Secular Cycles of Radial Increment and Vitality of English Oak in the Oak Groves of the Southern Forest-Steppe

Radial increment of the stem of old-growth English oak trees can be a source of information about their growth and development at the final stage of life and longevity potential. The aim of the study has been to assess the vitality of oak during ontogenesis based on the analysis of the secular cyclicity of the radial stem increment. The objects have been groups of trees differing in crown development dynamics in 90- and 270-year-old upland and 130- and 230-year-old floodplain oak groves of the Tellerman Experimental Forestry of the RAS. The increments have been measured from the images of stem core samples scanned at 1200 dpi. The vitality of the oak has been assessed by the early wood increment and its relationship with the crown development. The secular cycles of radial increment caused by the development and self-thinning of stands have been identified. In upland oak groves that have regenerated in clearings, the first branch of the cycle is descending, in floodplain oak groves that have begun to grow under the canopy, it is ascending. The first branch of the secular cycle continues until the age of the oak tree is 150–170 years, the second – up to 200–250 years. The coefficients of variation of the early and late wood radial increment vary within the range of 15–37 and 34–75 %, respectively, and largely depend on secular cyclicity. The results obtained indicate the priority of crown development over stem growth in the dynamics of the secular cycle. Thus, on the descending branches of the cycles, the relative decrease in the late wood increment is significantly greater than that of the early wood – by 1.2–3.8 times, while on the ascending branches its relative increase is only 1.0–1.7 times higher. During the transition from the 1st to the 2nd branch of the secular cycle, the average early wood increment has increased in the overwhelming majority of cases, while the ratio of the average late and early wood increment has decreased. In a secular cycle, the maximum variation in the early wood increment has been equal to the width of a row of vessels. The current vitality of the studied old-growth oak trees has been assessed as satisfactory. Under conditions of lesser rot damage, the longevity of the English oak could have been much higher.