Boron deficiency causes growth disturbance in spruce forests on slash-burnt areas

The final report on "Growth disturbance in spruce forests on burn-beaten area" is released (in Finnish)

A regional study on the phenomenon of growth disturbance was started in northern Savo in the late 1990s funded by the Employment and Economic Development Centre and the Ministry of Agriculture and Forestry. The more extensive project “Growth disturbance in spruce forests on burn-beaten area” was started in 2000 to explore the reasons, distribution, economic impact and control methods of the growth disturbance in mineral soil forests. The five-year project was accomplished as collaboration between the Finnish Forest Research Institute, the Faculty of Forestry at the University of Joensuu, and the regional Forestry Centre Northern Savo, funded by the Ministry of Agriculture and Forestry.

Boron deficiency is recognized in many parts of the world as a cause of growth disturbance. Also in Fennoscandia, the boron concentration in solid rock is low. In Finland boron is deficient especially in areas far away from the sea, where boron concentration of rain water is lower than at the coast. It is likely that the traditional burn-beaten cultivation and forest grazing husbandry have also increased boron deficiency in mineral soils, which, in turn, adds to the risk of growth disturbance in currently thriving forests. In peatlands boron deficiency was identified as the main reason for growth disturbance as early as in the 1970s.

Boron-derived growth disturbance can disappear for a while, but it often reappears. Multiple-shoots and bushy tops are signs of earlier disturbance. Alternation of disturbed and normal growth most likely depends on variation in weather conditions. During dry years trees obtain only minimal amounts of boron and signs of drought are first seen in trees in which the boron concentration is already low.

Boron deficiency is not the only reason for growth disturbance. Damage as a result of frost, diseases and animal interference, especially to plantations and young stands is difficult to identify retrospectively. However, these disturbances disappear when the stand grows. Disturbance caused by Thekopsora areolata in fully-grown stands and young regeneration forests is sometimes difficult to tell apart from an early-stage disturbance due to boron deficiency. Boron fertilisation does not reduce damages caused by Thekopsora areolata. In contrast, however, boron fertilisation does seem to improve slightly frost-resistance characteristics of trees. However, frost is not likely to be a very significant cause for growth disturbance.

White birch suffers growth disturbance too. Birch forests regenerated especially on fine-textured mineral soils and peat-based fields comprise often crooked and multiple-shoots and bushy tops. Boron deficiency can only explain a part of such damage. The main reasons are presumably due to other problems in soil nutrition and water resources.

For the purpose of determination of fertilisation need, the current recommendations for boron concentration in needles are still valid. On the other hand, the performed seedling tests indicate that more boron is needed to guarantee undisturbed growth of roots than of the shoot. However, this observation still needs to be investigated further.

The results of the research project as well as practical experience show that boron added to fertilisation, when carried out sufficiently early, ensures healthy development of spruce forests in fertile, boron-deficient mineral soils. Other defects found in spruce forests are usually temporary and do not cause considerable growth disturbance.