Impacts of managing loblolly pine plantations for biofuels production on site productivity and sustainability

Background/Question/Methods

Utilizing forest biomass for biofuel production has emerged as a promising approach as an alternative to fossil fuels. However, very little research has focused on how to simultaneously grow biomass for biofuel production while still managing for high quality wood products.  Furthermore, the potential effect that this could have on long-term soil productivity is critical for the viability of such an intensive management system.  As a result of this gap in scientific knowledge, a long term study has been established in a recently harvested 35 year old loblolly pine plantation that investigates the effects of intercropping and biomass management on site productivity and sustainability. The study consists of the following treatments with and without logging residuals for a total of six treatments replicated four times: 1) traditional loblolly pine plantation, 2) loblolly pine with switchgrass (intercropped) between beds, and 3) loblolly pine establishment with flat-planted pine trees between crop tree beds. Pure stands of switchgrass were also established as a comparison to the other treatments. The site preparation for each of these treatments varied in intensity and number of entries. A first step in evaluating the conversion of monoculture plantations to a dual-crop system was to evaluate the potential impacts associated with land clearing for establishing switchgrass and the removal of coarse woody debris on the ‘without logging residuals’ treatments. Soil compaction measurements were obtained using a soil penetrometer to a depth of 30 cm at 2.5 cm intervals before and after treatment installation. Ion exchange membranes were used to evaluate nutrient availability in the soil surface. Tree measurements and clip plots were used to access tree and switchgrass productivity.

Results/Conclusions

There were no significant differences in soil compaction before and after treatment installation. The average reading across all treatments at the soil surface was 201 kPa which increased to 1539 kPa and 1923 kPa at 15cm and 30cm depths, respectively. Treatments that included switchgrass had lower available N. There was no significant effect of switchgrass intercropping or intensive biomass removal on tree productivity in year two of the study. There was no effect of biomass removal on switchgrass productivity. The intercropping of switchgrass and the removal of biomass in this loblolly pine plantation is sustainable thus far (two years after treatments installed).