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The paper presents results of an assessment of needle age classes in Scots pine partial populations in the period of three and five years after outplanting in a lowland site in relation to conditions created by climatic elements. The method is based on Skuhravý’s (1987) description when the state of needle age classes of the third whorl (from the apex) is evaluated just once. Five-year summary of meteorological data was assessed according to the criteria of determination of periods of heat waves and dry spells. Differences in numbers of living needle age classes in the groups of Scots pine populations in the period of three and five years after their outplanting in permanent lowland sites proved the highest total number of living needle classes in the group of local lowland populations. On the contrary, the lowest total number of living needle classes was found out in the group of mountain populations. Critical climatic periods are overlaps of heat waves and dry spells that illustrate the complexity of moisture and temperature conditions of lowland sites. Although the Scots pine is a xerophytic species, it responded to the cumulation of adverse factors by accelerated needle senescence. Its physiological response to heat and dry stress consists in needle shedding, i.e. reduction in the assimilating area. As mountain populations are not usually exposed to heat waves, the impact of this phenomenon on them was the highest. On the other hand, the local (lowland) populations carrying information on the patterns of local climate from the reproduction period do not undergo any marked reduction in needle age classes.

Needle longevity as a criterion of response to a climatic fluctuation (so called heat wave) in Scots pine populations at early phases of ontogeny