Comparative hydraulic performance of piñon and juniper in a rainfall manipulation experiment

Background/Question/Methods

From 2000-2003, extreme drought  across the Southwestern US resulted in widespread tree mortality: piñon pine (Pinus edulis) experienced up to 95% mortality while juniper (Juniperus monosperma) mortality was 25% or less at surveyed sites. Field data have shown repeatedly that piñon typically exhibits isohydric regulation of leaf water potential, maintaining relatively constant leaf water potentials even as soil water potentials fluctuate, while juniper is anisohydric, allowing leaf water potential to decline during drought. The goal of this study was to elucidate functional consequences of these two contrasting hydraulic strategies. The study was conducted in the context of a rainfall manipulation experiment in piñon-juniper woodland at the Sevilleta National Wildlife Refuge and LTER in central New Mexico, USA, sampling trees in irrigation (~150% ambient rainfall), drought (50% ambient), cover control (ambient rainfall with similar drought infrastructure) and ambient control plots. To quantify tissue and shoot level hydraulic performances we measured sapwood area-specific (K_S, kg•m^-1•s^-1•MPa^-1) and leaf area-specific (K_L, g•m^-1•s^-1•MPa^-1) hydraulic conductivity in similar sized distal branches, and we calculated Huber Value (HV, sapwood area to leaf area ratio) to compare shoot level allocation.

Results/Conclusions

Samples collected at predawn and midday both exhibited significant trends between species and across treatments. Between species, juniper possessed significantly higher K_S compared to piñon in all plots except irrigation, and higher K_L than piñon in all plots. Across treatments, irrigated juniper exhibited higher K_S and K_L relative to ambient and droughted plants, while irrigated piñon exhibited higher K_S relative to ambient, drought and cover control plants, and irrigated and ambient piñon had higher K_L than droughted and cover control plants. Juniper did not modify HV across treatments, while irrigated piñon had significantly lower HV compared to all other plots. Thus, under current climatic conditions in the Sevilleta, piñon and juniper achieve similar shoot hydraulic performances, but through different strategies: juniper maximizes xylem conductivity, while piñon maximizes xylem supply to leaves. If climate change in the Southwestern US results in increased aridity, piñon could be vulnerable to extirpation from its current distribution in lower elevation piñon-juniper woodlands, as juniper demonstrates superior hydraulic capability at both the tissue and shoot level under drought conditions.