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Origin of Deciduous and Evergreen Habits in Temperate Forests
Daniel I. Axelrod
Vol. 20, No. 1 (Mar., 1966), pp. 1-15
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2406145
Page Count: 15
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Deciduous hardwoods first appeared at lower middle latitudes in the Northern Hemisphere during the early Cretaceous, associated with broadleafed evergreens in areas of warm temperate climate. Since deciduous hardwoods migrated to higher latitudes later in the Cretaceous, the deciduous habit presumably orginated at lower middle latitudes marginal to the tropical zone. Evidence suggests that initially the deciduous habit may have been an adaptation to moderate drought in the cooler part of the year, a relation that still exists in the climates of pronounced temperateness (Taiwan, southern Mexico, Yunnan) that support forests most like those of the late Cretaceous and early Tertiary. Evolution of the deciduous habit appears to provide an example of preadaptation and zonal evolution. By it, deciduous hardwoods of ultimately moist tropical to warm temperate derivation were able to live (a) in more intemperate ("continental") climates in both moist and dry regions, (b) in cool climates at higher latitudes which have marked photoperiodicity, and (c) in the cold to cool temperate subfreezing climates which appeared late in the Cenozoic. Deciduous hardwoods are essentially absent at middle to higher southern latitudes, regions where broadleafed evergreen forests have dominated throughout recorded angiosperm history. The time-space relations of austral Cretaceous angiosperm floras show that broadleafed evergreens migrated from low into high southern latitudes, indicating a source at lower warmer latitudes. Although deciduous hardwoods evolved in climates with a dry season marginal to the tropical zone in the Southern Hemisphere, southward migration in the Cretaceous and Tertiary may not have been favored by the moist, cloudy, rainforest climate of pronounced temperateness that seems to have resulted from the smaller land areas there as compared with boreal regions. Broadleafed evergreens evidently were more highly adapted to these climates which were not greatly unlike those in which they appear to have evolved, and they still inhabit them, ranging to the limit of tree growth in southern latitudes.
Evolution © 1966 Society for the Study of Evolution