Intensive Culture of Hybrid Poplars in Minnesota

Don E. Riemenschneider and Daniel A. Netzer, USDA Forest Service USDA Forest Service, Rhinelander, WI

William E. Berguson, Natural Resources Research Institute, Duluth, MN

Intensively cultured poplar plantations are well known in the Southern and Northwestern United States where they have made significant contributions to industrial fiber supplies. Intensively cultured plantations are less well known in the North Central United States, with interest historically centered on production of alternative fuels. But, several driving forces have recently combined to increase interest in intensively cultured plantations in the North Central region, especially in Minnesota. In response, about 6,000 acres of hybrid poplar plantings have been established as of 1996. A few operational scale plantings are as old as age six years, but most are three-years-old or younger. Support for research on cultural methods, site selection, genetics, and other areas has increased with the formation of the Minnesota Hybrid Poplar Research Cooperative. In the following sections we describe the forces that have caused increased interest in intensively cultured plantations, current cultural methods, current acreage and expected yields, and our current research emphases.

Driving Forces

Increased attention to the need for intensively cultured poplar plantations in Minnesota has been catalyzed by three driving forces. First, increased needs for non-timber resources on public forests has resulted in reduced harvests, the perception that harvests will be reduced in the future, or both. Second, industrial expansion in paper and oriented strand board (OSB) production has increased demand for aspen and other species that can be substituted for aspen. Third, the supply of natural aspen is unevenly distributed throughout all age classes and reduced availability is anticipated within the next 20 years or so. The aforementioned forces have combined within the last 10 years to drive the average price for aspen stumpage in Minnesota from about $3.50 per cord in 1986 to about $20.00 per cord in 1996 (Figure 1). In fact, some recent aspen sales in northern Minnesota have gone much higher, in excess of $40.00 per cord. The combined effects of all forces has made intensively cultured poplar plantings an attractive alternative wood supply.

Current Cultural Practices

Cultural methods almost always involve the use of herbicides. Some of the uses described in the following are for research purposes only and therefore subject to categorical exclusion. The reader should always ensure that a proposed commercial application is within the label. Site preparation protocols involving herbicides differ by region within the State of Minnesota. Initial site preparation is a good example. Fall tillage, marking planting rows by mechanical scribing, and the application of preemergent herbicides such as Oust™ or Lorox™ is common in the Alexandria area. Spring tillage is less acceptable, but is sometimes the only choice if access to sites in the fall is not possible. Chemicals can be applied in the spring prior to planting but application rates, especially for Oust™, must be reduced. Lorox™ is commonly applied immediately after planting.

Further north, sites enrolled in the CRP often have deep sod layers and sites benefit from prescribed burning before tillage. Cultivation in the spring is often necessary in the north because heavier northern soils can be seriously compacted over the winter. There is some debate regarding universal herbicide prescriptions across Minnesota. But, Lorox™ is commonly applied after planting in the north. In any case, an aggressive combination of chemical and mechanical vegetation management strategies is most commonly practiced to achieve a completely weed-free condition prior to planting in the spring. One possible exception to this rule is a minimum tillage strategy with poplars following a soybean crop. Some success using single trench tillage has been observed in this scenario.

Current clone selection is limited and the most commonly planted clones are DN-34 (P. deltoides x P. nigra cv. Eugenii) and NM-6 (P. nigra x P. maximowiczii). Other recommended clones are DN-2, DN-5, DN-70, DN-182, NE-222 (P. deltoides x P. caudina) and I-45/51 (P. deltoides x P. nigra) (Hansen et al. 1994). Nursery supplies of all clones is increasing although current demand is high in relation to supply especially as planting season nears. The scarcity of suitable clones has lead us to place a high priority on state and regional breeding efforts (see subsequent section).

Dormant stems are harvested from stool beds, usually in December, then cut into 10 inch cuttings and stored either frozen or refrigerated until shipment. Cutting quality guidelines are currently limited to size categories with an acceptable caliper range of 3/8” (~1.0 cm) to ¾” (~1.9 cm). Shipment by refrigerated transport is recommended. Cuttings are soaked in water at room (ambient) temperature to stimulate root growth. Soaking lasts for about 5 days, but cuttings should be observed frequently so they can be planted before adventitious roots are fully emerged and subject to damage.

Cuttings are mostly planted by hand in Minnesota. Fields can be scribed in two directions using a variety of mostly custom-built tractor-drawn tools. Two-directional scoring yields even spacing in both directions which eases cultivation throughout the first growing season. One-directional scoring often yields uneven within-row spacing that can limit cultivation to one direction, especially when cuttings are planted by machine. Then, weed growth within the row is more difficult to control. Poor within-row weed control can, in turn, increase reliance on chemical strategies. The most serious noxious weed problem in our plantings has been various species of thistle that are unaffected by Oust™. We have controlled thistle (Cirsium spp.) by mid-season spot application of Stinger™. Actively growing poplar trees are not killed by Stinger™ but we have observed some transient stem and leaf deformation, perhaps as a result of the chemical possessing limited auxin activity.

Proper use of chemicals usually results in a weed-free plantation through mid-July of the first year. Then, aggressive mechanical tillage is required because weeds will inevitably outgrow newly planted trees through this part of the summer. We have observed that, in some plantings, as much as 2/3 of total first year growth in height, stem caliper, and leaf number occurs after August 1, thus it is important to continue cultivation throughout the first growing season.

Cultural practices in years 2 and 3 are much the same as in year 1. Fall application of Oust™ and/or Roundup™ is recommended after leaf drop. Spring application at reduced rates prior to flushing, or even greening, of the buds is an alternative. Mechanical cultivation continues throughout year 2. Crown closure occurs late in year 2 or during year 3 at which time weed control treatments may be suspended. Mechanical cultivation for non-weed management objectives such as soil aeration may be continued, however we have little solid research that documents whether such cultivation is beneficial.

Plantations are not commonly irrigated in Minnesota. But, we are aware that corporate interest in this practice is surfacing. Whether the marginal benefits of irrigation, which can be manifold considering the ease with which mineral nutrients and pesticides can be applied in addition to water, outweigh marginal costs has yet to be determined.

Acreage and Expected Yields

Minnesota now contains approximately 6,000 acres of intensively cultured hybrid poplar plantations, mostly attributable to three activities. First, almost 2,000 acres have been planted in the vicinity of Alexandria, Minnesota with support from the Biofuels Feedstock Development Program of Oak Ridge National Laboratories (ORNL) and with landowner assistance from the Minnesota DNR and WesMin RC&D. Plantings are up to three years old and new plantings have been established as recently as 1996. Lands are predominantly enrolled in CRP, with landowner costs partially subsidized by the CRP, Minnesota DNR tree planting incentives, and operational support from ORNL. Second, an additional 2,000 acres have been established in the vicinity of Crookston, Minnesota. Landowners in the Crookston project are mostly under long-term contract with Minnesota Power and Light and receive technical assistance from the University of Minnesota, Crookston and the Agricultural Utilization and Research Institute, Crookston. Lastly, about 2,000 acres have been planted by various fiber-using companies. Land ownership in this class mostly includes acreage that has been under long-term corporate ownership or that has been recently acquired specifically for the deployment of intensively cultured plantations. Some uncontracted plantation establishment by individual landowners is found in addition to the aforementioned activity. The ultimate grower-consumer relationship is difficult to define at this time. A mosaic of many different strategies involving land ownership, risk sharing, and cash flow agreements may be the most likely outcome.

Most intensively cultured plantings in Minnesota are three years old or less. However, some large-plot yield trials were established in 1987 and 1988 (Figure 2) and limited yield estimates are available (Table 1). Data demonstrate the importance of both site selection and clone selection (Table 1). Indications are that choice of the proper site can be as important as selection of the proper clone (Table 1). Mean annual increment shows indications of culmination in some plots at the end of age 8 or 9 years. However, we believe that additional study is required for three reasons. First, analysis has shown that leaf nitrogen content in most plots has declined to about 2%, which may be insufficient to sustain rapid growth past mid-rotation. None of the studies described here have been fertilized after year four, but mineral fertilization is planned in the future, after which growth increment will be reevaluated. Second, our equations relating tree diameter and height to total above-ground biomass have not been recently re-calibrated. Experience has shown that biomass equations developed in our region inevitably underestimate biomass when the calibration population is younger than the population of current interest. Sampling is now being done to develop new equations. Last, culmination of mean annual increment in our research plots appears to be both site and clone dependent. Knowledge of the true regional range in culmination, and thus the ability to forecast yields and construct prudent silvicultural recommendations, can only be determined after culmination has been clearly demonstrated over a wide range of sites and clones. Overall, current research has demonstrated that yields based on the use of best practices can approach 5 dry tons per acre per year (Table 1).

Current Research Emphasis

Landowners and industry in Minnesota are fortunate because a research infra-structure has been in place in Minnesota for many years. In fact, much of the intensive culture technology that has been deployed in the Northwest and South was originally developed by USDA Forest Service, North Central Forest Experiment Station and cooperators at the University of Minnesota with support from various programs at ORNL.

A new hybrid poplar research cooperative has been organized with funding from the Minnesota State Legislature and Minnesota wood-using industries. Current research, supported by the new cooperative and ORNL, has several emphases. First, we are developing new clones through breeding and selection. Over 10,000 new clones of eastern cottonwood, F1 hybrids between eastern cottonwood and other species, and advanced generation backcross populations have been produced with the objective of increasing the genetic diversity of hybrid poplar production populations. Selection criteria include growth potential, adventitious rooting ability, and resistance to Septoria canker. Studies of rooting ability are of special interest because a large component of our selection program is devoted to pure eastern cottonwood, which roots erratically in our region under commercial planting conditions. Resistance to Septoria canker is also critical because that disease limits deployment of our fastest-growing selections. Early selections from nursery tests are entered into an ORNL-supported regional clone trial program with test locations in Minnesota, Wisconsin, Iowa, and Michigan.

Second, we are studying new vegetation management strategies that include use of more effective and environmentally benign chemicals, along with mechanical tillage methods. Third, we are revisiting previously established guidelines for planting stock quality and planting methods with the objective of increasing early growth while reducing planting costs. Fourth, we continue to monitor growth and yield of large plot clonal trials established as early as 1988. Research partners in this cooperative include University of Minnesota, Crookston; The Agricultural Utilization and Research Institute, Crookston; The Natural Resources Research Institute, Duluth; The University of Minnesota, Department of Forest Resources, St. Paul, and the USDA Forest Service, North Central Forest Experiment Station, St. Paul.

Summary

Deployment of intensively cultured hybrid poplar plantations is feasible in Minnesota. Yields are lower that those achieved with similar methods in the North West and South, but costs are lower as well. We have demonstrated that a combination of wise site and clone selection can produce potential yields of 4.7 dry tons per acre per year. A combination of factors including competing forest uses, industrial expansion, and the age distribution of native aspen stands has caused increased interest in the deployment of intensively cultured plantations to increase fiber supply. Several groups have established approximately 6,000 acres of plantations, mostly three years old or less, in Central to North Western Minnesota. A substantial research infrastructure exists to support the sustained development of intensive culture technology. Current research emphases include genetics, cultural improvements, vegetation management strategies, and growth and yield studies.

Literature Cited

1. Hansen, E.A., Ostry, M.E., Johnson, W.D., Tolsted, D.N., Netzer, D.A., Berguson, W.E., and Hall, R.B. 1994. Field performance of Populus in short rotation intensive culture plantations in the north-central U.S. Res. Pap. NC-320. U.S. Department of Agriculture, Forest Service, North Central Forest, Experiment Station. St. Paul, MN. 13p.

File posted on March 17, 1998; Date Modified: February 21, 1999