Poplar is a generic term used to describe trees in the Populus genus. The genus includes cottonwoods, and aspen. Hybrid poplars are trees that are developed by crossing two different species of poplars. The most common poplars used to develop crosses include the eastern and black cottonwood, aspen, the Lombardy, European black, balsam and white poplars. The crossing often results in trees that display the best traits of each parent species. They are usually hardier, grow faster, and are more widely adapted than either parent. The various hybrid poplars varieties are known as accessions and they are numbered. One of the most successful accessions is OP-367 which was developed by crossing eastern cottonwood (Populus deltoids) with European black poplar (Populus nigra). Poplars developed from these crosses are commonly known as DN crosses. The other accessions used in the trial (15-029, 50-197, 52-225) are known as TD crosses because they are developed from the black cottonwood (Populus trichocarpa) and eastern cottonwood (Populus deltoids).
Poplar trees are grown for a wide variety of uses. The uses include: home landscaping, environmental cleanup, and production of paper, wood, and biomass products. During the late 1990’s, large hybrid poplar tree plantations were established in the Northwestern United States for the production of pulpwood for paper mills. However, since that time the market value of hybrid poplar pulpwood has declined dramatically. As such, new uses for poplar products are being developed. These uses include: saw logs used to make furniture frames, pallets, wood trim, and core materials that are overlain with veneers. Poplar wood is being used to produce plywood, oriented strand board (wafer board) and laminated strand lumber (LSL) which is used for beams and headers. Current testing is evaluating hybrid poplars as a source of bio-fuels to produce energy.
Cooperative Extension faculty initiated two hybrid poplar survival and production trials in 1998 and 1999. The first trial was established in Diamond Valley, north of Eureka and the second on the Newlands Agricultural Experiment Station in Fallon, Nevada.
The Diamond Valley trial was planted on June 11. 1998. The trial consisted of 100 trees each of three accessions for a total of 300 trees. The accessions planted at the Diamond Valley site included OP- 367, 50-197, and 52-225. Each accession was replicated five times. The trees were planted as cuttings approximately 8 inches long and 3/8 inch in diameter. They were planted on eight foot spacing within and between rows. The cuttings were planted with one bud showing above the soil line and all buds facing up. They were irrigated using drip irrigation. The trees were protected from grazing deer by 48” tall growth tubes. An active fertilization, weed and rodent control program was established at the same time to maximize growth of the trees.
The Fallon trial was established on April 27, 1999 with the planting of 294 (98 each accession) trees. The accessions planted in Fallon included OP-367, 50-197, and 15-029. The trees were planted on five foot spacing within and between rows. Each accession was replicated seven times. They were planted in the same fashion as the Diamond Valley trial. The Fallon trees were irrigated using flood irrigation on the same schedule as the adjacent alfalfa (approximately every 2 weeks). Weeds were controlled using Roundup® and mechanical methods. No fertilizer was applied. In April of 2002, ten trees of each accession were dug using a 24” tree spade and moved approximately 100 yards to test the ability of the trees to withstand transplanting as potential landscape stock.
The trees were thinned by removing every other tree April of 2003. The purpose of the removal was to increase the tree spacing and determine biomass production after four years. The total green weight of each accession was recorded during the thinning process.
The planting costs for each tree on the Diamond Valley site are displayed in table 1.
The trees begin budding on June 25, 1998, and by the end of July, the trees were beginning to grow out of the tops of the tree shelters that were 4 feet in height. The irrigation to the trees was reduced by approximately 50 percent beginning the first of September in an attempt to “harden off” the trees for winter. The first killing frost occurred on September 19th when the temperatures dipped to 27 degrees Fahrenheit. The irrigation system was then shut down to avoid damage to the system. The survival and production evaluation was completed on October 12, 1998. The survival percentage was in excess of 99% with only two trees failing to grow out of the 300 planted.
The production evaluation was limited to determining the average tree heights at the end of the first growing season. Table 2 is the average tree height of the three hybrid poplar accessions planted in Diamond Valley during June 1998 and measured during October 1998.
Irrigation of the trees planted in Diamond valley resumed in May of 1999. The vast majority of the top growth of the trees winterkilled. By the October 1999 evaluation, most of the trees had sprouted from the roots but were considered a loss. The project was abandoned that year.
The planting costs for the Fallon trial were much lower than that experienced in Diamond valley. No tree shelters or drip irrigation supplies were required as the trees were flood irrigated and no deer were present. The planting technique was also less labor intensive as the only action required was to press a 3/8” hole in the soil and press the cutting into the hole. Table 3 is the estimated cash cost of planting hybrid poplars in Fallon.
The trees were evaluated for survival and production in November of 1999. The information collected included the number alive, and the height. Table 4 displays the results of the first year evaluations.
The trees fared well in the winter of 1999-2000 with only 1 additional death and rapid growth occurring with all accessions. Table 5 displays the evaluation information collected for the 2000 growing season.
Tree survival continued to be excellent over the winter of 2000-2001 with only one additional death. By the fall of 2001, the trees were large enough to begin collecting data on tree diameters one foot above the ground. Table 6 displays the evaluation information collected for the 2001 growing season.
Tree survival, heights and diameters were again recorded following the 2002 growing season. No additional trees had winterkilled and the growth rate continued to be exceptional. Table 7 presents the results of the 2002 growing season.
The greenwood biomass from each accession was determined in April of 2003 by cutting and weighing every other tree in the plantation. The data was converted to average weight per tree (green weight) in order to calculate different yields/acre based on tree spacing. Table 8 is the average tree weight from the 4-year-old stand growing in Fallon and tons per acre dry weight on 5 X 5 spacing. The moisture percentage of the harvested trees is assumed to be 50%.
The hybrid poplar trees planted in Eureka were a complete failure as over 98% died the first winter. Hybrid poplars are grown in climates colder than Eureka throughout the United States and the authors speculate that the tree shelters remained on the trees too long and were irrigated too late in the season which delayed the onset of dormancy. Because of the abnormal warmth, provided by the shelters the trees failed to go dormant before winter temperatures dropped into the killing range. The trial was not repeated.
The Fallon trial was much more successful with four-year mortality ranging from a low of 0 percent to a high of only 7 percent. The accession OP-367 averaged 9.3 feet of growth per year, while 50-197 and 15-029 grew at 7.1 and 6.7 feet per year respectively. The growth in diameter was less impressive due to competition for light and space resulting from the 5’ by 5’ spacing.
The biomass produced by the trees was comparable to that experienced by hybrid poplar trees growing in the Northwest. Accession OP-367 averaged 6.8 tons per acre per year. Accession 50-197 and 15- 029 averaged 4.8 and 5.2 tons per acre per year over the four-year growth period.
The hybrid poplar trial is continuing in Fallon. Results will be updated as the data is developed.
Extension's Communication Team
Davison, J. and Riggs, W., 2005, Hybrid Poplar Production 1998- 2003 in Eureka and Churchill Counties, Extension | University of Nevada, Reno, FS-05-25
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