1. We investigated the population dynamics of the keystone symbiotic N-fixing species Alnus tenuifolia (thinleaf alder) and the patterns of primary succession on the Tanana River floodplains of interior Alaska, USA. The goals of this study were to characterize (i) the variation in the population structure of thinleaf alder and its influence on ecosystem function; (ii) the role of a fungal disease outbreak in driving the population dynamics of thinleaf alder; (iii) the patterns of landscape-scale thinleaf alder recruitment; and (iv) the variation in successional pathways across the landscape.
2. Soil N concentrations and pools increased tenfold with thinleaf alder stand age due to the accumulation of N-rich organic material. Alder stem density varied threefold among early successional stands, and was directly related to soil N.
3. The current outbreak of a fungal disease caused widespread branch dieback and mortality of alder. Young stems were disproportionately susceptible to disease-induced mortality. Overall disease incidence and mortality of young stems were positively related to alder stem density.
4. Thinleaf alder age structures revealed that landscape-scale recruitment was pulsed over time.
5. Multiple pathways of primary succession were found using indirect gradient analysis and associated environmental characteristics were described.
6. Synthesis. The population dynamics of thinleaf alder and the inter-relationship with disease-driven disturbance can strongly influence soil N accumulation and ecosystem function in primary succession on an active glacial floodplain. The temporal pattern of thinleaf alder recruitment across the landscape appears to reflect the influence of the hydrologic regime on silt bar development and alder dispersal limitation and population dynamics. Differential life history traits explain the predominant successional pathway, but an alternative successional pathway suggests this pattern can be altered by stochastic events, disturbance, environmental variation or other factors.