Woody riparian species response to different soil moisture conditions: laboratory experiments on Alnus incana (L.) Moench

Greenhouse experiments were conducted to study the response of Alnus incana to a variety of water table treatments in two different sediment types from the Drac and Isere Rivers in the Rhone River Basin, France. The aim was to establish which water regimes promoted greatest growth rates following germination. The apparatus used (known as a 'rhizopod') consists of a central water well connected to sixteen soil-filled tubes of 1.2 m height. Design modifications included a pump system attached to each rhizopod allowing automatic and different water-table drawdown rates to be applied, and insertion of tensiometers to monitor soil suctions during the experiment. The drawdown rates applied to the five rhizopods used in the first experiment were: 3 cm/day; 3 cm/day + weekly application of 1 cm rainfall; 1 cm/day; 0.5 cm/day; and 0 cm/day. The highest growth rates were measured in well-drained sandy loams experiencing water level decline rates of 1 cm/day and 3 cm/day. These high growth rates correspond with soil suction values of between 80 and 110 centimetres of water. Lowest growth rates were experienced in soils remaining wet for more than 7 continuous days or in coarse sandy and gravelly sells which experienced water level decline rates of greater than or equal to 1 cm/day.