Productivity and thinning effects in hybrid aspen (Populus tremula L. × P. tremuloides Michx.) stands in southern Sweden

Abstract

Summary The productivity of hybrid aspen stands in southern Sweden, originating from clone selections performed during the 1980s, was investigated. Thirteen former research and demonstration sites were included. Repeated measurements of tree diameters and heights were taken, from which yields were estimated. The results indicate that the mean annual increment, obtained without any artifi cial addition of fertilizers or irrigation, will exceed 20 m 3 of stem wood ha � � a � � during a 20 – 25-year rotation period. This corresponds to a total average woody biomass production, including branches, of over 8 tonnes dry matter ha �� a �� . These production levels show that hybrid aspen is a competitive alternative for short rotation forestry in Sweden, and that the clone selection programme has greatly enhanced productivity, compared with published growth rates of the material previously used. For comparison, two stands that regenerated via root suckers from material used in the breeding programme up to the 1960s were included. They showed higher growth than the 13 planted stands, mainly due to higher stem density and faster height development. This indicates that even further increases in hybrid aspen yield are possible in stands derived from suckers of the most recently selected clones. Thinning studies, including three different weights of thinning, were carried out on fi ve of the sites. Five years after the start of the treatments, stands given no thinning showed signifi cantly higher current annual increment, but signifi cantly smaller mean stem diameter than stands in which thinning was applied. However, the faster diameter development (and thus higher volumes of valuable wood assortments) associated with heavier thinnings may compensate economically under current market conditions for the loss in total volume production.