Authors: G.T. Creber, J.E. Francis Keywords: Antarctica, arctic, Glossopteris, greenhouse climate, high latitude, palaeobotany, photosynthesis, photoperiodism, polar light regime, secondary wood Abstract:
For many millions of years in the geological past, Earth was in a ‘greenhouse’ condition when the tropical zone was more extensive than now, extending from about 40° N to 40° S. Within this zone the fossil wood shows no growth rings or only weakly developed ones, as in the tropics today. The temperate zones occupied the remainder of the planet reaching the poles to the exclusion of the present day boreal zones. Consequently, abundant tree growth was possible at the highest latitudes where the annual solar energy input regime is very different from those at lower latitudes. Towards the poles the growing season has 4380h of solar energy input, much of it being received in 24h periods of continuous daylight. That this makes for excellent tree growth is evidenced by the wide growth rings, containing as many as 200 cells between one ring boundary and the next, as recorded in the fossil wood. It is clear that if efforts to prevent global warming are not successful, then the polar regions will become very important for forestry as the ice caps recede. Furthermore there will be a positive feedback effect; as the albedo of tree crowns is so much greater than that of ice, much more solar energy will be retained in the atmosphere.
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