Responses of some Southern Hemisphere tree species to a prolonged dark period and their implications for high-latitude Cretaceous and Tertiary floras

Abstract

Seedlings of some Southern Hemisphere tree species were grown under experimental conditions to investigate their tolerance of long periods of darkness. The taxa used have either a fossil record at high latitudes during the Cretaceous or early Tertiary, or their biogeography suggests that they occurred at high latitudes during this period. The plants were subjected to an approximation of cool/cold, dark polar winters (10 weeks at 4°C) and warm, dark polar winters (15°C). These were compared with plants grown under natural Hobart winter daylengths at 15°C. Most of the plants were able to tolerate this period of darkness. In general, tissue death was less in the 4°C dark treatment than in the 15°C dark treatment. Fourteen of the sixteen species examined survived the 4°C dark treatment with relatively little damage. No significant deterioration in chlorophyll levels was detected at the end of the dark treatment in the plants tested. A negative correlation was recorded between tissue damage of dark-treatment plants and starch levels of light-treatment plants of the same species. There was a weak positive correlation between tissue damage and the dark respiration rate in evergreen species. These preliminary results have several implications for understanding plant survival at high latitudes during the Cretaceous and Tertiary. Firstly, the tolerance of this experimental dark period suggests that woody plants could have survived natural prolonged dark periods at this time, but with a greater tolerance of cool-cold winters than of mild winters. Secondly, the differential tolerance shown by these species suggests that the long dark winters could have acted as a filter of species movements between the high latitude regions of Gondwana. Thirdly, the lengthening winter days as the Australian Plate moved northwards into lower latitudes during the Tertiary may have influenced the decline ofspecies and species habits, e.g. deciduousness, that were highly suited to survival of polar winters.