Resources near the soil surface become available to plants typically in the form of pulses, i.e. short bursts of relatively high resource availability, followed by long periods with practically no availability. These pulses are evoked by warm-season rainfall events, which instantaneously wet the soil surface and initiate biogeochemic nutrient processing that release plant nutrients into solution. To make use of such resource pulses, plants must be ready for resource uptake within hours of the rainfall event. This requires the maintenance of a functional root system in the shallow soil.

In grasses and shrubs of the Colorado Plateau, we observed significant water uptake less than 24 hours after the addition of deuterium-enriched water (Schwinning et al. 2001), suggesting that many of the dominant life forms of this region maintain uptake-roots near the soil surface, even in the midst of summer. There are, however, large quantitative differences, between species, in the extent of resource pulse use.

Shrub species of the Colorado Plateau are also very variable in their capacity for nitrogen uptake, as was shown in an experiment where pulses of enriched nitrate-nitrogen and deuterium-enriched water were applied to the soil surface (Gebauer and Ehleringer 2000). Surprisingly, the capacity of shrub species to take up nitrogen was uncorrelated with their capacity to take up water, suggesting that the rates of water and nitrogen uptake by surface roots are largely uncoupled. This may give the opportunity for greater niche separation and reduced competition in this community, as species diversify in the timing of their greatest demand for water and nutrients.