Seed dispersal, germination and early seedling establishment of Populus alba L. under simulated water table declines in different substrates
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Populus alba L. is an autochthonous species dominating the overstory of the floodplain forests across the Mediterranean region. In contrast to some other Populus spp., very little is known about its regeneration strategies. Poplars yearly disperse huge amounts of wind- and water-dispersed, non-dormant, short-lived tiny seeds that need the bare, open and moist substrates created by fluvial-geomorphic events to germinate and establish. To survive, the growing roots must keep pace with the falling water table and associated soil moisture zone. Using a greenhouse experimental facility, 9-day-old P. alba seedlings were subjected to five hydrological treatments (permanent saturation, drawdown rates of 1, 2.5, 5 cm day−1 and immediate drainage) in two different substrates (coarse and sandy), and their survival and growth were evaluated. Also, P. alba seed dispersal was monitored in the field, and seed germinability and longevity were tested in the laboratory. No seedlings survived the water table declines in the coarse substrate although survival was high (85%) under saturated conditions. In the sandy soil, survival was significantly greater in the permanent saturation (87%) and 1 cm day−1 (88%) treatments than in the 2.5 cm day−1 (58%), 5 cm day−1 (25%) and immediate drainage (22%) treatments. The lowest root and shoot growth rates occurred under the saturated and immediate drainage conditions. Seed dispersal lasted 6–8 weeks and peaked in mid-April, initial seed germinability was high (92%) and seed longevity was relatively long (half viability period, 30 days). The creation of nursery sites and release of controlled floods in mid-April, followed by water table declines of less than 1 cm day−1 in coarse substrates and less than 5 cm day−1 in sandy substrates are recommended for enhancing the initial establishment of P. alba seedlings.