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Evolution and Biogeography of Madrean-Tethyan Sclerophyll Vegetation

Daniel I. Axelrod
Annals of the Missouri Botanical Garden
Vol. 62, No. 2 (1975), pp. 280-334
DOI: 10.2307/2395199
Stable URL: http://www.jstor.org/stable/2395199
Page Count: 55
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Evolution and Biogeography of Madrean-Tethyan Sclerophyll Vegetation
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Abstract

Broadleaved evergreen sclerophyllous taxa occupied a subhumid belt across much of North America-Eurasia by the middle Eocene. They originated from alliances in older laurophyllous forests that adapted to spreading dry climate. Since the continued trend to aridity finally restricted sclerophyllous vegetation to subhumid areas separated by drier tracts, it now occurs in areas with summer rain as well as in summer-dry mediterranean climates. Taxa of chaparral and macchia habit are common undershrubs in sclerophyll woodlands, to which they are seral. Shrublands spread only recently, though the adaptive structural features of the taxa are ancient and probably not pyrogenic. The history of Madrean-Tethyan sclerophyll vegetation illuminates three biogeographic problems. First, related taxa that link the Mediterranean-California areas are part of the larger problem of ties between these areas and those of summer rainfall, of taxa now in summer-rain areas that were in presently summer-dry areas into the early Pleistocene, and of the more numerous taxa that linked sclerophyllous vegetation of the Madrean-Tethyan regions during the Tertiary. The ties between summer-dry and summer-wet areas are relicts of the Neogene; taxa now in mediterranean-climate areas adapted functionally to these new climates during the Pleistocene; and most trans-Atlantic links owe to migration across a narrower ocean with more numerous islands, to a broader zone of subhumid climate, and to a more easterly trending Appalachian axis with numerous dry edaphic sites. Second, by the mid-Oligocene spreading dry climate had confined a formerly continuous temperate rainforest to southern Mexico, the West coast and the Appalachian area. Winter cold and summer drought exterminated it in the West, whereas in the East winter cold eliminated most evergreen dicots, leaving a dominantly deciduous hardwood forest there. The temperate "Appalachian disjuncts" in southern Mexico are therefore ancient, and did not migrate south to enter a forest previously without deciduous hardwoods, as others maintain. Third, the Canarian laurel forest derived its taxa from those in laurophyllous forests that covered northern Africa into the middle Miocene, not by southward migration from southern Europe in the Pliocene. Since many shrubs in the surviving laurel forest also contribute to macchia on bordering slopes, the ancient origin of their typical adaptive structural features is clearly implied.

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