Population dynamics of Fraxinus nigra in response to flood-level variations, in Northwestern Quebec

Population dynamics of black ash (Fraxinus nigra) were investigated in five stands with different exposures to spring flooding at the southwestern lim it of Quebec's boreal forest. Fifty-four 100-m(2) plots were sampled along 25 transects laid out perpendicular to Lake Duparquet, In each plot, all bl ack ash >5 cm in diameter at breast height (dbh) were aged using dendrochro nological methods. An age-height regression was used to calculate the age o f those individuals <5 cm in dbh. Stump sprouts had a significantly faster height and diameter growth rate than individuals established from sexual or igin. Two stands in this study showed deficits in sexual recruitment. One s tand, located on a deep organic layer, also showed recent colonization by F . nigra that may have started during the 1930s. This stand had the highest stem density, and sprouting was restricted to the younger age classes. Ordi nation analysis of dispersion indices characterizing the age distribution o f F. nigra indicated a definite pattern toward higher sexual recruitment in plots and stands that were less exposed to flooding. In contrast, poor sex ual recruitment was observed on more exposed sites. However, this situation was compensated for by abundant vegetative recruitment, Analysis of the pe riods of growth release and suppression of 153 trees revealed that F. nigra population dynamics are not regulated by major disturbance events. A gener al upward trend in radial growth and F. nigra expansion toward higher eleva tion sites observed since the end of the 19th century may be due to changes in Lake Duparquet's hydrological regime. Increases in the severity of floo ding events may result in the decrease in seedling abundance observed in th e more exposed stands. F. nigra populations were found to be resistant to l ong-term natural water fluctuations due to their ability to regenerate both by seeds and by sprouts.