Abstract:
Plant succession is among the fundamental concepts in ecology. Studies addressing plant recruitment, successional mechanisms, and the role of root-colonizing fungi with focus on dark-septate endophytes (DSE) were
conducted in the laboratory and on the forefront of receding Lyman Glacier (North Cascade Range, Washington, U.S.A.). Primary successional studies primarily focus on biotic interactions although initial recruitment is controlled abiotically. In a field study, we found that plant distribution is not random but determined by sites that allow seed trapping and protect germinating seeds from desiccation. An additional hypothesis that early successional plant individuals form centers of establishment for subsequent colonizers was tested by assaying the effect of willow shrubs on invading plants. Results suggested that while the canopy had either neutral or inhibitory effects on new seedling establishment, the soil beneath the canopies favors establishment. The role of fungi in succession is poorly known. We present results of eight years of floristic surveys of the ectomycorrhizal macrofungi at the site. The data show that the communities of ectomycorrhizal fungi differ between primary and secondary successional sites. Plants on the glacier forefront were frequently colonized by nonmycorrhizal DSE. We reviewed the literature on DSE and listed reports of about 600 colonized plant species from the tropics to arctic and alpine habitats. Although they appear to be ubiquitous, the ecological role of DSE is poorly understood: results of past experiments are inconsistent and do not explain the role of DSE in their natural habitats. We further investigated the
host-DSE association and its relevance to plant fitness on the glacier forefront in laboratory and field studies. We conclude that some DSE may function as typical mycorrhizal associates but that the research tools for studying these associations are inadequate and biased. A field study on the distribution of discrete RAPD-phenotypes (individuals) showed that DSE colonize several plant species under natural conditions. A single fungal individual can
colonize adjacent plants, potentially providing a mycelial pathway for resource flow as hypothesized for ectomycorrhizal systems.
