Epsilon Archive for Student Projects

Abstract

European ash Fraxinus excelsior L. is a large deciduous tree species common throughout Europe. It can grow on a wide range of sites and is often found in mixed broadleaved forests. Ash rarely occurs in pure stands, which probably explains why silviculture of the species has received little attention in the past. If managed carefully, ash stands can produce a valuable timber on relatively short rotations. Ash has also been an important tree out of an ecological and cultural perspective since way back in time. But now the future of the ash is threatened
by ash dieback, a disease first discovered in Poland 1992. Since then ash dieback has rapidly spread to most parts of Europe, causing high levels of mortality in all age classes. The understanding of the disease is still limited and there are only few guidelines suggesting how infected stands should be managed.

The main purpose of this study was therefore to investigate the influence of silviculture on ash dieback stands and provide guidelines on how these stands should be handled both from quality and health aspects. Measurements and assessments were carried out in four experimental stands with young ash in Denmark during the summer of 2013. During 2005-2007 the experimental plots were installed and thinned to following four stem densities: (1) unthinned control plots (1700-500 trees/ha), (2)1500 tr/ha, (3) 500 tr/ha and (4) 100-150 tr/ha. Results showed that no silvicultural treatment resulted in stands where most trees had small diameters, low yearly diameter growth, poor quality due to sweeps and crooks on the stem and primary crowns without leaves. Sample plots thinned to 1500 tr/ha gave the most optimal results, with a significantly higher share of trees with good quality, no epicormic shoots and good primary crown score compared to the other treatments. However, field observations strongly suggest that water drainage conditions play an important role for the results. In the majority of the plots a clear trend was seen with more dead and dying trees at the bottom of dips and healthier trees on slopes and on higher level ground.

Active management recommendations in stands with many healthy trees are selective thinnings of intermediate strength, where bad trees are removed. In stands with extensive attacks by ash dieback, it may be a good idea to replace pure ash stands with a mixed forest. A more passive approach where dead and dying trees are left could also be a good alternative. This increases the amount of dead wood, important for biodiversity and could possibly prevent rash fellings of ash dieback stands. Currently the future of the ash is very uncertain, but with more knowledge of ash dieback and how infected stands should be handled, the hope of saving this valuable broadleaved tree species increases.