Investigations on European larch were started in Lithuania in 2007. From the historical sources, it is evident that the larch trees were planted in Lithuania as early as the beginning of the 19th century (Januškevičius, 2004). In addition, the ecology of larch was investigated in several studies (for example, Jankauskas, 1954). However, there was little information on the effects of environmental factors on the radial growth of larch in Lithuania. Furthermore, although the tree-ring widths of European larch have been investigated in several studies in Lithuania (Kaminskaitė, 2002; Kaselytė, 2003; Pukienė and Bitvinskas, 2000), the results are based on one or few research plots, and they are controversial. Therefore, there was a need to assess the radial growth of European larch over a wide network of experimental plots in Lithuania. The aims of this study were to (1) establish such a network including all known growing places of larch in Lithuania, (2) construct a regional tree-ring chronology of larch and (3) assess the tree-ring growth dynamics and compare it with other coniferous species (Scots pine and Norway spruce) at the same stands.
Our investigation was based on 25 experimental plots (351 larches) (Vitas and Žeimavičius, 2010; Vitas, 2015). Tree-ring widths were measured using image analysis (flatbed scanner and Cybis CooRecorder programme), and local chronologies were compiled. The average age of investigated larches is 107 years, and the oldest tree is 157 years old. The average tree-ring width is 2.18 mm, and the average mean sensitivity is 0.35. The average correlation coefficients decrease from 0.50–0.55 to 0.30–0.40, whereas the distance increases from 50 to 200–250 km. However, the similarity between site chronologies is usually statistically reliable, even if the plots are located 100–200 km apart from each other. Hence, a similar growth pattern is characteristic of larch growing throughout Lithuania.
The compiled regional tree-ring chronology spans from 1850 to 2008. The analysis of signature years has revealed that the formation of narrow rings is linked to hot/dry summers and wide rings to warm winters and springs. The tree-ring patterns of larch and Norway spruce, growing in a mixed stand, are similar (r = 0.26–0.51, p≤0.01), while the similarity with Scots pine is much lower (r = 0.16–0.20, p = 0.03–0.13).
A regional chronology shows a high inter-annual variation. The high mean sensitivity of larch may be explained by the frequent occurrence of light rings (Vitas, 2018). Light rings are characterised by a latewood zone with thin-walled incomplete lignification cells (low latewood density). Light rings have been reported among conifers in northern latitudes and form as a result of biotic (defoliation) and climatic factors. Our study has shown that European larch light rings in Lithuania are associated with narrower rings than their adjacent rings and a decreased latewood proportion in the tree ring. Furthermore, the study has shown that long-lasting droughts in spring and summer and excessive wet periods in summer, especially in August, are the main climatic drivers inducing the light rings of larch. However, not all light rings could be explained with climatic extremes, suggesting that there are also non-climatic triggers of the light rings in larch.