MECHANISMS OF PRIMARY SUCCESSION FOLLOWING DEGLACIATION AT GLACIER BAY, ALASKA

In primary succession following deglaciation at Glacier Bay, Alaska, w e tested the hypothesis that the major effect of initial nitrogen-fixi ng colonizers is to facilitate establishment of late-successional domi nants and that other possible causes of successional change (e.g., lif e history factors governing seed rain and competitive interactions amo ng species) need not be invoked. Environment changed dramatically thro ugh the first 200 yr of succession. Soil organic matter increased 10-f old in the upper mineral soil with corresponding increases in soil moi sture, total nitrogen (N), and capacity to support plant growth and de clines in bulk density, pH, and total phosphorus (P). Plant growth in pioneer soils tended to be simultaneously limited by both N and P, as well as by unknown factors (perhaps lack of mycorrhizae), whereas only P limited growth in older soils. Light availability to seedlings decl ined through succession. Early-successional species (Epilobium latifol ium, Dryas drummondii) had smaller seeds, younger age at first reprodu ction, shorter life-span, and shorter height at maturity than did mid- successional (alder, Alnus sinuata) and late-successional species (sit ka spruce, Picea sitchensis). Seed rain of alder and spruce was neglig ible in the pioneer stage, increased prior to the stage in which a spe cies was dominant, and was greatest in the stage in which a species do minated. Vegetation in each successional stage inhibited germination a nd initial establishment of sown alder and spruce seeds (except a tend ency of the ''black-crust'' algal/microbial community in the pioneer s tage to enhance survivorship). Removal of the surface litter layer gen erally enhanced germination and survivorship, particularly of alder. C omparisons of germination in the greenhouse and the field indicated th at climatic or indirect vegetation effects (e.g., differential seed pr edation) and allelopathy also reduced germination and establishment in vegetated communities. Naturally occurring spruce seedlings grew most rapidly in the Dryas and alder stages and most slowly in the spruce s tage. Similarly, growth of spruce seedlings transplanted into each suc cessional stage was facilitated by the Dryas (nonsignificantly) and al der stages but inhibited by the spruce stage, relative to earlier succ essional stages. Facilitation of growth of natural and transplanted sp ruce seedlings by Dryas and alder stages was associated with higher N and P uptake and tissue nutrient concentrations, whereas nutrient upta ke and concentration in spruce seedlings declined in the spruce stage. By contrast, transplanted alder seedlings grew rapidly and accumulate d most nutrients in the pioneer stage and were strongly inhibited by s ubsequent stages. The facilitative effect of Dryas and alder comes pri marily from inputs of organic matter and associated N. Addition of ald er litter stimulated nutrient uptake and growth of transplanted spruce seedlings in the pioneer and Dryas stages, whereas shading had no eff ect on growth of spruce seedlings. Root trenching and planting of spru ce near isolated alders indicated that, although the net effect of ald er is facilitative, alder also inhibits growth of spruce seedlings thr ough competition for soil resources. Strong root competition also occu rs in the spruce stage. Alder competitively inhibits Dryas, primarily by shading but also through the physical and allelopathic effects of i ts litter. In general, both at Glacier Bay and elsewhere, life history traits determine the pattern of succession. Changes in competitive ba lance accompanying successional changes in environment provide the mec hanism for changes in species dominance. Initial site conditions (and facilitation, where present) influence the rate of change and final st ate of community composition and productivity. We conclude that no sin gle factor or mechanism fully accounts for primary succession at Glaci er Bay.