Effects of Tree Shelters and Weed Control on Blue Oak Growth and Survival

Oaks ā€˜nā€™ Folks ā€“ Volume 7, Issue 4 ā€“ May, 1992


Blue oak (Quercus douglasii) is one of several species of native California oaks that is reported to be regenerating poorly in some locations. Recent studies have indicated that two of the most important factors limiting both natural recruitment success and field performance of planted seedlings are 1) limited soil moisture induced by competing plants, and 2) browsing or insect defoliation. A variety of approaches have been used to offset these factors including several types of cages and alternative methods of weed control. This study compared the effects of a relatively new type of protector (rigid tree shelters) with standard screen cages on the field performance of blue oak seedlings. It also examined the response of seedlings to varying levels of weed control including no weed removal and weed-free circles with diameters of 2, 4, and 6 feet.

Twenty one-year-old container seedlings were planted for each of the eight treatment combinations in February, 1991. From planting until the end of summer, weeds in the circles were removed by hoeing. The survival, height, height increment, caliper and caliper increment were evaluated at the end of the first field growing season.

After one year seedlings protected with tree shelters were significantly taller than those in screen cages (46 vs 29 cm), had significantly greater height increment and significantly greater survival (90 vs 71%). However, there were no differences in caliper or caliper increment between these groups. The size of weed control areas also had significant, but less dramatic, effects. In general, seedlings receiving no weed control or 2-foot diameter circles had significantly less height, height increment, caliper and caliper increment than those with 4- or 6-foot diameter weed-free circles. There were no significant differences in survival between the four weed control treatments, though seedlings from treatments with the greatest weed control had the highest average survival. There was, however, a significant interaction between weed control and type of protector for height increment; seedlings with good weed control tended to grow much taller in shelters, while those with little or no weed control remained stunted regardless of the protection used.

This study indicates that rigid tree shelters can promote increased survival and height growth of blue oak seedlings. However, shelters had no effect on caliper and consequently the seedlings tended to be tall and spindly. Stem elongation of this type may partially be due to lower light levels within the shelters. It is not yet clear how this growth form will affect subsequent field performance or the ability of the seedlings to grow upright once the shelters are removed. This study also found that the size of weed control significantly affected performance, with 4-foot diameter circles best in terms of costs and benefits. Larger circles added little to growth or survival, while smaller circles or no weed control resulted in less survival and smaller seedlings.


prepared and edited by John M. Harper and Richard B. Standiford