Joshua Tree Range - California [ds3020]

Joshua tree is a visually distinctive plant found in California’s Mojave Desert and adjacent areas. The unique silhouette and tall stature of Joshua tree relative to typical surrounding vegetation make it one of the most recognizable native plants of California deserts. There are two species of Joshua tree in California, western Joshua Tree (Yucca brevifolia) and eastern Joshua tree (Yucca jaegeriana). Eastern Joshua tree (Yucca brevifolia ssp. jaegeriana) distribution is represented in the data incidentally, but the primary purpose of this dataset is to illustrate the distribution of western Joshua tree. Western Joshua tree is distributed in discontinuous populations in the Mojave Desert and in a portion of the Great Basin Desert. Western Joshua tree is often noted as being abundant near the borders of the Mojave Desert in transition zones. No attempt was made to map Joshua tree distribution outside of California, and therefore the data are limited to geographic areas within California. CDFW possesses vegetation maps that cover a large portion of the California deserts where Joshua tree generally occurs. CDFWs Vegetation Classification and Mapping Program (VegCAMP) uses a combination of aerial imagery and fieldwork to delineate polygons with similar vegetation and to categorize the polygons into vegetation types. In 2013, an effort was made to create a vegetation map that covers a large portion of the California deserts. The vegetation data from this project includes percent absolute cover of Joshua tree and in some instances only Joshua tree presence and absence data. Western Joshua tree and eastern Joshua tree were lumped together as one species in these vegetation maps. A rigorous accuracy assessment of Joshua tree woodland vegetation alliance was performed using field collected data and it was determined to be mapped with approximately 95 percent accuracy. This means that approximately 95 percent of field-verified, polygons mapped as Joshua tree woodland alliance were mapped correctly. While Joshua tree woodland alliance requires even cover of Joshua tree at greater than or equal to 1 percent, the vegetation dataset has polygons recorded with less than 1 percent cover of Joshua tree as well as simple presence and absence data. The CDFW used Joshua tree polygons from vegetation mapping combined with additional point data from other sources including herbarium records, Calflora, and iNaturalist to create the western Joshua tree range boundary used in the March 2022 Status Review of Western Joshua Tree. CDFW reviewed publicly available point observations that appeared to be geographic outliers to ensure that incorrectly mapped and erroneous observations did not substantially expand the presumed range of the species. In a limited region, hand digitized points were used where obvious Joshua tree occurrences that had not been mapped elsewhere were present on aerial photographs. Creating a range map with incomplete presence data can sometimes be misleading because the absence of data does not necessarily mean the absence of the species. Some of the observations used to produce the range map may also be old, particularly if they are based on herbarium records, and trees may no longer be present in some locations. Additionally, different buffer distances around data points can yield wildly different results for occupied areas. To create the the western Joshua tree range boundary used in the March 2022 Status Review of Western Joshua Tree, CDFW buffered presence locations, but did not use a specific buffer value, and instead used the data described above in a geographic information system exercise to extend the range polygons to closely follow known occurrence boundaries while eliminating small gaps between them.约书亚树（Joshua tree）是一种视觉特征鲜明的植物，分布于美国加利福尼亚州莫哈韦沙漠及毗邻区域。相较于周边典型植被，其独特的轮廓与高挑株型使其成为加州沙漠地区最具辨识度的本土植物之一。加利福尼亚州境内分布有两种约书亚树：西约书亚树（Yucca brevifolia）与东约书亚树（Yucca jaegeriana）。其中东约书亚树（Yucca brevifolia ssp. jaegeriana）的分布仅在本数据中附带体现，本数据集的核心目的为展示西约书亚树的分布范围。西约书亚树的种群呈不连续分布，见于莫哈韦沙漠与大盆地沙漠的部分区域，在莫哈韦沙漠边界的过渡地带，其种群通常较为繁盛。本数据集未对加州以外区域的约书亚树分布进行制图，因此数据范围仅限加州境内。加利福尼亚州鱼类与野生动物部（California Department of Fish and Wildlife, CDFW）拥有覆盖加州多数沙漠区域的植被制图数据，而约书亚树的主要分布区正涵盖其中。该部门的植被分类与制图项目（Vegetation Classification and Mapping Program, VegCAMP）结合航空影像与野外实地调查，勾绘出植被特征一致的多边形区域，并将其归类为不同植被类型。2013年，相关团队完成了覆盖加州多数沙漠区域的植被制图工作。该项目获取的植被数据包含约书亚树的绝对盖度百分比，部分区域仅记录了约书亚树的存在与缺失情况。在上述植被制图中，西约书亚树与东约书亚树被合并为单一物种进行标注。研究团队利用野外采集数据，对约书亚树灌丛植被群丛（Joshua tree woodland vegetation alliance）的制图结果开展了严格的精度验证，结果显示其制图准确率约为95%——即95%经野外核实、被标注为约书亚树灌丛群丛的多边形区域均绘制准确。需注意的是，约书亚树灌丛群丛要求约书亚树的盖度≥1%，但本植被数据集中同时收录了盖度<1%的多边形区域，以及仅记录存在与缺失情况的数据集。CDFW结合植被制图获取的约书亚树多边形数据，与标本馆记录、Calflora、iNaturalist等其他来源的点位数据，共同构建了用于2022年3月西约书亚树现状评估的分布范围边界。CDFW对公开的点位观测数据进行了核查，剔除地理异常值，以避免错误标注与无效观测结果不当扩大该物种的推测分布范围。在部分有限区域内，当航空影像中存在未被此前制图覆盖的明确约书亚树分布点时，研究团队采用手动数字化的点位数据进行补充。由于存在数据不全的情况，构建分布范围图有时可能产生误导：数据未覆盖区域并不一定代表该物种不存在。此外，用于构建分布范围图的部分观测数据可能较为老旧（尤其是基于标本馆记录的观测），部分点位的约书亚树可能已不复存在。同时，为数据点设置不同的缓冲距离，可能会对物种占据区域的测算结果产生极大差异。为构建2022年3月西约书亚树现状评估所用的分布范围边界，CDFW对存在点位进行了缓冲处理，但未采用固定的缓冲值，而是通过地理信息系统分析工作，结合前述数据将分布范围多边形延伸至贴合已知的分布边界，同时消除其间的微小间隙。