Ecological resilience and resistance to cheatgrass (Bromus tectorum) invasion in southwestern U.S. drylands: 1980-2019

Spatially explicit and climate sensitive information on the responses of ecosystems to global change processes is required for strategic planning and prioritization of conservation and restoration. Indicators of ecological resilience and invasion resistance (R&R) are used increasingly to understand the influence of climate regimes on ecosystem responses to global change processes and determine effective management strategies. We developed multiclass indicators of R&R based on proxy soil temperature and moisture regimes, or climate regimes, for southwestern U.S. drylands that accounted for precipitation seasonality and climate change at a scale of 1 kilometer (km) for the time period from 1980-2019. The climate variables that we used, mean annual temperature, precipitation, and monsoon index, allowed us to construct climate sensitive and ecologically meaningful climate regimes that mirrored the soil temperature and moisture regimes in the U.S. National Cooperative Soil Survey data and Ecological Site Descriptions that are widely used by managers in the western U.S. The climate regimes reflected the dominant ecological types and plant functional groups and allowed us to develop meaningful indicators of R&R. Particularly important was identification of a monsoon index value (0.30) to delineate areas with an ustic (summer moist) precipitation regime as indicated by the frequency of C4 vs. C3 grass species. The climate regimes included four temperature regimes (cryic, frigid, mesic, and hypermesic), both ustic and xeric/aridic moisture regimes, and multiple moisture subclasses based on average precipitation zones (PZ). These climate regimes were used to develop two spatial data layers (rasters in GeoTIFF format) representing R&R for the time period 1980-2019.Previous indicators of ecological resilience and resistance to invasion (R&R) focused on the sagebrush biome and relied on soil temperature and moisture regimes mapped as part of the U.S. National Cooperative Soil Survey or climate and water availability indicators derived from ecohydrological process models. However, proxy soil temperature and moisture regimes (climate regimes), and their associated R&R indicators, based on atmospheric climate variables are likely preferable to those based on soil climate because (1) soil temperature and moisture regimes from the Soil Survey are static and challenges exist in evaluating climate change effects based on these indicators, (2) discontinuities exist in soil map units across state boundaries, and (3) many areas, notably some U.S. National Forests, do not yet have published soil surveys. The finer-scale climate regimes and multiclass R&R indicators that we developed allowed us to account for precipitation seasonality and thus to provide more accurate information on the likely responses to disturbances and climate suitability to the widespread invasive annual, cheatgrass (Bromus tectorum), across the southwest drylands. We expect the climate regimes and R&R indicators will provide valuable information for strategic planning and prioritization and will be useful to develop prioritization strategies for fire prevention and management, invasive species management, conservation of species habitat, and to evaluate restoration success.For more information about these data, see Chambers et al. (in review).

为了战略规划和优先考虑生态系统对全球变化过程的响应，以及保护和恢复的优先级，需要具有空间明确性和气候敏感性的信息。生态韧性及抗入侵性（R&R）指标日益被应用于理解气候体系对生态系统响应全球变化过程的影响，并确定有效的管理策略。我们基于代用土壤温度和湿度体系，或气候体系，针对美国西南干旱地区，开发了R&R的多级指标，这些指标考虑了降水季节性和气候变化，其时间跨度为1980年至2019年，尺度为1公里。我们使用的气候变量，包括年平均温度、降水量和季风指数，使我们能够构建出对土壤温度和湿度体系具有敏感性和生态意义的气候体系，这些气候体系与在美国国家合作土壤调查数据和生态区描述中广泛应用的土壤温度和湿度体系相呼应。气候体系反映了主要的生态类型和植物功能群，并使我们能够开发出有意义的R&R指标。尤其是识别出季风指数值（0.30），以划分具有ustic（夏季湿润）降水体系的区域，这由C4与C3草本植物种类的频率来指示。气候体系包括四种温度体系（寒带、冷带、温带和超温带）、ustic和xeric/aridic湿度体系，以及基于平均降水区（PZ）的多个湿度子类。这些气候体系被用于开发两个空间数据层（GeoTIFF格式的栅格），代表1980-2019年期间的R&R。先前关于生态韧性和抗入侵性（R&R）的指标主要关注蒿草生物群落，并依赖于美国国家合作土壤调查中的土壤温度和湿度体系，或从生态水文过程模型中得出的气候和水可用性指标。然而，基于大气气候变量的代用土壤温度和湿度体系（气候体系）及其相关的R&R指标，可能比基于土壤气候的指标更可取，原因如下：（1）土壤调查中的土壤温度和湿度体系是静态的，基于这些指标评估气候变化效应存在挑战；（2）土壤图单元在州界之间存在不连续性；（3）许多地区，特别是美国一些国家森林，尚未发布土壤调查。我们开发的更精细尺度的气候体系和多级R&R指标，使我们能够考虑降水季节性，从而提供更准确的关于对干扰的潜在响应和气候适宜性的信息，特别是在西南干旱地区的广泛入侵性一年生植物，例如赖草（Bromus tectorum）。我们预期气候体系和R&R指标将为战略规划和优先考虑提供宝贵信息，并将有助于制定火灾预防和管理的优先策略，入侵物种管理，物种栖息地保护，以及评估恢复成功。有关这些数据的更多信息，请参阅Chambers等（待审稿）。