Floristic diversity responses in young hybrid aspen plantations to land-use history and site preparation treatments

Floristic diversity was studied in 7- to 8-year-old commercial hybrid aspen (Populus tremula L. × Populus tremuloides Michx.) plantations on abandoned agricultural sites with a different land use (grassland or crop field) and site preparation (whole-area ploughing or strip tillage) history. The aim of the study was to investigate how the understorey vegetation had developed in such repeatedly disturbed communities and which environmental variables had significantly affected this. A total of 204 vegetation plots (2 m × 2 m) were established within 51 experimental areas; vegetation descriptions were compiled, concentrations of total N, extractable P and K, and pH of the soil humus layer were determined, and canopy cover of the trees was estimated. Weighted average Ellenberg values for light, moisture, pH, and nitrogen, as well as several life-history characteristics, were calculated for the vegetation plots. Altogether 191 vascular plant species were described: on average 16.7 ± 0.4 species per plot and 28.6 ± 1.1 species per experimental area. Former land use and site preparation method had a significant impact on the position of vegetation plots in detrended correspondence analysis (DCA) ordination, confirmed also by the multiresponse permutation procedure (MRPP). Soil characteristics were significantly correlated with DCA axes. Former land use and site preparation method also affected the species composition. All sites were dominated by competitor species; ruderals were represented in a higher proportion in former fields and whole-area ploughed sites. Species richness and Simpson's diversity index were higher in plantations where strip tillage had been used for site preparation and lower on sites with higher nutrients concentrations in the humus layer. Generally, overstorey vegetation, characterized in the current study using canopy cover, had not started to affect understorey vegetation in young plantations. Application of less intensive site preparation methods is recommended in order to support higher species richness and lower share of ruderal species.