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River management practices have altered the hydrological regimes of many rivers and also altered the availability of regeneration niches for riparian species. We investigated the impact of changed hydrological regimes on the sex ratios and the Spatial Segregation of the Sexes (SSS) in the dioecious species Salix myrsinifolia Salisb.–phylicifolia L. and S. lapponum L. by studying the free-flowing Vindel River and the regulated Ume River in northern Sweden. We surveyed sex ratios of these species in 12 river reaches on the Vindel River and in 17 reaches on the Ume River. In addition, we surveyed the sex and location above mean river stage of 1,002 individuals across both river systems to investigate the SSS of both species. Cuttings were collected from male and female individuals of S. myrsinifolia–phylicifolia from both rivers and subjected to four different water table regimes in a greenhouse experiment to investigate growth response between the sexes. We found an M/F sex ratio in both river systems similar to the regional norm of 0.62 for S.myrsinifolia–phylicifolia and of 0.42 for S. lapponum. We found no evidence of SSS in either the free-flowing Vindel River or the regulated Ume River. In the greenhouse experiment, hydrological regime had a significant effect on shoot and root dry weight and on root length. Significantly higher shoot dry weights were found in females than in males and significantly different shoot and root dry weights were found between cuttings taken from the two rivers. We concluded that changed hydrological regimes are likely to alter dimensions of the regeneration niche and therefore to influence sex ratios and SSS at an early successional stage, making it difficult to find clear spatial patterns once these species reach maturity and can be sexed.

We compared the plant understorey between stands of introduced Pinus contorta and native P. sylvestris in boreal northern Sweden using a chronosequence of 24 paired stands of P. contorta or P. sylvestris. We located one 1000 m2 sample plot in each stand in which we recorded tree variables, number of vascular plant and lichen species, and cover of the understorey of each of the groups vascular plants, bryophytes and lichens. We tested for differences between the two chronosequences and assessed relationships between the understorey and stand variables. Tree variables did not differ between stands but stands of P. contorta had more than three times greater percent ground cover of needle litter compared to stands of P. sylvestris. Naturally regenerated individuals of P. contorta occurred in five out of 12 stands of P. contorta older than 25 years. The understorey was similar between stand types in terms of species richness, cover, and species composition but the entire species pool was larger for stands of P. sylvestris than for stands of P. contorta. Thirty-eight species were only found in stands of P. sylvestris; the corresponding figure for stands of P. contorta was six species. Shade-intolerant understorey species were disfavoured in stands of P. contorta. The development at the stand level of understorey (vascular plant and lichen) species richness and cover over time was similar between stand types. In both types of stands, the number of lichen species and the percent ground cover of lichens and bryophytes increased with stand age; the corresponding values for vascular plant species did not. The results propose a more homogeneous flora in stands of P. contorta compared to stands of P. sylvestris, suggesting lower understorey species richness at the landscape scale. The larger shading effect of needles in the canopy and on the ground in stands of P. contorta, which is supposed to imply less habitat variation, might contribute to this difference.