Change in Yield Between First and Second Rotations in Willow (Salix spp.) Biomass Crops is Strongly Related to the Level of First Rotation Yield

Yield is a critical factor in the development and understanding of willow biomass crops which experience multiple harvests after planting. Small changes in yield can have substantial impacts on economic and environmental assessments. Studies have reported increases between first rotation yield (FRY) and second rotation yield (SRY); however, there is minimal agreement on the variation of this increase. This study analyzes FRYs and SRYs of commercial willow cultivars in 360 research plots across five sites in the Northeast and North Central USA. Mean FRYs were 9.6 Mg ha(-1) year(-1) and mean SRYs were 7.9 % greater at 10.3 Mg ha(-1) year(-1). The relative change in yield between rotations was high for plots with low FRYs, but decreased as FRY increased and was negative when FRYs were highest. Therefore, applying a single yield increase factor to all willow crops may result in errors. Linear and logistic regression modeling were used to predict the magnitude of yield change across the range of FRYs and the probability for increasing/constant yields. Results showed that FRY alone predicts yield change with an R (2) of 0.635, and adding cultivar and site/management factors increases R (2) to 0.697. One study limitation is that many plots with the highest FRYs came from a drought influenced site. A meta-analysis of literature data revealed that this pattern of decreasing relative gains as FRYs increase is widespread though not previously addressed. Acknowledging this pattern should provide more accurate yield estimates over multiple rotations.