The role of soil factors in distribution of oak in understory in different forests in Latvia

Latvia is located in the hemi-boreal forest zone, the transition between the boreal coniferous and temperate broadleaved woodland. Forests are dominated by coniferous tree species (Pinus sylvestris and Picea abies) mixed with deciduous species (mainly Betula pendula, Populus tremula, Alnus glutinosa and Alnus incana), and broad-leaved tree species such as Quercus robur and Fraxinus excelsior are less abundant (Hytteborn et al. 2005). Broad-leaved forests in Latvia including oak forests are situated in their northmost distribution border that increases biodiversity of tree species as already it is observed to other species in north latitude – white spruce Picea glauca, black spruce Picea mariana, pubescent oak Quercus pubescens (Lloyd and Fastie, 2002; Weber et al., 2008).
In anthropogenic pine forests regeneration is often by other tree species, the most common of which is Quercus robur (Weber et al., 2008; Matias and Jump, 2012; Olsson et al., 2013, Zerbe, 2002) and Betula pendula. It is considered the cause is eutrophication of forest environment by soil enrichment with nutrients (Laiviņs, 1998; Blaško et al., 2013; Stevens et al., 2010), as well air pollution and climate change stimulate eutrophication (Laiviņš un Rūsiņa, 2007).
In study three different groups of soil sampling plots were compared. The first group was in antropogenic pine forest in Riga, the capital city of Latvia. The second group was forests in Vidzeme, the region of Latvia, the dominant species in plots were littleleaf linden Tilia cordata, Norway spruce Picea abies, wych elm Ulmus glabra, European ash Fraxinus excelsior, European aspen Populus tremula, pedunculate oak Quercus robur, silver birch Betula pendula, Scots pine Pinus sylvestris, and oak in understory. The third group were oak forests with dominant tree species oak Quercus robur mixed with Norway spruce Picea abies, European aspen Populus tremula and silver birch Betula pendula.
In general, Quercus robur is ecologically very plastic tree species and can form stands on very different soils, which differ in soil texture, presence of free carbonates and humification degree of organic layer (Ikauniece et al., 2012; Bobiec et al., 2011). Also oak in understorey is found frequently on very diverse soils. Therefor it is considered soil factor is not main influencing factor on oak regeneration in understory in forests of Latvia.