Physiological and morphological responses of red alder and sitka alder to flooding

Red alder (Alnus rubra Bong.) and sitka alder (A. viridis ssp. sinuata Regel] Löve & Löve) are nitrogen-fixing woody species that grow sympatrically along the Pacific coast of North America. Red alder is found in poorly drained lowlands, as well as in soils of moist upland slopes, whereas sitka alder generally colonizes well-drained soils. To identify factors that contribute to flood tolerance, we conducted greenhouse experiments subjecting both species to a 20-day flood and 10-day recovery and red alder to a 50-day flood and 20-day recovery. We determined the effect of this stress on nitrogenase activity, root and nodule alcohol dehydrogenase (ADH) activity, lenticel and adventitious root development, relative growth rate (RGR), and leaf gas exchange. After 24 h of flooding, nitrogenase activity could not be detected in either species. Limited nitrogenase activity did return in red alder at the end of a 10-day recovery following the 20-day flood, but sitka alder showed no recovery of nitrogenase activity. After 50 days of continuous flooding, red alder nitrogenase activity returned to pretreatment levels. Red alder root and nodule ADH activity was more than twice that of sitka alder under flooded conditions. Sitka alder showed extensive root mortality and leaf abscission over the same 20-day flooding period. Flooded red alder exhibited an initial decline in root RGR, but recovered between days 10 and 20 with the formation of adventitious roots. Furthermore, initiation of adventitious roots in red alder coincided with an increase in stomatal conductance without a similar recovery of carbon dioxide exchange rate. Sitka alder formed few adventitious roots, lost much of its root and leaf biomass, and showed no restoration of growth during flooding or recovery. Different responses of red and sitka alder to flooding serve as a partial explanation for the different patterns of distribution of these species and suggest some adaptations of red alder that permit flood tolerance.