Piñon pine (Pinus edulis) and single-seed juniper (Juniperus monosperma) woodlands of the southwestern US are a model system for studies contrasting functional traits between tree species of divergent hydraulic strategies. Decades of field research have demonstrated the susceptibility of piñon to prolonged periods of drought, as widespread mortality has been observed and attributed to a lethal combination of water stress and subsequent bark beetle invasion. Despite an abundance of research focused on the mechanisms leading up to mortality, we do not possess a deep understanding of how ecohydrology is altered after trees die. In a previous study, we observed a decrease in juniper transpiration following large-scale pinon mortality, which suggested that piñon may be playing a unique role that benefits juniper within intact woodlands. To evaluate this hypothesis, we implemented a manipulation experiment, which killed all mature piñon or all juniper trees within 1000 m2 plot areas. We girdled trees in the fall of 2017, and complete mortality was achieved within ~8 months. Pre- and post-mortality dynamics of ecosystem structural changes and the hydraulic responses of surviving trees to this rapid mortality event were monitored, including soil moisture and water potentials across canopy and inter-canopy locations on the landscape.
Results/Conclusions
Here we present over two years of continuous sap flux measurements within the bole (n=54) and roots (n=108) of piñon and juniper. In the two consecutive growing seasons following mortality, we observed that the residual living trees of each species upregulated transpiration with respect to control plots consisting of mixtures of live pinion and juniper. We found that soil water content and water potential gradients increased within pinion-mortality treatments. Additionally, our measurements coincided with a drought event in the summer of 2018, during which time we observed hydraulic redistribution (HR) exhibited by 40% of juniper roots. This was often presented as sustained negative flows in course roots during both day and night amidst the driest periods. Following a rewetting of the soil, these roots began to exhibit sustained positive nocturnal flows. We did not, however, see any evidence of HR by piñon, suggesting that the roots of this species may be hydraulically disconnected from the soil at periods of enhanced water stress. While total volume of water moved in this process is low, we postulate that HR in juniper is functioning to preserve fine roots, rather than to translocate and accumulate water throughout soil layers.