Writer: D'Lyn Ford
LAS CRUCES - In New Mexico, landscape trees don't exactly have it made in the shade. Sunny, dry conditions make it hard for some trees to survive, limiting homeowners' options.
"Many people want to know what type of shade tree to plant in their yards," said Rolston St. Hilaire, a landscape design researcher at New Mexico State University. "Currently, we have a limited palette of shade trees for southern New Mexico."
St. Hilaire is trying to identify shade trees that can best handle moderate drought conditions, which can be caused by more than lack of water. Trees growing in pots or in the city surrounded by lots of concrete also have trouble surviving.
In an initial greenhouse study, St. Hilaire worked with graduate student Coye Balok to identify drought-tolerant trees that could thrive for years to come. They tested seven trees, all available at local nurseries: golden rain tree, Arizona ash and five different types of native oak.
Balok took measurements for four months, watering the trees and allowing them to lose half of their water to simulate drought.
"I had to come in between 4 and 5 a.m. to take water potential measurements from each leaf from each tree of each species," he said. "So I would have to take a predawn measurement and then I would come in the same day between 10:30 and 11 and take a midday water potential measurement, showing us how well the leaf rehydrates in the span of one day."
The leaves can tell a lot about which trees can weather New Mexico's weather. Typically, thicker, waxier and hairier leaves mean less water loss. Leaves with lots of hairs--called trichomes--feel velvety.
In addition, the number and size of the leaves' pores--known as stomates--play a role in water regulation.
To analyze these leaf characteristics, Balok and St. Hilaire studied scanning electron and light microscope images of the seven species.
Results from the greenhouse and microscope work showed that even though the commonly used Arizona ash is the most velvety, it may not be the best choice for New Mexican landscapes. The tree showed signs of transpiring or losing water quicker than the other species.
On the other hand, the researchers said the golden rain tree may be a good bet, because it seems to adjust well to drought conditions.
In general, the oaks were better at regulating water loss. The Texas red oak had the highest ratio of root growth to top growth, which is an important trait for tolerating drought. And with lots of trichomes and stomates, the Chinkapin oak also did well controlling water loss.
Before making any final recommendations, St. Hilaire plans to test the species that did the best in the greenhouse in an outdoor study to gauge how well the trees respond to southern New Mexico's sunlight and soil characteristics.
A third stage of the study would be to develop water-use recommendations for the trees that best tolerate drought.
For his part, Balok defended his master's thesis earlier this month and is set to graduate in May. He already has a job as a nursery manager. Balok said he won't miss those predawn trips to the greenhouse.
In addition to his research, St. Hilaire teaches courses in NMSU's agronomy and horticulture department about ornamental plant identification, landscape design, landscape construction and computer-aided landscape design.
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