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A widely accepted standpoint contends that plant growth near the cold edge of the species range, such as treelines, does not depend on the annual temperature seasonality (i.e. difference between maximum and minimum temperature values) but rather on the warmth of summer season. In contrast to this expectation, we show that the growth of treeline Norway spruce ( Picea abies) is well explained by temperature seasonality as a single climatic determinant. To do so, the tree-ring data of spruce trees growing on alpine treeline in Lapland was compared with long climate records. Biennial time-series of temperature seasonality capture both the decadal and abrupt growth fluctuations with a correlation coefficient of r = 0.601. We also show that the archetypal association between summer temperature and treeline tree growth may in fact be by far a more complex relationship than previously thought. Spruce growth appears responsive to late- June (r = 0.494) and mid-July (r = 0.310) temperatures but unresponsive to temperatures during the early July, that is, during the grand period of the tracheid formation. Climatic warming may enhance the treeline spruce growth unless the warming is concentrated on unresponsive interval in the midst of the growing season. Water relations did not play significant role as agents of P. abies growth. [ABSTRACT FROM AUTHOR]