Mercury concentrations and export from small central Canadian boreal forest catchments before, during, and after forest harvest

Northern boreal forests are a strong sink for mercury (Hg), a global contaminant of significant concern to wildlife and human health. Mercury stored in forest soils can be mobilized via runoff and erosion, and under suitable conditions can be methylated to its much more bioaccumulative form, methylmercury. Forest harvesting can affect the mobilization and methylation of Hg, though the direction and magnitude of the impact is unclear or conflicting across previous studies. This study examined 5 harvested and 2 reference watersheds in northwestern Ontario, Canada, before, during, and after harvest to quantify changes in stream total and methylmercury concentration and loads and identified potential landscape and management factors that contribute to differences in stream response. In watersheds where streams were buffered by natural vegetation (≥30 m), no significant changes in total Hg or methylmercury concentrations or loads were observed. Significant increases in methylmercury concentrations and loads were observed downstream of a stream crossing in a watershed where the relatively small stream was unmapped and therefore only buffered by a 3 m machine exclusion zone. These results show that when current best management practices that minimize soil and water disturbance are followed, harvest can have a minimal impact on total and methylmercury loads, even in extensively harvested watersheds. However, there is a need for improved mapping of small streams to ensure best management practices are applied adequately across the landscape.

Land use change and forest management effects on soil carbon stocks in the Northeast U.S

BACKGROUND

In most regions and ecosystems, soils are the largest terrestrial carbon pool. Their potential vulnerability to climate and land use change, management, and other drivers, along with soils' ability to mitigate climate change through carbon sequestration, makes them important to carbon balance and management. To date, most studies of soil carbon management have been based at either large or site-specific scales, resulting in either broad generalizations or narrow conclusions, respectively. Advancing the science and practice of soil carbon management requires scientific progress at intermediate scales. Here, we conducted the fifth in a series of ecoregional assessments of the effects of land use change and forest management on soil carbon stocks, this time addressing the Northeast U.S. We used synthesis approaches including (1) meta-analysis of published literature, (2) soil survey and (3) national forest inventory databases to examine overall effects and underlying drivers of deforestation, reforestation, and forest harvesting on soil carbon stocks. The three complementary data sources allowed us to quantify direction, magnitude, and uncertainty in trends.

RESULTS

Our meta-analysis findings revealed regionally consistent declines in soil carbon stocks due to deforestation, whether for agriculture or urban development. Conversely, reforestation led to significant increases in soil C stocks, with variation based on specific geographic factors. Forest harvesting showed no significant effect on soil carbon stocks, regardless of place-based or practice-specific factors. Observational soil survey and national forest inventory data generally supported meta-analytic harvest trends, and provided broader context by revealing the factors that act as baseline controls on soil carbon stocks in this ecoregion of carbon-dense soils. These factors include a range of soil physical, parent material, and topographic controls, with land use and climate factors also playing a role.

CONCLUSIONS

Forest harvesting has limited potential to alter forest soil C stocks in either direction, in contrast to the significant changes driven by land use shifts. These findings underscore the importance of understanding soil C changes at intermediate scales, and the need for an all-lands approach to managing soil carbon for climate change mitigation in the Northeast U.S.

Interdecadal variation in sediment yield from a forested mountain basin: The role of hydroclimatic variability, anthropogenic disturbances, and geomorphic connectivity

Variation in sediment yield may reflect a signal of disturbances in the upstream landscape, modified by sediment routing. This study, conducted in a forested drainage basin in the inland Pacific Northwest, USA, sought to generate a better insight into the interdecadal variability of sediment yield in mountain landscapes in response to environmental change during the last century. To this end, we examined: (1) sediment yield fluctuations; and (2) their association with streamflow and land use changes; as well as (3) streamflow links to climate variability modes; and (4) the influence of sediment delivery from hillslope sources to streams (lateral connectivity) and its downstream routing through the stream network (longitudinal connectivity) on land use signal at the basin's outlet. Sediment yield between 1910 and 2017, estimated based on reconstructed fluvial delta growth, displayed an order of magnitude variability, which indicates a substantial geomorphic sensitivity. The interpretation of temporal patterns and an exploratory statistical analysis pointed to land use-related sediment supply changes as the primary driver of these fluctuations, dominating system behavior before changes in environmental regulations and practices in the mid-1970s. Hydroclimatically controlled streamflow variability appeared to be more prominent in the subsequent period. Our connectivity analysis suggested that a considerable portion of coarse sediment mobilized by harvest and road construction may still reside within the channel network. In light of previous research in this landscape system, we speculate that, despite limited anthropogenic pressures in the recent decades, its characteristics and behavior continue to be conditioned by land use legacies. Overall, this study contributes to the growing understanding of profound anthropogenic transformation of the earth surface. Specifically, it demonstrates that historical resource extraction may have left a lasting imprint even in relatively remote mountain landscapes. Given the ongoing rapid environmental change, such understanding is crucial for watershed management, conservation, and restoration.

Forest biomass accumulation is an important source of acidity to forest soils: Data from Swedish inventories of forests and soils 1955 to 2010

The input of acidity to Swedish forest soils through forestry between 1955 and 2010 is compared with the acid input from atmospheric deposition. Depending on region, input of acidity from forestry was the minor part (25-45%) of the study period's accumulated acid input but is now the dominating source (140-270 mol_c ha^-1 year^-1). The net uptake of cations due to the increase in standing forest biomass, ranged between 35 and 45% of the forestry related input of acidity while whole-tree harvesting, introduced in the late 1990s, contributed only marginally (< 2%). The geographical gradient in acid input is reflected in the proportion of acidified soils in Sweden but edaphic properties contribute to variations in acidification sensitivity. It is important to consider the acid input due to increases in standing forest biomass in acidification assessments since it is long-term and quantitatively important.

Forest management impacts on stream integrity at varying intensities and spatial scales: Do biological effects accumulate spatially?

The effects of forest harvesting on headwaters are quite well understood, yet our understanding of whether impacts accumulate or dissipate downstream is limited. To address this, we investigated whether several biotic indicators changed from smaller to larger downstream sites (n = 6) within three basins that had intensive, extensive or minimal forest management in New Brunswick (Canada). Biofilm biomass and grazer abundance significantly increased from upstream to downstream, whereas organic matter decomposition and the autotrophic index of biofilms decreased. However, some spatial trends differed among basins and indicated either cumulative (macroinvertebrate abundance, predator density, sculpin GSI) or dissipative (autotrophic index, cotton decomposition) effects downstream, potentially explained by sediment and nutrient dynamics related to harvesting. No such among-basin differences were observed for leaf decomposition, biofilm biomass, macroinvertebrate richness or sculpin condition. Additionally, results suggest that some of the same biological impacts of forestry observed in small headwaters also occurred in larger systems. Although the intensive and extensive basins had lower macroinvertebrate diversity, there were no other signs of biological impairment, suggesting that, overall, current best management practices protect biological integrity downstream despite abiotic effects.

Forest management impacts on stream integrity at varying intensities and spatial scales: Do abiotic effects accumulate spatially?

Though effects of forest harvesting on small streams are well documented, little is known about the cumulative effects in downstream systems. The hierarchical nature and longitudinal connectivity of river networks make them fundamentally cumulative, but lateral and vertical connectivity and instream processes can dissipate the downstream transport of water and materials. To elucidate such effects, we investigated how a suite of abiotic indicators changed from small streams to larger downstream sites (n = 6) within three basins ranging in forest management intensity (intensive, extensive, minimal) in New Brunswick (Canada) in the summer and fall of 2017 and 2018. Inorganic sediments, the inorganic/organic ratios and water temperatures significantly increased longitudinally, whereas nutrients and the fluorescence index of dissolved organic carbon (DOC; indication of terrestrial source) decreased. However, some longitudinal trends differed across basins and indicated downstream cumulative (inorganic sediments, the inorganic/organic ratios and to a lesser extent DOC concentration and humification) as well as dissipative (temperatures, nutrients, organic sediments) effects of forest management. Overall, we found that the effects previously reported for small streams with managed forests also occur at downstream sites and suggest investigating whether different management practices can be used within the extensive basin to reduce these cumulative effects.

Does forest harvest increase the mercury concentrations in fish? Evidence from Swedish lakes

A number of studies have evaluated the effects of forest harvest on mercury (Hg) concentrations and exports in surface waters, but few studies have tested the effect from forest harvest on the change in fish Hg concentrations over the course of several years after harvest. To address this question, mercury (Hg) concentrations in perch (Perca fluviatilis) muscle tissue from five lakes were analyzed for two years before (2010-2011) and three years after (2013-2015) forest harvest conducted in 2012. Fish Hg concentrations in the clear-cut catchments (n=1373 fish specimens) were related to temporal changes of fish Hg in reference lakes (n=1099 fish specimen) from 19 lakes in the Swedish National Environmental Monitoring Programme. Small (length<100mm) and large perch (length≥100mm) were analyzed separately, due to changing feeding habitats of fish over growing size. There was considerable year-to-year and lake-to-lake variation in fish Hg concentrations (-14%-121%) after forest harvest in the clearcut lakes, according to our first statistical model that count for fish Hg changes. While the effect ascribed to forest harvest varied between years, after three years (in 2015), a significant increase of 26% (p<0.0001) in Hg concentrations of large fish was identified in our second statistical model that pooled all 5 clearcut lakes. The large fish Hg concentrations in the 19 reference lakes also varied, and in 2015 had decreased by 7% (p=0.03) relative to the concentrations in 2010-2011. The majority of the annual changes in fish Hg concentrations in the clearcut lakes after harvest were in the lower range of earlier predictions for high-latitude lakes extrapolated primarily from the effects of forest harvest operations on Hg concentrations in water. Since the risk of forest harvest impacts on Hg extends to fish and not just surface water concentrations, there is even more reason to consider Hg effects in forestry planning, alongside other ecosystem effects.

Integrate carbon dynamic models in analyzing carbon sequestration impact of forest biomass harvest

Biomass is an attractive natural energy resource for mitigating climate change. However, the loss of carbon sequestration as an ecosystem service due to biomass harvest has not been considered in previous studies. To assess the impact of biomass harvest on carbon sequestration, carbon dynamics in the forests and the atmosphere were integrated. The impact of forest biomass harvests on carbon sequestration was assessed based on the difference between carbon sequestration after harvest and carbon sequestration without harvest. A Chapman-Richards function and the forest vegetation simulator (FVS) were used to simulate the growth of a forest stand. The carbon dynamics in the atmosphere were simulated by the Bern2.5CC carbon cycle model. Characterization factors of the impact were calculated in three time horizons: 20-, 100- and 500-year. According to the simulations, postponement of harvest and low harvest intensity could prolong the compensation period. The annual impact on carbon sequestration was mostly negative over a short time and became positive in the end of compensation period. The highest characteristic factors of the impact on carbon sequestration were found in rotation length of 100years with the time horizon of 500-year in the Chapman-Richards simulation and in the lowest harvest intensity with the time horizon of 500-year in the FVS simulation. Based on the results, increasing growth rate, postponing harvest, reducing harvest intensity and increasing length of time horizon could reduce the impact of forest harvest on carbon sequestration. The method proposed in this study is more proper to assess the impact on carbon sequestration, and it has much wider applications in forest management practice.

Organic horizon and mineral soil mercury along three clear-cut forest chronosequences across the northeastern USA

Mercury (Hg) is a globally distributed pollutant trace metal that has been increasing in terrestrial environments due to rising anthropogenic emissions. Vegetation plays an important role in Hg sequestration in forested environments, but increasing tree removal for biofuels and wood products may affect this process. The long-term effect of clear-cutting on forest soil Hg remains uncertain, since most studies are limited to measuring changes for < 10 years following a single harvest event. The chronosequence approach, which substitutes space for time using forest stands of different ages since clear-cutting, allows for investigation of processes occurring over decades to centuries. Here, we utilized three clear-cut forest soil chronosequences across the northeastern USA to understand Hg accumulation and retention over several decades. Total Hg concentrations and pools were quantified for five soil depth increments along three chronosequences. Our results showed Hg concentrations and pools decreased in the initial 20 years following clear-cutting. Mineral soil Hg pools decreased 21-53% (7-14 mg m^-2) between 1-5-year-old stands and 15-25-year-old stands but mineral soil Hg pools recovered in 55-140-year-old stands to similar values as measured in 1-5-year-old stands. Our study is one of the first to demonstrate a decrease and recovery in Hg pool size. These changes in Hg did not correspond with changes in bulk density, soil C, or pH. We utilized a simple two-box model to determine how different Hg fluxes affected organic and mineral soil horizon Hg pools. Our simple model suggests that changes in litterfall and volatilization rates could have caused the observed changes in organic horizon Hg pools. However, only increases in leaching could reproduce observed decreases to mineral soil Hg pools. Further studies are needed to determine the mechanism of Hg loss from forest soils following clear-cutting.

Effects of disturbance and vegetation type on total and methylmercury in boreal peatland and forest soils

Mercury (Hg) concentrations in freshwater fish relates to aquatic Hg concentrations, which largely derives from soil stores of accumulated atmospheric deposition. Hg in catchment soils as a source for aquatic Hg is poorly studied. Here we test if i) peatland soils produce more methylmercury (MeHg) than forest soils; ii) total Hg (THg) concentrations in top soils are determined by atmospheric inputs, while MeHg is produced in the soils; and iii) soil disturbance promotes MeHg production. In two small boreal catchments, previously used in a paired-catchment forest harvest manipulation study, forest soils and peatlands were sampled and analysed for Hg species and additional soil chemistry. In the undisturbed reference catchment, soils were sampled in different vegetation types, of varying productivity as reflected in tree density, where historical data on precipitation and throughfall Hg and MeHg fluxes were available. Upper soil THg contents were significantly correlated to throughfall inputs of Hg, i.e. lowest in the tree-less peatland and highest in the dense spruce forest. For MeHg, top layer concentrations were similar in forest soils and peatlands, likely related to atmospheric input and local production, respectively. The local peatland MeHg production was documented through significantly higher MeHg-to-THg ratios in the deeper soil layer samples. In the disturbed catchment, soils were sampled in and just outside wheeltracks in an area impacted by forest machinery. Here, MeHg concentrations and the MeHg-to-THg ratios in the upper 5 cm were weakly significantly (p = 0.07) and significantly (p = 0.04) different in and outside of the wheeltracks, respectively, suggesting that soil disturbance promotes methylation. Differences in catchment Hg and MeHg streamwater concentrations were not explained by soil Hg and MeHg information, perhaps because hydrological pathways are a stronger determinant of streamwater chemistry than small variations in soil chemistry driven by disturbance and atmospheric inputs of Hg.