Management of Irrigated Hybrid Poplar Plantations in the Pacific Northwest

Charles A. Wierman, Boise Cascade Corporation, Wallula, Washington

ABSTRACT

Increasingly reduced timber availability in the Pacific Northwest has necessitated the search for alternative sources of wood fiber. Short rotation, intensively cultured plantations of hybrid poplar have the potential to mitigate some of this wood supply shortage.

Boise Cascade Corporation has embarked on a project to establish about 8,000 ha of irrigated hybrid poplar plantations in the Columbia River Basin of eastern Oregon and Washington. These tree farms will provide approximately 15 to 20 percent of the raw material supply for the company's pulp and paper mill at Wallula, Washington. The fiber will be used in the production of uncoated free sheet white papers. In addition to increasing the fiber available to the mill, the hybrid poplar provide other benefits including: reduced bleaching and improved paper qualities of brightness, opacity and print ability.

Fiber farms are developed on existing agricultural land. Traditional irrigation systems are converted to drip irrigation. Planting sites are prepared with mechanical cultivation and pre-emergent herbicides. Planting stock is unrooted dormant cuttings produced in stool beds. Planting is done by hand during the month of April. Crop tending consists of irrigation, weed control, fertilization and pest control. A six year rotation is planned at which time the trees are expected to be approximately 18-20 m in height and 15-20 cm in diameter at breast height.

INTRODUCTION

The supply of wood fiber in the Pacific Northwest from both public and private land has been severely curtailed by preservationist activities. This artificial supply shortage, coupled with continued societal demand for wood products, has caused the price of wood to increase dramatically. Therefore, the cost of producing fiber from intensively managed hybrid poplar plantations has become economically justifiable.

The concept of fiber farming is based on the assumption that the trees are an agricultural crop. And, indeed, they have been deemed so by the legislative and regulatory authorities. This crop is established, grown, and harvested using the same intensive, mechanized philosophy applied in more traditional crops. The short rotations and high yields achieved are dependent on this aggressive management approach, and can not be attained through less intensive practices.

The goal of Boise Cascade Corporation's fiber farming project is to supply 15 to 20 percent of the raw material for the company's pulp and paper facility at Wallula, Washington. This volume of wood will require approximately 8,000 ha of plantations. The project began in 1990, with the first planting done in 1991. The first harvest of trees is scheduled for 1997. Harvesting will be done on a six year cycle, with the expectation that the rotation can be shorted over time with improved genetics and agricultural practices. Year round harvesting is planned to minimize storage time for the chips. A sustainable, even flow of wood is required, which will entail harvesting and planting about 1,500 ha annually. Hybrid poplar was chosen as the species to grow based on its adaptability to the local environment, fast growth, and wood characteristics.

LOCATION

Boise Cascade's fiber farms are located in the arid regions of eastern Oregon and Washington. Annual rainfall is approximately 15 to 20 cm, of which half falls as snow. Temperatures are extreme, regularly reaching 40 degrees C in the summer, and falling to below -20 degrees C in the winter. Soils are alkaline, with a pH of 7 to 8. Soil texture is extremely sandy, with very little organic matter or water holding capacity. Natural vegetation is a steppe-sage habitat dominated by sagebrush and grasses, with occasional cactus. The growing season is quite long, usually having up to 200 frost free days. The northern latitude provides additional hours of sunlight. Agricultural crops grown in the area all require irrigation, and include: corn, wheat, potatoes, alfalfa, asparagus, onions, carrots, grapes, apples, cherries, peaches, and a variety of minor crops.

IRRIGATION

Irrigation was brought to the Columbia Basin beginning with Bureau of Reclamation projects in the 1950's. These projects were open canal systems which transported water, diverted from rivers or impoundments, through prime agricultural land. Individual farms then tapped in to this vast supply. More recently, wells have been developed in some areas which supply ample water from deep aquifers. Pumping stations have also been established on rivers and lakes which supply water to independent farms or irrigation districts.

Traditional irrigation systems include: flood, rill, hand lines, solid set, wheel lines, and center pivots. Drip irrigation is relatively new to the area, and used mostly in vineyards and orchards. However, drip irrigation is the most practical method of irrigating fiber farms due to the height growth and dense canopy of the trees. It is also a very efficient method of irrigation in that less power is required due to low operating pressures and there are almost no evaporative losses.

Irrigation systems are designed specifically for the tree crop. The drip tube rows are about three meters apart. Emitters are spaced at one meter intervals along the tube. Trees are planted at every other emitter which gives an approximate planting density of 1,500 trees per ha. Irrigation blocks are designed for hydraulic efficiency based on the existing equipment and land topography. The irrigation blocks range from 100-600 ha. They are then subdivided into 15 to 20 ha planting blocks which are generally rectangular. The underground lateral pipes are 500 to 800 m long, and the above ground individual drip tube rows average 300 m.

A variety of drip irrigation system designs and equipment has been tested. Each situation may be somewhat unique, but standardization of the systems is operationally desirable. In general, Boise Cascade's preferred drip irrigation design includes: sand filtration which is essential to keep emitters from plugging, buried (1 m), telescoping PVC laterals with end flush outs, a shutoff valve, screen filter and pressure regulator on each line, pressure compensating emitters, and a pressure sensor at the end of each line. These materials may be provided by a number of manufacturers. Experience in a particular application will determine the best suppliers.

Maintenance of the drip tube is a continuous process. Each tube must be flushed periodically to remove sediment. Animal damage occurs routinely and must be monitored and repaired. Plans are to remove the above ground portions of the system before harvest, and then to reuse it for subsequent rotations.

The irrigation season lasts from April to October. Water is applied on a daily basis to keep the soil moisture slightly below field capacity. Weekly soil moisture measurements are taken

using neutron probes. Irrigation schedules are then determined based on changes in soil moisture and tree demand based on predicted weather patterns. The systems are capable of delivering up to ten mm of water per day which is the estimated maximum evapotranspiration demand of the trees. Annual water application varies between 50 and 90 mm depending on the site and age of the trees.

SITE PREPARATION

Hybrid poplar establishment usually follows other crops. Protection of the site is important due to high wind erosion hazard in the area, so, unless sufficient organic residual is left after harvest of the prior crop, a cover crop of winter wheat is established. The site is surveyed for the construction of the drip irrigation system, and the reference stakes are used to determine the tree row locations. Each row is mechanically marked by cutting a twenty cm slit in the soil. A two m band centered over the tree row is treated with Treflan pre-emergent herbicide, and rotovated during the winter or spring prior to planting. Immediately before planting, a broadcast herbicide treatment of glyphosate or 2,4-D may be applied to the site depending on the weed community that is present.

PLANTING

Hybrid poplars which perform best in the Columbia Basin are crosses between Populous Trichocarpa and Populous Deltoides or Populous Deltoides and Populous Nigra. Cloning individual hybrid genotypes is very easy due to their ability to form roots and shoots from both dormant wood and actively growing plants. Clones selected for operational planting are field tested for growth rate, cold hardiness, insect and disease resistance, and wood characteristics. Five clones have been used operationally to date in 4,500 ha of plantations. Deployment strategy is to match the clones to site conditions, and then use the fastest growing clone available for that site.

The planting stock used is unrooted, dormant cuttings produced in stool beds. Stools are established at a density of 11 per square mm. Production after the year of establishment is about 375,000 cuttings per ha. Stool beds are usually replaced after 4 or 5 years. Cutting production begins in January and is finished by the first of March. The annual sprouts from the stools are cut with hydraulic shears in the field, and transported to a facility where they are cut into 20 cm lengths by a set of parallel circular saws. The sticks are then graded by diameter, 10 to 15 mm being acceptable. A viable bud within 2.5 cm of the top is also required. The sticks are then oriented in the same direction, and placed in plastic bags of 50 cuttings. The bundles of 50 are then placed in a larger plastic bag, and boxed in batches of 500 trees. The cuttings go from the stool to the freezer the same day to prevent desiccation.

Planting begins when the soil temperature reaches 13 degrees C, usually in early April. Cuttings are kept frozen at -2 degrees C until 3 or 4 days prior to planting. At that time they are placed at the planting site to defrost and initiate root development. No treatments, such as soaking or growth hormones, are applied to cuttings.

Mechanical planting has been attempted, but as of yet has not been successful. The difficulty is placing the cutting directly at an emitter, which is necessary to assure survival. Hand planting quality has proved satisfactory, since the planting spot is marked by the wet spot from the irrigation. Production rates are very high at about 5,000 cuttings per person day. The sticks are planted to a depth where the top bud is left above the soil line. Irrigation is applied prior to planting and continuously thereafter to enhance root development.

Initial survival in the plantations is 90 to 95 percent. This stocking level is supplemented about one month after planting by an interplanting done during an irrigation maintenance operation. Stocking levels at the end of the first year average about 98 percent.

CROP TENDING

Intensive crop management is essential to producing the expected yields at harvest. The hybrid poplars are sensitive to many factors which influence growth such as competition, nutrition, and pests. Control of these environmental conditions to the greatest extent possible is necessary to obtain the maximum benefit from the clonal genetics.

Weed control is the biggest agronomic challenge facing the fiber farm. A variety of sources contribute to the difficulty in weed control including: lack of rainfall to incorporate herbicides, the sensitivity of the hybrid poplar to damage by herbicides, and difficulty with mechanical treatments because of the drip tube. Many site preparation herbicides require water, either from overhead irrigation or rainfall, to incorporate the herbicide into the soil or to activate it. Drip irrigation and the local climate do not provide the necessary water. Therefore, a tilled in herbicide is the most effective treatment, but to date a compound with a wide efficacy range on the local weed community has not been found. Once the trees are planted, any herbicide applied to the leaves can cause severe damage. Growing season weed control is limited to treating the row centers with a shielded spray or a wick application. Grass control herbicides can be used in a broadcast application. During the dormant season, a broadcast treatment is applied to remove annuals that germinate before the trees leaf out. Cultivation of any sort has generally not been successful due to damage done to the drip tube.

Maintaining the optimum nutrition for the trees is important in obtaining maximum growth. Fertilizer is applied annually throughout the growing season. Chemigation, i.e., application of treatments through the irrigation water, is the standard practice, although some treatments require application by ground equipment or by air. A mixed fertilizer containing zinc, phosphorous, sulfur and nitrogen is applied in April. This is followed by continuous light applications of nitrogen from May through the end of July. Desirable levels of foliar nutrients have been determined. Regular foliage samples are taken to determine if these targets are being achieved. If not, supplemental treatments are applied either on a broadcast or spot basis. Due to the alkalinity of the soil, micro nutrient deficiencies are the most common. These include: zinc, calcium, and manganese.

There are a number of pests that attack hybrid poplar in eastern Oregon and Washington. Insects are by far the most important, but are limited mainly to leaf beetles and stem borers. These insects can be easily controlled with systemic insecticides applied directly through the irrigation water. Results of a treatment can be seen in a few days, and the effect lasts several months. The hot dry climate is unfavorable to fungal diseases, but melamsora meduse leaf rust has occurred late in the fall after the growing season. Animal damage occurs sporadically, and can be important in localized areas. Deer browse when the trees are young can cause multiple stems and significant height reductions. Beaver damage occurs along most water ways, so an active trapping program is necessary. Mice cause the most severe animal damage by girdling young trees in the winter. This happens in weedy areas when there is a snow cover. Weed control is the best preventive measure, but baiting will reduce the incidence of damage if applied before snowfall.

RESULTS

During the first three years of operation the fiber farm has exceeded initial expectations. An annual inventory is taken on the entire farm to precisely determine growth rates and standing wood volume. This information is kept in a detailed Management Information System (MIS), which is linked to a Geographic Information System (GIS) containing layers on planting blocks, irrigation blocks, soils, elevation, etc.

The stocking rates described above are more than adequate to obtain full site occupancy. Height growth rates vary slightly from year to year depending on weather conditions. Average first year height has been about 2.5 m. Second year height has averaged 6.4 meters. Third year height has averaged 10.7 m. While there is some variation around these average values, the range is remarkably small. This uniformity can be attributed to the use of clonal material, uniform site conditions, irrigation and intensive management. Diameter (DBH) is not measured in the first year. Second year DBH averages 5.3 cm and third year DBH averages 10.4 cm.

SUMMARY

Fiber farming of hybrid poplar in the Columbia Basin is proving to be a successful venture for Boise Cascade Corporation. Tree growth is exceeding expectations. Costs to grow the crop are predictable and controllable. The operating environment is favorable from a legislative and regulatory point of view. The additional wood supply at a reasonable cost is attractive to the mill. The short white fibers of poplar wood provide benefits to the mill in addition to fiber supply which include: improved brightness, opacity and print ability in the uncoated free sheet paper grades as well as reduced bleaching.

File posted on March 5, 1996; Date Modified: February 21, 1999