Sir,

Growth patterns for coniferous trees in northern Scandinavia and for Larix sibirica in the polar Urals summarized by Reference AdamenkoAdamenko (1963) over the past 250 yr. are remarkably similar to the growth of Picea engelmannii Parry in the Canadian Rockies over the same period (Reference Bray and StruikBray and Struik, 1963; Bray, 1963). The Canadian growth patterns are more nearly synonymous with those of the polar Urals than of Scandinavia.

I had not seen Adamenko’s paper when my summary of the Canadian data was published (Bray, 1963), and I wish to suggest that his summary reinforces some conclusions I had reached at that time. Conditions for forest tree growth in these three regions were most favourable in the mid to late eighteenth century and following the first two decades of the twentieth century. These two intervals are synchronous with the two periods of maximum mean yearly sunspot number since exact European observations began in A.D. 1700. Conversely, the periods 1656–1723, 1799–1833 and, to a lesser degree, 1879–1913 were intervals of minimum forest growth and minimum sunspot activity. The most probable explanation for this apparent correlation of tree growth and sunspot activity is summer temperature, which is apparently higher, on average, during periods of maximum mean yearly sunspot activity.

Summary data given by Reference BrayBray (1965, table 2) record the tendency for the minimum sunspot activity periods (1656–1723 and 1799–1833) to have apparently been periods of snow accumulation with subsequent major glacier advance in the Coastal Range and Rockies in north-west North America. Periods of maximum sunspot activity (1756–1798 and 1914–present) were, on average, intervals of glacial retreat or stagnation.