Growth-independent mortality of Lithuanian forest tree species

Abstract

Growth-independent tree mortality (mortality of trees from the upper layer of the stand canopy or mortality besides self-thinning) was investigated in north-temperate forests of central Europe (Lithuania). Data from long-term tree observations on 420 experimental plots and data from 900 permanent forest monitoring plots were used. Investigations covered stands of 20–120 years of age with a variety of the following tree species: Scots pine (Pinus sylvestris L.), Norway spruce [Picea abies (L.) Karst.], birch (Betula pendula Roth. and B. pubescens Ehrh.), aspen (Populus tremula L.), common ash (Fraxinus excelsior L.), pedunculate oak (Quercus robur L.) and alder (Alnus glutinosa L. and Alnus incana L.). Periodic measurements were carried out every 5–10 years in experimental plots and annually in forest monitoring plots. The average annual growth-independent mortality rate of all tree species in Lithuania was 0.55% (on experimental plots) and 0.62% (on forest monitoring plots). The lower mortality rate was typical for Scots pine, pedunculate oak and common ash trees (tree species with a higher maturity age), while the higher rate was typical for birch, Norway spruce, and aspen (species with a lower maturity age). The mortality of species with the lower mortality rate increased significantly with increasing age. Especially high mortality (2–3%) was recorded in older stands (birch > 60–70 years, Norway spruce > 70–80 years, and aspen > 80–90 years). More than 60% of dead trees had symptoms of wind damage. Stem insects and root diseases were identified as mortality causes for 14.3% and 16.5% of dead trees, respectively. Significant fluctuations in tree mortality rate over time were caused by extreme meteorological conditions (wind, drought) and insect invasions. Under such conditions tree mortality increased by up to 10 times.