Flowering and seed production of aspen in Scotland during a heavy seed year

Abstract

In 1996, profuse flowering of aspen ( Populus tremula L. occurred throughout Scotland. In response, studies were initiated in Perthshire and Strathspey to investigate aspects of flowering, seed production and seed storage of aspen. The proportions of clones flowering were high, averaging 72 per cent in Strathspey and 77 per cent in Perthshire. The sex ratios were 1.8 male clones to 1 female clone in Strathspey and 1.3 male clones to 1 female clone in Perthshire, which are within the range of ratios recorded for aspen on the Continent. Easy and reliable methods of sexing aspen trees in the field during flowering are described. Seed production per catkin on four female clones in Tayside ranged from six to 60 seeds per catkin. Seed production was clearly controlled with the number of male clones in the vicinity of the female clones. A simple field technique for artificially pollinating female flowers is described. Artificial pollination resulted in seed production averaging between 159 and 460 seeds per catkin, i.e. approximately a 12-fold increase over natural pollination. The viability of seed from the two study areas was very high ranging from 94 to 98 percent. The numbers of seed per kilogram varied between 7 million and 10 million viable seeds kg -1 . Seed stored at 6 per cent moisture content following rapid drying, and at -2°C, retained levels of viability in excess of 95 per cent for at least one year. The results present excellent opportunities for increasing the supply of planting stocks of aspen from Scottish seed and fundamentally change current perceptions of the ecological status of aspen in Scotland.