Rocky Mountains Douglas fir (Pseudotsuga
menziesii var. glauca (Beissn.) Franco) and green
Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) are
introduced trees in Lithuania. Douglas fir was introduced in
Europe by D. Douglas in 1825-1827 and firstly planted in Lithuania
approximately 90-100 years ago together with other introduced
trees (Янушкевичюс и др. 1990).
According to the inventory data, about 60 sites of mature Douglas fir trees were reported during the 20th century in Lithuania. Rocky Mountains Douglas fir is more common than green Douglas fir in them (Januškevičius 2004, Ramanauskas 1973, Snarskis & Galinis 1974, Tauras 1989). There is only a sparse number of sites where mature Douglas fir trees grow in Lithuania at present. Trees have died off due to unknown reasons in several parks, e.g. Kuršėnai Park, Šereitlaukis Park, Bebrujai forest, Pienionys Park, Vyžulionys Park and several other sites. Mature trees grow mainly in western and central part and are rare in eastern Lithuania (Januškevičius 2004, Ramanauskas 1973, Žeimavičius 1995, Žeimavičius 2002). The first dendrochronological research on Douglas fir in Lithuania was performed in 1984. Research has shown a strong negative influence of colds in winter and spring for radial growth of Douglas fir (Žeimavičius 1995, Жеймавичюс & Будрюнас 1990). Investigations at the end of the 20th century have indicated a decline and increased mortality of trees (Žeimavičius 2002).
For the purpose of current investigation 19 research plots of
Douglas fir were selected in Lithuania. As a result 19 local
chronologies on the radial growth of Douglas fir were constructed.
The longest chronologies contain 95 years and encompases from two
to 23 trees. We found a very high similarity (correlation) among
sites (p<0.01). The masterchronology from 187 trees (15 plots)
was constructed (Vitas & Žeimavičius 2006).
Multiple regression has demonstrated the positive impact of air temperature in winter and spring and also a strong positive influence of precipitation in June (east Lithuania). Analysis of pointer years revealed the minimal number of negative pointer years in the seaside lowlands and Žemaičiai uplands regions. This shows the more favourite growing conditions for the Douglas fir in the west Lithuania (Vitas & Žeimavičius 2006).
Comparison of main climatic factors limiting the radial growth of Douglas fir with the factors strongly connected to other conifer species, which naturally grows in Lithuania have shown that Douglas fir takes the middle position between Norway spruce and Scots pine. The radial growth of spruce is strongly limited by summer droughts and pine is sensitive to low air temperature at the end of winter and spring (Vitas & Žeimavičius 2006).
Previous research conducted on Douglas fir in Lithuania has indicated the decline of trees from 1988 to 1999 (Žeimavičius 2002). It was found that more than 50% recorded mature trees have died during this period and in 1999 there remained only 59 trees compared to 118 trees in 1988. Trees died off and were cut in three growing places during 1990-2000: Pienionys Park, Vyžulionys Park and Bebrujai Forest. The growth patterns of Douglas fir in Vyžulionys Park and Bebrujai Forest show the sharp decrease of the radial growth in 1992 compared to 1991 (Vitas & Žeimavičius 2006). Such decrease in the radial growth of tree persisting for several years is operated by drastic changes in eco-physiological conditions, which inhibit the cambial activity. The radial growth of several species has shown negative pointer years as a consequence of extreme drought in 1992. This is typical especially to Norway spruce growing in Lithuania. This decrease in the radial growth of Douglas fir could be also connected to the invasion of pests. It is well known that the invasion of pests lead to high mortality of trees already declined by extreme climatic conditions. Thus we think that extreme droughts, which have played as a predisposing factor for the decline of Douglas fir (Vitas & Žeimavičius 2006). For further reading, please visit our bibliography page.