Infradian Rhythms in Seasonal Growth Dynamics of Scots Pine Trees’ Stem, Branches, and Needles

The aim and objectives of the research are to study seasonal changes in morphometric traits of needle, stem, and branch growth in Scots pine (Pinus sylvestris L.) trees, to assess the influence of endogenous and environmental factors on growth processes, and to generalize the ideas for the nature of fluctuations (oscillation period is more than one day) in the growth rate of woody plant. Seasonal dynamics of the needle growth and the apical growth of stem and branches of young (14–16-year old) Scots pine trees, growing in the forest park of a big city (Yekaterinburg, Russia) were studied. The relationship between the growth dynamic and air temperature and precipitation was assessed. Four stages of the apical growth of stem and branches were distinguished: (1) preliminary, (2) intensive, (3) additional, and (4) final growth. Changes in the rates and accelerations of stem- and branch growth during stages one and two are synchronous and occur as successive oscillations with a period of 4–8 days, which corresponds to infradian rhythm. During stage three, stem growth rate oscillations ceases. The basic patterns of changes in accelerations and rates of stem- and branch growth seasonal dynamics are the same, but oscillations in accelerations are more suitable for determination of their periods. Seasonal changes in needle growth rate are divided into two stages: basic and terminal growth. Three maxima were observed during the first stage, each of which corresponds in time to air-temperature peaks. The dynamics of needle growth accelerations occur as successive quasi-periodic oscillations. During the basic growth stage, there are seven to eight oscillations with a period of 4–10 days. The existence of infradian oscillations in the growth rate of stem, branches, and needles indicates the formation in the apical shoot and needle meristems of big groups of synchronously dividing and/or elongating cells. This assumption underlies the proposed qualitative model, explaining the cause of fluctuations in the growth rate observed as longitudinal type waves (growth waves). The development of the shoot apical meristem begins with the activation of cell divisions in the organizing center, from which newly formed cells move to the peripheral zone, where their proliferation and elongation continue. Shoot elongation results from cell elongation after the division stage. The duration of the division–elongation cycle of individual cells in a group is approximately the same. The beginning and end of the cycle are synchronized for all cells in the group. The size of the group determines the amplitude of growth rate fluctuations. After the elongation stage, part of the group ceases proliferation and begins differentiation. The remaining cells retain the ability to divide and participate in the formation of the next group. The existence of fluctuations in growth rate indicate that the relative number of cell groups, dominated by division or elongation, changes at time intervals corresponding to infradian rhythms. This sequence of events is maintained until the stage of growth cessation. Needle growth occurs similarly to the shoot growth. It is driven by the activity of the intercalary meristem cells, located in the basal part of the needle. Seasonal changes in the growth rate of Scots pine’s stem, branches, and needles are highly positively correlated with ambient temperature and not with precipitation. The relationship between growth accelerations and temperature is weak. A comparative analysis of seasonal dynamics of air temperature and shoot and needle growth rates suggests that temperature changes may influence the synchronization of growth rate fluctuations. The lack of correlation between growth accelerations and temperature supports the hypothesis that endogenous factors play a key role in the generation of shoot and needle growth waves.