CONE SEROTINY IN JACK PINE - ONTOGENIC, POSITIONAL, AND ENVIRONMENTAL-EFFECTS

The main objective of this study was to determine empirically if the absence of serotinous (seed retaining) cones in young jack pine (Pinus banksiana Lamb.) was due to environmental causes, particularly to ground-radiated heat, or to the juvenility of the trees. Twenty-five jack pine populations were sampled in the southern part of the boreal forest, in Abitibi, Quebec. The age, diameter at breast height (DBH), and number of serotinous and nonserotinous cones per branch were determined for 30 trees per population. A subset of 10 other trees per population was felled to determine the number of serotinous and nonserotinous cones per cone age-class and per branch. DBH, age, and height of these trees were also measured. The results showed that DBH is more strongly related than age or height to the appearance of the first serotinous cone. Most trees that did not bear any serotinous cones had not reached a DBH of 7 cm. Until individuals had reached a DBH of 10 cm, die proportion of serotinous cones increased from year to year. The proportion of serotinous cones greater-than-or-equal-to 4 years old was significantly lower than that of younger cones, indicating that old cones tended to open, particularly on those trees with a DBH greater than 10 cm. The proportion of serotinous cones on branches below 260 cm was significantly lower than that of cones situated above that height. The same trends were observed for cones where serotiny was determined in the laboratory, suggesting that the thermal environment is not entirely responsible for serotinous cone opening. We suggest that the age of branches affects the opening of cones and that the absence of serotinous cones in small trees is related to their juvenility. Seed release resulting from the absence of serotiny in small jack pine trees and the opening observed in old cones may play an important role in serotinous populations when the time interval between two fires is longer than the life-span of one jack pine generation.