Population biology and stand development of birch (Betula pubescens Ehrh.) on disturbed sites in Iceland

Abstract

Deforestation has reduced native birch woodlands and shrublands in Iceland from an estimated 25,000 to 40,000 km^2 at the time of settlement 1100 years ago to about 1,250 km^2presently. The objective of this research was to describe and quantify aspects of birch colonization and stand development on severely eroded sites, which could be used to develop strategies for landscape restoration. Patterns of birch colonization were inferred from age/size structure and spatial distribution of birch populations on disturbed sites in southern and northeastern Iceland. In addition, long-term changes in birch cover were quantified from aerial photographs. Increases in birch abundance occurred as existing birch clusters expanded and coalesced and produced seeds which germinated and initiated birch clusters that subsequently served as foci for additional colonization. Ectomycorrhizal infectivity, as estimated by the most probable number technique, was twenty-five to thirty-five times lower in soils from disturbed sites than observed for soil collected from a developing birch stand. Even so, mycorrhizae did develop on roots of birch seedlings grown on these disturbed soils in a controlled environment. The distribution of seedlings was quantified along belt transects radiating from birch clusters and isolated birch plants and related to seed rain and ground cover. Seed dispersal curves tended to be unimodal with a log-linear decrease in seed density with increased distance from the seed source. Patterns of seedling density seemed to reflect the interaction between seed rain and the availability of safe-sites. Field experiments were designed to evaluate the suitability of some common microsites (mineral soil, gravel, rock, moss cushions, dwarf-shrub and grass) for birch seedling establishment. Germination from a one-time dissemination and seedling survival were monitored for two years. Seedling emergence was greater in non-vegetated than vegetated microsites, but seedling survival was generally lower in non-vegetated microsites. The majority of seedling mortality occurred during winter on all microsites. Suitability of non-vegetated microsites for seedling establishment was augmented by low-level fertilization that served to enhance overwinter survival. The results are discussed in relation to strategies for restoration of birch woodlands on eroded and disturbed sites.