Recent tree-limit history of Piceaabies in the southern Swedish Scandes

Abstract

The recent history of Piceaabies (L.) Karst. at its altitudinal tree limit has been studied in the southern Swedish Scandes. Altitudinal transects (131) were evenly distributed over a tract of mountains of ca. 40 × 200 km. The age of spruces growing at the tree limit and downhill were estimated by annual ring counts. The spruce tree limit had risen (on average by ca. 50 m altitudinally) in ca. 70% of the studied transects as a result of the subsequent growth in height of old, established, formerly stunted individuals. Their growth in height accelerated during the 1930's, in response to the general climatic warming. A rise in the tree limit because of the establishment of new individuals (after 1915) was noted in only 7% of the studied transects. Most of the spruces growing in the tree-limit ecotone established around the 1860's and the 1940's, which were epochs with relatively snowy winters. After 1860, spruce establishment was not correlated with the summer mean temperature. Successful regeneration of spruce at the tree limit is dependent of a deep and stable snow cover and the requisite balance between precipitation–meltwater and evaporation being maintained in the early summer. The importance of air temperatures in May for successful growth and natural regeneration was evident. High air temperature in May is detrimental, since it promotes a too early initiation of growth and a consequent increased risk of frost damage. The spruce populations at the tree limit are recruited both from local seed parents and from long distance dispersal of seed from trees growing at lower altitudes.