Long-period fluctuations of radial increment and vitality of pedunculate oak in Tellerman oak forests

Fluctuations in the growth rate and vitality of oak forests in the forest-steppe led in the XXth century to their decline and drying out with a periodicity of about 25–45 years. In the context of climate change, the relevance of studying the wave dynamics of oak forests in order to analyse the risks of oak mortality and possibilities to reduce them is increasing. The article presents the results of a study on long-term fluctuations since the XIXth century of radial growth of early and late trunk wood in 270-year-old upland and 230-year-old floodplain oak forests in comparison with cycles in 90-year-old upland and 130-year-old floodplain oak forests. The age trend is described as a century-long cycle, the two branches of which differ in period and amplitude. In old-age oak forests, 5 cycles of radial growth of the oak trunk were identified, in younger oak forests – only two. The vitality of the oak was assessed by the type of crown development, which was identified by the width of the early wood growth. The first 1–2 cycles are 50–70-year-old, the longest, asynchronous in upland and floodplain old-age oak forests, caused by wave self-thinning of stands. Then the duration of the cycles decreases to 25–40 years: in the floodplain oak forest – from the end of the XIX century, in the upland – from the beginning of the XX century. These fluctuations are synchronous in all studied oak forests as a result of drought and insect damage to the leaves, their minima coincide with the waves of oak drying out. Thinning has increased the resistance of the 90-year-old oak grove to long-period fluctuations. The CV of the growth indices in it is less than in other oak forests by an average of 1.3 times, and the CV of the long-period intra-century component by 2 times. Thinning of the stands’ canopy during the dying out of oak in the 1960s–1970s obviously increased the stability of oak forests during the next long-period cycle. The risk of oak decline and mortality increases at the minimums of intra-century cycles and depends on their amplitude. The risk also increases near the minimums of century-long cycles – in upland oak forests after 130 years of age, in floodplain forests after 60 and 190 years. An increase in the probability of oak mortality is expected in upland oak forests only no earlier than in the early 2030s.