Finnish Forest Research Institute  Metla

Media release 5.4.2005

The quality of the growing site determines the likely benefit of fertilization
- the risk of nutrient problems can be assessed

The present classification of forest habitats is not sufficiently accurate for defining acute or potential nutrient deficiencies in forest stands. The Finnish Forest Research Institute (Metla) has developed reliable methods for detecting nutrient deficiencies. Certain indicators, such as the level of decomposition and thickness of peat, occurrence of certain vegetation species, or the tree’s visible reactions to deficiencies, e.g. needles turning yellow, can be used to predict the development of the nutrient status of trees. Information on the origin of the peatland is also essential to facilitate reliable assessment of the nutrient status and need for fertilization of the peatland stand.

A Metla research project has developed a method to assess the effects of fertilization on different peatland stands. The research also reveals the limits to which fertilization is potentially profitable. For example, a nitrogen-rich fen with moderate tree cover, but deficient in potassium and phosphorus is worth fertilizing. On the other hand, in nitrogen-poor bogs the benefit is less profitable and the result is negative. Hence, the biggest increases of growth are gained in nitrogen-rich fens. The research shows that their growth response increases 2-3 times for 20-30 years compared with unfertilized stands. The internal rate of return for the fertilization investment can be as much as 15 %. Of the tree species, the response by Scots pine (Pinus silvestris) is stronger than with Norway spruce (Pices abies) which, in turn, is stronger than that with downy birch (Betula pubescens). The research thus provides more accurate selection criteria for fertilization targets.

Various levels of potassium and phosphorus deficiencies are found in approximately 15-20% of the total peatland drainage area. According to this research, in actual wetlands and swamps as well as in thinly-peated areas trees are often able to make use of the growth medium’s mineral nutrients well enough to avoid the danger of serious nutrient imbalance. The effect of fertilization in such conditions is insignificant. In order to ensure successful fertilization in aged drainage areas where vegetation has changed, it is essential to be able to distinguish peatland areas that are rich or poor of nitrogen and to recognize the original peatland type (fen or bog). The likelihood of nutrient deficiency is clearly increased, when peat thickness exceeds 40 cm.

Accurate information on nutrient status of the peatland stand through needle analysis
Peat analysis has proven to be a fairly good tool to measure the nutrient content of peatland stands in forest drainage areas and in areas that have been boggy fields or peat digging grounds. The most important indicators for the nutrient content of the growth medium are its nitrogen content, level of decomposition and the type of peat. Needle analysis, on the other hand, reveals the nutrient status of the peatland stand at a given time.

Traditionally, needle samples for a nutrient status analysis have been collected during winter time. This study shows that samples collected during early autumn can also be used to gain reliable results for a nutrient status study of coniferous trees. Results show that needle and leaf analysis is suitable for identification of nutrient deficiencies and determination of optimal nutrient contents not only in pine but also in spruce and downy birch.

Drained peatlands have been fertilized since the 1950s, mainly with phosphorus and potassium but also with nitrogen and trace elements. By the early 1990s the amount of fertilization decreased drastically, but during the past few years it has started to increase again when measured in fertilized areas. The clearest growth has occurred in the use of ”health fertilizers” supported through public funds (KEMERA funds). Health fertilizers are used to correct the nutrient composition of the peatland stand in sites with nutritional imbalance. The research project on nutrient economy was part of the Ecologically and economically sustainable forestry on drained peatlands (SUO) research programme carried out by Metla in 1999-2003.

Additional information:

  • Senior Researcher Mikko Moilanen, Finnish Forest Research Institute/Muhos Research Station,
    tel. +358 10 211 3743 or +358 50 391 3743, email: mikko.moilanen @ metla.fi

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