Differences in litter cover and understorey flora between stands of introduced lodgepole pine and native Scots pine in Sweden

Abstract

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 m 2 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.