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
The goal of this project is to determine the nature and magnitude of changes in the hydrologic properties of arid soils with increasing amounts of pedogenic calcium carbonate. The amount and morphology of the calcium carbonate in arid soils varies laterally and vertically with changes in the age of the soils, thus the hydrologic properties also vary systematically The calcium carbonate cements soil particles changing the apparent texture of the soil horizon and thus other soil properties such as structure, porosity, moisture retention, and unsaturated and saturated hydraulic conductivity also change significantly. There has been no systematic study of the impact of increasing amounts of calcium carbonate on the hydrologic properties of semi-arid soils. The ultimate goal of this study is to provide a basis for developing more accurate pedotransfer functions, which are the main methods for obtaining soil hydrologic properties of rangeland soils.
Selection of Surfaces: Three terraces of different ages were chosen at the outlet of a small watershed basin at the base of Sierra Ladrones in North West Sevilleta National Wildlife Refuge. These surfaces have shown varying stages of calcic horizons.
Digging Pits: 3 Pits up to a meter deep were dug on each surface.
Describing the Soils: the soil profile in each pit was described using USDA soil survey guidelines.
Soil Sampling: From every pit, soil samples were collected every 10 cm. Also soil peds were collected from every horizon for bulk density analysis.
Infiltration Experiment: In order to check the soil hydraulic conductivity, a tension disk infiltrometer was used on every soil horizon in each pit.
Laboratory Analysis: The soil samples were split and sieved for laboratory analysis
CaCO3 Content: The total inorganic carbonate content was calculated using Chittick’s apparatus
Bulk Density: The bulk density of the soil peds was calculated using the Clod’s apparatus.
PSDA: Particle size distribution analysis was carried out with the presence of carbonate on the 2mm sample.
Carbonate Digestion: The carbonate was digested to remove the amount of carbonate from the sample. PSDA was performed again on the soil samples without the carbonate.
Information on Collection Sites:
Study Area 1:
Study Area Name: Surface 1(Pit 1)(Young Surface)
Study Area Location: Outlet of the small watershed basin at the base of Sierra Ladrones
Study Area Description:
Elevation: 1623 m
Geology: Quaternary Sierra Ladrones Formation
Soils: Laborcita-Pilabo-Lemitar complex
Climate: Semi arid, Rainfall ~ 250 mm
North Coordinate: 34° 24.5’
West Coordinate: 106° 58.1'
Study Area 2:
Study Area Name: Surface 2 (Pit 2)(Intermediate Surface)
Elevation: 1615 m
Geology: Quaternary Sierra Ladrones Formation
Climate: Semi-arid, Rainfall ~250 mm
North Coordinate: 34° 24.491'
West Coordinate: 106° 58.046'
Study Area 3:
Study Area Name: Surface 3 (Pit 3) (Oldest Surface)
Elevation: 1633 m
Climate: Semi-arid, Rainfall~250 mm
North Coordinate: 34° 24.405'
West Coordinate: 106° 58.020'
Other Field Crew Members: Ritchie Andre and Ramirez Carlos
Maintaining high rates of water loss during times of high resource availability could allow establishing woody desert perennials to grow quickly by allowing them to take advantage of the fleeting but abundant monsoonal moisture typical of warm deserts like the Chihuahuan. However, a plant cannot endlessly increase water loss in order to grow faster --there are hydraulic constraints on rates of water loss. The hydraulic properties of each particular plant xylem and soil microsite, as well as the AR:AL absorbing root area to transpiring leaf area ratio) interact to set limits on rates of water loss. If transpiration rates become too high, cavitation may limit the ability of the xylem to supply water to the leaves. The main objective of this study was to test two hypotheses on a population of Larrea tridentata at the Sevilleta LTER in central New Mexico (1) do small plants grow faster and use water less conservatively than large, and (2) are there differences in the hydraulic constraints on small and large plants. Measurements were made every six weeks in the spring, summer and fall from April 2002 - August 2003. Field measurements of shoot growth, gas exchange and plant and soil water potentials were made to determine growth rates and water use. Measurements of leaf specific conductance determined the ability of the xylem to supply water to the leaves. Excavation findings were used to estimate (AR:AL). Xylem vulnerability curves and soil texture analysis were used to determine the hydraulic properties of the plant xylem and soil. A model determined where the limiting conductance occurred in the plant-soil continuum.
For gas exchange measurements a LiCor 6400 portable gas exchange system was used. Measurements took place in June, August and September 2002 and May, June, and August 2003. Two measurements were made on each plant at approximately 7-9:30AM and 10-12:30PM. One branch tip was chosen and marked on each 10 small and 10 large plants. The same branch tip was used for measurement throughout the day unless it broke, at which time another branch tip was chosen and marked. Stomatal Ratio was set to one because stomates are present on both sides of the leaf in this species. Because of the small size of the leaves, an energy balance approach was used to calculate the leaf temperature in the chamber.
Chamber temperature and humidity were controlled at ambient and reference CO2 was set to 400ppm. Using natural light plants were clamped into the chamber, oriented in their original direction, chamber conditions were allowed to stabilize. Leaf area was set to one during measurement. Because of the small leaves of the species, a branch tip had to be measured. Measured branch tips were cut and returned to the lab where their leaf area was measured.
A Vista Scan Scanner was used to create an image of the leaves. The bitmap image was then analyzed for number of pixels using Scion Image. A regression equation was developed which converted pixel number into leaf area in cm2.The gas exchange data was then recalculated to adjust for leaf area.
For plant water potential a Scholander Pressure bomb was used to measure branched stem tips consisting of 15-20 leaves and a woody base. Predawn water potential samples were collected between 4AM and 5AM. Midday water potentials were collected between 11AM and 1PM. Samples were placed into a plastic baggie with a moist paper towel during transport. Samples were collected in May, June, August, September and December 2002 as well as January, March, May, June and August 2003.
For whole plant hydraulic conductance measurements large sections of the xylem were measured using a vacuum canister to generate known vacuum pressures. The plant was attached to a water filled container on a balance via tiagon tubing. Changes in weight on the balance, and thus flow rate (mg/s) through the plant were measured by a computer using a program written in Turbo Pascal. Each sample was measured at four or five pressures and the change in flow rate with pressure was calculated as the total hydraulic conductance of all tissues contained in the sample. All samples were immediately placed into a plastic bag with a wet paper towel and transported to the lab where they were measured within 48 hours of collection.
For large plants an entire stem of the plant was cut at the base. The stems were cut under water at the lab, to about 30cm. For small plants the entire plant was excavated, and any roots larger than about 2mm in diameter were kept intact. Back at the lab, most of the root system was cut off under water, leaving the root collar and the initial un-branched portion of the main root which obviously supplied the entire plant. For both sizes, all green material was removed from the tips of the branches, leaving only woody stems.
The green portion of the plant included all leaves and sometimes large (up to 150mm) branched sections. For root hydraulic conductance measurements root segments +AD4-50cm were cut in the field and transported to the lab wrapped tightly in 3 plastic bags containing wet paper towel. Segments were re-cut underwater and the ends shaved off with a razor blade. They were then placed on a manifold and flow through the segment was measured. Segments were then flushed for 15 minutes with distilled water at 100kPa. Flow was then re-measured. Percent loss of conductivity is calculated as the difference between pre and post flush flow divided by post flush flow and multiplied by 100. Due to time constraints only one flush was performed on each sample. For soil water potential monitoring soil thermocouple psychrometers were placed under 4 large and 4 small plants at 30cm and 45cm below the soil surface. Measurements were made at or around 2AM when temperature gradients were at a minimum. A Campbell datalogger reported millivolt output which was converted to MPa using calibrations determined in the lab. Calibration involved regression of millivolt output against solutions of known salt concentration for each psychrometer.
Additional Information on the personnel associated with the Data Collection / Data Processing Joy Francis, a post-doc with Jim Gosz, was instrumental in setting up this study.
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