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Sap flow gauges were used to measure the flow of water through the trunk of a tree.

Sap flow gauges were used to measure the flow of water through the trunk of a tree.

Researchers working in the experimental apple orchards at Cornell University, New York, are developing an irrigation scheduling model to help apple growers in the Northeast know how much water their trees use.

To apple growers in the arid West, this probably seems common. But to growers in the eastern United States, it is uncharted territory. Historically, most orchards have not been irrigated, but every year, more growers see the need for supplemental water.

The project is driven by growers’ needs for high yields of large high-quality apples, according to Dr. Alan Lakso, who has spent his career studying how trees do their basic work of turning sunlight and water into fruit and wood. One of his colleagues is Dr. Terence Robinson, also at Cornell, whose work with the tall spindle apple orchard design—and grower adoption of it—has increased sunlight capture and yields, but also the need for irrigation. Growers are planting apples on dwarfing rootstocks and at higher densities, making supplemental water more important.

Lakso and graduate student Danilo Dragoni, a specialist in microclimatology, and Rick Piccioni, an instrumentation specialist, have developed a computerized model, and Robinson is working with New York growers to test the model and help them implement it.

Lakso and Robinson hope that by later this year apple growers will be able to go to the Northeast Regional Climate Center Web site at Cornell, www.nrcc.cornell.edu, click on a specialty page for apples and a weather station near them, and find out how much water their trees have used in the previous days and weeks.

“We’re still working out some of the bugs in the system,” Lakso said. One bug has to do with the need for good data on sunlight. In the cloudy, humid Northeast, sunshine is rarely 100 percent of possible, but many of the weather stations don’t measure the amount of sunlight received. Most measure rainfall, temperature, relative humidity, wind speed, and leaf wetness, and these basic ingredients help in predicting disease risk and insect development.

Some of the newer stations do record data on sunlight received, but light meter readings can drift and need frequent calibration. “You have to have good sunlight data to work with since the sunlight is the energy driving water use,” he said. At Geneva, the researchers did directly measure sun energy received at different times of the day.

Water use estimations

Irrigation models are built by calculating water use—evaporation and transpiration—from a reference crop, usually grass, using daily weather data, Lakso said. Then, the water needs of another crop, like apples, is estimated to be a fraction of the standard grass (called the  “crop coefficient”). The model of the standard grass reference provides a consistent response to the weather.

“Our problem is that for a humid climate, the grass reference doesn’t work as well as it does in an arid climate,” Lakso said. Water loss from grass is controlled mostly by sun, not as much by air temperature and humidity. But apple trees aren’t flat structures like grass. Moreover, apples have the ability to adjust their water loss by opening and closing pores in the leaf (stomata). “For several reasons, apples trees are dependent on humidity and stomatal conductance as well as sunlight,” Lakso said.  “So, apple water use is more responsive to humidity than the standard grass. That requires a different apple-­specific model.”

In New York, apple trees need supplemental water at two critical times: When they are young and rapid shoot growth is needed to fill their space, and in mature trees during July and August when summer water stress can keep apples from sizing properly.

“In our erratic climate, irrigation is not always needed,” Lakso said. “It’s a form of crop insurance. A lot of growers understand the value of irrigation, especially with the young orchards they are planting at high densities on dwarfing rootstocks where shallow roots don’t explore much of the soil. Also, heavy crops already struggle to size fruit, so water stress must be limited in those cases.”

Measuring tree water use

A typical apple orchard in the Northeast is a mixture of tree canopies, bare strips of shaded soil where vegetation is controlled, and grass alleyways. However, with trickle irrigation, water is applied only under the trees; alleys get adequate water from natural rainfall.

In gathering data for the moisture loss equation, Lakso’s team used a couple of techniques to directly measure water loss from trees: sap flow gauges, which measure the flow of water through the trunk of a tree, and whole-tree gas-exchange chambers, in which trees were enclosed in clear plastic balloons and the water loss from each tree measured directly.

The studies were done at the Geneva Experiment Station on eight-year-old Royal Empire apples on M.9 rootstock trained to central leader at a density of 792 trees per acre.

The final model is quite complicated but specific to apples. It includes factors for sunlight, humidity, temperature, wind speed, and tree leaf area.  Adjustments for immature-sized trees and different times of the season are being developed.

In the final analysis, the studies showed what many growers suspected: Apple trees in humid climates transpire less (about 15 percent less) than those in arid climates like Washington and California, but water use is much more variable because the weather is not as uniform.  On warm, dry days, a New York orchard is ­much like a Washington orchard in water use, but on cool, humid days, it uses much less.