Bioclimatic classification of US vegetation along a coast-to-coast macrotransect crossing central United States. I: Mediterranean vegetation

This report presents the first of two parts of a bioclimatic classification of the vegetation of the United States. Using a geographical information system, 987 weather stations were located along a longitudinal macrotransect from the shores of the Atlantic to Pacific on four maps: Map of the Physiographic Divisions of the Conterminous US, US Potential Natural Vegetation Map, US Ecoregion Map, and Terrestrial Ecosystems-Isobioclimates Map of the Conterminous United States. Based on these maps, bibliographic resources and field data, we deduced the potential natural vegetation (PNV) of each weather station; then, we assigned the different PNV types to alliance or association levels using the US National Vegetation Classification (USNVC). In a next step, USNVC groups were related with similar level phytosociological syntaxa described in the study area. The bioclimatic distribution of the USNVC units defined was then interpreted using the bioclimatic classification proposed in successive approximations by S. Rivas-Martínez. The distribution of USNVC alliances was mainly linked to the macrobioclimates (Mediterranean, Temperate, and Tropical) of the longitudinal gradient examined, though some edaphic factors induced the appearance of specialized plant groups. Herein, we present our data for the Mediterranean macrobioclimate, in which 53 alliances and 28 isobioclimates were identified.

本报告介绍了美国植被生物气候分类两部分内容的第一部分。本研究借助地理信息系统（Geographic Information System, GIS），沿大西洋至太平洋的纵向大样带，在四张地图的基础上布设了987个气象站点，分别为：美国本土地貌分区图、美国潜在自然植被（Potential Natural Vegetation, PNV）图、美国生态区地图以及美国本土陆地生态系统-等生物气候图。基于上述地图、文献资料与野外实测数据，我们推导得到了每个气象站点对应的潜在自然植被；随后依据美国国家植被分类系统（US National Vegetation Classification, USNVC），将各类PNV类型归至群丛联盟（alliance）或群丛（association）分类等级。后续步骤中，我们将USNVC分类单元与研究区内已记载的同等级植物社会学句法单元进行关联匹配。借助S. Rivas-Martínez提出的逐次逼近式生物气候分类法，对所定义的USNVC单元的生物气候分布特征开展解译。USNVC群丛联盟的分布主要与本次研究考察的纵向梯度下的三大类大生物气候带（地中海气候带、温带气候带与热带气候带）紧密相关，但部分土壤因子亦可催生特化植物类群。本报告展示了地中海大生物气候带的相关研究数据，该区域中共识别出53个群丛联盟与28个等生物气候单元。