Evolution and Biogeography of Madrean-Tethyan Sclerophyll Vegetation

Axelrod, D.

Broadleaved evergreen sclerophyllous taxa occupied a subhumid belt across much of North American-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 with summer rain as well as in summer-dry mediterranean climates. Taxa of chaparal and macchia habit are common underschrubs 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 all these new climates during the Pleistocene; and most trans-Atlantic links owe to migration across a narrower ocean with more numberous islands, to a broader zone of subhumid climate, and to a more easterly trending Appalacian 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, wheras in the East winter cold eliminated most everegreen dicots, leaving a dominantly deciduous hardwood forest there. The temperate 'Appallachian 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.

Evolution and Biogeography of Madrean-Tethyan Sclerophyll Vegetation

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