The distribution of trees is generally determined by their ecological characteristics. Both internal and external factors, e.g. climate, influence the growth of trees. Many dendroclimatological studies have shown that climate explains a relatively large part of the temporal variability in tree-ring width (Fritts, 2001). The relationship between climate and tree-ring width occurs because plant growth is affected by specific conditions in the habitat environment. In addition, the nature of soil where trees grow may affect their response to climate (Linderholm, 2001; Linderholm et al., 2002; Vitas & Erlickyte, 2007; Weber et al., 2007).
Scots pine (Pinus sylvestris L.) has a very wide ecological amplitude and successfully occupies dry habitats and sandy soils as well as peatland habitats. Therefore, it has a wider geographical distribution than any other pine species (Ohlson, 1995; Richardson & Rundel, 2000).
Tree rings of Scots pine have been successfully used in dendroclimatology, mainly regarding trees growing on dry soils. Dendroclimatological investigations of tree rings in Latvia (Spalte, 1975, 1985; Skudra, 1982; Elferts, 2007a, 2007b), Lithuania (Bitvinskas, 1978; Vitas, 2004; Vitas & Erlickyte, 2007), Estonia (Laanelaid, 1982; Laanelaid & Eckstein, 2003), Poland (Cedro, 2001, 2006), and Scandinavia (Linderson, 1992; Linderholm, 2001; Linderholm et al., 2002) show that Scots pine growing on dry soil is sensitive to cold winters in the area around the Baltic Sea. Several studies treat the growth of Scots pine on peatlands. These have focused mainly on bogs and little is known about Scots pine growth on fens. Most of the previous studies examine relationships of pine growth on drained fens from a wood production improvement perspective (Pyateckij, 1963; Zalitis, 1967, 1968). There are only a few studies where the age and size of Scots pine as well as some habitat factors (nutrient characteristics of the peatlands and climate) are examined in relation to the natural regeneration of this tree species on fens (Agren et al., 1983; Ohlson, 1995).
In pristine peatlands, e.g. fens, the most important differences in comparison to mineral soil sites are a high water table level (Boggie, 1972; MacDonald & Yin, 1999), poor soil aeration, and low nutrient availability and uptake, which control seedling survival (Agren & Zackrisson, 1990; Ohlson & Zackrisson, 1992; Sarkkola et al., 2004), and extremely slow growth of trees (Ohlson, 1995; MacDonald & Yin, 1999; Fritts, 2001; Hokka & Ojansuu, 2004). Water table depth is regulated by several factors, of which temperature and precipitation are the most important in controlling seasonal and yearly variation. This means that both of these factors influence tree growth directly and indirectly through regulation of the water table by evapotranspiration (Fritts, 2001; Linderholm et al., 2002). Climatic amelioration has been of great importance with regard to the comparatively successful regeneration of pine and spruce on peatlands (Agren et al., 1983).
Ohlson (1995) observed that on fen sites the growth rate of Scots pine is greater and much more variable than on bog sites. Therefore, we suggest that Scots pine growing on fens can be more useful in dendroclimatology compared to pine growing on bogs. The aim of this study was to determine the climatic factors influencing the radial growth of Scots pine growing on dry and wet soils near Lake Engure in Latvia.
MATERIAL AND METHODS
Lake Engure is situated at the western coast of the Gulf of Riga, from which it is separated by a 1.5-2-km wide dune area. It is the largest relict lake in the coastal area of Latvia and has remained there since the time of the Litorina Sea (Viksne, 1997; Pakalne & Kalnina, 2005). It is a protected nature area, founded in 1957 as a restricted ornithological area (nowadays a nature park). …