Terrestrial Ecosystems - Isobioclimates of the Conterminous United States

The U.S. Geological Survey (USGS) has generated and mapped isobioclimate classes for the contiguous United States. These isobioclimate classes were created as part of an effort to map standardized, terrestrial ecosystems for the nation using a classification developed by NatureServe (Comer and others, 2003). Ecosystem distributions were modeled using a biophysical stratification approach developed for South America (Sayre and others, 2008) and now being implemented globally (Sayre and others, 2007). Bioclimate regimes strongly influence the differentiation and distribution of terrestrial ecosystems, and are one of the key input layers in the ecosystem delineation process. The Rivas-Martínez methodology used to produce these classes was developed from a consideration of bioclimatology and its relationship to phytogeography (Rivas-Martínez, 2004; Rivas-Martínez and others, 1999, 2004). This approach develops a number of bioclimatic indices calculated from a variety of data on temperature and precipitation (e.g. average temperature of the coldest month, total precipitation of the warmest four-month period, a continentality index and a thermicity index). Daymet data, which was developed from 18-year (1980-1997) climatological records and is available at a spatial resolution of 1 kilometer, was used as the source data for these indices (Thornton, 1997). Once calculated the values of these indices are compared to established thresholds for the differentiation of thermotypic (warm/cold gradients) and ombrotypic (wet/dry gradients) regions, and the results are used in sets of decision rules to identify classes. The classification is implemented in four levels: macrobioclimates, bioclimates, thermotypes (thermoclimatic belts) and ombrotypes (ombroclimatic belts). The final isobioclimates dataset represents areas of the 127 unique thermotype-ombrotype combinations that were mapped for the contiguous United States.美国地质调查局（U.S. Geological Survey, USGS）为美国本土生成并绘制了等生物气候类别（isobioclimate classes）制图。该类别的研发是美国标准化陆地生态系统绘制工作的一部分，采用了NatureServe（Comer等人，2003）开发的分类方案。生态系统分布的建模采用了专为南美洲开发的生物物理分层方法（biophysical stratification approach）（Sayre等人，2008），目前该方法已在全球范围内推广应用（Sayre等人，2007）。生物气候格局（bioclimate regimes）对陆地生态系统的分异与分布具有显著影响，是生态系统划界流程中的关键输入图层之一。用于生成此类别的Rivas-Martínez方法，源自对生物气候学及其与植物地理学（phytogeography）关系的研究（Rivas-Martínez，2004；Rivas-Martínez等人，1999、2004）。该方法通过温度与降水的多维度数据，计算得到多项生物气候指数，包括最冷月平均气温、最暖四个月总降水量、大陆度指数（continentality index）以及热度指数（thermicity index）。本研究的源数据来自Daymet数据集（Daymet）：该数据集基于1980-1997年共18年的气候记录生成，空间分辨率为1千米（Thornton，1997）。计算得到的指数值将与既定阈值进行比对，以区分热型（thermotypic，暖/冷梯度）与湿型（ombrotypic，湿/干梯度）区域，随后通过一系列决策规则识别对应的气候类别。该分类体系分为四个层级：宏生物气候、生物气候、热型（即热气候带，thermoclimatic belts）以及湿型（即湿气候带，ombroclimatic belts）。最终的等生物气候数据集，涵盖了为美国本土绘制的127种独特热型-湿型组合所对应的区域。