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The abandonment of agricultural areas is usually accompanied by the stochastic encroachment of shrubs and trees or deliberate afforestation, but this leads as a rule to the decline of grassland plant diversity. Future climate change may increase precipitation and air humidity in temperate and boreal forest zones, and thus change vegetation dynamics.

We conducted a five-year experiment on the Free Air Humidity Manipulation experimental facility (FAHM) with planted deciduous trees to examine how both the grassland afforestation with deciduous trees and the increase of air humidity in interaction with forest soil supplementation and cover of herbaceous layer affect the species richness, composition and cover of bryophyte layer. The effects of the same factors on the cover of herbaceous layer were also analysed. The species of bryophytes and cover of herbaceous and bryophyte layer were surveyed in September of 2007-2011.

During these early years of forest succession, herbaceous layer cover decreased and bryophyte cover increased, although the direct suppressing effect of tree canopy was observed on both layers. The input of forest soil as diaspore deposit of bryophytes had some positive effect on the cover of bryophytes, while the increased air humidity had quite limited effect. The density of tree canopy (defined in our study through Leaf Area Index) first promoted the species richness of bryophytes, particularly of species with grassland preference. However, together with the herbaceous layer it became suppressing in the last year.

The species richness of bryophytes declined during the first years and then stabilised. The decline relied mainly on the disappearance of grassland species. The richness of bryophytes was supported by forest soil supplementation only in the initial year.

The bryophyte species composition changed rapidly in first two years mainly due to decline of short-lived grassland species and perennial species started to dominate.

Our results show that bryophyte succession during the afforestation is largely driven by natural loss of grassland specialists after tree canopy closure, while diaspore supplementation from forest and improved air moisture level had small effects.