A multi-scale perspective of water pulses in dryland ecosystems: climatology and ecohydrology of the western USA

Abstract

In dryland ecosystems, the timing and magnitude of precipitation pulses drive many key ecological processes, notably soil water availability for plants and soil microbiota. Plant available water has frequently been viewed simply as incoming precipitation, yet processes at larger scales drive precipitation pulses, and the subsequent transformation of precipitation pulses to plant available water are complex. We provide an overview of the factors that influence the spatial and temporal availability of water to plants and soil biota using examples from western USA drylands. Large spatial- and temporal-scale drivers of regional precipitation patterns include the position of the jet streams and frontal boundaries, the North American Monsoon, El Niño Southern Oscillation events, and the Pacific Decadal Oscillation. Topography and orography modify the patterns set up by the larger-scale drivers, resulting in regional patterns (10(2)-10(6) km2) of precipitation magnitude, timing, and variation. Together, the large-scale and regional drivers impose important pulsed patterns on long-term precipitation trends at landscape scales, in which most site precipitation is received as small events (< 5 mm) and with most of the intervals between events being short (< 10 days). The drivers also influence the translation of precipitation events into available water via linkages between soil water content and components of the water budget, including interception, infiltration and runoff, soil evaporation, plant water use and hydraulic redistribution, and seepage below the rooting zone. Soil water content varies not only vertically with depth but also horizontally beneath versus between plants and/or soil crusts in ways that are ecologically important to different plant and crust types. We highlight the importance of considering larger-scale drivers, and their effects on regional patterns; small, frequent precipitation events; and spatio-temporal heterogeneity in soil water content in translating from climatology to precipitation pulses to the dryland ecohydrology of water availability for plants and soil biota.

Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland

“…There was little difference in the general behaviour of the soil moisture dynamics for the shrub-and bare-cover profiles with the exception that the bare-cover profile had larger increases in soil moisture for a given event (data not shown), which was consistent with the influence of canopy interception in shrub-cover micro-sites (Loik et al, 2004). Only large rain events (one day with >25 mm or back-to-back days each with >15 mm rainfall) generated considerable wetting at 15 cm and little changes were seen at 30 cm (Figure 2(b)).…”

Soil enzyme responses to varying rainfall regimes in Chihuahuan Desert soils

“…Precipitation in the Southwestern USA mainly occurs in small, pulsed events (Loik et al 2004) and is predicted to become more variable in the future (Diffenbaugh et al 2008, Gutzler andRobbins 2011). Given that dryland systems are sensitive to changes in precipitation (HeislerWhite et al 2009, Thomey et al 2011, Sala et al 2012 it is unclear how availability of soil resources, often made available via extracellular enzymes, will change under future patterns of precipitation.…”

Water relations and photosynthesis along an elevation gradient for Artemisia tridentata during an historic drought

“…Drought is common and can be persistent in many parts of the United States where synoptic meteorological patterns, topography, local features, and microsite conditions affect precipitation patterns and soil water (Knapp et al 2015b;Loik et al 2004;Sun et al 2015). Long-term drought has affected the central and southeast United States in recent years (Berdanier and Clark 2015;Knapp et al 2015a), and California in 2015 is experiencing a 1-in-1200 year drought (Griffin and Anchukaitis 2014;Williams et al 2015).…”

“…Many plant and ecosystem patterns and processes are affected by precipitation characteristics, which determine soil water availability and vegetation productivity (Knapp et al 2015b;Loik et al 2004;Sala et al 2015). Combined with other stresses such as invasive species, pests, and wildfire, drought affects the distribution and function of key species, populations, and ecosystems (Abatzoglou and Kolden 2011;Anderegg et al 2015).…”

“…vaseyana (Rydb.) Beetle (Asteraceae) is widespread throughout the Great Basin Desert, occurring at elevations from 800 to 3100 m (NCRS 2004), a gradient over which conditions differ considerably (Loik et al 2004). Artemisia tridentata possesses deep roots for accessing water in soil layers that are recharged with winter precipitation, which primarily occurs as snowfall (Richards and Caldwell 1987), and shallow roots which allow it to access smaller summer precipitation pulses (Loik 2007).…”