Efficient biomass production through management like thinning is crucial for increasing the supply of renewable and carbon neutral feedstock. However, change in growth rates may alter feedstock properties and affect subsequent bioenergy conversion, material and chemical production. This study evaluated the effects of thinning treatments and stem diameter on the fuel, elemental, and structural composition of stemwood and bark from second-rotation poplar plantation (original stand: 1100 stumps ha−1). Two different thinning methods were applied: row thinning (removing all stems for every other row of plantation and reducing stump density to 550 stumps ha−1) and stem thinning (retaining only the single largest stem per stump). The results showed that thinning method and stem diameter affect fuel and lignocellulosic composition. Single-stem trees at high stump density had the best fuel traits, with low ash and high volatile matter to fixed carbon (VM/FC) ratios, reflecting reduced growth competition. Smaller stems contained more ash and VM/FC in bark. Carbon, hydrogen, and nitrogen contents were not affected by treatments. Single-stem trees had higher hemicelluloses and lower lignin, indicating more complete cell wall development, while crowded, multi-stem conditions increased lignin. Highest extractives were found in bark from low-density single-stem trees. Both total biomass and structural components yields were highest for single-stem trees without row thinning. It highlights the benefits of stem thinning. This study suggests that both quality and quantity of biomass from second-rotation poplar plantation can be influenced by thinning treatments and stem diameter, with potential implications for bioenergy and bio-based chemicals or fuels.