PHYTOSOCIOLOGICAL INVESTIGATIONS IN A FAST-GROWING PLANTATION OF POPLARS AND WILLOWS ON FORMER ARABLE LAND

The ground vegetation- no longer treated with herbicides- of a biomass plantation with balsam poplars, aspen and willows on former arable land was chararacterized in the 5th growing season (i. e. summer before the 1st harvest) and in two consecutive seasons and compared with the field layer of adjacent wheat fields and green fallow areas. The total number of species in the plantation stands amounted to 145 as compared with 17 in the field. Short living species and those carrying green foliage during winter predominate, creating a distinct spring and autumn aspect. Species typical of weed associations of arable fields are still present, others belonging to meadow, shrub and thicket associations are immigrating. Coverage, as a rule, varied from 27-75% with extremes between 5-100%, the amount of dry shoot matter in mid-summer from 0.19 to 1.96 t/ha. Both parameters were dependent on the tree species in the upper storey and were inferior in the northern part of the plantation (where better drained soils prevail) and beneath the fast growing poplars as compared with the southern compartment of experimental area (with more wet soils) and beneath the slow growing willows. The average indicator values of the ground vegetation with regard to soil reaction and N status of the soil were closely correlated with the results of respective chemical determinations. In the two years following the first harvest both the number of species and the coverage increased. Fertilization with N, P, K, Mg and Ca, applied either singly or in different combinations, hitherto failed to significantly influence the composition and the biomass of the ground vegetation. The soils are still well supplied with nutrients as a consequence of the intensive fertilization during the agricultural use of the land. Depending on the coverage and growth the aboveground biomass of the soil flora accumulated 7-35 kg N, 1-7 kg P, 2-16 kg Ca, 5-47 kg K and 0.5-4 kg Mg per hectare.