The aim of this study was to quantify the effects of fertiliser N on C stocks in trees (stems, stumps, branches, needles, and coarse roots) and soils (organic layer +0-10 cm mineral soil) by analysing data from 15 long-term (14-30 years) experiments in Picea abies and Pinus sylvestris stands in Sweden and Finland. Low application rates (30-50 kg N ha À1 year À1 ) were always more efficient per unit of N than high application rates (50-200 kg N ha À1 year À1 ). Addition of a cumulative amount of N of 600-1800 kg N ha À1 resulted in a mean increase in tree and soil C stock of 25 and 11 kg (C sequestered) kg À1 (N added) (''N-use efficiency''), respectively. The corresponding estimates for NPK addition were 38 and 11 kg (C) kg À1 (N). N-use efficiency for C sequestration in trees strongly depended on soil N status and increased from close to zero at C/N 25 in the humus layer up to 40 kg (C) kg À1 (N) at C/N 35 and decreased again to about 20 kg (C) kg À1 (N) at C/N 50 when N only was added.In contrast, addition of NPK resulted in high (40-50 kg (C) kg À1 (N)) N-use efficiency also at Nrich (C/N 25) sites. The great difference in N-use efficiency between addition of NPK and N at Nrich sites reflects a limitation of P and K for tree growth at these sites. N-use efficiency for soil organic carbon (SOC) sequestration was, on average, 3-4 times lower than for tree C sequestration. However, SOC sequestration was about twice as high at P. abies as at P. sylvestris sites and averaged 13 and 7 kg (C) kg À1 (N), respectively. The strong relation between N-use efficiency and humus C/N ratio was used to evaluate the impact of N deposition on C sequestration. The data imply that the 10 kg N ha À1 year À1 higher deposition in southern Sweden than in northern Sweden for a whole century should have resulted in 2.0 ± 1.0 (95% confidence interval) kg m À2 more tree C and 1.3 ± 0.5 kg m À2 more SOC at P. abies sites in the south than in the north for a 100-year period. These estimates are consistent with differences between south and north in tree C and SOC found by other studies, and 70-80% of the difference in SOC can be explained by different N deposition.

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An increasing body of evidence indicates that the association between different plant species may lead to a reduction in insect herbivory, i.e. associational resistance. This might be due to a top-down regulation of herbivores by increased numbers of natural enemies or to a disruptive bottom-up influence of lower host plant accessibility. In particular, the richer plant communities release more diverse plant odours that may disturb olfactory-guided host choice and mating behaviour of insect herbivores, i.e. the "semiochemical diversity hypothesis". However, this hypothesis has been rarely tested experimentally in natural habitats, notably forest ecosystems. We tested the effects of non-host volatiles (NHV) on mate and host location by the pine processionary moth (PPM) at the scale of individual pine trees with branches of non-host tree (birch) at their base. Pheromone trap catches and the numbers of larval nests were both reduced by non-host presence under treated pine trees, confirming an associational resistance mediated by NHV. In both males and females, the antenna could detect several birch volatiles, including methyl salicylate (MeSa). MeSa inhibited the attraction of the PPM male to pheromone traps, as did bark and leaf chips from birch trees. Our test of three doses of MeSa at the habitat scale (50 m forest edges) showed that the reduction in the numbers of male PPM captured in traps and in larval nests was MeSa dose-dependent. These results show that odours released by deciduous non-host trees can reduce herbivory by a forest defoliator in conifers, providing support to the "semiochemical diversity hypothesis" as a mechanism of associational resistance.

We quantified allozyme diversity and quantitative genetic variation within and between six populations of the locally rare Scabiosa canescens and the more common and widespread S. columbaria (Dipsacaceae). For the within-population component, we quantified allozyme diversity as the expected heterozygosity (H e ), followed by a sib analysis to estimate the heritability (h 2 ) for eight vegetative and floral characters. There was no consistent association between h 2 and H e , or between h 2 and current population size. Regarding the betweenpopulation component, we compared the overall level of population differentiation for the allozymes (F ST ), assumed to be selectively neutral, with the corresponding estimates for the quantitative traits (Q ST ) to determine if the marker genes and the polygenic characters have been influenced by the same evolutionary force(s). Differences between estimates of F ST and Q ST for S. canescens were too small to be declared as significant, indicating that neutral phenotypic evolution has led to the observed differentiation in this species. Our data indicate higher levels of phenotypic differentiation in S. columbaria with values of Q ST exceeding the allozymic F ST for all traits. This finding indicates a great potential for S. columbaria to respond to local selection pressures, perhaps explaining why this species has a broader ecological amplitude than S. canescens.

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Bumblebees foraging on the self-incompatible Anchusa officinalis fly between near neighbour plants and between near neighbour inflorescences within plants. Although many-flowered plants attracted most bumblebees these plants received fewer visits on a per flower basis than smaller plants, and each bumblebee visited a smaller proportion of the flowers. The calculated effective visitation rate per flower was highest on plants of an intermediate size. If pollen-carryover was assumed to be limited the most efficient plant was predicted to be smaller since the proportion of fertilized flowers per bumblebee visit is expected to decrease further on the largest plants in relation to the total flower number. These predictions were tested by measuring fruit-set in the field. The percentage fruit-set decreased with plant size at all sizes that were investigated. That the most efficient plant was small indicates that pollen-carryover was indeed limited. However, the low percentage fruit-set associated with large size did not present a serious problem since the total estimated seed production per plant still increased with size. Selection favoring smaller plants may be low or absent in Anchusa.