Finnish Forest Research Institute  Metla

Media release 16.12.2005

Warm July increases the height growth of Scots pine during the following year

The warmer it is in July, the better the height growth of Scots pine in Lapland during the following season. Contrary to this, warm weather in November has an adverse effect on height growth after two years. Precipitation has no effect on growth. These findings were reported in a study made at the Finnish Forest Research Institute.

Correlations between the height growth of Scots pine and monthly-level climate indicators were explored in this study in five stands in Lapland, located in Rovaniemi, Sodankylä, Laanila, Kaamanen and Kevo. The study period ranged from the 1950s until 1996.

The most important factor controlling annual height growth is the ambient temperature of the previous July. The warmer the July, the better the height growth during the following year. The correlation was the strongest in the three most northern stands. The average annual height growth in the Rovaniemi experimental stand was approximately 25 cm and that in Kevo, approximately 15 cm. A difference of one degree in July temperatures led to an average change of 1.8 cm in the height growth for the following year. Hence, the variation range in height growth caused by differences in annual July temperatures was as much as 10-15 cm.

The height growth of Scots pine is predetermined in that the growth units are formed in the buds during the previous summer, mainly in July. Thus the July weather conditions are decisive for height growth. How quickly the shoots develop to their full size will depend on the conditions and especially the temperature during the early summer. However, the final shoot length is already determined in the previous year.

There is a negative correlation between the average November temperature two years earlier and height growth: a warm November reduces the number of growth units, resulting in shorter height growth on year later. A warm late autumn is harmful to trees, most likely because high temperatures increase respiration of trees, which reduces their energy resources. Precipitation has no effect on height growth.

The results presented above concern only the northern boreal forests in Lapland. At more southern latitudes, the significance of temperature is reduced and that of precipitation is increased.

The results of the study are applicable in dendroclimatology that aims to clarify climatic conditions of the past using samples of old wood. For example, by examining old tree trunks lifted from mires and lake floor mud, researchers are able to conclude what kind of weather conditions prevailed thousands of years ago. Together with time series of tree-ring growth, tree-ring density and carbon isotopes, height growth is an excellent indicator of the past climate.

Publication: Hannu Salminen & Risto Jalkanen (2005). Modelling the effect of temperature on height increment of Scots pine at high latitudes. Silva Fennica 39(4), 497–508.

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