Responses of red alder and black cottonwood seedlings to flooding

Red alder ( Alnus rubra Bong.) and black cottonwood ( Populus trichocarpa Torr. & Gray) seedlings were monitored to evaluate response during a 20-day period of artificial flooding and a 20-day recovery period following flooding. During the flooding period, both species showed changes in nutrient uptake and transport, initiated stemderived adventitious roots that became aerenchymatous, and exhibited hypertrophied lenticels. Flooded red alder seedlings also showed reduced height and leaf area growth and developed lower-stem hypertrophy. Flooded black cottonwood seedlings exhibited root dieback, aerenchyma in below ground root tips, and changes in root hydraulic conductance and xylem pressure potential. Contrary to expectations, however, stomatal closure following flooding was not observed in either species. Flooded red alder seedlings increased growth rapidly when drained, and by the end of the recovery period, formerly flooded and non-flooded red alder seedlings differed only minimally in this respect. In contrast, several characteristics of black cottonwood – including growth rate and nutrient content – still differed between formerly flooded and non-flooded seedlings at the end of the recovery period. Based on observed treatment differences at the end of the experiment, red alder seedlings were judged to be more tolerant of flooding than black cottonwood.