Seasonal changes in the hydrogen isotope ratios of xylem waters were measured to determine water sources used for growth in desert plants of southern Utah. While all species used winter-spring recharge precipitation for spring growth, utilization of summer rains was life-form dependent. Annuals and succulent perennials exhibited a complete dependence on summer precipitation. Herbaceous and woody perennial species simultaneously utilized both summer precipitation and remaining winter-spring precipitation, with herbaceous species much more reliant on the summer precipitation component. Several of the woody perennials exhibited no response to summer precipitation. Currently, precipitation in southern Utah is evenly partitioned between [...]