'Fast' and 'slow' water handling strategies explain pinyon pine decline and juniper expansion

Semiarid forests in the western USA have undergone large-scale shifts in dominance due to decreasing pinyon pine abundance and increasing juniper abundance. This shift has been attributed to fast water loss in aboveground pinyon tissues during a long-term drought. Water loss, or 'outputs', however, are only a part of a plant's water budget. Water inputs, or root water uptake, are also important, but rarely well understood. We used depth-controlled tracer experiments, root water potentials, and soil water flow models to estimate pinyon and juniper inputs (root water uptake) and stomatal conductance as a proxy for water outputs. Measurements were made over two years at three sites that crossed an aridity gradient, Utah, USA. Pinyon roots were good at 'following' water through the soil, but not good at absorbing water from dry soil. Pinyon tracer uptake profiles (1-90 cm) changed more from month to month than juniper (coefficient of variation = 0.25 ± 0.02 and 0.16 ± 0.02, respectively) in ways that provided 12% more water over time. However, because pinyon roots had higher water potentials, the net water uptake advantage for pinyon was smaller: over six sampling dates, pinyon absorbed 2% more tracer than juniper. Aboveground, pinyon stomatal conductance was 25% greater than juniper. Results indicate that pinyon both absorbs and releases water faster than juniper. These 'fast' aboveground and belowground strategies place pinyon at higher risk of rapid transitions from growth to mortality than previously thought from aboveground research alone and suggests continued pinyon mortality in the future.