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The
Blue Oak Project In Fall 1990, Doug McCreary, a researcher at the University of California, Sierra Foothill Research and Extension Center, along with Mike Connor and Jerry Tecklin installed a project designed to test various means of re-establishing blue oak trees in the 1960's (Tecklin, J., Conner, M. and McCreany, D., "Rehabilitation of a Blue Oak Restoration Project"). Tecklin, the primary author of the study, describes the site as a northeast-facing slope, at about 1100 feet in elevation. The soil is a very rocky loam. After 25 years of moderate grazing, one blue oak tree, a few shrubs and a "handful" of stunted re-sprouted blue oaks (less than 60 cm tall) remained. The planting consisted of 1,440 blue oaks. The research team planned to evaluate the effectiveness of three types of planting stock (acorns, 3-month-old seedlings, and 1-year-old seedlings). They also planned to evaluate 5 methods of weed control: scalping - removal of the turf layer where the seedling would be planted; plastic mulch mats; one herbicide application per year; two herbicide applications per year; and a control - no weed control. These treatments were tested in every combination in order to determine the most effective and cost-effective establishment method. The entire plot was fenced to exclude the cattle that graze the Sierra Foothill Research and Extension Center as part of other rangeland research work. The team expected to have a field of sapling-size trees after five or six years. However, as often the case with the best laid plans of mice and men (or, in this case, voles and men), things did not work out that way. Initial Results Almost immediately, two predators stressed the planting: grasshoppers and voles. The oak seedlings provided a ready food source for the grasshoppers and voles. The grasshoppers defoliated many of the seedlings, generally in mid-summer when the grasshopper populations peaked. Because cattle were excluded, vegetative cover and vole habitat increased. The voles gnawed a large percentage of the oak seedlings off at the ground line. Damage caused by both predators forced the young oaks to continually re-sprout from below the point of girdling. Seedlings that lacked the energy reserves needed to re-sprout eventually died. After two growing seasons, mortality had reached an unacceptable level, and the growth of surviving seedlings was extremely disappointing. Surviving seedlings averaged just 16 cm - about 6-inches in height. The research team had enough data to show differences in the performance of the different types of planting stock and the benefits of thorough weed control. However, none of the treatments produced satisfactory results. Remediation Heading into the growing season, the team decided something had to be done to protect the seedlings from voles and to give the seedlings a boost. They decided to test two remedial measures. Both used treeshelters to protect the seedlings. Treeshelters are translucent plastic tubes, about four inches in diameter, and a range in height from two to five feet. They are extruded from plastic polymers. with a unique twin-walled design to increase their strength / weight ratio. The team replanted the blocks hit hardest by mortality with acorns and seedlings, and protected the replants with 4-foot tall treeshelters. Four-foot tubes were chosen because, in addition to protection from voles, they would provide protection against deer. Glyphosate was sprayed around each tube to kill weeds. The progress of these replants would then be compared to the original seedlings in other blocks. The research team also installed 4-foot tall tubes over surviving seedlings from the original planting, to see if they could be rejuvenated. McCreay refers to this practice as "retrofitting" seedlings. Eighty-three pairs of surviving seedlings from the original planting were matched, with one seedling in each matched pair receiving a tube. The progress of the 873 seedlings with the tubes installed would be compared to 83 unsheltered pairs. Successful Results Measurements were taken three years after the treatments were installed. The results are dramatic. In the first treatment, where new seedlings were planted, shelters were installed and glyposate was sprayed for weed control. The replants are on average seven times taller compared to the original seedling - despite being two years younger. The survival rate for the plants at 88 percent for seedlings and 75 percent for acorns, while survival of unsheltered seedlings in the same treatment block has stabilized at just 16 percent. The shelters provided another, unexpected benefit. Grasshoppers did not seem to attack the protected seedlings until after the seedlings emerged from the tubes. This gave the seedlings one or two "grasshopper-free" growing seasons in which to get better established. As a result, by the time grasshoppers did defoliate the tree seedlings, they had greater energy reserves and were able to quickly "refoliate" - and often put on a late season growth flush. Only a few of the unprotected seedlings, which were defoliated by grasshoppers while still knee-high, were able to refoliate in this manner. The second remediation treatment, the "retrofitted" seedlings (surviving seedlings from the original planting in 1990) show similar results. The survival rate of retrofitted seedlings is 98 percent, with none of the trees showing signs of vole damage. Survival in the best block of original seedlings is 76 percent - and with 23 percent of those seedlings show definite signs of vole damage. The research team expects seedling morality to increase. Plus, while the unprotected trees have only doubled in height in the three years since remediation, the protected seedlings have grown to seven times their original height. As with the replants, the retrofitted seedlings were protected from grasshoppers until they emerged. As a result, they were bigger and more vigorous when they faced grasshopper defoliation, and were far more likely to refoliate and continue growing. Applying Lessons Learned From Other Studies Based on personal experience and the manufacture's guidelines, McCreary plans to leave the tube on the trees for at least another year, even though the trees emerged 1-2 years ago. Early on in his work with these tubes, McCreary noticed that when seedlings emerged from the tubes, they had relatively thin, minimally tapered stems. If the tubes are removed at the point, the young trees are often unable to support themselves. They should be left in place to support the young trees and provide continued vole and deer protection. When trees emerge and are exposed to the wind, the swaying of the trunk triggers growth responses that reallocate energy into stem thickness rather than height growth. Emergent trees quickly develop the stem strength and thickness necessary to support them. One manufacturer recommends leaving them in place until the base of the tree reaches 2 inches in diameter. Based on experience, McCreary feels this is a good rule of thumb. A New Twist: Cattle In April 1997, the protected replanted and retrofitted oak seedlings had safely achieved sapling size so the research team decided to allow cattle back onto the site. This was done to demonstrate the feasibility of re-establishing oaks without taking the land out of livestock production. While this phase of experiment is only a few weeks old as of this writing, the results are encouraging. The cattle are contentedly grazing the grass and have not bothered the protected blue oaks. The unsheltered seedlings are still only knee-high. The results mirror those of many other projects, both in Europe ant the United States, where these tubes have effectively protected young trees from cattle and other livestock. It is sometimes necessary to use a tube taller than the 4-feet to protect tree seedlings from cattle. It may also be necessary to use a second stake driven on prevent cattle from rubbing against the tube and spinning it around. In Europe, where land is at a premium and must serve the dual purpose of livestock production and forest regeneration, this practice is standard. Conclusion The Blue Oak Project at the University of California's Sierra Foothill Research and Extension Center is a striking example of the difference protection for trees can make in forest regeneration efforts. When the oak seedlings were originally panted in 1990, researches did everything "by the book". They prepared the site properly, planted high-quality stock according to standard practices, and controlled weeds (except in the "no weed control" test blocks). Despite this investment of time and money, the project was headed toward failure due to the devastating effects of grasshoppers and voles. The tubes provided protection from cattle, deer and rodents. They also protected the seedlings from grasshoppers until the seedlings had strong energy reserves to withstand defoliation. Survival increased and growth was accelerated, which led to the success of the Blue Oak Project. For more information, contact David Bogue, Treessentials® Company.
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