SEED DYNAMICS OF BETULA-ALLEGHANIENSIS IN A DECIDUOUS FOREST OF NORTH-EASTERN NORTH-AMERICA

For yellow birch in a sugar maple-yellow birch forest near Quebec, no viable seeds were shed before the end of August in 1988, although by then the cumulative seed rain totalled 222 ± 103 seeds m-2. Viability of seeds in the seed rain was low in August 1988 (3%), but increased through the season to a peak at the end of October (74.4%), at the time of maximum seedfall (7377 ± 99 seeds m-2 wk-1). Seed dispersal of the 1988 seed crop continued until the following spring and early summer. Seedfall during the 1988-89 winter period accounted for 24% of the viable seeds disseminated from August 1988 to July 1989. Autumn 1989 seed rain was 15 times lower than of autumn 1988. Seed viability was significantly lower in autumn 1989 than in autumn 1988 (20% and 53%, respectively). Seeds were contagiously dispersed over the site. Seasonal dynamics of the seed bank mostly involved the litter layer. Seed number (in 1988) in the litter layer declined from June (192 ± 26 viable seeds m-2) through July (107 ± 18 viable seeds m-2) to August (77 ± 14 viable seeds m-2), but increased in October (1297 ± 143 viable seeds m -2) as a result of inputs from the seed rain. Newly emerged seedlings were more numerous on the study site in autumn 1989 than in autumn 1988. In both 1988 and 1989, seedlings were contagiously dispersed. Further, 1988 and 1989 spatial patterns for newly emerged seedlings were similar, related to specific microsite requirements for successful seedling establishment. At the beginning of the 1988 season, there were 828 ± 45 viable seeds m-2 in the top 15 cm of soil. By the end of October 1988, there were 1859 ± 162 viable seeds m-2, although by then the cumulative seed rain amounted to 2073 ± 160 viable seeds m-2. Successful seedling establishment accounted for <1% of the seasonal deficit of 1042 viable seeds m-2, this implies that >99% of the seed losses from the seed bank were due to predation, attack by pathogens, depletion of seed reserves and unsuccessful seedling establishment. -from Authors