Joakim Hjältén

Biodiversity Differences between Managed and Unmanaged Forests: Meta-Analysis of Species Richness in Europe

Past and present pressures on forest resources have led to a drastic decrease in the surface area of unmanaged forests in Europe. Changes in forest structure, composition, and dynamics inevitably lead to changes in the biodiversity of forest-dwelling species. The possible biodiversity gains and losses due to forest management (i.e., anthropogenic pressures related to direct forest resource use), however, have never been assessed at a pan-European scale. We used meta-analysis to review 49 published papers containing 120 individual comparisons of species richness between unmanaged and managed forests throughout Europe. We explored the response of different taxonomic groups and the variability of their response with respect to time since abandonment and intensity of forest management. Species richness was slightly higher in unmanaged than in managed forests. Species dependent on forest cover continuity, deadwood, and large trees (bryophytes, lichens, fungi, saproxylic beetles) and carabids were negatively affected by forest management. In contrast, vascular plant species were favored. The response for birds was heterogeneous and probably depended more on factors such as landscape patterns. The global difference in species richness between unmanaged and managed forests increased with time since abandonment and indicated a gradual recovery of biodiversity. Clearcut forests in which the composition of tree species changed had the strongest effect on species richness, but the effects of different types of management on taxa could not be assessed in a robust way because of low numbers of replications in the management-intensity classes. Our results show that some taxa are more affected by forestry than others, but there is a need for research into poorly studied species groups in Europe and in particular locations. Our meta-analysis supports the need for a coordinated European research network to study and monitor the biodiversity of different taxa in managed and unmanaged forests.

Effects of Simulated Herbivory and Intraspecific Competition on the Compensatory Ability of Birches

To improve our knowledge regarding plant responses to herbivory we sub? jected juvenile birches (Betula pubescens) to different types of simulated herbivory, i.e., removal ofthe top (1 cm) ofthe main stem or defoliation (including removal of main stem top), and to different levels of intraspecific competition (i.e., three stand densities). Treat? ments were also repeated during the following year. Birch responses to these treatments were measured as changes in plant height and dry mass. Defoliated plants showed reduced growth irrespective of the level of intraspecific competition. Topped plants in the low-density plots showed enhanced growth, i.e., over- compensation, whereas topped plants growing in the high- and medium-density plots were unaffected by the treatments. Our results indicate that we should expect a continuum of plant responses to herbivory. Thus, there is little value in asking whether herbivory has a positive or negative effect on plant growth. Instead, emphasis should be placed on identifying conditions under which we would expect different responses. Further, we suggest that overcompensation should not generally be considered as an adaptive response that has evolved as a consequence of natural selection due to herbivory, but rather as an indirect consequence of selection for competitive ability. Because apical dominance (assumed to be an adaptation to competition) restricts growth, the growth rate will increase if apical dominance is removed.

Herbivore avoidance by association: vole and hare utilization of woody plants

The probability that an individual plant will be attacked by a herbivore depends not only on the characteristics of the individual plant, but also on the quality and abundance of its neighbours. However, plants have been reported to receive protection from herbivory both when associated with plants of higher and lower palatability (the attractant-decoy and repellent-plant hypotheses, respectively), and there are no mechanistic explanation for these different outcomes of plant spatial association. We used patch-use theory (marginal-value theorem) to predict under which circumstances we should expect plants to gain protection from herbivory due to association with other plants

Sexual Dimorphism and Biotic Interactions

In the last decade, an increasing number of studies have suggested that males and females of dioecious plants may be differentially affected by competition, herbivory, and parasitism. In dioecious plant species, there is a fundamental sexual difference in patterns of reproductive allocation. Males may allocate more resources to reproduction than females during flowering, but total reproductive allocation is typically greater in females than in males (Chap. 6). These sexual differences in schedules of reproductive allocation have in several species been found to be associated with differences in vegetative phenology, rate of growth, morphology, and chemical composition (Lloyd and Webb 1977; Chaps. 6 and 7), which are characters that are likely to affect competitive ability and interactions with herbivores and parasites (e.g., Crawley 1983; Keddy 1989; Marquis 1992; Dix and Webster 1995).

Effects of management on coarse woody debris volume and composition in boreal forests in northern Sweden

Forest management practices have led to a reduction in the volume and a change in the composition of coarse woody debris (CWD) in many forest types. This study compared CWD volume and composition in reserves and two types of managed forest in the central boreal zone of Sweden. Ten areas were surveyed, each containing clear-cut, mature managed and old-growth stands, to determine the volume of standing and lying CWD in terms of species composition, decay class and size class. Volumes of CWD on clear-cuts and in mature managed forests were high compared with previous studies. Old-growth forests (72.6 m3 ha−1) contained a greater volume of CWD than mature managed forests (23.3 m3 ha−1) and clear-cuts (13.6 m3 ha−1). Differences were greatest for the larger size classes and intermediate decay stages. Despite stand ages being up to 144 years, CWD volume and composition in managed forests was more similar to clear-cuts than to old-growth forests.

Terrestrial trophic dynamics in the Canadian Arctic

The Swedish Tundra Northwest Expedition of 1999 visited 17 sites throughout the Canadian Arctic. At 12 sites that were intensively sampled we estimated the standing crop of plants and the densities of herbivores and predators with an array of trapping, visual surveys, and faecal-pellet transects. We developed a trophic-balance model using ECOPATH to integrate these observations and determine the fate of primary and secondary production in these tundra ecosystems, which spanned an 8-fold range of standing crop of plants. We estimated that about 13% of net primary production was consumed by herbivores, while over 70% of small-herbivore production was estimated to flow to predators. Only 9% of large-herbivore production was consumed by predators. Organization of Canadian Arctic ecosystems appears to be more top-down than bottom-up. Net primary production does not seem to be herbivore-limited at any site. This is the first attempt to integrate trophic dynamics over the entire Canadian Arctic.

Saproxylic and non-saproxylic beetle assemblages in boreal spruce forests of different age and forestry intensity.

Current clear-cutting forestry practices affect many boreal organisms negatively, and those dependent on dead wood (saproxylics) are considered as particularly vulnerable. The succession of species assemblages in managed forest habitats regenerating after clear-cutting is, however, poorly known. We compared beetle assemblages in three successional stages of managed boreal spruce forests established after clear-cutting and two types of older spruce forests that had not been clear-cut. We also assessed whether saproxylic and non-saproxylic beetle assemblages show similar biodiversity patterns among these forest types. Beetles were collected in window traps in nine study areas, each encompassing a protected old-growth forest (mean forest age approximately 160 years, mean dead wood volume 34 m3/ha), an unprotected mature forest (approximately 120 years old, 15 m3/ha), a middle-aged commercially thinned forest (53 years old, 3 m3/ha), a young unthinned forest (30 years old, 4 m3/ha), and a clearcut (5-7 years after harvest, 11 m3/ha). Saproxylic beetles, in particular red-listed species, were more abundant and more species rich in older forest types, whereas no significant differences among forest types in these variables were detected for non-saproxylics. The saproxylic assemblages were clearly differentiated; with increasing forest age, assemblage compositions gradually became more similar to those of protected old-growth forests, but the assemblage composition in thinned forests could not be statistically distinguished from those of the two oldest forest types. Many saproxylic beetles adapted to late-successional stages were present in thinned middle-aged forests but absent from younger unthinned forests. In contrast, non-saproxylics were generally more evenly distributed among the five forest types, and the assemblages were mainly differentiated between clearcuts and forested habitats. The saproxylic beetle assemblages of unprotected mature forests were very similar to those of protected old-growth forests. This indicates a relatively high conservation value of mature boreal forests currently subjected to clear-cutting and raises the question of whether future mature forests will have the same qualities. Our results suggest a high beetle conservation potential of developing managed forests, provided that sufficient amounts and qualities of dead wood are made available (e.g., during thinning operations). Confirming studies of beetle reproduction in dead wood introduced during thinning are, however, lacking.

Sampling saproxylic beetle assemblages in dead wood logs: comparing window and eclector traps to traditional bark sieving and a refinement

The use of saproxylic beetle community as a metric to evaluate nature conservation measures in forests requires efficient methods. We first compare traditional bark sieving to a potential improvement (extracting beetles from whole bark with Tullgren funnels) to determine the most efficient. Secondly we compare this most efficient bark sampling to eclector and window traps. At the species, family, and functional group levels, we consider species richness, abundance and practical aspects. Traditional bark sieving missed >50% of the individual beetles compared to whole bark sampling so we recommend the latter. Window traps caught large numbers of mobile saproxylic beetles, but a high proportion of non-saproxylics results in high sorting cost; bark sampling and eclector traps had a high proportion of saproxylics and obligate saproxylics. Compared to bark sampling, eclector traps are non-destructive, and monitor the whole saproxylic assemblage (i.e. also beetles inside the wood). Overall, window traps are useful because they capture saproxylic beetles attracted to dead wood and sample the local species pool, whereas eclector traps capture the saproxylics that actually emerge from a particular piece of dead wood, and thus are suited to detailed studies. Overall, we suggest that a combination of these two best methods is highly complementary.

Selection of deciduous trees by free ranging voles and hares in relation to plant chemistry

In two field experiments the feeding preference of voles (Microtus agrestis and Clethrionomys glareolus) and hares (Lepus timidus), offered winter dormant shoots/branches of three deciduous tree species, were determined and related to nutritional quality and concentration of phenolics. Both the voles and the hares consumed more rowan (Sorbus aucuparia)/aspen (Populus tremula) (vole and hare experiment, respectively) than birch in the patches containing birch (Betula pubescens) together with rowan/aspen and slightly but not significantly more birch than alder (Alnus incana) in the birch/alder patches. Further-more, the overall concumption in both patch types was highest in rowan/aspen (vole and hare experiment, respectively) and lowest in alder, with birch being intermediate

Plant sex and hare feeding preferences

SummaryTo evaluate the general extent to which sex-related differences in palatability occur in boreal dioecious woody plants, males and females of five dioecious woody plant species were presented to free-ranging mountain hares (Lepus timidus) during winter. Hares strongly preferred branches from male plants when feeding on Populus tremula and Salix caprea and weakly preferred male S. pentandra. However, they did not show any sex-related preference when feeding on the other two species studied (Myrica gale and Juniperus communis). Nitrogen concentration and, to some degree, digestibility showed strong relationships with hare food preferences. By contrast, the concentration of phenolics was only weakly related to feeding preference. Phenolics could, nevertheless, still be important if only one or a few specific compounds deter hare feeding. These results indicate that sex-related differences in plant palatability in the boreal forest might be more widespread than previously believed, particularly for species of the family Salicaceae. Thus, herbivores might be responsible for the female-biased sex ratios found in willow populations in northern Scandinavia (e.g. Elmqvist et al. 1988).