Joshua Tree Range - California [ds3020]

<div style='text-align:Left;'><div><div><p><span>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. </span></p><p><span>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. </span></p><p><span>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. </span></p></div></div></div>

约书亚树是一种形态极具辨识度的植物，分布于美国加利福尼亚州莫哈韦沙漠及周边区域。相较于周边典型植被，约书亚树独特的轮廓与高挑的身姿使其成为加州沙漠地区最易识别的本土植物之一。加州境内共有两种约书亚树：西部约书亚树（Yucca brevifolia）与东部约书亚树（Yucca jaegeriana）。本数据集仅附带呈现东部约书亚树（Yucca brevifolia ssp. jaegeriana）的分布信息，核心目标为展示西部约书亚树的分布范围。西部约书亚树以不连续种群的形式分布于莫哈韦沙漠以及大盆地沙漠的部分区域。通常而言，西部约书亚树在莫哈韦沙漠边缘的过渡地带种群较为繁茂。本数据集未尝试绘制加州境外的约书亚树分布范围，因此数据仅覆盖加州境内的地理区域。
加州鱼类与野生动物部（California Department of Fish and Wildlife，简称CDFW）拥有覆盖加州沙漠大部分区域的植被制图数据，该区域正是约书亚树的主要分布地。CDFW的植被分类与制图项目（Vegetation Classification and Mapping Program，简称VegCAMP）结合航空影像与野外调查，勾勒出植被特征相似的多边形区域，并将其归类为不同植被类型。2013年，研究团队完成了一幅覆盖加州沙漠大部分区域的植被制图。该项目获取的植被数据包含约书亚树的绝对盖度百分比，部分数据集仅包含约书亚树的存在/不存在记录。在上述植被制图中，西部约书亚树与东部约书亚树被合并为单一物种进行标注。研究团队基于野外采集的数据，对约书亚树灌丛植被联盟开展了严格的精度验证，结果表明制图准确率约为95%。这意味着，经野外验证的、被标注为约书亚树灌丛植被联盟的多边形区域中，约95%的标注结果准确无误。尽管约书亚树灌丛植被联盟要求约书亚树的盖度不低于1%，但本植被数据集仍包含盖度低于1%的约书亚树多边形记录，以及仅标记存在/不存在的数据集。CDFW将植被制图获取的约书亚树多边形数据，与标本馆馆藏记录、Calflora数据库以及iNaturalist平台等其他来源的点位数据相结合，以此生成了用于2022年3月《西部约书亚树现状评估》的西部约书亚树分布范围边界。CDFW对公开获取的、看似地理异常的点位观测数据进行了审核，以确保错误标注或有误的观测记录不会不合理地扩大该物种的推定分布范围。在部分有限区域内，研究团队通过人工数字化的方式录入点位数据：当航空影像中出现未在其他制图中被标注的明显约书亚树分布区域时，即采用此类点位。
基于不完全的存在数据绘制分布范围图有时会产生误导，因为数据缺失并不代表该物种在该区域不存在。用于生成分布范围图的部分观测数据可能年代久远——尤其是基于标本馆记录的数据——部分点位的约书亚树可能已不复存在。此外，对数据点位采用不同的缓冲区距离，会使物种栖息区域的划分结果产生巨大差异。为生成用于2022年3月《西部约书亚树现状评估》的西部约书亚树分布范围边界，CDFW对存在点位进行了缓冲区处理，但未采用固定的缓冲区数值，而是通过地理信息系统工具结合前文所述的数据，将分布范围多边形延伸至贴合已知的物种出现边界，同时消除多边形之间的微小间隙。