Sources of spatial and temporal heterogeneity in the colonization of an alpine krummholz environment by the weedy subalpine plant Chamerion angustifolium (fireweed)

Plant-plant interactions may increase or reduce the impact of abiotic stress on species distributions, depending on the balance between competition and facilitation. We report on the role of a willow (Salix) shrub canopy in mediating the importance of drought stress for performance at seedling and adult life stages of the herbaceous wildflower, Chamerion angustifolium L. (fireweed). Willows form the vegetative boundary (krummholz) between subalpine and alpine life zones in the Rocky Mountains. Chamerionaangustifolium is a weedy pioneer species common in subalpine vegetation and advancing upward into the krummholz in the central Rocky Mountains. A mosaic of soil moisture variation dictated spatial patterns of seed germination and seedling survival in C.aangustifolium in the absence of willows. However, willows restricted the capacity of C.aangustifolium to exploit natural patches of soil moisture during establishment. Seed germination, seedling survival, and net establishment success increased markedly with soil water supply in the open, but not under the willow canopy. For mature plants, willows restricted photosynthesis at the peak of the growing season when rainfall was frequent, but ameliorated the daily decline in leaf water status under drought late in the season. Results suggest that the impact of willows on desiccation stress in alpine populations of fireweed varies with life history stage and phenology.