Topoclimatic Refugia for Last Stands: Empirical Examples and Modeling Approaches

Abstract:

Maintaining regional biogeographic distributions – the presence of species or assemblages within defined geographic areas – may be the only realistic conservation strategy given the magnitude of projected warming. In California, consistent projections of altered phenology driven by warmer temperatures and landscape aridification driven by increased climatic water deficit (CWD) will play out over complex mesoclimatic and topoclimatic gradients. Topoclimatic refugia provide for “last stands” of species within large landscapes. Range contractions during warming/drying periods eventually end in small mesic pockets determined by fine-scale topography and hydrology. Topoclimatic refugia can be mapped using models of insolation, cold-air pooling, flow accumulation, and soil depth. Arrays of temperature sensors can quantify lengths and correlations of intersecting topoclimatic gradients. Species respond along these gradients according to their life-history. Bay checkerspot butterfly populations persist through phenological extremes when late hostplant senescence on steep north-facing slopes allows springtime larval survival. The following winter growing larvae can crawl tens of meters upslope onto warmer slopes, advance their emergence by weeks, and provide a buffer against early hostplant senescence. The refugia consist of steep north-facing slopes within tens of meters of warmer upslope areas. Pockets of low CWD on low insolation slopes with deeper soils that accumulate upslope water provide mesic refugia; currently, these sites often support outlying stands of mesic vegetation. CWD tolerance of existing vegetation near arid limits will be exceeded as temperatures rise, but stands currently well away from the limits may persist longer. If CWD limits are exceeded, short range shifts (< 1 km) may allow persistence in adjacent areas. Importantly, species may already be present but rare in or near potential refugia in complex topography.