COMPARATIVE FLOOD TOLERANCE OF BIRCH ROOTSTOCKS

The potential for enhancing flood tolerance of birches by using better adapted rootstocks was evaluated. Survival, growth, and physiological responses were compared among flooded and nonflooded container-grown Japanese birch (Betula platyphylla var. japonica Hara. 'Whitespire') t rees grafted onto each of four rootstocks: paper birch (B. papyrifera Marsh), European birch (B.pendula Roth.), river birch (B. nigra L.), a nd 'Whitespire' Japanese birch. Separate studies were conducted in Fal l 1991 and Spring 1992. Results showed no consistent differences in ne t photosynthesis (P(n)) or survival among nonflooded plants regardless of rootstock or season, nor were any symptoms of graft incompatibilit y evident. Flooding the root system for as long as 44 days revealed co nsiderable differences among the four rootstocks, with similar trends for fall and Spring. Plants on river birch rootstocks typically had on e of the highest P(n) rates and stomatal conductance (g(s)) and, in ce rtain cases, greater mean shoot growth rates and survival of plants su bjected to prolonged flooding. Although plants with European birch roo tstocks had survival rates similar to those of plants with river birch rootstocks, plants on European birch rootstocks had lower P(n) under prolonged flooding, fewer late-formed roots, lower root-tip density af ter flooding, more abscissed leaves, and greater inhibition of shoot g rowth of plants flooded the previous fall. Paper and Japanese birch ro otstocks were most sensitive to flooding and had the lowest survival r ate after flooding. However, plants on paper birch rootstocks were the only plants whose P(n) did not increase significantly when flooding e nded; they had the most abscissed leaves during spring flooding and th e greatest inhibition of shoot growth in the spring after flooding the previous fall. The four rootstocks ranked from most to least flood to lerant were river > European > Japanese > paper.