Sediment production following severe wildfire and post-fire salvage logging in the Rocky Mountain headwaters of the Oldman River Basin, Alberta

In 2003, the Lost Creek fire burned 21,000 ha of nearly contiguous crown land forests in the headwater regions of the Oldman River Basin, Alberta. Seven small watersheds with various levels of land disturbance (burned, post-fire salvage logged, unburned) were instrumented and monitored for four years to measure stream discharge, sediment concentration, and sediment yields for a range of dominant flow periods characteristic of the region (baseflow, spring melt, and stormflow). Stream discharges reflected runoff regimes consistent with high regional precipitation and the high relief physiographic setting of the study area. Suspended sediment concentrations and yields were significantly higher in both burned and post-fire salvage logged watersheds than in unburned watersheds and were strongly influenced by topographic and hydro-climatic controls. Sediment availability was much higher in both the burned and post-fire salvage logged watersheds but it varied strongly with flow condition, particularly during the snowmelt freshet and high flow events. Because of increases in wildfire frequency and severity over recent decades, understanding the range of impacts from both wildfire and post-disturbance management strategies such as salvage logging is likely to become increasingly important for land managers.     

The Effect of Wildfire on Soil Mercury Concentrations in Southern California Watersheds

Mercury (Hg) stored in vegetation and soils is known to be released to the atmosphere during wildfires, increasing atmospheric stores and altering terrestrial budgets. Increased erosion and transport of sediments is well-documented in burned watersheds, both immediately post-fire and as the watershed recovers; however, understanding post-fire mobilization of soil Hg within burned watersheds remains elusive. The goal of the current study is to better understand the impact of wildfire on soil-bound Hg during the immediate post-fire period as well as during recovery, in order to assess the potential for sediment-driven transport to and within surface waters in burned watersheds. Soils were collected from three southern California watersheds of similar vegetation and soil characteristics that experienced wildfire. Sampling in one of these watersheds was extended for several seasons (1.5 years) in order to investigate temporal changes in soil Hg concentrations. Laboratory analysis included bulk soil total Hg concentrations and total organic carbon of burned and unburned samples. Soils were also fractionated into a subset of grain sizes with analysis of Hg on each fraction. Low Hg concentrations were observed in surface soils immediately post-fire. Accumulation of Hg coincident with moderate vegetative recovery was observed in the burned surface soils 1 year following the fire, and mobilization was also noted during the second winter (rainy) season. Hg concentrations were highest in the fine-grained fraction of unburned soils; however, in the burned soils, the distribution of soil-bound Hg was less influenced by grain size. The accelerated accumulation of Hg observed in the burned soils, along with the elevated risk of erosion, could result in increased delivery of organic- or particulate-bound Hg to surface waters in post-fire systems.     

Wildfire impacts on hillslope sediment and phosphorus yields

Purpose  

The impact of wildfire on (a) slope hydrological processes, (b) soil erodibility, and (c) post-fire hillslope sediment and phosphorus (P; dissolved and particulate) yields are quantified for natural forest areas of the burned Evrotas River basin, Peloponnese, Greece. Further, the geochemical partitioning of P in burned and unburned sediment is evaluated by sequential extraction to assess potential bioavailability of particulate P (PP) in downstream aquatic ecosystems.     

Modeling scour and deposition in ephemeral channels after wildfire

The area burned by wildfire in the states of Arizona and New Mexico in the southwestern US has been increasing in recent years. In many cases, high severity burns have caused dramatic increases in runoff and sediment yield from burned watersheds. This paper describes the potential and limitations of the HEC6T sediment transport model to describe changes in channel scour and deposition following the Cerro Grande fire near Los Alamos, New Mexico. Following the fire, Pueblo Canyon, near Los Alamos, was subject to a peak flow two orders of magnitudes higher than any discharge in the 7-year period of record, and twice the initial post-fire estimate of the 100-year event. HEC6T requires that the limits of scour and deposition on a cross-section be specified prior to application. This was achieved by using geomorphologic principles, predicted post-burn hydrology and long-term estimates of channel change derived from air photos, to estimate post-fire channel widths. Because significant quantities of silt and clay were present in the runoff, erosion shear stress and erosion rate parameters for cohesive sediments had to be obtained experimentally. After a sensitivity analysis, an optimization routine was used to estimate the optimal model parameter values for sensitive parameters. HEC6T was able to accurately model the change in cumulative sediment volume change derived from Airborne Laser Swath Mapping (ALSM, often called Lidar) taken before and after the large post-fire event. One discrepancy between the HEC6T model prediction and the ALSM-estimated change was that the ALSM-estimated change showed the greatest amount of deposition in a portion of the canyon with increasing slope, which the HEC6T model did not predict. Any sediment transport model will predict increased sediment transport capacity with increasing energy slope, so that it was considered to be beyond the capability of any sediment transport model to predict this deposition. Therefore, HEC6T simulated the overall changes in scour and deposition within reasonable expectation of the capabilities of physically-based sediment transport modeling indicating that it is capable of modeling sediment transport in ephemeral channels following wildfire.     

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.     

Exploring the factors influencing the hydrological response of soil after low and high-severity fires with post-fire mulching in Mediterranean forests

Despite ample literature, the influence of the individual soil properties and covers on the hydrological response of burned soils of forests has not clearly identified. A clear understanding of the surface runoff and erosion rates altered by wildfires and prescribed fires is beneficial to identify the most suitable post-fire treatment. This study has carried out a combined analysis of the hydrological response of soil and its driving factors in burned forests of Central-Eastern Spain. The pine stands of these forests were subjected to both prescribed fire and wildfire, and, in the latter case, to post-fire treatment with mulching. Moreover, simple multi-regression models are proposed to predict runoff and erosion in the experimental conditions. In the case of the prescribed burning, the fire had a limited impact on runoff and erosion compared to the unburned areas, due to the limited changes in soil parameters. In contrast, the wildfire increased many-fold the runoff and erosion rates, but the mulching reduced the hydrological response of the burned soils, particularly for the first two-three rainfalls after the fire. The increase in runoff and erosion after the wildfire was associated to the removal of the vegetation cover, soil water repellency, and ash left by fire; the changes in water infiltration played a minor role on runoff and erosion. The multi-regression models developed for the prescribed fire were accurate to predict the post-fire runoff coefficients. However, these models were less reliable for predictions of the mean erosion rates. The predictions of erosion after wildfire and mulching were excellent, while those of runoff were not satisfactory (except for the mean values). These results are useful to better understand the relations among the hydrological effects of fire on one side and the main soil properties and covers on the other side. Moreover, the proposed prediction models are useful to support the planning activities of forest managers and hydrologists towards a more effective conservation of forest soils.     

Comparison of Coleoptera assemblages from a recently burned and unburned black spruce forests of northeastern North America

Several insect groups have adapted to fire cycles in boreal forests, and can efficiently use new habitats created by fire. Our study aimed at producing a first characterization of post-fire Coleoptera assemblages of black spruce forests of eastern North America. For two years, we sampled Coleoptera using flight-interception traps in burned stands of contrasting age and structure in a 5097-ha wildfire and in neighbouring unburned mature stands. More than 40 species were exclusively captured in burned stands. Time elapsed since fire and proximity of unburned forests were the most significant parameters affecting Coleoptera assemblages. Stand age and structure had limited effects on assemblage structure; the Scolytid Polygraphus rufipennis Kirby was the only common species to clearly favor older stands. Fire-associated Coleoptera assemblages found in our study area were clearly distinct from those found in similar unburned stands; we should thus be conservative in our management approach concerning recently burned stands.     

多年连续计划烧除对云南松林坡面水土流失的影响

为探讨多年连续计划烧除对云南松林坡面水土流失的影响,科学评价云南松林计划烧除后森林生态系统的服务功能,2019年在滇中新平县照壁山云南松林设置计划烧除区域和未烧除区域的径流小区,对降雨及产流产沙变化进行了定位观测,建立了降雨、径流和泥沙的数值关系,结果表明:(1)2019年研究区降雨量为568.6 mm,仅为多年平均降雨量939.79 mm的60.5%,降雨主要集中雨季5—10月,雨季降雨量占年降雨总量的73.8%。雨季降雨51场次,其中产流降雨28场,占雨季降雨总量的93.7%。(2)多年连续计划烧除增强了降雨转化为林地坡面径流的能力,且小雨量级最能凸显计划烧除促进林地坡面径流泥沙的产生。(3)计划烧除小区和未烧除小区的径流主要与降雨量显著相关,计划烧除小区的径流与降雨呈幂函数关系,未烧除小区的径流与降雨呈多项式关系。(4)计划烧除小区和未烧除小区的泥沙均与降雨量、径流量显著相关,且泥沙与降雨、径流的关系均呈幂函数形式。表明多年连续计划烧除改变林地水文特征,提高了降雨转化为林地坡面径流的能力,提高了径流挟带泥沙的能力,才使计划烧除林地的径流泥沙比未烧除林地多。     

Reproductive success of the black-backed woodpecker (Picoides arcticus) in burned boreal forests: Are burns source habitats?

The black-backed woodpecker (Picoides arcticus) is considered a fire specialist throughout its breeding range. Given its high abundance in recent burns, it has been hypothesized that post-fire forests are source habitats for this species. We conducted a 3-year post-fire study to evaluate the temporal occupancy and reproductive success of black-backed woodpeckers in high-severity burned black spruce forests of central Quebec, Canada. We examined how reproductive success varied temporally and spatially within a burned landscape and investigated the potential source or sink status of this woodpecker population over time. Woodpecker nest density was high in the year after fire but declined significantly over the 3-year period. Based on 106 nests, nest success declined from 84% the first year after fire to 73% and 25%, respectively, for the second and third years after fire. Nest density and reproductive success were higher in areas with high proportions of burned mature forests than in areas dominated by burned young forests. Reproductive success was also higher in proximity to unburned forests. Comparison of annual productivity with a range of survival estimates indicated that these burned forests likely functioned as source habitats for the first 2 years following fire, although this status varied as a function of pre-fire forest age. Our results suggest that post-fire forests may contribute significantly to population levels in fire-prone ecosystems. Forest management practices that reduce the amount of mature and over-mature forests can affect the quality of post-fire habitats important to the black-backed woodpecker and other fire-associated species.     

Characterization and spatial distribution of particulate and soluble carbon and nitrogen from wildfire-impacted sediments

Purpose

The heavily forested Cache la Poudre (CLP) watershed in northern Colorado, USA, was impacted by the High Park wildfire in 2012. The wildfire burned land and vegetation immediately adjacent to the CLP River where blackened, ashy sediment samples were collected from five sites upstream of the City of Fort Collins drinking water intake to evaluate the spatial distribution and characteristics of burned sediments, along with quantifying and characterizing soluble compounds following a leaching experiment.

Materials and methods

At each site, samples were collected from three locations: (1) the edge of the bank adjacent to the water edge (downbank), (2) 1 m upslope of location 1 (midbank), and (3) 2 m upslope of location 1 (upperbank). All solid sediment samples were analyzed for elemental composition, and a subset of solid sediment samples were analyzed with ¹³C solid-state nuclear magnetic resonance spectroscopy. Sediments were mixed with the background CLP River water collected from upstream of the wildfire and allowed to leach for 6 and 24 h to determine the quantity and quality of water-soluble constituents. Filtered samples were analyzed for dissolved organic carbon (DOC), iron, manganese, and inorganic nutrient concentrations, by optical properties, and for disinfection byproduct (DBP) formation.

Results and discussion

Percent carbon and nitrogen content of the solid sediments were good predictors of leachate DOC concentration. The mean fluorescence index was higher for wildfire-impacted sediment leachates (1.50) compared to the background CLP River water (1.37), which may be due to changes in DOM molecular weight and oxidation of organic matter. All sediment leachates showed consistently higher haloacetonitrile and chloropicrin yields (DBP concentration/DOC concentration) compared to background CLP River water, whereas carbonaceous DBPs did not.

Conclusions

The collected sediments showed that burned material accumulated downstream near the river and was composed of inputs from burned soil and biomass along with the mobilization of unburned terrestrial material. The leachates of these sediments have different characteristics compared to the background CLP River water, indicating that DOM leached from sediments following a wildfire may increase aquatic DOC concentrations and N-DBP formation.

     

Spatial and temporal controls on post-fire hydrologic recovery in Southern California watersheds

The current study investigates the spatial and temporal dynamics of post-fire vegetation and the subsequent influence on seasonal and annual hydrologic responses in chaparral-dominated watersheds. Post-fire climatology, burn severity, slope aspect, and vegetation behavior are evaluated for two basins burned during the 2003 Old Fire in the San Bernardino Mountains in Southern California. Climate and discharge data are used to evaluate seasonal and annual variability of post-fire hydrologic fluxes. Data obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) are used to estimate Enhanced Vegetation Index (EVI) and differenced Normalized Burn Ratio (dNBR). A Savitzky–Golay filtering technique and an integrated EVI annual fraction are utilized to assess vegetation recovery under a range of potential controls. Vegetation recovery is highly variable in both watersheds and is related to slope aspect (solar and water availability), initial biomass levels, and burn severity. South and west facing slopes show higher pre-fire EVI (biomass) and significant loss of vegetation cover after fire. Vegetation in both watersheds responds to an extreme wet season during the second post-fire year, however recovery rates are not sustained. North and east aspects show the quickest biomass gain relative to pre-fire conditions by the end of the study period (WY 2010), while the west and south slopes show lower biomass recovery. High burn severity areas show the slowest recovery across all slope aspects, with these regions just approaching 90% of pre-fire biomass by the end of the seven-year post-fire period. The variable rate of vegetation recovery across the watersheds results in significant changes in annual and seasonal discharge throughout the post-fire period. Runoff ratios remain elevated in both systems and there is increased dry season flow for much of the study period, indicating that plant water consumption and flowpaths are not back to pre-fire behavior by the end of WY 2010.     

Soil microbial response following wildfires in thermic oak-pine forests

The ecosystem response to wildfire is often linked to fire severity, with potentially large consequences for belowground biogeochemistry and microbial processes. While the impacts of wildfire on belowground processes are generally well documented, it remains unclear how fire affects the fine-scale composition of microbial communities. Here, we investigate the composition of soil bacterial and fungal communities in burned and unburned forests in an attempt to better understand how these diverse communities respond to wildfire. We explored the belowground responses to three wildfires in Linville Gorge, NC, USA. Wildfires generally increased soil carbon content while simultaneously reducing soil respiration. We employed amplicon sequencing to describe soil microbial communities and found that fires decreased both bacterial and fungal diversity. In addition, wildfires resulted in significant shifts in both bacterial and fungal community composition. Bacterial phylum-level distributions in response to fire were mixed without clear patterns, with members of Acidobacteria being representative of both burned and unburned sites. Fungal communities showed consistent increases in Ascomycota dominance and concurrent decreases in Basidiomycota and Zygomycota dominance in response to burning. Indicator species analysis confirmed shift to Ascomycota in burned sites. These shifts in microbial communities may reflect differences in the quality and quantity of soil organic matter following wildfires.     

Determining the effects of wildfire on sediment sources using 137Cs and unsupported 210Pb: the role of landscape disturbances and driving forces

Purpose

Wildfires represent one of the major natural disturbances within forested landscapes and have potential implications for the quality and function of downstream aquatic ecosystems. This study aimed to determine if a wildfire in a mountainous, forested watershed in British Columbia, Canada, caused a change in the dominant sediment source in the immediate 1?C2?years following the wildfire, and if the sediment sources changed over the medium term (3?C7?years) as the landscape recovered.

Materials and methods

Source materials (surface soil, subsurface soil and channel bank material) and fluvial (suspended and channel bed) sediment samples were collected over the period 2004 to 2010 from a watershed burnt by a wildfire in 2003, and from an adjacent watershed that was not impacted by the fire. Samples were analysed for the fallout radionuclides (FRNs) caesium-137 (¹³⁷Cs) and unsupported lead-210 (²¹⁰Pb_un). An unmixing model was used to calculate the relative source contributions of the fluvial sediment samples.

Results and discussion

¹³⁷Cs and ²¹⁰Pb_un were concentrated in the upper layers of surface soils in both watersheds and were statistically different to concentrations in subsurface and channel bank material. In the burnt watershed, FRN concentrations were greatest in the ash layer. Sediment sources as determined by the unmixing model were 100?% subsurface/channel bank material in the unburnt watershed, while in the burnt watershed 8.5?±?2.5?% was derived from surface soils. In both watersheds, there were no major changes in the relative contributions from surface soil and from subsurface/channel bank material over the period 2004 to 2010. Thus, while the wildfire did cause a change in sediment sources, it was fairly subtle and did not conform to the effects following wildfire described for other studies in contrasting environments, which typically document a major increase in hillslope contributions relative to channel bank sources.

Conclusions

There was a limited response in terms of fine-grained sediment sources (and also sediment fluxes) in the burnt watershed. The reason for this muted response to a severe wildfire is likely to be the lack of precipitation, especially winter precipitation and the associated snowmelt, in the first year following the wildfire. Thus while the landscape was primed for erosion and sediment transport, the lack of a driving force meant that there was a limited immediate post-fire sediment response.     

Wildfire effects on the properties and microbial community structure of organic horizon soils in the New Jersey Pinelands

The frequency and intensity of wildfires are expected to increase in the coming years due to the changing climate, particularly in areas of high net primary production. Wildfires represent severe perturbations to terrestrial ecosystems and may have lasting effects. The objective of this study was to characterize the impacts of wildfire on an ecologically and economically important ecosystem by linking soil properties to shifts in microbial community structure in organic horizon soils. The study was conducted after a severe wildfire burned over 7000 ha of the New Jersey Pinelands, a low nutrient system with a historical incidence of fires. Soil properties in burned and non-burned soils were measured periodically up to two years after the fire occurred, in conjunction with molecular analysis of the soil bacterial, fungal and archaeal communities to determine the extent and duration of the ecosystem responses. The results of our study indicate that the wildfire resulted in significant changes in the soil physical and chemical characteristics in the organic horizon, including declines in soil organic matter, moisture content and total Kjeldahl nitrogen. These changes persisted for up to 25 months post-fire and were linked to shifts in the composition of soil bacterial, fungal and archaeal communities in the organic horizon. Of particular interest is the fact that the bacterial, fungal and archaeal communities in the severely burned soils all changed most dramatically during the first year after fire, changed more slowly during the second year after the fire, and were still distinct from communities in the non-burned soils 25 months post-fire. This slow recovery in soil physical, chemical and biological properties could have long term consequences for the soil ecosystem. These results highlight the importance of relating the response of the soil microbial communities to changing soil properties after a naturally occurring wildfire.     

An evaluation of three wood shred blends for post-fire erosion control using indoor simulated rain events on small plots

The assessment teams who make post-fire stabilization and treatment decisions are under pressure to employ more effective and economic post-fire treatments, as wild fire activity and severity has increased in recent years across the western United States. Use of forest-native wood-based materials for hillslope mulching has been on the rise due to potential environmental, erosion control efficacy, and economic incentives. One concern regarding use of woody materials prepared on or near burned sites is the wide range in the size distribution of the shredded materials. We tested three blends of shredded woody materials, each blend containing different amounts of fine (less than 2.5 cm in length) woody particles. The blends (AS IS with 24% fines, MIX with 18% fines, and REDUCED with 2% fines) were applied at 50 and 70% ground cover to 5-m² plots containing burned soil placed at 40% slope and evaluated through simulated rain events which consisted of a rain only, a rain plus low flow, and a rain plus high flow period. The REDUCED blend was the optimum for both runoff and sediment concentration reduction under conditions of rainfall and rainfall plus concentrated flow. There was no difference between application rates of 50 and 70% for either of the rainfall plus concentrated flows tested. Our recommendation was that 50% ground cover of the REDUCED blend was adequate for both rainfall and sediment reduction compared to a bare soil. The other two blends were effective in reducing runoff but not sediment concentration compared to a bare soil. The wood shred manufacturing and blending process resulted in two statistically different relationships between application rate and ground cover; relationships were controlled by the amount of fines.     

Nutrient losses in eroded sediment after fire in eucalyptus and pine forests in the wet Mediterranean environment of northern Portugal

Nutrients sorbed onto eroded sediment from small bounded plots installed in newly burned and unburned Eucalyptus globulus and Pinus pinaster forests in the Águeda Basin, north-central Portugal were measured over an 18-month period. The data are used to determine: (i) the effects of fire on nutrient loss, (ii) the importance of fire-induced losses on soil fertility, and (iii) temporal variations in nutrient losses. Fire increased losses of total nitrogen, exchangeable potassium and available phosphorus by 3–4 orders of magnitude. This is attributed to increased erosion and high nutrient concentrations at the soil surface in the burned forests, where burning of organic matter and vegetation increased nutrient availability. Enhanced rates of loss were sustained for at least 3 years, resulting in much greater post-fire nutrient losses than reported in drier regions of the Mediterranean. Losses of available P had the greatest potential for reductions in soil fertility.     

Reaction of lizard populations to a catastrophic wildfire in a central Arizona mountain range

In April 1996, the Lone Fire denuded over 90% of the vegetation in 130 km² around the Four Peaks area of the Mazatzal Mountains in central Arizona. To understand the reaction of a relatively immobile guild of species to a wildfire, we pit-trapped lizards from 1996 to 1999 in both burned and unburned interior chaparral and Madrean evergreen forest. In 26,214 trap nights, we found relative abundance was up to 10 times greater in burned than unburned vegetation. Species richness and diversity values were also greater in burned sites. Our data indicate a rapid settlement of burned areas primarily by individuals that survived the fire. Increased capture rate, diversity and richness values in 1998 and 1999 indicate that many species of lizards may even prefer early successional stages in chaparral and Madrean evergreen forests. Resident species of Teiidae and Sceloporus undulatus were more adapted to disturbed habitats than other resident Phrynosomatidae or Crotophytidae.     

The Effect of Fire on Mercury Cycling in the Soils of Forested Watersheds: Acadia National Park,Maine, U.S.A.

This study compares mercury (Hg) and methylmercury (MeHg) distribution in the soils of two forested stream watersheds at Acadia National Park, Maine, U.S.A. Cadillac Brook watershed, which burned in 1947, has thin soils and predominantly deciduous vegetation. It was compared to the unburned Hadlock Brook watershed, with thicker soil and predominantly coniferous vegetation. Soils in both watersheds were primarily well drained. The fire had a significant impact on the Cadillac watershed, by raising the soil pH, altering the vegetation, and reducing carbon and Hg pools. Total Hg content was significantly higher (P > 0.05) in Hadlock soils (0.18 kg Hg ha^-1) compared to Cadillac soils (0.13 kg Hg ha^-1). Hadlock O horizon had an average Hg concentration of 134±48 ng Hg g^-1 dry weight, compared to 103±23 ng Hg g^-1 dry weight in Cadillac O horizon. Soil pH was significantly higher in all soil horizons at Cadillac compared to Hadlock soils. This difference was especially significant in the O horizon, where Cadillac soils had an average pH of 3.41±0.22 compared to Hadlock soils with an average pH of 2.99±0.13.To study the mobilization potential of Hg in the O horizons of the two watersheds, batch adsorption experiments were conducted, and the results were modeled using surface complexation modeling. The results of Hg adsorption experiments indicated that the dissolved Hg concentration was controlled by the dissolved organic carbon (DOC) concentration. The adsorption isotherms suggest that Hg is more mobile in the O horizon of the unburned Hadlock watershed because of higher solubility of organic carbon resulting in higher DOC concentrations in that watershed.Methylmercury concentrations, however, were consistently higher in the burned Cadillac O horizon (0.20±0.13 ng Hg g^-1 dry weight) than in the unburned Hadlock O horizon (0.07±0.07 ng Hg g^-1 dry weight). Similarly, Cadillac soils possessed a higher MeHg content (0.30 g MeHg ha^-1) than Hadlock soils (0.16 g MeHg ha^-1). The higher MeHg concentrations in Cadillac soils may reflect generally faster rates of microbial metabolism due to more rapid nutrient cycling and higher soil pH in the deciduous forest. In this research, we have shown that the amount of MeHg is not a function of the total pool of Hg in the watershed. Indeed, MeHg was inversely proportional to total Hg, suggesting that landscape factors such as soil pH, vegetation type, or land use history (e.g., fire) may be the determining factors for susceptibility to high Hg in biota.     

Patterns of microarthropod abundance in oak-hickory forest ecosystems in relation to prescribed fire and landscape position

We examined patterns of microarthropod abundance in oak-hickory (Quercus-Carya) forest ecosystems in southern Ohio (USA) in relation to landscape position and fire frequency. Abundances of various suborders of Acari and Collembola were determined in samples taken in June 1999 in three forested watersheds, one that had been burned annually for four consecutive springs (1996-1999), one that had been burned periodically (1996 and 1999), and an unburned control. Microarthropod abundance was significantly lower in the annually burned watershed than the periodically burned and control watersheds. Since both the periodically burned and annually burned watersheds were burned in April 1999, the lower microarthropod abundance in the annually burned watershed was not simply an immediate effect of burning. At the landscape scale, the abundance of oribatid mites was greater in xeric than intermediate or mesic landscape positions. Within any single watershed, there was no significant linear relationship between litter mass and microarthropod abundance. However, when all three watersheds were combined, there was a significant, positive relationship between litter mass and microarthropod abundance, mainly due to the annually burned watershed where there was very low litter mass and low microarthropod abundance. Both fire frequency and landscape position have significant effects on microarthropod abundance; however, those effects cannot be robustly predicted based solely on forest floor litter mass differences.