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 (KS, kg•m-1•s-1•MPa-1) and leaf area-specific (KL, g•m-1•s-1•MPa-1) hydraulic conductivity in similar sized distal branches, and we calculated AS:AL (sapwood area to leaf area ratio) to compare shoot level allocation.
Samples collected at predawn and midday both exhibited significant trends between species and across treatments. Between species, juniper possessed significantly higher KS compared to piñon in all plots except irrigation, and higher KL than piñon in all plots. Across treatments, irrigated juniper exhibited higher KS and KL relative to ambient and droughted plants, while irrigated piñon exhibited higher KS relative to ambient, drought and cover control plants, and irrigated and ambient piñon had higher KL than droughted and cover control plants. Junipers did not modify AS:AL across treatments, while irrigated piñon had significantly lower AS:AL 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 Southwest results in increased aridity, piñon could be vulnerable to extirpation from its current distribution in lower elevation PJ woodlands, as juniper demonstrates superior hydraulic capability at both the tissue and shoot level under drought conditions.
One shoot from each target tree was harvested between 0430-0545h and between 1200-1400h, to get predawn and midday water potential (referred to hereafter as ΨPD and ΨMD, respectively). Samples were placed in plastic bags containing a small segment of moist paper towel to prevent further dessication, which were placed in coolers out of direct sunlight in the interim time between collection and processing (between 15-60 minutes). Water potential (ΨW) [u1]was measured using a pressure chamber (PMS, Corvallis, OR).
After ΨW was measured, shoots were placed in humid plastic bags and allowed to equilibrate for 24 hours in a refrigerator. Shoots were then trimmed underwater to remove peripheral embolized tissue and inserted into a steady state flow meter to measure hydraulic conductivity, Kh, kg•m-1•s-1•MPa-1 (see Hudson et al. 2010 for a full explanation of method). In brief, the steady state flowmeter operates on the Ohm’s Law analogy of hydraulic transport (Tyree 1997), and solves for Kh by knowing the pressure gradient and the flow rate of sap surrogate (20 mM KCl, Zwieniecki et al. 2001) through the flowmeter, and measuring the pressure drop across the sample stem segment. Hydraulic conductivity was calculated as flow through the sample segment divided by the pressure gradient across the sample segment. Sapwood cross-sections and distal leaf areas were measured for each sample to normalize Kh at tissue level (KS, sapwood area specific hydraulic conductivity, kg•m-1•s-1•MPa-1) and shoot level (KL, leaf area specific hydraulic conductivity, g•m-1•s-1•MPa-1). AS:AL was calculated for each species by dividing each sample’s sapwood area by distal leaf area.
Instrument Name: Pressure Chamber
Manufacturer: PMS Instrument Company
Model Number: 1505D
Instrument Name: Gage model pressure transducer (0-15 psig range)
Manufacturer: Omega Engineering, INC.
Model number: PX26-015GV