Braathen Dendrokronologiska Undersökningar

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Annual variations in climate and annual tree-ring growth

Every annual ring in wood can be coupled to the specific year when it was developed if the tree during a certain number of years had an annual growth so that the variation in climate can be traced in the growth. Because the annual variations of climate never exactly repeat themselves during a certain minimum sequence of years, a corresponding sequence of annual growth also is unique for those trees which are sufficiently affected by the variations of climate. In Sweden the growth is affected up to 70% by general factors like temperature, light and precipitation and to about 30% by local and internal factors like competition to neighbouring trees, conditions of substrata and hormonal factors.
 
In arid areas of the globe the degree of access to water is a dominant regulating factor of growth while in our latitudes the water table is filled up every cold season and is able to supply plants which have a deep root system with water even during very dry summer seasons. Concerning trees temperature is a more restricting factor the further we get to the north. Oak (Quercus Robur and Quercus Petrea) is rare north of the river Dalälven and beech (Fagus Silvatica) is rare north of the province of Götaland. Elm (Ulmus Glabra), ash (Fraximus Excelsior) and linden (Tilia Cordata) also have limits to the north which easily can be identified. In spring those species need a rather high initial temperature in order to start their internal circulation and develop foliage. For that reason they will get a shorter season of growth the further to the north they are until they reach a limit where a favourable production of seed has major difficulties. Pine (Pinus Silvestris), spruce (Picea Abies) sallow (Salix) and birch (Betula) exist all over the country, but they reach their own boundaries of climate with increasing altitude in mountain areas. Birch reaches the highest altitude and is easily established even in poor and shallow substrata. The evaporation from the foliage of birch is strong and birch therefore is sensitive to dry and warm summers. The growth of annual rings tends to be more dependent on local factors than on general factors. This makes birch difficult or nearly impossible to date.

At a certain initial temperature t0 the tree starts to grow. With increasing temperature the growth increases exponentially until evaporation becomes a limiting factor. At t1 the increase is decreased. At t2 the pace of growth reaches its maximum and at t3 a collapse occurs and the pace of growth decreases considerably with increasing temperature.
 
The weather systems which move over northern Europe mostly cover large areas. The connecting annual variations of climate therefore may be traced in the variations of growth of the trees in places which are far distant from each other. An example of this is that in 1902 the annual growth was low in the upper part of Lapland (north), Bohuslän (west), Skåne (south) and Gotland (east). In these provinces the mean summer temperature, the amount of sun exposure and precipitation are separate from each other, but the variations from one year to the other agree to a high percentage.
 
The variations of mean summer temperature from 1860 to 1975 in Gothenburg (Göteborg) and Stockholm agree with each other to more than 97%.
 
The local conditions for the growth of separate trees may vary considerably. Trees on marsh grounds may suffer from lack of nourishing substances, a high water table may prevent expansion of the root system towards the depth and trees in the neighbourhood may cause damage on branches if they are felled by winds. Diseases and severe competition to other trees are examples on inhibiting factors of growth. Disturbed patterns of growth decrease the possibility of tree-dating.

Growth diagrams for three arbitrarily chosen pines (Pinus Silvestris) from western Sweden. The growth lacks severe anomalies, and the correlation to the reference chronology from the Södertörn-area (South of Stockholm) exceeds 60 %.