SEXUAL VERSUS ASEXUAL COLONIZATION BY PHRAGMITES AUSTRALIS: 25-YEAR REED DYNAMICS IN A MEDITERRANEAN MARSH, SOUTHERN FRANCE

Colonization by common reed (Phragmites australis) is often assumed to be driven mainly by vegetative growth of rhizomes. Seedlings are rarely observed in the field despite the annual flowering, the large seed production, and germination potential. We considered that this was mainly a consequence of the rare occurrence of the regeneration window of reed but, provided the suitable conditions were created, reed could efficiently colonize empty space by seedling establishment. We analyzed the relative importance of sexual and asexual propagation in a reed-dominated marsh in Southern France using past aerial photographs to reconstruct the reedbed dynamics over the last 25 years and a full-scale experiment simulating a rarely occurring low water level in spring. We observed phases of slow colonization corresponding to vegetative growth and periods of rapid closure of mudflats by reed. Reedbed dynamics were a combination of slow vegetative growth and a few phases of seedling establishment. The preservation of stable water levels favored colonization by vegetative growth. The experimental spring drawdown led to a 25% increase in reed area with up to 40 m progressions. Such rare events with low water levels in spring favor sexual colonization. Sexual reproduction plays an important role in the pioneer stage of reedbeds, allowing both fast progression and establishment of genetically diverse stands.