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Short Rotation Biomass Crops for Alaska

  • Updated Short Rotation Biomass Photo 1
  • Shortrotationbiomassphoto2
  • Updated Short Rotation Biomass Photo 3

Project Summary

The project is designed to assess growth rates of balsam poplars under short rotation on an experimental land-fill in Southcentral Alaska to determine their applicability as a sustainable heating fuel for rural Alaskan communities.

Project Need

Balsam poplar grows naturally in Alaska, and is found generally north of the Alaska Range. However, very little information is known about the growth rates of balsam poplar under short rotation management.

Understanding the growth rates of balsam poplar when used as a biomass resource is important when planning a biomass energy project. Knowing the length of the harvest intervals of a stand of trees can help decide if the stand can provide enough wood to be a sole resource, or if it would be better used as a supplimental resource to another source of biomass.

Project Description

In 2004, 144 trees, including balsam poplar, were planted on an experimental land-fill cap on Elmendorf Air Force Base to observe the evapotranspiration effects of the trees in removing ground water from a landfill site. In 2010, those trees were harvested, fertilizer applied to half of the trees, and the regrowth of the trees from the roots (coppicing) was observed for two years.

In 2012, the two year re-spouted old tree stems were measured for diameter at 12” and 51” above the ground. The trees were then harvested and weighed for total wet biomass. The trees were dried at 105oC until bone dry, and reweighed for total dry weight.

The measurements taken were used to create an equation that can predict the amount of biomass in a standing tree.

Project Results

 The biomass collected in the first harvest (7+ years growth) measured 9.6 ton/ac, and after two years the regrowth measured 5 ton/ac or 2.4 ton/ac/yr. The trees in the second rotation grew faster due to the well established root system.

The trees in the fertilized area did not grow any faster than those in the non-fertilizes areas, however more weeds were observed. The dry weight of the stems were plotted against the diameter of the stems to create an equation that can be used to measure trees of the same species and age to determine the amount of biomass in the live tree. This can be a useful tool when deciding when to harvest a tree for biomass energy.

The balsam poplar wood was also tested for energy content using a bomb calorimeter. The biomass harvested in the second rotation contained an average of 8460 BTU/lb, or the equivalent to 631 gallons per acre of diesel using a straight conversion.

Project Conclusions

Short rotation woody biomass may be a viable option for some areas of Alaska. Managing the local poplar, aspen, willow, and other hardwood species for short rotation can be an alternative to planting a crop. These trees, if managed well, can suppliment slower growing species like spruce and birch. Using trees close to the community can also decrease the distance needed to travel and associated fuel costs to harvest wood.

More research is needed to understand the optimal rotation length between harvests. A longer rotation time of 10 to 20 years may allow enough biomass to accumulate per acre to provide a majority of the biomass fuel needed annually.

Photo 1:  Four month old balsam poplar after harvest . Courtesy of A. Byrd, ACEP/UAF.

Photo 2: Calipers measure the diameter of a 2 year old balsam poplar stem. Courtesy of A. Byrd, ACEP/UAF.

Photo 3: Bomb calorimeter used to measure wood energy.  Courtesy of A. Byrd, ACEP/UAF.