Species

The majority of the plant species on the Colorado Plateau use both summer and winter-derived soil water, though in different proportions. Although precipitation is about equally distributed between the fall/winter and spring/summer months, winter precipitation appears to be far more important as a water source to plants in this ecosystem than warm season precipitation.

In part this may be due to the high evaporative losses associated with summer rainfall. According to our estimates, more than 90% of total summer precipitation may be lost by soil evaporation without contributing to primary productivity.

Contrary to our initial expectation, plant functional types that specialize on the exploitation of summer rain, such as Hilaria jamesii are not the greatest consumers of summer rain in this ecosystem, because of their comparatively low leaf area cover.

It is often the soil moisture generalists, capable of extracting water both near the surface and deeper down, such as Gutierrezia sarothrae, Coleogyne ramosissima and Artemesia filifolia, which profit the most from spring and summer rains. Their capacity to extract water from deeper soil layers allows these plants to maintain a relatively high leaf area cover into summer, an important prerequisite for capitalizing on brief pulses of soil moisture. This double advantage may explain the widespread dominance of shrubs with dimorphic root systems throughout the cold desert ecosystems of the Colorado Plateau and the Great Basin Desert. Although shubs like Gutierrezia sarothrae can supply more than 50% of their water demand from shallow soil moisture after rain, they do not appear to compete for this water source, possibly because plant water uptake accounts for only a small fraction of total evapotranspiration following rain events. Similarly, soil nutrient dynamics following summer showers may be dominated by microbial and abiotic processes, leaving little opportunity for nutrient competition. Thus, cold desert shrubs appear to compete predominantly for stored winter water and possibly for soil nutrients released in early spring. Based on a given availability of stored water in the deeper soil, and a given soil moisture pulse frequency, optimally adjusted plant phenotypes can be predicted from considering water transport constraints through soils and plants. These constraints prevent phenotypes from simultaneously maximizing the exploitation of winter-derived and summer-derived soil moisture.

For example, to maximize winter-water uptake a plant must have most roots in deeper soil layers and a relatively high root/shoot ratio with relatively high drought tolerance to maintain water uptake into summer when this the winter-water store becomes increasingly depleted.

By contrast, to maximize summer-water uptake a plant must have a predominantly shallow root system and a small root/shoot ratio, combined with traits of drought evasion, to extract shallow soil moisture at the fastest rate possible, when it is available, while avoiding desiccation during the periods between soil moisture pulses. These hydrologic tradeoffs generate a range of opportunities for functional diversification and, in interaction with rainfall variability, may contribute to maintaining plant functional diversity within ecosystems.

By better understanding the hydrologic tradeoffs associated with extraction of winter- and summer-derived soil water, we are beginning to understand the influence of regional precipitation patterns and precipitation change on the composition of desert vegetation.