Jan Bengtsson

Biodiversity, disturbances, ecosystem function and management of European forests

Abstract We review the effects of human impact on biodiversity of European forests in the light of recent views on disturbances and succession in ecosystems, and discuss recent ideas on how biodiversity affects ecosystem functions such as productivity and ecosystem stability. With this as a background we discuss how to better manage European forests for both production and biodiversity. We argue that the next generation of forestry practices need to understand and mimic natural disturbance dynamics much better than the present ones. Of particular importance is the fact that most species in European forests have evolved in forests that were to a large extent influenced by large grazers, first by megaherbivores and later, in historic times, by domestic animals. We highlight several areas where new knowledge and management tools are urgently needed: (i) How do species survive and adapt to the natural disturbance regimes in different regions and forest types? (ii) How can new and imaginative forest management practices be devised that take natural disturbance regimes into account? (iii) How does forest biodiversity affect ecosystem function and stability in a changing world, in particular in the light of predicted climate changes? (iv) How are ecological processes at different levels and scales related to diversity, and how do different management practices affect biodiversity? (v) How can efficient agroforestry methods be developed to preserve biodiversity? (vi) What is the role of humans and human behaviour for sustainable management of ecosystems?

Higher levels of multiple ecosystem services are found in forests with more tree species

Forests are of major importance to human society, contributing several crucial ecosystem services. Biodiversity is suggested to positively influence multiple services but evidence from natural systems at scales relevant to management is scarce. Here, across a scale of 400,000 km2, we report that tree species richness in production forests shows positive to positively hump-shaped relationships with multiple ecosystem services. These include production of tree biomass, soil carbon storage, berry production and game production potential. For example, biomass production was approximately 50% greater with five than with one tree species. In addition, we show positive relationships between tree species richness and proxies for other biodiversity components. Importantly, no single tree species was able to promote all services, and some services were negatively correlated to each other. Management of production forests will therefore benefit from considering multiple tree species to sustain the full range of benefits that the society obtains from forests.

Which species? What kind of diversity? Which ecosystem function? Some problems in studies of relations between biodiversity and ecosystem function

Abstract I examine a number of problems that need to be identified and accounted for when examining the relationships between diversity and ecosystem function. Among these are measures of diversity and complexity in ecosystems: species richness, diversity indices, functional groups, keystone species, connectance, etc, all of which may be difficult to relate to ecosystem function. Several important distinctions, when testing diversity–function relationships empirically, are discussed: Diversity of functional groups, diversity within functional groups vs. total diversity; manipulating variables such as body-size distributions vs. manipulating diversity per se; effects of diversity vs. effects of biomass; and diversity–function relations under stable vs. changing conditions or perturbations. It is argued that for the management and development of sustainable ecosystems, it is probably more important to understand the linkages between key species or functional groups and ecosystem function, rather than focusing on species diversity. This is because there are possible mechanistic relations between what species do in ecosystems and ecosystem function. Diversity, being an abstract and aggregated property of the species in the context of communities and ecosystems, lacks such direct relations to ecosystem functions.

Carbon and nitrogen in coniferous forest soils after clear-felling and harvests of different intensity

Abstract Soil carbon and nitrogen contents were determined at four coniferous forest sites in Sweden (Scots pine and Norway spruce in southern and northern Sweden, respectively) over a period of 15–16 years after clear-felling. Three levels of logging-residue harvesting had been applied: (i) conventional stem harvest (residues left on site); (ii) harvesting all above-ground tree parts except needles; (iii) above-ground whole-tree harvesting (no residues remaining). The forest floor and the top mineral soil layers (0–20 cm) were examined, excluding coarse woody detritus (i.e. dead branches and roots). When measured, 15–16 years following clear-felling, the content of C and N in the humus layer had decreased markedly at all sites, whereas increased amounts were frequently observed in the mineral soil. Total C pools decreased by 22% at the northern site with Norway spruce, and by 17% at the southern spruce site. The corresponding figures for N were 22% and 13%. No changes in the total pool were detected at the southern site with Scots pine, whereas at the northern pine site C decreased by 7% and N increased by 7%. Changes in C and N storage were accompanied by decreases in the C N ratio in the humus layer and increases in the C N ratio in the top 5 cm of the mineral soil. No general effect of harvest intensity on soil C or N pools was shown in analyses over all sites, but there was a site and treatment interaction for N, indicating site-specific effects on this element. At the northern spruce site, whole-tree harvesting as well as removal of woody residues only (i.e. needles left on site) resulted in a greater reduction of N pools in the humus compared with conventional harvest. In contrast, at the southern spruce site conventional harvesting resulted in a greater reduction in total N pools compared with the other treatments. Whole-tree harvesting increased C N ratios in the humus and 0–5 cm mineral soil layers compared with the other treatments. This effect was detected in the humus layer only 8 years after harvest at the southern sites, whereas at the northern sites it was observed in the mineral soil 16 years after felling. The implications of these results on N mineralization rates are discussed.

Effects of different forest harvest intensities on the pools of exchangeable cations in coniferous forest soils

Abstract Effects of harvest intensity on exchangeable cations (1 M NH 4 Cl extracts) were examined in four coniferous forest soils in Sweden, 15–16 years after clear-felling. Logging residues were harvested at three intensities, applied experimentally in a randomized block design ( n = 4): (i) conventional stem-only harvest, (ii) branches and stems harvested but needles left on the ground and (iii) whole-tree harvesting where all above-stump biomass was harvested. Two of the sites were located in northern Sweden, and the other two were in southern Sweden. In each region one site was occupied by a pure Scots pine stand ( Pinus sylvestris L.) and the other by a pure Norway spruce stand ( Picea abies (L.) Karst.). Whole-tree harvesting resulted in a lower level of base saturation, especially in the humus layer. Compared with base saturation levels in the stem-only harvest treatment, the reductions were 19%, 16% and 8% at the southern sites with spruce and pine and at the northern spruce site, respectively. At the northern pine site no significant treatment effect was detected. Analyses across all study sites of element pools in the humus and 0–20 cm mineral soil layers indicated that reductions in base saturation generally were associated with depletions of exchangeable pools of K, Ca and Mg. In addition, effective cation exchange capacity per unit area was in general lower after whole-tree harvesting compared with the other treatments. Effects on exchangeable pools of Mn and Zn were similar to those observed for base cations. Exchangeable pools of acidic cations were generally not affected, but at the southern sites pools of acidic cations increased with harvest intensity. No significant differences in soil pH(H 2 O) were detected between treatments. A relatively high proportion of the Ca left on site in the form of slash was recovered in the soil ca. 15 years after felling. This was not the case for K. Furthermore, the nutrient load recovered in the soil was lower at the southern site with Norway spruce than at the other sites. Apart from these inter-site differences, the results were consistent across all study sites.

Agricultural intensification and biodiversity partitioning in European landscapes comparing plants, carabids, and birds

Effects of agricultural intensification (AI) on biodiversity are often assessed on the plot scale, although processes determining diversity also operate on larger spatial scales. Here, we analyzed the diversity of vascular plants, carabid beetles, and birds in agricultural landscapes in cereal crop fields at the field (n = 1350), farm (n = 270), and European-region (n = 9) scale. We partitioned diversity into its additive components alpha, beta, and gamma, and assessed the relative contribution of beta diversity to total species richness at each spatial scale. AI was determined using pesticide and fertilizer inputs, as well as tillage operations and categorized into low, medium, and high levels. As AI was not significantly related to landscape complexity, we could disentangle potential AI effects on local vs. landscape community homogenization. AI negatively affected the species richness of plants and birds, but not carabid beetles, at all spatial scales. Hence, local AI was closely correlated to beta diversity on larger scales up to the farm and region level, and thereby was an indicator of farm- and region-wide biodiversity losses. At the scale of farms (12.83-20.52%) and regions (68.34-80.18%), beta diversity accounted for the major part of the total species richness for all three taxa, indicating great dissimilarity in environmental conditions on larger spatial scales. For plants, relative importance of alpha diversity decreased with AI, while relative importance of beta diversity on the farm scale increased with AI for carabids and birds. Hence, and in contrast to our expectations, AI does not necessarily homogenize local communities, presumably due to the heterogeneity of farming practices. In conclusion, a more detailed understanding of AI effects on diversity patterns of various taxa and at multiple spatial scales would contribute to more efficient agri-environmental schemes in agroecosystems.

Biodiversity and species redundancy among litter decomposers

We discuss biodiversity in relation to ecosystem processes, particularly litter decomposition. Three hypotheses concerning the relations between organism groups, diversity and decomposition rates are proposed and tested against data from a two-year straw decomposition experiment. Barley straw mass loss and chemical composition, soil temperature and moisture, and the abundance of bacteria, fungi (total and FDA-active), protozoa, nematodes, microarthropods and enchytraeids were monitored.

Bumble bees (Bombus spp) along a gradient of increasing urbanization.

Background Bumble bees and other wild bees are important pollinators of wild flowers and several cultivated crop plants, and have declined in diversity and abundance during the last decades. The main cause of the decline is believed to be habitat destruction and fragmentation associated with urbanization and agricultural intensification. Urbanization is a process that involves dramatic and persistent changes of the landscape, increasing the amount of built-up areas while decreasing the amount of green areas. However, urban green areas can also provide suitable alternative habitats for wild bees. Methodology/Principal Findings We studied bumble bees in allotment gardens, i.e. intensively managed flower rich green areas, along a gradient of urbanization from the inner city of Stockholm towards more rural (periurban) areas. Keeping habitat quality similar along the urbanization gradient allowed us to separate the effect of landscape change (e.g. proportion impervious surface) from variation in habitat quality. Bumble bee diversity (after rarefaction to 25 individuals) decreased with increasing urbanization, from around eight species on sites in more rural areas to between five and six species in urban allotment gardens. Bumble bee abundance and species composition were most affected by qualities related to the management of the allotment areas, such as local flower abundance. The variability in bumble bee visits between allotment gardens was higher in an urban than in a periurban context, particularly among small and long-tongued bumble bee species. Conclusions/Significance Our results suggest that allotment gardens and other urban green areas can serve as important alternatives to natural habitats for many bumble bee species, but that the surrounding urban landscape influences how many species that will be present. The higher variability in abundance of certain species in the most urban areas may indicate a weaker reliability of the ecosystem service pollination in areas strongly influenced by human activity.

HABITAT DIVERSITY OR AREA PER SE? SPECIES RICHNESS OF WOODY PLANTS, CARABID BEETLES AND LAND SNAILS ON ISLANDS

SUMMARY (1) We examine patterns of species richness of woody plants, carabid beetles and land snails, respectively, on seventeen undisturbed forested islands (area range: 0-6-75 ha) in Lake Malaren, Sweden, in relation to area, isolation, vegetation structure, habitat diversity, habitat heterogeneity, and other factors. (2) The slopes of the species-area relations (z-values in the power model) are 0 10 for woody plants, 0-16 for land snails, 0-36 for carabid beetles, and 0-62 for forest birds. The z-values differ significantly from zero and also differ between all organism groups except woody plants and land snails on one hand, and land snails and carabid beetles on the other. (3) Island area is the best single predictor of species richness in the organism groups examined. Since habitat variables and island area are uncorrelated on the studied islands, we can reject the habitat diversity hypothesis as an explanation for the species-area relations found. (4) Total densities of woody plants and land snails were not correlated with island area, while the total density of carabid beetles was positively correlated with island area. (5) Islands with a high proportion of wet forests have relatively higher species richness of carabid beetles, and the number of land snail species relates positively to the proportion of deciduous forest. The patterns found are discussed in relation to general species richness theories.

Changes in forest-floor chemistry caused by a birch admixture in Norway spruce stands.

Abstract The aim of this study was to determine how soil chemistry and the distribution of fine roots (