culture. The Center for American Archeology specializes in Middle Woodland culture. Late Woodland period (500–1000 CE) The late Woodland period was a time of apparent population dispersal, although populations do not appear to have decreased. In most areas construction of burial mounds decreased markedly, as did long-distance trade in exotic materials. At the same time, bow and arrow technology gradually overtook the use of the spear and atlatl. Agricultural production of the "Three Sisters" (maize, beans, and squash) was introduced. While full-scale intensive agriculture did not begin until the following Mississippian period, the beginning of serious cultivation greatly supplemented the traditional gathering of plants. Late Woodland settlements became more numerous, but the size of each one (with exceptions) was smaller than their Middle Woodland counterparts. The reasons for this are unknown, but it has been theorized that populations increased so much that trade alone could no longer support the communities and some clans resorted to raiding others for resources. Alternatively, the efficiency of bows and arrows in hunting may have decimated the large game animals, forcing the tribes to break apart into smaller clans to better use local resources, thus limiting the trade potential of each group. A third possibility is a colder climate may have affected food yields, possibly affected by Northern Hemisphere extreme weather events of 535–536, also limiting trade possibilities. Lastly, it may be that agricultural technology became sophisticated enough that crop variation between clans lessened, thereby decreasing the need for trade. As communities became more isolated, they began to develop in their own unique ways, giving rise to small-scale cultures that were distinctive to their regional areas. Examples include the Baytown, Troyville and Coles Creek cultures of Louisiana; the Alachua and Weeden Island cultures of Florida; and the Plum Bayou culture of Arkansas and Missouri. Although the 1000 CE ending of the Late Woodland period is traditional, in practice many regions of the Eastern Woodlands adopted the full Mississippian culture much later than that. Some groups in the north and northeast of the current United States, such as the Iroquois, retained a way of life that was technologically identical to the Late Woodland until the arrival of Europeans. Despite the widespread adoption of the bow and arrow during this time, the peoples of a few areas appear never to have made the change. During Hernando de Soto's travels through the Southeastern Woodlands around 1543, for instance,