Evaluating the biomass production of coppiced willow and poplar clones in Michigan, USA, over multiple rotations and different growing conditions

Willow and poplar are both important crops for Short Rotation Intensive Culture (SRIC). Here, we report the results of biomass yield trials at three locations in Michigan, where the average annual yield of twelve willow and two poplar clones where compared over three- or four-year growth periods after coppicing. On average, fast-growing hybrid poplars (NM5 and NM6) were more productive than any of the willow clones tested by about 30%. Poplar and willow revealed different growth patterns after successive harvests; poplars grew quickly and consistently while willow growth was initially slower but increased over time. Willows at the intensively managed site in southern Michigan were usually 2–5 times more productive than at the two less intensively managed sites in southern Michigan and northern Michigan, but poplars at both southern sites were similarly more productive than at the northern site. The annual yield of clones was further standardized to account for differences in growing degree days (GDD) between sites in southern and lower Michigan, which caused differences in the relative ranking of some willow clones to change. The results suggest that GDD-standardized yield rates may be useful for comparisons of clonal performance over larger regions. Decomposition of clonal yield rates into individual stool yield and survival rates revealed a strong positive correlation (r2 = 0.94) between them, indicating that faster-growing clones also had higher survival rates. The best performing clones for biomass production were also identified.

► We evaluate biomass yield production rates of 12 willow and 2 poplar clones.
► Willows grew slower than poplars, but willow growth increased over multiple harvests.
► Survival and growth rates of clones were highly correlated.
► Rankings of clonal performance are sensitive to growing degree days.