Poplars are among the most frequently cultivated trees in European short rotation coppices in Europe for both energy and material use. In a randomized trial, 37 different poplar varieties were cultivated on a typical sand-dominated site under continental climatic conditions of the Northeast German Lowlands. Planted in 2012, the poplars were harvested every three years based on the mini-rotation system. Survival rates were determined after the first growing season in 2013 and again right before the three harvests in 2015, 2018, and 2021. Important plant physiological parameters (i. e. resprouting capacity, plant height, and diameter at breast height DBH) were also recorded for the three harvests, and the harvested biomass was calculated in Mg ha −1 y −1 DM (Megagramm dry mass per hectare and year). Survival rates decreased to varying degrees for all poplar varieties. The loss of individuals was particularly severe in the first year after planting and in the last rotation period. Resprouting capacity, however, increased continuously for all poplar varieties over the nine years. The yields of the first harvest in 2015 were not analyzed, since these results do not reflect variety-specific characteristics or site conditions, but rather depend on the technological framework. In the second rotation interval, yield increases attributed to plant height and DBH were generally recorded for all poplar varieties at the 2018 harvest. Seven poplar varieties achieved more than 11 Mg ha −1 y −1 DM and can be recommended for commercial use. However, many varieties only yielded around 8 Mg ha −1 y −1 DM. Six varieties, including newly bred varieties, even yielded less than 4 Mg ha −1 y −1 DM. At the third harvest, significant shifts in the ranking of the poplar varieties in terms of biomass yield were observed. For example, poplar varieties that had medium yields in the second harvest, such as AF 19, now achieved up to 17.13 Mg ha −1 y −1 DM. A total of 16 poplar varieties increased their yield compared to the second rotation interval, while 21 varieties produced poorer harvest results. The yield increases were primarily achieved by new Italian varieties (i. e. AF 19, AF 2, AF 13), whereas older, well-known varieties such as all Max varieties, Fritzi Pauley, Androscoggin, and Muhle Larsen suffered significant yield reductions. It was conspicuous that the varieties with the greatest yield increases had the lowest survival rates. Since exceptionally high temperatures coupled with significantly lower precipitation were recorded during the three years of the third rotation, it is reasonable to conclude that the poplar varieties with high survival rates and therefore high plant densities suffered from drought-induced growth stress. Therefore, when establishing or re-establishing SRC, lower plant densities and longer rotations should be favored, combined with new drought-tolerant poplar varieties. • SRC yield is impacted by drought, planting density and chosen poplar variety. • Reduced planting density may promote biomass yield by mitigating drought-induced stress. • Midi-rotation (6-10 years) achieves generally higher yields than mini-rotation (2-4 years). • Poplar varieties should be carefully selected to match the site characteristics for optimal results/yield.
Landgraf et al. (Sun,) studied this question.