Understorey vegetation and factors affecting it in young deciduous forest plantations on former agricultural land
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The establishment of forest plantations (including short-rotation forest plantations) is seen as one way to reemploy abandoned agricultural sites. The main goal of commercial forest plantations is the production of timber and woody biomass, however, the implications for biodiversity (including floristic diversity) can not be neglected. Due to the agricultural legacy the understorey of forests established on former agricultural land differs from the understorey of forests that were never cleared and these differences may persist for long periods. Since 1999 more than 700 ha of plantations with semi-exotic hybrid aspen (Populus × wettsteinii Hämet-Ahti) have been established on former agricultural land in Estonia that can be managed with 20- to 30-year rotations. Experience from other countries has indicated that environmental problems may occur in the plantations of exotic tree species. Among the native deciduous tree species silver birch (Betula pendula Roth) is considered to be a suitable tree species for the establishment of plantations in Estonia. However, abandoned agricultural sites become afforested also in the process of secondary succession with Betula spp among first arrivers. Studies conducted in former mining areas that have compared unassisted natural succession and the establishment of plantations have pointed out that naturally regenerated stands may support higher understorey species richness and diversity than plantations. Given the above mentioned, the following aims were set to the thesis: 1) to determine which site- and stand-related factors have affected understorey vegetation characteristics in young commercial forest plantations on abandoned agricultural land; 2) to study if plantations of semi-exotic hybrid aspen offer similar habitat for the understorey as plantations of native tree species silver birch; 3) to characterize the formation of the bryophyte layer; 4) to characterize the formation of forest understorey; 5) to compare understorey vegetation characteristics between naturally regenerated birch stands and silver birch plantations. Data on understorey, overstorey and site factors were collected from 73 experimental plots established in 24 hybrid aspen and 11 silver birch plantations and in 11 naturally regenerated birch stands. Two monitorings were carried out in plantations: the first monitoring in 7 to 9-year-old plantations and the second monitoring in 13 to 14-year-old plantations. Based on the results of the thesis, the observable trends in the understorey vegetation of young plantations were mostly driven by the former agricultural land use, site preparation method, and soil properties. With increasing stand age the influence of pre-establishment disturbances becomes less pronounced and the impact of overstorey-related factors increases. The majority of the vascular plant and bryophyte understorey vegetation characteristics were similar in young hybrid aspen and silver birch plantations and it may be concluded that semi-exotic hybrid aspen provides similar habitat for understorey as native silver birch. The proportion of forest species was low in the vascular plant understorey of plantations at the time of both monitorings. Although the number and cover of forest species slowly increased between the two monitorings, clear domininance of grassland species continued. The cover of the bryophyte layer was low in young plantations. Typical bryophytes were light-demanding perennials. The silvicultural management of plantations in the future should include measures that provide habitats for epixylic and epiphytic bryophyte species. The comparison of naturally regenerated stands and plantations showed that the species richness and diversity of bryophytes were higher in naturally regenerated birch stands, but the species richness and diversity of vascular plants did not differ between the two stand types. However, significantly higher numbers of forest species (both vascular plant and bryophyte) indicated that the formation of forest understorey had progressed further in naturally regenerated stands. This can be explained by the longer undisturbed succession and colonization period of naturally regenerated stands. Both natural and artificial regeneration can be recommended as possible alternatives for the establishment of new forests on former agricultural land.