Forest Floor Material Research Articles

Differentiating Potential Suspended Sediment Sources Using Radionuclide Tracers and Soil Organic Matter Analysis in a Headwater Catchment in Chuncheon, South Korea

Identifying potential sources of suspended sediment (SS) in headwater catchments is crucial for water quality management. To differentiate these potential SS sources, we investigated the distribution of two fallout radionuclides (FRNs), 137Cs and 210Pbex, using gamma spectrometry along with soil organic matter (SOM) analysis in a headwater catchment with five potential SS sources: cultivated land, non-harvested forest floor, eroded hillslope, harvested forest floor, and stream bank. The 137Cs and 210Pbex concentrations and the SOM content were considerably higher in the harvested forest floor materials than in the other four potential SS source materials. FRN concentrations revealed distinct properties according to the type of potential SS sources. Specifically, the combination of FRNs (with the effect of SOM content removed) associated with the mineral fraction and SOM showed distinguishable differences among the potential SS sources, except for no difference between cultivated land and eroded hillslope. Therefore, SOM and FRNs, or their combination, can be effective indices to differentiate or trace potential SS sources on various land use/land cover types within a catchment. Further field tests will allow the tracing techniques that bind FRNs with SOM to contribute to understanding SS transport from non-point sources within a catchment.

Biodegradability of unheated and laboratory heated dissolved organic matter.

Following wildfires, partially combusted biomass remains on the forest floor and erosion from the landscape can release dissolved pyrogenic organic matter (dPyOM) to surface waters. Therefore, post-fire alterations to dissolved organic matter (DOM) in aquatic systems may play a vital role in DOM stability and biogeochemical cycles. Dissolved PyOM biodegradation remains poorly understood and is expected to vary with combustion temperature and fuel source. In this study laboratory heating and leaching of forest floor materials (soil and litter) were used to compare the biodegradability of unheated, low (250 °C), and moderate (450 °C) temperature leachates. Inoculation experiments were performed with river microbes. Dissolved organic carbon (DOC) and nitrogen (DON), inorganic nitrogen, and DOM optical properties were monitored for 38 days. Inoculation experiments showed significantly greater DOC biodegradation of low and moderate temperature samples (64% and 71%, respectively) compared to unheated samples (32%). The greater DOC biodegradation may be explained by lower molecular weight DOM composition of heated leachates which was supported by higher initial E2/E3 ratios (absorbance at 250 nm/365 nm). Further, the observed decrease in the E2/E3 ratio after incubation suggests biodegradation of smaller compounds. This trend was greater for heated samples than unheated DOM. Specific ultraviolet absorbance increased after incubation, suggesting biodegradation of aliphatic compounds. Inoculated moderate temperature samples showed the greatest DON degradation (74%), followed by low temperature (58%) and unheated (51%) samples. Overall, results suggest that low and moderate temperature dPyOM was more biodegradable than unheated DOM, which may have implications for aquatic biogeochemical cycling, ecosystem function, and water quality in fire-impacted watersheds.

Changes in ecosystem nutrient pools through stand development following whole-tree harvesting of jack pine (Pinus banksiana) on sandy, nutrient poor soils in northern Lower Michigan

We used a 10-site, 42-year chronosequence of jack pine (Pinus banksiana) plantations to study the changes in ecosystem nutrient pools following whole-tree harvesting (WTH) on sandy soils. We developed locally-derived allometric equations to predict nutrient contents of jack pine as a function of diameter at breast height and used these to estimate the amount of nitrogen (N), calcium (Ca), potassium (K), magnesium (Mg), and phosphorus (P) stored in aboveground biomass. In addition, we quantified total N, Ca, K, Mg and P in surface organic horizons, plant-available nutrient pools in mineral soils and leaching losses of nutrients for four years post harvest. Finally, we constructed simple input–output nutrient balances accounting for harvest removals at varying ages, post-harvest leaching losses and nutrient inputs from atmospheric deposition and weathering under varying scenarios of harvest intensity and frequency. Nutrient accumulation in aboveground biomass followed a sigmoidal pattern with a period of rapid accumulation between ages 10–20 y, a period of slowing accumulation between 20 and 30 y to an apparent asymptote from 30 y onward. Our estimates of asymptotic aboveground nutrient stocks for these plantations were highest for N (263 kg ha−1), intermediate for K and Ca (112 and 95 kg ha−1) and lowest for Mg and P (22 and 17 kg ha−1). Nutrients in forest floor materials increased from our youngest stand (4 y post harvest) with no signs of slowing accumulation in older stands. We observed weak trends of increasing soil nutrient availability across our chronosequence; however, variability was high both within and among stands. The business-as-usual scenario of WTH and planting on a 50-y rotation had a positive input–output balance for all nutrients with the exception of K, where outputs over this rotation exceeded inputs by 54 kg K ha−1. Shifting to a bole-only harvest improved the nutrient balance for all elements considered and shifted K to a net positive balance of 32 kg K ha−1. Scenarios designed to maximize production of biomass energy with WTH on shorter rotations exacerbated the K imbalance, with net losses of 72 and 63 kg K ha−1 for 30- and 20-y rotations, respectively.

Legacy Effects of Sorption Determine the Formation Efficiency of Mineral-Associated Soil Organic Matter.

Sorption of dissolved organic matter (DOM) is one major pathway in the formation of mineral-associated organic matter (MOM), but there is little information on how previous sorption events feedback to later ones by leaving their imprint on mineral surfaces and solutions ("legacy effect"). In order to conceptualize the role of legacy effects in MOM formation, we conducted sequential sorption experiments with kaolinite and gibbsite as minerals and DOM derived from forest floor materials. The MOM formation efficiency leveled off upon repeated addition of identical DOM solutions to minerals due to the retention of highly sorptive organic molecules (primarily aromatic, nitrogen-poor, hydrogen-poor, and oxygen-rich molecules), which decreased the sorption site availability and simultaneously modified the mineral surface charge. Organic-organic interactions as postulated in multilayer models played a negligible role in MOM formation. Continued exchange between DOM and MOM molecules upon repeated sorption altered the DOM composition but not the MOM formation efficiencies. Sorption-induced depletion of high-affinity compounds from solutions further decreased the MOM formation efficiencies to pristine minerals. Overall, the interplay between the differential sorptivities of DOM components and the mineral surface chemistry explains the legacy effects that contribute to the regulation of fluxes and the distribution of organic matter in the soil.

Short-term intensive warming shifts predator communities (Parasitiformes: Mesostigmata) in boreal forest soils

Increasing global mean surface temperatures from climate change will coincide with longer and more frequent short-term, extreme warming events. Because of this, habitats like boreal forests are predicted to have new temperature regimes. Boreal forest soils contain a diverse array of microarthropods and nematodes, which include the main soil predators, mesostigmatic mites (Acari: Parasitiformes: Mesostigmata). Although extensive research exists on how climate warming affects oribatid mite and collembolan communities, fewer studies have examined how warming effects Mesostigmata communities. We tested the effect of short-term (three months), intensive warming (+8 °C) on Mesostigmata communities from the boreal forest using experimental mesocosms containing forest-floor material. We collected moss mats and underlying forest floor organic material from a boreal forest and incubated them within individual mesocosms at 12 °C and 20 °C for three months, where 12 °C represented the long-term average growing season temperature and 20 °C corresponds to the potential extreme surface temperature from climate warming for the region. We enumerated all extracted microarthropods and nematodes, and identified all Mesostigmata adults to the species-level. In total, we counted 24,080 nematode individuals, and 19,582 total microarthropod individuals, of which 3349 individuals (1899 adults and 1450 juveniles) were mesostigmatic mites, consisting of 14 species. Mesostigmatic juvenile and adult abundances, along with adult community-level body mass were higher under warming, which lead to a shift in community composition. Changes to Mesostigmata communities were driven by the greater abundances of parthenogenetic species, primarily Veigaia mitis (Berlese), under warming—a response that has been shown in oribatid mite communities, but not mesostigmatic communities, before. Overall, we found that warming shifted mesostigmatic mite communities in the boreal forest, which has wide ranging implications for the soil food web.

Soil texture affects the coupling of litter decomposition and soil organic matter formation

Incomplete knowledge on the environmental factors linking litter decomposition and the formation of soil organic matter (SOM) hampers the sustainable management of soil as a carbon (C) sink. Here, we explored the effect of soil texture on the fate of C from decomposing litter (Indiangrass; Sorghastrum nutans (L.) Nash) and the concurrent formation of SOM in mineral soils of different textures (sand- and clay-rich) and forest floor material. We quantified the amount of litter C respired, C remaining in the litter, and litter C retained in the soil/forest floor in a 186-day incubation employing stable isotope analyses (13C). We complemented our isotopic approach with the extraction of microbial biomarkers from the litter and soils/forest floor material and spectroscopic studies into the compositional changes of the incubated materials.We found that soil texture affected both the decomposition of litter and the retention of litter-derived C in the soil. The soil rich in clay provided conditions favorable for a more efficient microbial utilization of the litter material (high pH and high C use efficiency) as compared to the sand-rich soil and the forest floor. This resulted in lower amounts of litter C respired as CO2 (25.0%, vs. 55.6 and 56.1% in clay vs. sand and forest floor material, respectively) and higher amounts of litter C retained in the clay-rich soil (12.6% vs. 3.5 and 5.3% in clay vs. sand and forest floor material, respectively). High contents of silt- and clay-sized mineral particles in the clay-rich soil likely resulted in the ability to stabilize litter C in aggregates and organo-mineral associations, perhaps as microbial residues. This ability was low in the sand-rich soil and virtually absent in the forest floor, where the recalcitrance of the litter and native SOM was probably more relevant, and a larger portion of litter C may have been retained in the soil as relatively untransformed plant compounds. We emphasize that litter decomposition, the formation of SOM, and soil texture are tightly linked, such that any differences in soil texture alter litter decomposition and SOM formation patterns for the same litter.

Decomposition of wood stakes in the Pacific Northwest after soil compaction and organic matter removal

Forest operations can affect soil productivity by impacting the amount and distribution of surface organic matter (OM) and changing the properties of surface mineral soil. The North American Long-Term Soil Productivity Study (LTSP) was developed to address such long-term changes after pulse disturbances associated with clearcut harvesting, soil compaction, and OM removal across a wide spectrum of forest sites. From 2000 to 2002 we established a study to assess the impact of four soil disturbance treatments on OM decomposition in the mineral soil on seven LTSP study plots from southern British Columbia, Canada to southern Oregon, USA. The soil treatments were combinations of two levels of compaction and organic matter removal: 1) removal of only merchantable logs [OM0] , 2) removal of all the forest floor and woody material [OM2], 3) no soil compaction after harvest [C0], and 4) “severe” soil compaction after harvest [C2]. Aspen (Populus tremuloides Michx.) and pine (Pinus taeda L.) wood stakes were inserted into the mineral soil of each treatment plot and in an adjacent unharvested control stand to a depth of either 20 or 30 cm (depending on soil depth). At the end of three years the soil compaction and the OM removal treatments had relatively little effect on pine or aspen stake decomposition at the British Columbia study sites. In the United States, soil compaction increased decomposition on the two study sites with coarse-texture mineral soil with surface OM intact. Three key findings were: (1) a strong positive relationship (p = 0.0009) between stake water content and stake mass loss, (2) aspen and pine stake decomposition was greater in the clearcut-LTSP treatments than in the unharvested stands, (3) the rehabilitation of the compacted soil treatment on the British Columbia study sites generally increased wood stake decomposition. However, overall, there were no consistent wood stake decay trends across all sites and LTSP treatments.

Impacts of forest harvest removal and fertiliser additions on end of rotation biomass, carbon and nutrient stocks of Pinus radiata

Forest harvesting practices remove tree biomass and the nutrients that they contain. There is uncertainty around the impact of this nutrient removal on the long-term sustainable production of forests over multiple rotations. To explore the impact of harvest management practices on planted forest sustainability, three long-term trials in Pinus radiata D. Don forests were assessed at the end of their second rotation of 26 to 27 years to determine the impact of harvest residue removal and fertiliser additions on sustainable forest productivity. The harvest residue removal treatments were stem only (SO), whole-tree (WT), and whole-tree plus forest floor (FF) removal with and without urea-nitrogen fertiliser additions (cumulative range across three sites 950 – 3200 kg N ha−1) from early rotation out to mid-rotation (last fertiliser application age range across sites 8 – 22 years). The FF removal treatment reduced soil carbon and nitrogen stocks throughout the subsequent rotation, whereas forest floor carbon and nutrient stocks had recovered from the FF removal treatment by the end of the rotation. Forest productivity was reduced at one site with the FF removal treatment, taking 3 years longer for stems to reach 35 cm diameter at breast height than other residue removal treatments. Between WT and SO harvest residue treatments there were very limited impacts on soil carbon and nutrient stocks, no impact on forest floor mass, carbon and nutrient stocks and the sum of the above ground live biomass, carbon and nutrient stocks or tree productivity. Across all harvest removal treatments fertiliser addition increased soil carbon and nitrogen stocks for sites with low initial soil stocks. The increased soil carbon and nitrogen stocks from fertiliser addition were associated with forest productivity gains with the time to reach 35 cm (diameter at breast height) for one site reduced by 3 – 5 years. Fertiliser addition to sites with FF removal was able to mitigate soil carbon and nitrogen stocks only when very large amounts of fertiliser were added (cumulative amounts of 1150 and 3200 kg N ha−1). The end of rotation results from this long-term study demonstrate the importance of harvest residues and forest floor material at low fertility sites for long-term nutrient sustainability in planted forests.

Goethite-Bound Phosphorus in an Acidic Subsoil Is Not Available to Beech (Fagus sylvatica L.)

In forests, where supply of bioavailable P from easily weatherable primary minerals is small, plants are thought to recycle P efficiently by uptake of P released from decomposing forest floor material. Yet, a share of the P is leached into subsoil where it is strongly adsorbed onto reactive surfaces of pedogenic Fe and Al oxides. This raised the question whether P leached into subsoil is also recycled. In order to investigate the mobilization of P bound to hydrous Fe oxides, we conducted a mesocosm experiment in a greenhouse. Beech saplings were grown for 14 months in subsoil material (Bw horizon from the P-poor Luss forest) with added goethite-P adsorption complexes, either in inorganic (orthophosphate) or in organic (phytate) form. Four types of control mesocosms were run: soil only and soil mixed with either dissolved orthophosphate or dissolved phytate or goethite. At the end of the experiment, neither total P mass in trees nor P contents in leaves differed between the treatments. According to leaf nutrient contents, plant growth was strongly limited by P in all treatments. Yet, total P mass in trees did not increase over the course of the experiment. Thus, despite of its P demand, beech was not able to acquire P from goethite surfaces within two vegetation periods. Also P added in dissolved form to the soil as well as native soil P were not available. This suggests that once inorganic and organic P is bound to pedogenic metal oxides in mineral soil, it is not recycled, which can be an explanation for field data demonstrating quantitatively significant stocks of P in subsoil of P deficient forests.

Modelling the hydrologic effects of vegetation growth on the long-term trajectory of a reclamation watershed

Reclamation watersheds that integrate fen peatlands into the design require the inclusion of uplands that are capable of supporting forest development while concurrently supplying sufficient groundwater recharge to downgradient wetland ecosystems. This necessitates selecting materials with suitable soil hydraulic properties and identifying the appropriate thickness and layering to fulfill the dual function of uplands as water storage, and water conveyance features. Currently, these systems incorporate tailings sand - a mine waste material - overlain by a cover soil of fine forest-floor material. The developmental pathway of these uplands is currently unknown, and it is unclear whether these landforms will provide enough groundwater recharge once a climax vegetation community establishes. Therefore, this research attempts to estimate the maximum density of vegetation, and associated water balance fluxes of a constructed upland integrated into a peatland watershed. The numerical modelling software HYDRUS-1D simulated soil moisture dynamics using a 65-year meteorological record, and a plant water stress algorithm was used to estimate the maximum sustainable leaf area index that the upland could support. Based on the thickness of the cover soil, the upland could support an average leaf area index of 1.2. Under this vegetation density, average annual groundwater recharge was 83 mm, and predominantly supplied by snowmelt (64%). Given this quantity of recharge, the model indicates that the upland will continue to provide enough groundwater to offset the anticipated water deficit in the downgradient fen ecosystem. However, by altering the design of the upland, specifically the spatial arrangement and thickness of cover soil, the same recharge could be supplied while also allowing for a higher average vegetation density. Such a design could allow for the creation of watersheds with a higher proportion of peatland.

Microbial response to carbon and nutrient additions in boreal forest soils and coversoils used during post-mining reclamation

Two types of organic-matter-rich coversoils are used during reclamation in the oil sands region of Alberta: forest floor material (FFM) salvaged from upland forests, and peat material (PM) salvaged from boreal wetlands. In this study, we tested the hypothesis that carbon (C) and nutrient availability may limit microbial activity in these reclamation materials by measuring their response to either 13C-labeled glucose or NPKS addition. Coversoil materials were compared with two natural forest soils corresponding to target sites for reclamation. A shift in microbial community structure (determined using phospholipid fatty acid analysis) was detected after both additions, but it was stronger with glucose than NPKS, especially for the two reclamation materials. For all soils, the increase in microbial respiration was stronger after glucose than after NPKS addition. The majority of CO2 originated from soil organic matter (SOM) for the natural forest soils but from glucose for the reclamation materials. In PM, glucose addition triggered SOM mineralization, as shown by a positive priming effect. Despite the absence of a priming effect for FFM, microbial communities incorporated higher rates of glucose into their biomass and respired double the amount of glucose compared with the other materials. Furthermore, the overall microbial community structure in the FFM became more similar to that of the natural forest soil materials following glucose addition. These findings indicate that C and NPKS limitations were stronger for the two reclamation materials than for the two natural forest soils. Furthermore, microbial communities in the two reclamation materials responded more readily to labile C than to NPKS addition.

Surface and subsurface material selections influence the early outcomes of boreal upland forest restoration

Soil is an essential component supporting the growth and maintenance of forests, providing anchorage, water, and nutrients for a diversity of plants. In Canada's boreal forest, surface soils differ widely in their chemical and physical conditions, ranging from coarse to fine textured mineral soils in the uplands to organic soils in the lowlands. Industrial disturbances in the boreal region require the selective salvage of surface and sub-soils from low- and upland areas during open pit mine operations, which can be used to reconstruct different soil profiles by arranging the materials in variable layers and thicknesses. The type of materials and their configuration in the reconstructed soil profile can have a profound effect on early forest establishment in reclaimed areas. We compared the growth of trembling aspen (Populus tremuloides Michx.), jack pine (Pinus banksiana Lamb.), and white spruce (Picea glauca Moench.) on sites that had different reconstructed soils caps using varying surface soil materials (salvaged lowland peat and upland forest floor material (FFM)), placement depths (10 or 30 cm for peat, 10 or 20 cm for FFM), and subsoil material types determined by salvage depth (Bm, BC, and C). Early seedling establishment and growth and soil and climatic parameters were monitored over a five-year period. Generally, seedling growth was greatest on FFM and appeared to be related to phosphorous availability, while peat as a surface soil reduced growth, likely due to delayed soil warming in the spring and cooler soil conditions that potentially limited resource availability. However, the greater water holding capacity of the organic matter in peat also provided a benefit for seedling growth that was apparent during water limiting climatic conditions. The underlying subsoil material also influenced growth, particularly later in establishment when roots occupied the deeper subsoils. Aspen growth was greatest when the subsoil was shallow salvaged and represented a weathered subsoil (Bm) compared to the more deeply salvaged, less weathered subsoils BC and C. Aspen and pine seedlings, with their larger roots systems, may have benefited from small increases in the silt fraction of the subsoils, likely due to the greater water holding capacity of these otherwise coarse textured sandy soils. Spruce regeneration responded marginally to soil treatments because of its overall slow growth-strategy and tolerance to resource limitations. Our results indicate that different soil capping strategies affect the performance of forest regeneration during post-mining site reconstruction, and boreal forest species may vary in responses according to their ecological life strategies.

Lignin from white-rotted European beech deadwood and soil functions

In forest ecosystems, deadwood can improve carbon storage, nutrient availability, and water holding capacity in soils. Yet the effect of organic matter from deadwood such as lignin on these soil functions and their regulators are unknown. We hypothesized that carbon storage, exchangeable cations, and pore space increase with the quantity of lignin-derived phenolic acids from beech deadwood. We also hypothesize that the most pronounced differences occur in more advanced decay classes, in the forest floor at sites with moder forest floors, and in the Ah horizon at sites with mull forest floors. Cupric oxide-oxidation products were used to determine lignin concentration, composition, and oxidation from paired reference and test samples next to 42 downed European beech (Fagus sylvatica L.) deadwood logs in ten stands in Southwest Germany. Compared to reference points, the sum of vanillyl, syringyl and cinnamyl lignin-derived phenols increased next to beech deadwood (within 10–20 cm). The composition and oxidation of lignin-derived phenols also changed near beech deadwood: syringyl/vanillyl ratios increased while cinnamyl/vanillyl and aldehyde/acid ratios for vanillyl decreased. Water-extractable organic carbon (OC) and its aromaticity also increased next to beech deadwood as did total OC and particulate OC separated by density fractionation relative to total and mineral-bound OC. These changes occurred namely in the organic horizons of moder forest floors, and in the Ah horizon underneath mull forest floors. These observations indicated that phenols predominantly entered soil in fluxes of fragmented and dissolved organic matter from beech deadwood. Changes to soil nutrient availability and porosity were linked to increasing lignin-derived phenols from beech deadwood especially in nutrient-poor soils and near heavily decayed deadwood. This is evidence that soils close to beech deadwood, a substrate, are spatially limited pedogenic hot-spots that have increased soil carbon, available nutrients, and pore space depending on the forest floor and parent material.

Blended pulp mill, forest humus and mine residual material Technosols for mine reclamation: A growth-chamber study to explore the role of physiochemical properties of substrates and microbial inoculation on plant growth

A growth chamber trial was conducted to investigate the effects of blends of pulp and paper mill residuals and forest humus on soil properties, microbial communities and germination rate and biomass production of annual ryegrass (Lolium multiflorum) in both acid-producing and neutral to mildly alkaline mine tailings in a mine reclamation context. The organic residual amendments improved the nutritional status of the tailings substrates, and increased pH in acid-generating tailings, leading to higher germination rates and improved plant growth. A trace addition (<0.02% of sludge by dry weight) of natural forest floor material as a microbial inoculum to the sludge could increase plant biomass up to four-fold. The effects of sludge application on bioavailability of metals were variable, with the concentration of soluble copper (Cu) and nickel (Ni) increasing in some of the substrates following organic amendments. Addition of paper mill residuals to mine tailings modified the microbial communities observed in the oligotrophic tailings with the majority of DNA sequences in the sludge amended substrates being found to be closely related to heterotrophic bacterial species rather than the chemolithotrophic communities that dominate tailings environments.

Additive or synergistic? Early ectomycorrhizal fungal community response to mixed tree plantings in boreal forest reclamation.

Ectomycorrhizal fungi are an important component to ecosystem function in the boreal forest. Underlying factors influencing fungal community composition and richness, such as host identity and soil type have been studied, but interactions between these factors have been less explored. Furthermore, mixed-species stands may have additive or synergistic effects on ectomycorrhizal fungi species richness, but this effect is challenging to test on natural sites due to difficulty in finding monospecific and mixed-species stands with similar site conditions and history. Forest reclamation areas can provide an opportunity to explore some of these fundamental questions, as site conditions and history are often known and managed, with the added benefit that knowledge emerging from these studies can be used to evaluate the recovery of degraded forest landscapes. Here, we compared the richness and composition of ectomycorrhizal fungi in young single- and mixed-species stands established on a reclamation area designed to inform strategies to restore upland boreal forests disturbed by oil sands mining. Seedlings of three host tree species (Populus tremuloides, Pinus banksiana, Picea glauca) were planted in single- and mixed-species stands on three different salvaged soils (forest floor material, peat, subsoil). After four growing seasons, there was no difference in total ectomycorrhizal fungi species richness and composition in mixed- versus combined single-species stands indicating that an additive effect of host tree species prevailed early in development. However, there were compositional shifts in fungal communities across both the host tree species and the salvaged soil type, with soil type being the strongest driver.

Soil carbon pools and fluxes vary across a burn severity gradient three years after wildfire in Sierra Nevada mixed-conifer forest

Carbon (C) storage in soils contributes to the strength and stability of total ecosystem C sinks, but both aboveground and belowground C is vulnerable to loss during fire. The distribution of soil C and nitrogen (N) among various defined pools – e.g., active, slow and resistant C, and ammonium and nitrate as forms of inorganic N – determines the C storage capacity of forests and the nutrient availability for plant communities recovering from wildfires. Projections of increased wildfire severity due to a warming climate and frequent droughts raise concerns about parallel increases in fire's impacts on the sizes and mineralization kinetics of soil C and N pools, with potentially long-lasting effects on the strength of the forest C sink and on the ability of forests to recover from disturbance. Therefore, we sought to determine how the sizes and mineralization rates of soil C and N pools vary across a gradient of fire severity three years after the Chips Fire burned 30,500 ha of Sierra Nevada mixed-conifer forest. We measured total C and N in forest floor and mineral soil (0–5 cm), the pool sizes and mean residence times of the active, slow, and resistant C in mineral soil, and the pool sizes and mineralization rates of inorganic N in mineral soil. Forest floor total C was lower in areas that experienced high severity fire than in unburned reference areas, an effect likely attributable to greater combustion of forest floor material in high severity areas. Mineral soil C content did not vary with fire severity. Over a 300-day lab incubation, mineral soil CO2-C efflux rates were consistently lower in soils from areas that experienced high severity fire relative to unburned reference areas and were associated with longer mean residence times of the slow C pool. Forest floor N content was lower in high severity areas than unburned areas, whereas mineral soil total N did not vary with fire severity. Mineral soil ammonium and total inorganic N concentrations increased significantly with fire severity in field-fresh soils, but this trend was no longer apparent after a 300-day lab incubation, indicating that site-specific factors control N availability among fire severity levels. Our results indicate that future increases in wildfire severity in mixed-conifer forest may alter the strength of the forest C sink by impacting the amount C stored in forest floor, the stability of mineral soil C, and the availability of N to recovering plant communities.

Ecohydrological functioning of an upland undergoing reclamation on post‐mining landscape of the Athabasca oil sands region, Canada

AbstractEcohydrological functioning of natural Boreal forest in Canada's Boreal Plains is a product of interactions between soil hydrophysical characteristics and hydrogeochemical processes. These interactions create a moisture–nutrient gradient within the surface soils, increasing along low‐relief transitions from upland to riparian zone, and in turn influence the distribution of vegetation communities. It is not yet known if/when analogous ecohydrological functions can be achieved in constructed uplands following industrial disturbance, such as that following oil sands development. Hence, to assess this, we studied interactions between hydrogeochemical processes and vegetation colonization in a constructed upland relative to hydrophysical properties of 2 reclamation cover substrates during a typical continental climate's growing season. Our results indicated that in 3 years of postconstruction, the establishment of a moisture–nutrient gradient that supports vegetation colonization along slope positions was still limited by heterogeneity of cover substrates. Portions of the upland under peat–mineral mix were characterized by lower nutrient availability, high moisture content, and establishment of planted shrubs and trees. In contrast, forest floor materials plots were characterized by poor soil quality, but higher nutrient availability and greater colonization of invasive grasses and native shrubs. We suggest that the colonization of underdeveloped soils by invasive grasses may facilitate pedogenic processes and thus should be accepted by reclamation managers as a successional milestone in the recovery of ecohydrological functioning of constructed uplands. Poor soil structure under forest floor materials could not support edaphic conditions required by plants to efficiently utilize fertilizer, making this practise futile at the early stage of soil development.

Plant recolonization of reclamation areas from patches of salvaged forest floor material

AbstractQuestionUnderstorey development is a great challenge in the restoration of many forest sites, particularly when sources of vegetation propagules are scarce. Can placement of propagule‐rich soil patches within reclaimed landscapes otherwise covered with propagule‐poor material promote the dispersal of vegetation from the patches into the surrounding areas?LocationLarge reclamation site in the Canadian (Alberta) boreal forest.MethodPatches of propagule‐rich forest floor material were placed within a matrix of propagule‐poor peat material. Vegetation assessments (cover estimates, seed rain) were done surrounding these patches in the third and fourth growing seasons.ResultsThere was significant egress of species from the patches into the peat after four growing seasons, and overall species associated with the patches had higher cover in the peat than species that were associated with the peat itself. While wind‐dispersed herbaceous species from the patches were found at the leading edge of the egressing community, most species used vegetative propagation, resulting in short egress distances. Several patch‐associated species were found in seed rain collected on the peat areas but were not observed in this material, suggesting seedbed limitations.ConclusionDespite the relatively short distance of egress, this experiment suggests that placement of propagule‐rich soil material within reclaimed landscapes will promote egress into adjacent propagule‐poor soil material.

Decomposition rates of surface and buried forest-floor material

Mechanical site preparation is assumed to reduce soil C stocks by increasing the rate at which the displaced organic material decomposes, but the evidence is equivocal. We measured rates of C loss of forest-floor material in mesh bags either placed on the surface or buried in the mineral soil at four sites in different regional climates in British Columbia. During the 3-year incubation, buried forest-floor material lost between 5% and 15% more C mass than material on the surface, with the greatest difference occurring at the site with the lowest annual precipitation. Studies of the long-term fate of buried and surface humus are needed to understand the net effects of site preparation on soil C stocks.

Contribution of forest floor fractions to carbon storage and abundance patterns of arbuscular mycorrhizal fungal colonisation in a tropical montane forest

Forest floor carbon stocks, which include different components of litter, hemic and sapric materials, have not been empirically quantified in tropical montane forest, although they influence soil carbon (C) pools. To date, the contribution of arbuscular mycorrhizae in C sequestration potentials in tropical montane forests have not been clearly investigated. This study determined the amount of C stocks in the different decomposing layers of forest floor, mainly litter, hemic and sapric materials. The abundance of arbuscular mycorrhizal root colonisation differed among forest floor fractions. Forest floor was measured for depth, area density, dry mass and carbon fraction separately in Sungai Kial Forest Reserve, Pahang, Malaysia to calculate C stocks. Percentages of root colonisation in the hemic and sapric materials were investigated. The results showed that forest floor C stocks were significantly higher in hemic (5 Mg C ha−1) and sapric (7.7 Mg C ha−1) compared with the litter fragments (1.5 Mg C ha−1). Mycorrhizal root colonisation was significantly higher (75%) in the toeslope compared with the summit area in the hemic materials. Segregation of forest floor layers provided greater accuracy in forest floor C stocks reporting.