Pointer years

A pointer year (Swedish pekarår) is characterized by tree rings that are particularly narrow and occur in the majority of trees sampled within a site or an area. By taking into analyses 675 trees from the 49 sites studied (see map) we identified the major pointer year in oak over the 20th century (Table).

Year 1965 was negative and the strongest pointer year in oak, present in 32% of all trees sampled. Most of the negative pointer years were associated with exceptionally cold winters or springs (1868, 1940, 1956, and 1965), a period with spring-summer droughts (1869, 1947, 1992). Two positive pointer years (1882 and 1860) were not associated with neither temperature nor precipitation extremes. These years, however, had precipitation values reaching 83-94% thresholds of their distribution for at least two summer months.

A striking feature of the pointer years' list was a low proportion of trees representing even the most distinct growth anomalies (between 15 and 25% of the total amount of trees sampled). These values were 3-4 times lower than those reported for the Europe-wide pointer years for oak in Europe (Kelly et al. 2002). Since the selection on sites for our study included a variety of growing conditions and tree ages, these factor counld contribute to inhomogeneity in oak growth responces. Moreover, variation in site properties was superposed over West-East gradient in precipitation, the western parts of the region being considerably more oceanic than eastern parts.

Table. Pointer years in oak chronologies from the southern Sweden. A pointer year is defined as year with the growth index value located within 10% of distribution of index absolute values (in the lower 5% or higher 5% of its distribution). Years shown had growth anomalies in more than 15% of all trees in the dataset. For this time period each year was replicated by at least 150 trees.