The Topographic Signature of Ecosystem Climate Sensitivity in the Western United States

Abstract

It has been suggested that hillslope topography can produce hydrologic refugia, sites where ecosystem productivity is relatively insensitive to climate variation. However, the ecological impacts and spatial distribution of these sites are poorly resolved across gradients in climate. We quantified the response of ecosystem net primary productivity to changes in the annual climatic water balance for 30 years using pixel-specific linear regression (30-m resolution) across the western United States. The standardized slopes of these models represent ecosystem climate sensitivity and provide a means to identify drought-resistant ecosystems. Productive and resistant ecosystems were most frequent in convergent hillslope positions, especially in semiarid climates. Ecosystems in divergent positions were moderately resistant to climate variability, but less productive relative to convergent positions. This topographic effect was significantly dampened in hygric and xeric climates. In aggregate, spatial patterns of ecosystem sensitivity can be implemented for regional planning to maximize conservation in landscapes more resistant to perturbations. Plain Language Summary:It is well known that gradients in elevation and aspect can have a significant influence on the degree of water and energy available for plant growth and the sensitivity of ecosystems to wet or dry time periods. Little work has examined how hillslope topography and downslope movement of water to zones of convergent terrain can impact plant available water and vegetation growth. We quantified ecosystem response to the climatic water balance (ecosystem sensitivity) across a 30-year record and at a 30-m resolution across the western United States. Our results show that vegetation in zones of hillslope convergence, where moisture from upslope tends to accumulate, is less sensitive to droughts, especially in semiarid settings. Divergent hillslope positions were moderately sensitive to climate and less productive relative to convergent positions. Ecosystem response to topography was dampened in especially wet or dry climates due to significant moisture surplus or moisture deficit, respectively. These distributed measurements of ecosystem sensitivity are important considerations when describing local ecosystem-climate relationships and for identifying management priorities across landscapes. Zones of resistant vegetation are more likely to persist through future droughts, influencing the greater ecosystem's response to climate change.

Topographic, soil, and climate drivers of drought sensitivity in forests and shrublands of the Pacific Northwest, USA

“…However, variability in drought response can be substantial within biomes and within ecosystems (Fig. 2 ) 8 , 20 , 30 , underscoring the importance of biophysical processes governing soil water availability and plant physiologic responses to water stress 11 , 12 , 33 . Within-biome variability can be driven by endogenous factors (i.e., species composition) and by exogenous factors (climate, topographic, soil, and hydrologic characteristics).…”

“…Previous studies have characterized large-scale spatial patterns of vegetation responses to meteorological variability and have linked these responses to patterns of biomass and climate, generally finding stronger drought impacts in areas where productivity is water-limited 18 , 20 , 26 , 28 – 30 , but have not accounted for within-biome variability based on the effects of topography, soil, and landscape hydrology. These landscape characteristics produce spatial heterogeneity of soil water availability and evaporative demand during droughts, owing to processes such as soil drainage and water retention, water storage in weathered bedrock, lateral routing of soil water and groundwater, cold-air pooling, and shading from solar radiation 11 , 12 , 31 , 32 . Such landscape characteristics can thus affect vegetation community structure and plant physiological responses to drought 5 , 6 including vulnerability to drought-induced mortality 12 , 33 , and may produce localized areas of hydrologic buffering from climate variability and drought impacts, i.e., hydrologic refugia 11 , 31 .…”

“…These landscape characteristics produce spatial heterogeneity of soil water availability and evaporative demand during droughts, owing to processes such as soil drainage and water retention, water storage in weathered bedrock, lateral routing of soil water and groundwater, cold-air pooling, and shading from solar radiation 11 , 12 , 31 , 32 . Such landscape characteristics can thus affect vegetation community structure and plant physiological responses to drought 5 , 6 including vulnerability to drought-induced mortality 12 , 33 , and may produce localized areas of hydrologic buffering from climate variability and drought impacts, i.e., hydrologic refugia 11 , 31 . Understanding the biophysical processes that shape ecosystem responses to drought can help land managers better anticipate—and potentially mitigate—biodiversity and ecosystem-service losses as droughts intensify.…”

“…This region includes diverse topography, climates, and vegetation (from temperate rainforest to semi-arid shrublands), allowing us to compare drought-response patterns across large gradients of energy and water availability. Comparison across large climate gradients is useful because relationships between landscape topography and vegetation responses to drought can vary depending on climate conditions 11 . Like many areas of western North America, the Pacific Northwest region is projected to experience continued snowpack reductions, hotter and longer summers, and the intensification of seasonal droughts 2 , 34 , 35 .…”

“…Because previous studies have found stronger vegetation responses to climate variability with decreasing climatic wetness and biomass 20 , 28 , we anticipated that drought sensitivity would be greater in the shrub-steppe biome than in conifer forest. We also hypothesized that drought sensitivity would be reduced by soil characteristics that increase water infiltration and storage and by topographic features that concentrate runoff or suppress evaporative demand 11 , 31 , 33 . By examining the spatial drivers underlying drought sensitivity across biomes and drought-intensity levels, this study improves our ability to identify drought-sensitive ecosystems and anticipate ecosystem response to drought intensification under climate change.…”