Stem Cracks in Norway Spruce (Picea abies (L.) Karst.) in Denmark

1988

S. 53-69

8 Abb., 2 Tab., 16 Lit. Ang., Da, Zfsg. En, Da

Unselbständiges Werk

200006935

Stem cracks have in recent years been the object of some attention in North America, France, the Federal Republic of Germany, and, not least, in Scandinavia. This interest is due, among other things, to the fact that for the individual tree stem cracks represent structural weakening, inroads of pathogens, and loss of value. Since, furthermore, cracked trees belong to thebiggest trees in the stand (see Figures 2 and 7), even a few damaged trees ma y influence the stability and economy of the stand. The Danish Forest Experiment Station has during recent years registered stem cracks in connexion with regular measurements of provenance-, thinning-, tree species- and spacing experiments with Norway Spruce. In 4 out of 12 examined provenance experiments aged 19-23 years from seed, 100 cracked trees were found, which corresponds to an average crack incidence of 0.67% of the trees growing on the sites where stem cracks occurred (see Table 1). No stem cracks were observed in younger or older provenance experiments. In 8 older (37-90 years) and 2 younger (27 and 31 years) thinning experiments very few cracks were registered. Only one younger thinningexperiment showed a somewhat higher number of stem cracks at the age of 22 yea rs. The plotwise crack frequency varied here from 0 to 0.9%. In only one thinning experiment (outside the management of the Forest Experiment Station), where extremely heavy thinning treatments had been employed, a considerable number ofstem cracks were observed. In the most heavily thinned plot the stem crack rat e was 4.8% (see Table 2). No cracks were found in 3 fertilization experiments (49 years). In 17 tree-species experiments (17-53 years) there were only few, unsubstantial slits. The same applies to 3 younger (22-23 years) and 2 older (47 years) spacing experiments, but not to a younger spacing experiment (22 years),in which there were 85 stem-cracked trees. In the plot with the widest spacing (3.25 x 3.25 m) the crack rate was 18% (see Figure 8) . Based on a thinning exp eriment with 63 stem cracks distributed among 40 trees, exterior cracks have been analysed with regard to appearance and position on the stems, generally the crack follows the sinistrorse fibre direction in the stem, and the crack most often starts within the lowest 5 metres from the ground. The length of the crackvaried in this experiment from 0.3 to 4.8 m with an average length of 1.9 m (s ee Figure 3). However in a younger thinning experiment a crack length of 6.9 m has been found. The cracks are mainly facing westerly and northerly directions (see Figure 4). Summarily it may be said that the conditions which cause cracksare drought (especially late-summer drought) and forced growth during the youn ger stages of the life of the spruce.