Length and classification of natural and created forest edges in boreal landscapes throughout northern Sweden

Forest edges have numerous implications for structure and function of forest ecosystems. Previous studies on edge quantity have used broad classifications. However, edge influence is driven by the contrast in vegetation structure between adjoining ecosystems, and thus we need detailed site-specific data to assess the role of edges in forests. We studied the variability of sharp edges in 28 boreal landscapes (4 km × 4 km) across an 830 km gradient throughout northern Sweden. Our objectives were: (1) to compare the length of natural and created edges, (2) to classify edges in detail by edge origin, maintenance and forest attributes, and (3) to examine relationships between length of edge and landscape variables. Data were collected using stereo-interpretation of high spatial resolution colour infrared aerial photographs, in combination with line intersect sampling and plots. The length of edge varied from 12 to 102 m ha⁻¹ among landscapes, with an overall mean of 54 m ha⁻¹. Created edges dominated most landscapes (mean 33 m ha⁻¹) and had greater variability than natural edges (mean 21 m ha⁻¹). Maintained edges (e.g. roads, agricultural land) were more abundant than regenerating edges caused by logging. Thirty percent of total edges adjoined narrow linear features. Seventy percent adjoined wider patches and showed high variability (35 classes). Overall, high-contrast edges towards mature forest dominated, i.e. ones that may experience strong edge influence. The amount of edge increased with percent of landscape affected by disturbance, and decreased with latitude and elevation. This study shows that edges are both abundant and highly variable in boreal forests and that forestry is the main driver behind edge creation. Detailed classification of edges based on site-specific forest and patch attributes may help to estimate edge influence at landscape level, and can guide experimental design for examining the impact of edges on structure and function of forest ecosystems.     

Dissolved organic carbon and nitrogen leaching from Scots pine, Norway spruce and silver birch stands in southern Sweden

The effects of three common tree species - Scots pine, Norway spruce and silver birch - on leaching of dissolved organic carbon and dissolved nitrogen were studied in an experimental forest with podzolised soils in southern Sweden. We analyzed soil water collected with lysimeters and modeled water fluxes to estimate dissolved C and N fluxes. Specific UV absorbance (SUVA) was analyzed to get information about the quality of dissolved organic matter leached from the different stands. Under the O horizon, DOC concentrations and fluxes in the birch stands were lower than in the spruce and pine stands; annual fluxes were 21 g m⁻² y⁻¹ for birch and 38 g m⁻² y⁻¹ and 37 g C m⁻² y⁻¹ for spruce and pine, respectively. Under the B horizon, annual fluxes for all tree species ranged between 3 and 5 g C m⁻² y⁻¹, implying greater loss of DOC in the mineral soil in the coniferous stands than in the birch stands. We did not find any effect of tree species on the quality of the dissolved organic matter, as measured by SUVA, indicating that the chemical composition of the organic matter was similar in leachates from all three tree species. Substantial amounts of nitrogen was leached out of the soil profile at the bottom of the B horizon from the pine and birch stands, whereas the spruce stands seemed to retain most of the nitrogen in the soil. These differences in N leaching have implications for soil N budgets.     

Method to compute fiber directions in wood from computed tomography images

This paper describes a new method, called the CT-direction method, in which the fiber directions in wood in three-dimensional space are calculated from the pixel information on a series of two-dimensional computed tomography images. Local fiber directions are calculated from the principal directions of inertia of measurement spheres distributed throughout the body of the wood object. The calculated fiber directions are probably due to density streaks in the material, such as fiber bundles, which are directed in the fiber direction, and not the density of individual fibers, which are too small to be detected. The fiber directions vary locally, and density streaks from knots, growth rings, and compression wood influence the results, which adds spread to the results. The fiber directions are presented as spiral grain angles and conical angles and are compared with spiral grain angles measured with the tracheid-effect method. The comparisons show that the CT-direction method is a nondestructive way to measure fiber directions locally and in the interior of the body of a piece of wood.     

Forest edge quantification by line intersect sampling in aerial photographs

There is a need for accurate and efficient methods for quantification and characterisation of forest edges at the landscape level in order to understand and mitigate the effects of forest fragmentation on biodiversity. We present and evaluate a method for collecting detailed data on forest edges in aerial photographs by using line intersect sampling (LIS). A digital photogrammetric system was used to collect data from scanned colour infrared photographs in a managed boreal forest landscape. We focused on high-contrast edges between forest (height ≥ 10 m) and adjoining open habitat or young, regenerating forest (height ≤ 5 m). We evaluated the air photo interpretation with respect to accuracy in estimated edge length, edge detection, edge type classification and structural variables recorded in 20 m radius plots, using detailed field data as reference. The estimated length of forest edge in the air photo interpretation (52 ± 8.8 m ha⁻¹; mean ± standard error) was close to that in the field survey (58 ± 9.3 m ha⁻¹). The accuracy in edge type classification (type of open habitat) was high (88% correctly classified). Both tree height and canopy cover showed strong relationships with the field data in the forest, but tree height was underestimated by 2.3 m. Data collection was eight times faster and five times more cost-effective in aerial photographs than in field sampling. The study shows that line intersect sampling in aerial photographs has large potential application as a general tool for collecting detailed information on the quantity and characteristics of high-contrast edges in managed forest ecosystems.     

Pine weevil (Hylobius abietis) damage to cuttings and seedlings of Norway spruce

Damage caused by pine weevil (Hylobius abietus L.) to planted seedlings and cuttings of Norway spruce (Picea abies (L.) Karst.) was studied at five clearcut sites in south-eastern Sweden. The main objective was to compare the two types of stock in terms of attack frequency and mortality due to pine weevil feeding. Cuttings and seedlings with the same initial stem-base diameter (4 mm) were compared. Two sites were harvested and scarified shortly before planting, two were harvested shortly before planting, but were not scarified, and one was harvested 2 years before and scarified the autumn before planting. The total mortality 5 years after planting was highest, greater than 90%, at the new, non-scarified sites, and lowest, 23%, at the old, scarified site. More than 90% of the mortality was caused by pine weevil feeding. Attack frequency and pine weevil induced mortality were significantly higher among seedlings than among cuttings. Mortality due to pine weevil damage was 4–43% higher in seedlings than in cuttings after the fifth year. Of the cuttings and seedlings that were attacked in the first year, a significantly higher frequency of the seedlings were girdled. The higher resistance of cuttings to pine weevil damage may partly explain the more rapid growth of cuttings reported in other studies. However, the causes of their higher resistance need to be further investigated. The thicker bark and needles on the stem base of the cuttings could be important in this respect.     

An Estimation of the Potential Fluxes of Nitrogen Phosphorus, Cadmium and Lead from Sea Ice and Snow in the Northern Baltic Sea

The amount of nutrients (N and P) and trace elements (Cd and Pb)accumulated and incorporated into the snow and sea ice cover was assessed for the northern Baltic Sea, resulting in an estimate for the potential flux of nutrients and trace elements to underlying waters during sea ice and snow melt. From the atmosphere the sea ice and snow cover accumulate less than 3% of the annual N and P load, and about 5% of the annual Cd and Pb load to the Bothnian Bay. The atmospheric deposition of nutrients to the northern Baltic Sea has declined considerably during the last decade. The total accumulation in the snow and ice, from the atmosphere and seawater, make up to 6% of the annual nutrient and up to 40% of the annual Cd and Pb load into the Bothnian Bay. Thus sea ice plays an important, but still poorly understood role in chemical cycling, transformations, and budgets. The fate of substances accumulated and released from snow and sea ice merit further investigation, especially if sea ice is a source and a platform for transformation of accumulated substances, thus indirectly affecting their toxicity and/or bioavailability before they are released to the water column.     

Hydrogeochemistry of a stream draining sulfide-bearing postglacial sediments in Finland

The influence of hydrological conditions and overburden type on the stream-water quality in a catchment effected by fine-grained sulfide-bearing sediments was studied. Water samples for measurement of pH and electric conductivity were collected at 16 sites along the main course of the stream during autumn and spring high water flow and during low and intermediate water flow. On one occasion (spring high-water flow) samples for analysis of total organic carbon were collected at these sites. Seventeen water samples for analyses of chemical elements (ICP-MS and ICP-AES) were taken within the catchment basin during autumn high water flow. The pH and conductivity of snow, and the geochemistry of the sulfide sediments were also studied. The occurrence of fine-grained sediments within the catchment was estimated from topographic and geologic maps and by visit to areas were the overburden type could not be determined from the maps. The hydrological conditions had a strong influence on the water quality. During high water flow in spring and autumn the concentrations of ionic components (conductivity) and hydrogen ions in the stream water were high, while the concentrations were much lower during low and intermediate water flow and when the ground was frozen. Extensive leaching of the sulfide-bearing sediments, which had higher contents of S and metals than the fine fraction of till in the study area, resulted in elevated contents of Li, Ni, Zn, Co, B, Al, Mg, U, Sr, Ca, K and Na in the stream water during autumn high water flow. The contents of Cr, Ba, Pb, Si and Fe in stream water were not much influenced by the type of overburden, while V was depleted in drainage waters from the sulfide sediments. Humic substances flocculated in the water in the middle section of the stream, which may have been caused by di- and trivalent metal cations leached in large quantities from the oxidizing sulfide sediments.     

Saproxylic beetles in natural and man-made deciduous high stumps retained for conservation

Intensive forest management in Scandinavia has decreased the amount of dead wood required by saproxylic (wood-living) organisms. To reduce this problem, some dead wood is now retained during forest operations, often in the form of man-made high stumps (ca. 4 m high). Most often these stumps are cut with a harvester, although the stumps in this study were made with explosives. The aims of this study were to determine whether such stumps of aspen (Populus tremula) and birch (Betula spp.) could be used by red-listed saproxylic beetles (Coleoptera), and to examine how the fauna of man-made high stumps differs from that of natural stumps. We also studied how tree species, sun-exposure, stage of decay and trunk diameter influenced the fauna. In 169 samples of bark from high stumps 116 saproxylic species were found, of which 21 were red-listed. Many species, including red-listed ones, were more associated with man-made stumps than with natural stumps. However, in total, more species were found in the natural than in the man-made stumps. This is probably because man-made stumps provide a more homogeneous type of wood substrate than natural ones. Among the other variables the difference between aspen and birch was the most important. We conclude that man-made high stumps are valuable habitats for many saproxylic beetle species.