Intercropping giant miscanthus (Miscanthus × giganteus) with loblolly pine (Pinus taeda) for bioenergy feedstock production in Southeastern United States: Effects of shading on biomass yield

Background/Question/Methods

There is growing interest in utilizing dedicated biomass crops as feedstock for bioenergy production.  Giant miscanthus (Miscanthus × giganteus), a perennial, warm season grass is one potential high yielding, bioenergy crop.  In the Southeastern United States, forest-based biomass may be able to provide raw materials to help build a biomass-based energy industry.  Loblolly pine (Pinus taeda) planted on wide row spacing allows for the planting of dedicated energy crops between the trees.  The growing conditions in the inter-row area between trees can be favorable for grass growth early in the rotation, prior to canopy closure. However, as the stand ages, the trees will grow taller and leaf area increases.  A greenhouse study was conducted for 186 days (March-September, 2011) to examine the effect of varying light levels on the growth of giant miscanthus. Light levels inside the greenhouse were varied with shade cloth, allowing transmission of varying amounts of photosynthetically active radiation.  The objective of this study was to determine the effects of increased shading on the growth of Giant miscanthus.  During the growing season, daily photosynthetically active radiation (PAR) was measured with hourly averages collected by a data logger.  Above and below-ground biomass was harvested to determine biomass yield.

Results/Conclusions

Biomass yield varied significantly between shade levels with the lowest shade level resulting in 6.5 times greater biomass as compared to the heavest shade level.  Material harvested above ground was a good predictor of below-ground rhizome biomass.  The ratio of above to below-ground material averaged 4.2:1.  Specific leaf area and biomass yields along with recorded PAR values were utilized to find aPAR and light use efficiency.  Light use efficiency did not vary between treatments and averaged 4 grams of above ground biomass per intercepted megajoule (MJ).  This study demonstrated the high light use efficiency of Giant miscanthus under ideal growing conditions.  Giant miscanthus may be able to grow to full potential in an environment with daily maximum PAR levels above 650 µmoles m^-2 sec^-1.  Beer’s law predicts the maximum leaf area index of a tree canopy below which Giant miscanthus can grow unhindered is around 2.5 m² leaf surface m^-2 ground area.