In situ forcing of pollen maturation in Jack pine and Japanese larch male cones

Paper bags equipped with a polyethylene window were placed on branchlets 6 and 9 weeks before the time of pollen dispersal of Japanese larch and Jack pine, respectively. This technique produced a localized greenhouse effect around the branchlets, accelerating male cone development. For Jack pine trees, buds in bags with the window oriented upward had the shortest maturation time. In fact, it was possible to collect these pollen cones about 11 days before the unbagged control without decreasing the production of pollen. The mean germination rate of the pollen from the bags with the window pointed upward, however, was significantly lower than the control, by about 15%. Buds in bags with the window oriented downward had an accelerated development of about 9 days (compared to the control) but did not experience a decrease in pollen production or quality. This same technique caused the abortion of male cones of the Japanese larch trees. Thermal probes in the bags enabled us to follow the progression of temperature inside and outside the bags during the development of the male cone in each species. They revealed that the bags increased mean temperature, daily temperature range, and degree-day accumulation, and that this effect was most pronounced for Jack pine, when the polyethylene window was oriented upward.