Seasonal soil–water availability influences snakeweed root dynamics

We tested a hypothesis that variable precipitation may induce altered rooting patterns. A nursery study was conducted over 2 years to evaluate the effect of seasonally variable soil moisture on the rooting pattern of shallow-rooted shrub broom snakeweed (Gutierrezia sarothrae Britt and Rusby). Plants irrigated during the spring–summer, but grown under rainout shelters in the winter (S) produced more roots in the upper 30 cm of soil than plants receiving natural precipitation (N), or plants irrigated in the winter, but grown under rainout shelters in the spring–summer (W). Plants irrigated in the winter, but grown under the rainout shelter in the spring–summer had 45–47% more roots (p<0•05) extended below the 30 cm soil layers in the early spring than the other treatments. These well-developed deeper roots provided 75% of plant evapo-transpirational water use for the winter irrigation treatment. This altered root distribution also enabled the plants to produce above-ground biomass similar to that of plants receiving natural precipitation. Our data suggest that the root deployment pattern of snakeweed can be altered by variations in seasonal precipitation, which may help explain the competitive relationship between snakeweed and the shallow-rooted native grasses.