Areas of endemism of selected seed plants in southcentral/southwestern USA

Areas of endemism (AEs) are fundamental entities of analysis in biogeography and a key step for biogeographical regionalization. Even though many studies have contributed to the biogeographical knowledge of southern USA flora, no endemicity analysis (EA) has been conducted that would include a large number of native seed plant species from different families. A new analysis of plant spatial patterns is important as a first step for a future updated floristic regionalization of North America North of Mexico. It has become easier to accomplish owing to the increased availability of large-scale digitized distributional data and statistical methods of biogeographic analysis. Here we identify the AEs in SC/SW USA using digitized plant specimen data available from IDigBio. We built a database with 81,851-specimen point records of 400 selected mostly angiosperm species and applied the NDM/VNDM method of endemicity analysis. We then compare the established 26 AEs in the area of study with the floristic provinces in two comparatively recent regionalization systems of USA. To understand the spatial patterns, we pay some attention to the information on relationships of the endemic species found in phylogenetic literature.

Methods
Digitized herbarium data were downloaded from the Integrated Digitized Biocollections (iDigBio accessed January –April 2020), cleaned to eliminate duplicates and other erroneous records, and contrasted with those documented by the Biota of North America Project (BONAP; https://bonap.org, accessed April–May 2020), online databases (EOL: https://eol.org/), and literature (Turner at al., 2003; FNA).

特有分布区（Areas of endemism, AEs）是生物地理学研究的核心分析单元，同时也是生物地理分区工作的关键环节。尽管已有诸多研究增进了学界对美国南部植物区系的生物地理认知，但目前尚未有涵盖多科大量本土种子植物物种的特有性分析（Endemicity Analysis, EA）相关研究。作为未来更新墨西哥以北北美植物区系分区的首要步骤，开展全新的植物空间格局分析具有重要学术价值。得益于大规模数字化分布数据的日益丰富与生物地理分析统计方法的成熟，此类研究的实施门槛已大幅降低。本研究依托整合数字化生物标本馆藏（Integrated Digitized Biocollections, IDigBio）公开的数字化植物标本数据，对美国南部与西南部（SC/SW USA）的特有分布区进行识别。我们构建了包含400个选定物种（以被子植物为主）的81851条标本点位记录数据库，并应用特有性分析的NDM/VNDM方法开展研究。随后，将本研究划定的26个特有分布区与美国两套近年发布的生物地理分区系统中的植物区系省进行对比。为深入解析空间格局，本研究还参考了系统发育文献中关于特有物种亲缘关系的相关记载。

方法
本研究的数字化标本馆数据下载自整合数字化生物标本馆藏（iDigBio，2020年1—4月访问），经清洗以去除重复记录与其他错误条目，并与北美生物群项目（Biota of North America Project, BONAP; https://bonap.org，2020年4—5月访问）、在线数据库（生命百科全书Encyclopedia of Life, EOL: https://eol.org/）及相关文献（Turner等，2003；北美植物志Flora of North America, FNA）中的记录进行比对校核。