For this study, we wanted to evaluate the reproductive potential of northern red maple (Acer rubrum L.) populations to identify the possible factors responsible for the scattered distribution pattern of these northern populations.
Samara production and long-term establishment of seedlings were observed along a north-south transect crossing the transition zone between continuous and discontinuous stands of red maple (47°80'-49°27' N) in western Quebec.
Eleven populations of red maple were selected along a latitudinal gradient extending to the northern limit of the species. Seed traps were placed in each stand and distributed under the canopy of mature red maple trees. Seed abundance was tracked for 6 years from 1988 to 1993. Phenological observations were made in 1992 and 1993 at Roquemaure (Roq), a site located at the centre of the latitudinal gradient. Red maple trees were randomly selected within the population; counts of flower buds, pollinated buds and samaras produced were made in 1992-93. Samaras were collected from each branch immediately before dispersal and counted. During the summer of 1987, seedlings (< 1 cm d.b.h.) were collected and aged at each site in twenty 1 m2 quadrants and age of the seedlings (< 1 cm d.b.h.) was determined by counting the annual scars left by terminal buds.
Samaras were produced even at the northern limit but large yearly variations were observed. Over the 6-year period we counted 3 years (1989, 1990, 1993) when samara production was high, and 3 years (1988, 1991, 1992) when production was low. Phenological observations indicate that the occurrence of spring frosts at the time of flower bud flushing could contribute to decreasing the abundance of seeds. The age structure of southern localities had a relatively constant production of seedlings, as indicated by an inverse J-shaped distribution. However, the five northernmost localities show sporadic recruitment.
Populations at the northern limit are maintained essentially through vegetative reproduction and infrequent sexual recruitment. Our results indicate that regeneration within established stands through sexual recruitment is possible in all of the populations we studied. This potential becomes very low at more northerly sites and sexual reproduction alone would be unlikely to ensure successful stand regeneration. Without major disturbances in those stands, shade tolerant conifer species such as balsam fir (Abies balsamea) or black spruce (Picea mariana) would readily dominate the canopy. The discontinuous distribution of red maple stands at the northern limit is the consequence of either a random colonization of few sites during a better climatic period or remnants of a much larger distribution that has been constrained because of climatic deterioration.