Fluxes of nitrous oxide and methane on an abandoned peat extraction site: Effect of reed canary grass cultivation

Drained organic soils are among the most risky soil types as far as their greenhouse gas emissions are considered. Reed canary grass (RCG) is a potential bioenergy crop in the boreal region, but the atmospheric impact of its cultivation is unknown. The fluxes of N₂O and CH₄ were measured from an abandoned peat extraction site (an organic soil) cultivated with RCG using static chamber and snow gradient techniques. The fluxes were measured also at an adjacent site which is under active peat extraction and it is devoid of any vegetation (BP site). The 4-year average annual N₂O emissions were low being 0.1 and 0.01 g N₂O m⁻² a⁻¹ at the RCG and BP sites, respectively. The corresponding mean annual CH₄ emissions from the RCG and BP sites were also low (0.4 g and 0.9 g CH₄ m⁻² a⁻¹). These results highlight for the first time that there are organic soils where cultivation of perennial bioenergy crops is possible with low N₂O and CH₄ emissions.     

Decadal change in wetland–woodland boundaries during the late 20th century reflects climatic trends

Wetlands are important and restricted habitats for dependent biota and play vital roles in landscape function, hydrology and carbon sequestration. They are also likely to be one of the most sensitive components of the terrestrial biosphere to global climate change. An understanding of relationships between wetland persistence and climate is imperative for predicting, mitigating and adapting to the impacts of future climate change on wetland extent and function. We investigated whether mire wetlands had contracted, expanded or remained stable during 1960–2000. We chose a study area encompassing a regional climatic gradient in southeastern Australia, specifically to avoid confounding effects of water extraction on wetland hydrology and extent. We first characterized trends in climate by examining data from local weather stations, which showed a slight increase in precipitation and marked decline in pan evaporation over the relevant period. Remote sensing of vegetation boundaries showed a marked lateral expansion of mires during 1961–1998, and a corresponding contraction of woodland. The spatial patterns in vegetation change were consistent with the regional climatic gradient and showed a weaker co‐relationship to fire history. Resource exploitation, wildland fires and autogenic mire development failed to explain the observed expansion of mire vegetation in the absence of climate change. We therefore conclude that the extent of mire wetlands is likely to be sensitive to variation in climatic moisture over decadal time scales. Late 20th‐century trends in climatic moisture may be related primarily to reduced irradiance and/or reduced wind speeds. In the 21st century, however, net climatic moisture in this region is projected to decline. As mires are apparently sensitive to hydrological change, we anticipate lateral contraction of mire boundaries in coming decades as projected climatic drying eventuates. This raises concerns about the future hydrological functions, carbon storage capacity and unique biodiversity of these important ecosystems.     

Trends and uncertainties in Siberian indicators of 20th century warming

Estimates of past climate and future forest biomass dynamics are constrained by uncertainties in the relationships between growth and climatic variability and uncertainties in the instrumental data themselves. Of particular interest in this regard is the boreal-forest zone, where radial growth has historically been closely connected with temperature variability, but various lines of evidence have indicated a decoupling since about the 1960s. We here address this growth-vs.-temperature divergence by analyzing tree-ring width and density data from across Siberia, and comparing 20th century proxy trends with those derived from instrumental stations. We test the influence of approaches considered in the recent literature on the divergence phenomenon (DP), including effects of tree-ring standardization and calibration period, and explore instrumental uncertainties by employing both adjusted and nonadjusted temperature data to assess growth-climate agreement. Results indicate that common methodological and data usage decisions alter 20th century growth and temperature trends in a way that can easily explain the post-1960 DP. We show that (i) Siberian station temperature adjustments were up to 1.3 °C for decadal means before 1940, (ii) tree-ring detrending effects in the order of 0.6–0.8 °C, and (iii) calibration uncertainties up to about 0.4 °C over the past 110 years. Despite these large uncertainties, instrumental and tree growth estimates for the entire 20th century warming interval match each other, to a degree previously not recognized, when care is taken to preserve long-term trends in the tree-ring data. We further show that careful examination of early temperature data and calibration of proxy timeseries over the full period of overlap with instrumental data are both necessary to properly estimate 20th century long-term changes and to avoid erroneous detection of post-1960 divergence.     

Wood-feeding beetles and soil nutrient cycling in burned forests: implications of post-fire salvage logging

• 1 Rising economic demands for boreal forest resources along with current and predicted increases in wildfire activity have increased salvage logging of burned forests. Currently, the ecological consequences of post‐fire salvage logging are insufficiently understood to develop effective management guidelines or to adequately inform policy decision‐makers.
• 2 We used both field and laboratory studies to examine the effects of post‐fire salvage logging on populations of the white‐spotted sawyer Monochamus scutellatus scutellatus (Say) (Coleoptera: Cerambycidae) and its ecological function in boreal forest.
• 3 Monochamus s. scutellatus adults were relatively abundant in both burned and clear‐cut logged sites but were absent from salvage logged sites.
• 4 An in situ mesocosm experiment showed that the abundance of M. s. scutellatus larvae in burned white spruce bolts was linked to changes in total organic nitrogen and carbon in mineral soil.
• 5 Organic nutrient inputs in the form of M. s. scutellatus frass increased mineral soil microbial respiration rates by more than three‐fold and altered the availability of nitrogen. Changes in nitrogen availability corresponded with decreased germination and growth of Epilobium angustifolium and Populus spp. but not Calamagrostis canadensis.
• 6 Although the present study focused on local scale effects, the reported findings suggest that continued economic emphasis on post‐fire salvage logging may have implications beyond the local scale for biodiversity conservation, nutrient cycling and plant community composition in forest ecosystems recovering from wildfire.

     

Effects of wildfire and permafrost on soil organic matter and soil climate in interior Alaska

The influence of discontinuous permafrost on ground‐fuel storage, combustion losses, and postfire soil climates was examined after a wildfire near Delta Junction, AK in July 1999. At this site, we sampled soils from a four‐way site comparison of burning (burned and unburned) and permafrost (permafrost and nonpermafrost). Soil organic layers (which comprise ground‐fuel storage) were thicker in permafrost than nonpermafrost soils both in burned and unburned sites. While we expected fire severity to be greater in the drier site (without permafrost), combustion losses were not significantly different between the two burned sites. Overall, permafrost and burning had significant effects on physical soil variables. Most notably, unburned permafrost sites with the thickest organic mats consistently had the coldest temperatures and wettest mineral soil, while soils in the burned nonpermafrost sites were warmer and drier than the other soils. For every centimeter of organic mat thickness, temperature at 5 cm depth was about 0.5°C cooler during summer months. We propose that organic soil layers determine to a large extent the physical and thermal setting for variations in vegetation, decomposition, and carbon balance across these landscapes. In particular, the deep organic layers maintain the legacies of thermal and nutrient cycling governed by fire and revegetation. We further propose that the thermal influence of deep organic soil layers may be an underlying mechanism responsible for large regional patterns of burning and regrowth, detected in fractal analyses of burn frequency and area. Thus, fractal geometry can potentially be used to analyze changes in state of these fire prone systems.     

Estimating fine-root biomass and production of boreal and cool temperate forests using aboveground measurements: A new approach

Information of fine-root biomass and production is critical for quantifying the productivity and carbon cycle of forest ecosystems, and yet our ability to obtain this information especially at a large spatial scale (e.g., regional to global) is extremely limited. Several studies attempted to relate fine-root biomass and production with various aboveground variables that can be measured more easily so that fine-root biomass and production could be estimated at a large spatial scale, but found the correlations were generally weak or non-existed at the stand level. In this study, we tested a new approach: instead of using the conventional way of analysing fine-root biomass at the stand level, we analysed fine-root data at the tree level. Fine-root biomass of overstory trees in stand was first separated from that of understory and standardized to a common fine-root definition of < 2 mm or < 5 mm diameter. Afterwards, we calculated fine-root biomass per tree for a representative tree size of mean basal area for each stand. Statistically significant correlations between the fine-root biomass per tree and the diameter at the ground surface were found for all four boreal and cool temperate spruce, pine, fir and broadleaf forest types, and so allometric equations were developed for each group using a total of n = 212 measurements. The stand-level fine-root biomass of trees estimated using the allometric equations agrees well with the measurements, with r ² values of 0.64 and 0.57 (n = 171), respectively, for fine-roots < 2 mmand < 5 mm diameter. This study further estimated fine-root production as the product of fine-root turnover rate and fine-root biomass, and determined the turnover rate as a function of fine-root biomass, stand age, and mean annual temperature. The estimates of tree fine-root production agree well with reported values, with r ² value of 0.53 for < 2 mm and 0.54 for < 5 mm diameter (n = 162) at the stand level.     

Nitrogen mineralization in upland Precambrian Shield catchments: Contrasting the role of lichen-covered bedrock and forested areas

The upland boreal forest at the Experimental Lakes Area (northwestern Ontario, Canada) is characterized by treed soil islands interspersed within lichen and moss-covered bedrock outcrops. N mineralization was 2.5-fold and net nitrification was 13-fold higher on an areal basis over bedrock surfaces because of high mineralization rates under lichen and moss patches. The higher average soil temperature in lichen and moss patches could not account for the difference in mineralization rates. Lichens did not provide a significant additional source of N because they did not fix atmospheric N. A refractory conifer litter with a high C:N probably favours the immobilization of N in forest islands. Buried bag and in situ core incubations yielded similar net N mineralization rates but core incubations underestimated net nitrification rates. Both methods did not adequately measure dissolved organic N (DON) production rates because soil disturbance caused high initial DON concentrations. The higher export of mineral N from bedrock surfaces is probably a combination of the lower retention of N in precipitation and leaching of mineralized N from lichen and moss patches.     

Effects of climatic variability on the annual carbon sequestration by a boreal aspen forest

To evaluate the carbon budget of a boreal deciduous forest, we measured CO₂ fluxes using the eddy covariance technique above an old aspen (OA) forest in Prince Albert National Park, Saskatchewan, Canada, in 1994 and 1996 as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). We found that the OA forest is a strong carbon sink sequestering 200 ± 30 and 130 ± 30 g C m^–2 y^–1 in 1994 and 1996, respectively. These measurements were 16–45% lower than an inventory result that the mean carbon increment was about 240 g C m^–2 y^–1 between 1919 and 1994, mainly due to the advanced age of the stand at the time of eddy covariance measurements. Assuming these rates to be representative of Canadian boreal deciduous forests (area ≈ 3 × 10⁵ km²), it is likely they can sequester 40–60 Tg C y^–1, which is 2–3% of the missing global carbon sink. The difference in carbon sequestration by the OA forest between 1994 and 1996 was mainly caused by the difference in leaf emergence date. The monthly mean air temperature during March–May 1994, was 4.8 °C higher than in 1996, resulting in leaf emergence being 18–24 days earlier in 1994 than 1996. The warm spring and early leaf emergence in 1994 enabled the aspen forest to exploit the long days and high solar irradiance of mid-to-late spring. In contrast, the 1996 OA growing season included only 32 days before the summer solstice. The earlier leaf emergence in 1994 resulted 16% more absorbed photosynthetically active radiation and a 90 g C m^–2 y^–1 increase in photosynthesis than 1996. The concomitant increase in respiration in the warmer year (1994) was only 20 g C m^–2 y^–1. These results show that an important control on carbon sequestration by boreal deciduous forests is spring temperature, via the influence of air temperature on the timing of leaf emergence.