Early growth of Pinus nigra and Robina pseudoacacia stands: contributions to soil genesis and landscape improvement on lignite spoils in Ptolemaida

Two forest species planted on spoil heaps in Ptolemaida, Pinus nigra and Robinia pseudoacacia, were studied. The productivity of both species was high. The majority of trees in the pine stand were vigorous and had good quality trunks, while most Robinia trees suffered from strong competition from neighbouring trees and were forked in many branches. The addition of fly ash to spoil materials produced generally better growing conditions. Fourteen years after their establishment, the two stands had improved soil properties significantly more than had natural revegetation. Distinct surface, A1 and C horizons formed under both stands. The contribution to landscape aesthetic improvement by the evergreen Pinus nigra was greater than that by the deciduous Robinia, also a desirable species because of its initial fast growth, intense sprouting and rapid nitrogen-enriching properties. The study suggests that afforestation with mixed stands of evergreen pine species and nitrogen-fixing trees is probably the best solution for successful rehabilitation.