Climate Change Vulnerability Index: Ecosystems

Climate Change Exposure – This includes climate change stress and its likely effects on ecosystem-specific processes. These analyses include the identification of baseline climate conditions, including historical climatic variability, and future climate projections, including their departure from historical conditions. Where possible, changing climate variables are connected to dynamic processes, such as wildfire, hydrology, and sea level rise effects.

Climate Change Resilience – This includes the capacity of a system to absorb disturbance and reorganize while undergoing change to still retain essentially the same function, structure, identity, and feedback. For the HCCVI framework, we have broken measures into two categories: Sensitivity and Adaptive Capacity.

Sensitivity – This includes human alterations affecting ecological resilience. Analyses consider alterations to natural patterns and processes, such as landscape fragmentation, effects of invasive species, or human alterations to dynamic processes. These human alterations are considered here independently of climate change but likely interact with changing climate. These analyses could also include a temporal dimension, considering both legacies of past land use along with current or predicted future conditions.

Adaptive Capacity – This includes natural characteristics that affect the potential for resilience in a changing climate. Analyses consider aspects of natural species composition, such as the relative diversity within groups of species that provide critical functional roles (like nitrogen-fixing species), or the relative vulnerabilities of individual species that provide “keystone” functions (“ecological engineers” like prairie dog or beaver). They also consider natural variability in microclimate or geophysical diversity and how that varies across the distribution of a given natural community type.

Inter-Mountain Basins Big Sagebrush Shrubland occurs throughout much of the western U.S., typically in broad basins between mountain ranges, plains, and foothills between 800 and 2500 m elevation. Soils are typically deep, well-drained and non-saline. These shrublands are dominated by basin big sagebrush and Wyoming big sagebrush, with grasses like bluebunch wheatgrass, Indian ricegrass, blue grama, Idaho fescue, or needle and thread grass. In these landscapes, temperatures fluctuate widely each day, and across seasons, and much of annual precipitation falls as snow, with frequent growing-season drought.

Ecological interactions of climate, herbivory, and wildfire result in complex patterns of vegetation structure and species composition. Increasing fire frequency and severity can eliminate shrubs from the stands, as big sagebrush does not sprout after fire. Conversely, in the complete absence of fire, deep-rooted shrubs can shade out native grasses. At higher-elevation sites with less frequent wildfires, conifers trees and shrubs like pines and junipers may invade and eventually displace sagebrush.

Over time, these shrublands have been converted for agriculture in some areas, but for the most part, livestock grazing has been the common land use and fragmentation by rural road networks is common. Overgrazing has often resulted in the loss of native bunchgrasses and has been accompanied by wildfire suppression. The introduction of invasive annual grasses has resulted in large and frequent fires that support their further expansion. Vast areas of these shrublands have been severely altered by the introduction of invasive cheatgrass.

Applying the HCCVI to these shrublands includes the use of maps and expert analysis for climate exposure, and resilience.

Climate Exposure measures used 30-year averages of 19 common temperature and precipitation variables projected to the mid-21st century and compared them against their averages measured during the mid-20th century. This resulted in a relative exposure map highlighting areas of greatest expected climate stress to these shrublands in the Central Great Basin, Snake River Plain, and Colorado Plateau. A warming climate with more frequent droughts weakens big sagebrush shrubs and eliminates recruitment. Additionally, warmer/drier fuels may result in more frequent fires that could increase the rate of loss of mature stands through the conversion of these shrublands to annual grasslands that are adapted to frequent fire.

Resilience measures included those for Sensitivity, encompassing landscape fragmentation, invasive plant species abundance, and alteration to natural wildfire patterns. Maps of these factors indicate locations throughout the West where the current conditions of these shrublands are impacted by prior land uses, contributing moderate-high scores for Sensitivity. Adaptive Capacity measures included evaluation of different functional species groups (soil crusts, nitrogen-fixing plants, seed dispersers) and these vary from high in some and low in others.  Microclimate and geodiversity are moderate, as this type occurs in a broad range of topographic settings.

Looking out to the 2040-2070 timeframe, overall vulnerability for this shrubland type varies from low in some places to moderate and high in others. Driven both by measures of climate change exposure and sensitivity, moderate to high climate change vulnerability is anticipated for several large ecoregions, including the Snake River Plain, the western and eastern portions of the Central Basin and Range, and the eastern Columbia Basin.