosphere originate from the forest floor. In contrast, earthworms are recent invaders, having been introduced to the Canadian boreal during the 20th century. Their spread is mainly associated with anthropogenic activities. We examined stand-level effects of earthworms and wildfire on forest floor C by adapting an earthworm-C simulation model for the boreal and using it in combination with a forest C accounting model. Because the overall impact of an invasive species depends on its areal extent, we used a spatial model of earthworm spread to calculate the total predicted change in C storage at the landscape-level following earthworm invasions in northeastern Alberta. Depending on the ecological groups of earthworms modelled in stand-level simulations, the forest floor C stock was reduced by 49.7–94.3% after 125 years, although the majority of this reduction occurred 35–40 years after initiation of the invasion. Because earthworm activities reduced the amount of forest floor C available for burning, emissions from wildfire were lower in the presence of earthworms. Spatial modelling of earthworm effects within the 5,905,400 ha Alberta–Pacific Forestry Management Area projected that forest floor C stocks in the invaded stands decreased 50,875 Mg C by 2006, and 2,706,354 Mg C by 2056, compared with the same area if earthworms were not present. Loss of forest floor C averaged over the 50 year simulation was 10 g m² yr⁻¹; similar in magnitude to estimates for C loss in the Canadian boreal due to wildfire or harvesting. These results indicate effects of non-native earthworms on the forest floor should be included in predictions of forest ecosystem C budgets to ensure accurate attribution of emissions to heterotrophic respiration versus combustion.     

Nitrogen alters carbon dynamics during early succession in boreal forest

Boreal forests are an important source of wood products, and fertilizers could be used to improve forest yields, especially in nutrient poor regions of the boreal zone. With climate change, fire frequencies may increase, resulting in a larger fraction of the boreal landscape present in early-successional stages. Since most fertilization studies have focused on mature boreal forests, the response of burned boreal ecosystems to increased nutrient availability is unclear. Therefore, we used a nitrogen (N) fertilization experiment to test how C cycling in a recently-burned boreal ecosystem would respond to increased N availability. We hypothesized that fertilization would increase rates of decomposition, soil respiration, and the activity of extracellular enzymes involved in C cycling, thereby reducing soil C stocks. In line with our hypothesis, litter mass loss increased significantly and activities of cellulose- and chitin-degrading enzymes increased by 45-61% with N addition. We also observed a significant decline in C concentrations in the organic soil horizon from 19.5 ± 0.7% to 13.5 ± 0.6%, and there was a trend toward lower total soil C stocks in the fertilized plots. Contrary to our hypothesis, mean soil respiration over three growing seasons declined by 31% from 78.3 ± 6.5 mg CO₂-C m⁻² h⁻¹ to 54.4 ± 4.1 mg CO₂-C m⁻² h⁻¹. These changes occurred despite a 2.5-fold increase in aboveground net primary productivity with N, and were accompanied by significant shifts in the structure of the fungal community, which was dominated by Ascomycota. Our results show that the C cycle in early-successional boreal ecosystems is highly responsive to N addition. Fertilization results in an initial loss of soil C followed by depletion of soil C substrates and development of a distinct and active fungal community. Total microbial biomass declines and respiration rates do not keep pace with plant inputs. These patterns suggest that N fertilization could transiently reduce but then increase ecosystem C storage in boreal regions experiencing more frequent fires.     

The importance of dissolved organic carbon fluxes for the carbon balance of a temperate Scots pine forest

Efforts to increase our understanding of the terrestrial carbon balance have resulted in a dense global network of eddy covariance towers, which are able to measure the net ecosystem exchange of CO₂, H₂O and energy between ecosystems and the atmosphere. However, the typical set-up on an eddy covariance tower does not monitor lateral CO₂- and carbon fluxes such as dissolved organic carbon (DOC). By ignoring DOC fluxes eddy covariance-based CO₂ balances overestimate the carbon sink of ecosystems as part of the DOC drains into the inland waters and get respired outside the footprint of the eddy covariance tower. In this study we quantify 7 years (2000-2006) of DOC fluxes from a temperate Scots pine forest in Belgium and analyse its inter-annual variability. On average, 10 gC m⁻² year⁻¹ is leached from the pine forest as DOC. If the DOC fluxes are considered relative to the gross ecosystem carbon fluxes we see that DOC fluxes are small: 0.8 ± 0.2% relative to gross primary productivity, 1.0 ± 0.3% relative to ecosystem respiration, and (2.4 ± 0.4%) relative to soil respiration. However, when compared to net fluxes such as net ecosystem productivity and net biome productivity the DOC flux is no longer negligible (11 ± 7% and 17%, respectively), especially because the DOC losses constitute a systematic bias and not a random error. The inter-annual variability of the DOC fluxes followed that of annual water drainage. Hence, drainage drives DOC leaching at both short and long time scales. Finally, it is noted that part of the carbon that is leached from the ecosystem as DOC is respired or sequestered elsewhere, so the physical boundaries of accounting should always be reported together with the carbon budget.     

Determinants of litter mixing effects in a Swedish boreal forest

When the litter of a given species decomposes, it will often break down in the proximity of litters from other species. We investigated the effects of litters of 10 different species in a boreal forest of northern Sweden on each others' decomposition and N release rates; this was done through the use of litterbags containing two compartments separated by single mesh partition. Different litters could be placed on opposite sides of this mesh so that they were in contact with each other. Treatments consisted of all the possible pairwise combinations of the 10 species, with members of each pair placed in different compartments of the same litterbag. Litterbags were harvested after 1, 2 and 4 years in the field. Species differed significantly in their effects on decomposition and N loss rates of associated litters. Generally, litters from feather mosses and lichens showed the greatest promotion of decomposition on associated litters, while some vascular plant species, notably Empetrum hermaphroditum, showed the least. At year four, feather mosses also had the greatest positive effects on N loss from the litters of associated species. There were several instances in which litter of a given species decomposed at different rate when litter from its own species, rather than that of a different species, was placed in the adjacent litterbag compartment. This was particularly apparent in the second year, when across the entire data set, litters decomposed fastest when associated with their own litters. Generally, slowly decomposing litters had the greatest positive effects on decomposition of associated litters. It is proposed that in boreal forests slow decomposing litters (particularly those of feather mosses) may contribute to enhancing moisture attention in the litter layer, which in turn promotes the decomposition and N release of associated litters. Further, while litter mixing effects were clearly demonstrated in our study, they were also shown to be of secondary importance to the effects of species identity on decomposition.     

Net ecosystem productivity of boreal aspen forests under drought and climate change: Mathematical modelling with Ecosys

The net ecosystem productivity (NEP) of boreal aspen is strongly affected by comparative rates of annual potential evapotranspiration (E_a) and precipitation (P_a). Changes in E_a versus P_a during future climate change will likely determine changes in aspen NEP and consequently the magnitude of the carbon sink/source of a significant part of the boreal forest. We hypothesize that the effects of E_a versus P_a on aspen NEP can be modelled with a soil–root–canopy hydraulic resistance scheme coupled to a canopy energy balance closure scheme that determines canopy water status and thereby CO₂ uptake. As part of the ecosystem model ecosys, these schemes were used to model diurnal declines in CO₂ and latent heat (LE) exchange during a 3-year drought (2001–2003) at the Fluxnet-Canada Research Network (FCRN) southern old aspen site (SOA). These declines were consistent with those measured by eddy covariance (EC) at SOA, except that ecosystem CO₂ effluxes modelled during most nights were larger that those measured by EC or gap-filled from other EC measurements. Soil CO₂ effluxes in the model were close to, but sometimes smaller than, those measured by automated surface chambers at SOA. Diurnal declines in CO₂ exchange during the drought caused declines in annual NEP in the model, and in gap-filled EC measurements (model versus EC in g C m⁻²: 275 versus 367 ± 110 in 2001, 82 versus 144 ± 43 in 2002 and 23 versus 104 ± 31 in 2003). Lower modelled NEP was attributed to the larger modelled CO₂ effluxes. Ecosys was then used to predict changes in aspen net biome productivity (NBP = NEP − C lost from disturbance) caused by 6-year versus 3-year recurring droughts during 100-year fire cycles under current climate versus climate change projected under the IPCC SRES A1B scenario. Although NBP was adversely affected during recurring 6-year droughts under current climate, it recovered quickly during non-drought years so that long-term NBP was maintained at 4 g C m⁻² year⁻¹. NBP rose by 10, 108 and 126 g C m⁻² year⁻¹ during the first, second and third centuries under climate change with recurring 3-year droughts, indicating a gradual rise in sink activity by boreal aspen. However recurring 6-year droughts during climate change caused recurring negative NBP (C losses), gradually depleting aspen C reserves and eventually causing dieback of the aspen overstory during the third century of climate change. This dieback was followed by a large decline in NBP.We conclude that NBP of boreal aspen will rise gradually under current projections of climate change, except under prolonged (e.g. 6 years) recurring droughts, which would eventually cause aspen to die back and substantial amounts of C to be lost.     

Methane and nitrous oxide fluxes, and carbon dioxide production in boreal forest soil fertilized with wood ash and nitrogen

Wood ash has been used to alleviate nutrient deficiencies and acidification in boreal forest soils. However, ash and nitrogen (N) fertilization may affect microbial processes producing or consuming greenhouse gases: methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂). Ash and N fertilization can stimulate nitrification and denitrification and, therefore, increase N₂O emission and suppress CH₄ uptake rate. Ash may also stimulate microbial respiration thereby enhancing CO₂ emission. The fluxes of CH₄, N₂O and CO₂ were measured in a boreal spruce forest soil treated with wood ash and/or N (ammonium nitrate) during three growing seasons. In addition to in situ measurements, CH₄ oxidation potential, CO₂ production, net nitrification and N₂O production were studied in laboratory incubations. The mean in situ N₂O emissions and in situ CO₂ production from the untreated, N, ash and ash + N treatments were not significantly different, ranging from 11 to 17 μg N₂O m^?2 h^?1 and from 533 to 611 mg CO₂ m^?2 h^?1. However, ash increased the CH₄ oxidation in a forest soil profile which could be seen both in the laboratory experiments and in the CH₄ uptake rates in situ. The mean in situ CH₄ uptake rate in the untreated, N, ash and ash + N plots were 153 ± 5, 123 ± 8, 188 ± 10 and 178 ± 18 μg m^?2 h^?1, respectively.     

Temporal and spatial variability of soil respiration in a boreal mixedwood forest

Temporal and spatial variability of soil respiration (R_s) was measured and analyzed in a 74-year-old, mixedwood, boreal forest in Ontario, Canada, over a period of 2 years (August 2003–July 2005). The ranges of R_s measured during the two study years were 0.5–6.9 μmol CO₂ m⁻² s⁻¹ for 2003–2004 (Year 1) and 0.4–6.8 μmol CO₂ m⁻² s⁻¹ for 2004–2005 (Year 2). Mean annual R_s for the stand was the same for both years, 2.7 μmol CO₂ m⁻² s⁻¹. Temporal variability of R_s was controlled mainly by soil temperature (T_s), but soil moisture had a confounding effect on T_s. Annual estimates of total soil CO₂ emissions at the site, calculated using a simple empirical R_s–T_s relationship, showed that R_s can account for about 88 ± 27% of total annual ecosystem respiration at the site. The majority of soil CO₂ emissions came from the upper 12 to 20 cm organic LFH (litter–fibric–humic) soil layer. The degree of spatial variability in R_s, along the measured transect, was seasonal and followed the seasonal trend of mean R_s: increasing through the growing season and converging to a minimum in winter (coefficient of variation (CV) ranged from 4 to 74% in Year 1 and 4 to 62% in Year 2). Spatial variability in R_s was found to be negatively related to spatial variability in the C:N ratio of the LHF layer at the site. Spatial variability in R_s was also found to depend on forest tree species composition within the stand. R_s was about 15% higher in a broadleaf deciduous tree patch compared to evergreen coniferous area. However, the difference was not always significant (at 95% CI). In general, R_s in the mixedwood patch, having both deciduous and coniferous species, was dominated by broadleaf trees, reflecting changing physiological controls on R_s with seasons. Our results highlight the importance of discerning soil CO₂ emissions at a variety of spatial and temporal scales. They also suggest including the LFH soil layer and allowing for seasonal variability in CO₂ production within that layer, when modeling soil respiration in forest ecosystems.     

Soil compaction and forest floor removal reduced microbial biomass and enzyme activities in a boreal aspen forest soil

Soil enzymes are linked to microbial functions and nutrient cycling in forest ecosystems and are considered sensitive to soil disturbances. We investigated the effects of severe soil compaction and whole-tree harvesting plus forest floor removal (referred to as FFR below, compared with stem-only harvesting) on available N, microbial biomass C (MBC), microbial biomass N (MBN), and microbial biomass P (MBP), and dehydrogenase, protease, and phosphatase activities in the forest floor and 0–10 cm mineral soil in a boreal aspen (Populus tremuloides Michx.) forest soil near Dawson Creek, British Columbia, Canada. In the forest floor, no soil compaction effects were observed for any of the soil microbial or enzyme activity parameters measured. In the mineral soil, compaction reduced available N, MBP, and acid phosphatase by 53, 47, and 48%, respectively, when forest floor was intact, and protease and alkaline phosphatase activities by 28 and 27%, respectively, regardless of FFR. Forest floor removal reduced available P, MBC, MBN, and protease and alkaline phosphatase activities by 38, 46, 49, 25, and 45%, respectively, regardless of soil compaction, and available N, MBP, and acid phosphatase activity by 52, 50, and 39%, respectively, in the noncompacted soil. Neither soil compaction nor FFR affected dehydrogenase activities. Reductions in microbial biomass and protease and phosphatase activities after compaction and FFR likely led to the reduced N and P availabilities in the soil. Our results indicate that microbial biomass and enzyme activities were sensitive to soil compaction and FFR and that such disturbances had negative consequences for forest soil N and P cycling and fertility.     

Bacterial diversity and community structure along different peat soils in boreal forest

Bacteria in peat forest soil play important role in global carbon cycling. The distribution of bacteria population in different peat soils as a whole and how forest management practices alter the bacterial populations are still poorly known. Using pyrosequencing analysis of 16S rRNA gene, we quantified the diversity and community structure of bacteria in eight peat forest soils (pristine and drained) and two mineral forest soils from Lakkasuo, Finland with either spruce-dominant or pine-dominant tree species. In total, 191,229 sequences which ranged from 15,710 to 22,730 per sample were obtained and affiliated to 13 phyla, 30 classes and 155 genera. The peat forest soils showed high bacterial diversity and species richness. The tree species seems to have more strong impact on the bacterial diversity than the type of peat soil, which drives the changes in bacterial community structure. The dominant taxonomic groups across all soils (>1% of all sequences) were Proteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, Planctomycetes and Verrucomicrobia. The relative abundance of bacteria phylum and genus differed between soil types and between vegetation. Significant differences in relative abundance of bacteria phyla were only found for Gemmatimonadetes and Cyanobacteria between the pristine and the drained peat forest soils. At genus level, the relative abundance of several genera differed significantly between the peat soils with same or different tree species, including Burkholderia, Caulobacter, Opitutus, Mucilanginibacter, Acidocella, Mycobacterium, Bradyrhizobium, Dyella and Rhodanobacter.     

Effects of tree harvesting, forest floor removal, and compaction on soil microbial biomass, microbial respiration, and N availability in a boreal aspen forest in British Columbia

The effects of timber harvesting and the resultant soil disturbances (compaction and forest floor removal) on relative soil water content, microbial biomass C and N contents (C_mic and N_mic), microbial biomass C:N ratio (C_mic-to-N_mic), microbial respiration, metabolic quotient (qCO₂), and available N content in the forest floor and the uppermost mineral soil (0-3 cm) were assessed in a long-term soil productivity (LTSP) site and adjacent mature forest stands in northeastern British Columbia (Canada). A combination of principal component analysis and redundancy analysis was used to test the effects of stem-only harvest, whole tree harvest plus forest floor removal, and soil compaction on the studied variables. Those properties in the forest floor were not affected by timber harvesting or soil compaction. In the mineral soil, compaction increased soil total C and N contents, relative water content, and N_mic by 45%, 40%, 34% and 72%, respectively, and decreased C_mic-to-N_mic ratio by 29%. However, these parameters were not affected by stem only harvesting or whole tree harvesting plus forest floor removal, contrasting the reduction of white spruce and aspen growth following forest floor removal and soil compaction reported in an earlier study. Those results suggest that at the study site the short-term effects of timber harvesting, forest floor removal, and soil compaction are rather complex and that microbial populations might not be affected by the perturbations in the same way as trees, at least not in the short term.     

Monoterpenes and higher terpenes may inhibit enzyme activities in boreal forest soil

Plant secondary compounds, including terpenes, potentially play an important role in controlling the decomposition process in boreal forest soil. However, the role of terpenes is not well understood, and their direct influence on enzyme activity is not well-known. The aim of this study was to examine the possible effects of common monoterpenes and higher terpenes on the activity of enzymes crucial in C, N, P, S cycling, i.e. β-glucosidase, chitinase, protease, acid phosphatase and arylsulfatase. Monoterpenes (α-pinene, carene, myrcene), diterpenes (abietic acid and colophony), and triterpene (β-sitosterol) were used. Studies were done in two environments, in vitro (studies without soil) and in vivo (studies with soil). Soil experiments were conducted using humus layers of two different birch stands, the first N-poor with high organic matter content and the second N-rich with a lower organic matter content. In general, all the terpenes studied showed inhibitory potential against enzymes in in vitro studies. In the soil incubation studies, both of the measured enzymes, chitinase and β-glucosidase, showed some decrease in activity when exposed to different terpenes. Our study suggests that terpenes modify the enzyme machinery in boreal forest soil.     

Evaluation of carbon exchange in a boreal coniferous stand over a 10-year period: An integrated analysis based on ecosystem model simulations and eddy covariance measurements

In order to assess the capacity of the boreal forest ecosystem to intercept atmospheric carbon over a period of years, a climate-driven growth model (FinnFor, process-based) was applied to calculate the seasonal and inter-annual variability of net ecosystem CO₂ exchange (NEE) and component carbon fluxes (gross primary production - GPP and total ecosystem respiration - TER) against a 10-year (1999-2008) period of eddy covariance (EC) measurements in a Scots pine (Pinus sylvestris L.) stand in Eastern Finland. Furthermore, the role of climatic factors, leaf area index (LAI) and physiological responses of trees regarding the ecosystem carbon fixation processes were evaluated. An hourly time-step was used to simulate the carbon exchange based on measured tree/stand characteristics and meteorological input for the experimental site, and the dynamic LAI was used throughout the 10-year simulations. The model predicted well the annual course of NEE compared to the measured values for most of the years, with the development of LAI (2.4-3.3 m² m⁻², as simulated). The simulated NEE over the study period shows that, on average, 62% of the variation refers to daily and 88% to monthly measured NEE. Both modeled and measured daily NEE showed similar responses to the temperature, photosynthetically active radiation and vapor pressure deficit during the growing seasons. In the simulation, the annual amount of GPP varied from 720.8 to 910.4 g C m⁻² with a mean value of 825.3 g C m⁻², and the annual mean TER/GPP ratio was 0.79, close to the measured value. Carbon efflux from the forest floor was the dominant contributor to the forest ecosystem respiration. The inter-annual variation of GPP mostly corresponded to the development of LAI, temperature sum and total incoming radiation over the 10-year simulation period. It was suggested that the process-based model could be applied to study the carbon processes for natural and management-induced dynamics of Scots pine forest ecosystem over longer periods across a wider climate gradient in the boreal zone.     

Decreases in soil moisture and organic matter quality suppress microbial decomposition following a boreal forest fire

Climate warming is projected to increase the frequency and severity of wildfires in boreal forests, and increased wildfire activity may alter the large soil carbon (C) stocks in boreal forests. Changes in boreal soil C stocks that result from increased wildfire activity will be regulated in part by the response of microbial decomposition to fire, but post-fire changes in microbial decomposition are poorly understood. Here, we investigate the response of microbial decomposition to a boreal forest fire in interior Alaska and test the mechanisms that control post-fire changes in microbial decomposition. We used a reciprocal transplant between a recently burned boreal forest stand and a late successional boreal forest stand to test how post-fire changes in abiotic conditions, soil organic matter (SOM) composition, and soil microbial communities influence microbial decomposition. We found that SOM decomposing at the burned site lost 30.9% less mass over two years than SOM decomposing at the unburned site, indicating that post-fire changes in abiotic conditions suppress microbial decomposition. Our results suggest that moisture availability is one abiotic factor that constrains microbial decomposition in recently burned forests. In addition, we observed that burned SOM decomposed more slowly than unburned SOM, but the exact nature of SOM changes in the recently burned stand are unclear. Finally, we found no evidence that post-fire changes in soil microbial community composition significantly affect decomposition. Taken together, our study has demonstrated that boreal forest fires can suppress microbial decomposition due to post-fire changes in abiotic factors and the composition of SOM. Models that predict the consequences of increased wildfires for C storage in boreal forests may increase their predictive power by incorporating the observed negative response of microbial decomposition to boreal wildfires.     

An experimental study of charcoal degradation in a boreal forest

Degradation rates of pyrogenic carbon (PyC) under natural environmental conditions are largely unknown. Here we present results from a field experiment monitoring the change in mass, C- and N concentrations of a variety of charcoal types in a Norwegian boreal forest over a period of 20 months. The charcoal types represent different feedstock tree species, production temperature regimes, and placements in the forest, i.e. above ground, in the humus layer or in contact with the mineral subsoil. The types of charcoal had different initial C concentrations mainly depending on their production temperature. Nevertheless, all types of charcoal at all placements in the forest showed an initial drop in their C concentrations, which subsequently rose back to reach near initial values in part of the charcoal types. In part of the charcoal types, N concentrations decreased throughout the experiment, exhibiting considerable variation among feedstock species, production temperature regime, and placements in the forest. C/N ratios rose especially in charcoal made from wood of Scots pine (Pinus sylvestris L.), and charcoal that had been stored in contact with the mineral subsoil showed the most rapid mass gain. Our results confirm the important influence of production temperature and feedstock type on the degradation of charcoal, but they also show that microbial activity and environmental conditions play significant roles in charcoal degradation and thus for the fate of pyrogenic carbon under natural conditions.     

Areal activities and stratification of hydrolytic enzymes involved in the biochemical cycles of carbon, nitrogen, sulphur and phosphorus in podsolized boreal forest soils

A novel approach allowing on-site high throughput enzyme activity measurements by fluorogenic model substrates was applied to study the functioning of enzymes involved in biochemical cycling of nutrients in boreal forest soil ecosystems. The examined enzymes comprised α-glucosidase, β-glucosidase, β-xylosidase, β-cellobiosidase, N-acetyl-glucosamidase, acetate-esterase, butyrate-esterase, phosphomonoesterase, sulphatase and aminopeptidase, whereby spatial and seasonal variation of their activity was investigated over nine seasons in 2 years. The studied sites of boreal podzolized soil of Pinus sylvestris and Picea abies forest were located in central Finland. Activity of all enzymes except sulphatase was highest in the humus layer in all seasons. Maximum sulphatase activity was located below the humus layer in the soil column. Annual activities of acetate-esterase, butyrate-esterase, β-glucosidase and phosphomonosterase calculated to in situ temperature during the year were 480-700, 690-950, 110-190 and 110-200 mol m⁻² year⁻¹, respectively. They were up to 100 fold higher than the other six measured activities. The overall turnover capacity of the enzymes was >1000 mol of ester linked carbon, >700 mols carbon from different carbohydrates, 100-200 mol of ester linked phosphate, 10-40 mol of ester linked sulphate m⁻² year⁻¹. Winter time (November-April) contributed from 7 to 32% to the annual turnover capacity indicating important enzyme activities also during a cold period of the year. Clear-cutting of the tree stand did not adversely affect enzyme activities related to the cycling of carbon, nitrogen, sulphur and phosphorus during the year. The pH optimum for hemicellulose and cellulose hydrolysing enzymes was pH 3-4 and the pH optimum of phosphomonoesterase, sulphatase, aminopeptidase and N-acetyl-glucosamidase was 4-5. This shows that the hydrolytic activities were adapted to the acid pH-values of the soils. The soil hydrolytic potential was many fold higher as compared to the actual amount of litter it received in the P. sylvestris and P. abies forests.     

Mass loss measurements and statistical models to predict decomposition of leaf litter in a boreal aspen forest

Abstract

In a southern boreal aspen forest located in Saskatchewan, Canada, we examined decomposition rates of leaf litter from trembling aspen (Populus tremuloides Michx.), hazel (Corylus cornuta March.), and a mixture of different species over a six‐month period. Mass loss was measured in the field using the litter bag method. The greatest mass losses occurred during the first month regardless of litter type. On average, mass loss during the first 28 days was 3.2 g#lbkg^‐1#lbd^‐1 for the aspen leaves, 4.4 for hazel leaves and 4.9 for the mixture. The initial rapid loss of weight is attributed to leaching and decomposition of water soluble material. The decomposition rates of the leaf litter were related to water‐soluble organic carbon and nitrogen content, and C:N ratio. Several models were used to describe mass loss of the aspen, hazel, and mixed leaf litter at the early stages of decomposition. A single model was not found to be appropriate to describe decomposition of all leaf‐litter types. A second order model provided the best fit for the aspen litter decomposition, while the logarithmic model best described the decomposition of hazel and mixed leaf litter.     

Carbon dioxide and energy fluxes from a boreal mixedwood forest ecosystem in Ontario, Canada

A long-term flux measurement station has been established in a 74-year-old mixedwood forest ecosystem, located approximately 80 km west of Timmins in northern Ontario, as part of the Fluxnet-Canada Research Network (FCRN). Measurements of energy, water vapour, and carbon dioxide fluxes have been made continuously since August 2003 using the eddy covariance technique, along with ancillary meteorological variables. The spatial structure of the site was evaluated using a variety of sources and techniques, including remote sensing, showing that this forest is mixed but relatively homogeneous. The canopy top height is remarkably constant at between 30 and 32 m. The basal area varies from 18 to 27 m² ha⁻¹, and the aboveground biomass ranges from 82 to 122 Mg ha⁻¹. In this paper, we summarize the diurnal and seasonal patters of carbon dioxide exchange and water loss from September 1, 2003 to August 31, 2004. Net ecosystem productivity (NEP) is strongly related to temperature. Atmospheric vapour pressure deficit (VPD) in this ecosystem exerted strong biophysical control on the daily gross ecosystem productivity (GEP) and evapotranspiration. Seasonal change in shortwave albedo, as a result of the presence of mixed deciduous and coniferous species, was clearly evident. Albedo changes were comparable to the seasonal pattern of NEP. The dormant season lasts more than 6 months of the year at this station. This forest was a moderate sink of carbon over the measurement period. Annual values of GEP, ecosystem respiration (R), and NEP were 1075, 919, and 156 ± 35 g C m⁻², respectively.     

Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand

Continuous half-hourly measurements of soil CO₂ efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration (R_s) on soil temperature (T_s) and water content (θ). Daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ in February to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-July. Daily mean T_s at the 2-cm depth was the primary variable accounting for the temporal variation of R_s and no differences between Arrhenius and Q₁₀ response functions were found to describe the seasonal relationship. R_s at 10 °C (R_s10) and the temperature sensitivity of R_s (Q_10Rs) calculated at the seasonal time scale were 3.8 μmol m⁻² s⁻¹ and 3.8, respectively. Temperature normalization of daily mean R_s (R_sN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of R_s during the growing season. Daily R_sN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer (θ_fc, 0.30 m³ m⁻³): (1) R_sN was strongly reduced when θ decreased below θ_fc, which reflected a reduction in microbial decomposition, and (2) R_sN slightly decreased when θ increased above θ_fc, which reflected a restriction of CO₂ or O₂ transport in the soil profile.Diurnal variations of half-hourly R_s were usually out of phase with T_s at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime R_s10 and Q_10Rs parameters calculated from half-hourly nighttime measurements of R_s and T_s at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m⁻² s⁻¹ and 5.5 to 1.3, respectively. On average, daily nighttime R_s10 (4.5 μmol m⁻² s⁻¹) and Q_10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low R_s10 and Q_10Rs values at low θ. Overall, the use of seasonal R_s10 and Q_10Rs parameters led to an overestimation of daily ranges of half-hourly R_s (ΔR_s) during drought conditions, which supported findings that the short-term temperature sensitivity of R_s was lower during periods of low θ. The use of daily nighttime R_s10 and Q_10Rs parameters greatly helped at simulating ΔR_s during these periods but did not improve the estimation of half-hourly R_s throughout the year as it could not account for the diurnal hysteresis effect.     

Potential activities of enzymes involved in N,C, P and S cycling in boreal forest soil under different tree species

The aim of this study was to compare the potential activity of enzymes involved in N, C, P and S cycling in the humus layer under three tree species: silver birch, Norway spruce and Scots pine. For arylsulphatase and protease the highest activities were found under birch, whereas beta-glucosidase activity was highest under pine. Beta-glucosaminidase and acid phosphatase showed similar activities regardless of tree species. Our studies show that soils under these species may differ enzymatically from each other. Enzyme activity studies under different tree species need more attention as the activity of different enzymes influences on soil nutrient availability in boreal forest soil.