A comparison of three age determination methods for suppressed Norway spruce: implications for age structure analysis

Abstract

Three methods for age determination of suppressed Norway spruce were compared: (1) tree-ring counting in core-like strips; (2) pith node counting and (3) a method based on dendrochronological cross-dating (named within-tree cross-dating). The methods were tested on 20 suppressed spruces (age 100–140 years) from a boreal forest in Northern Sweden with known disturbance history. In all studied trees, the oldest pith dates were found below the root collar, the point which is normally aimed for when coring trees in field, thus, ring counts in the root collar level gave on average 20–26 years younger ages than pith node counting and within-tree cross-dating. Pith node counting and within-tree cross-dating gave largely identical pith dates but when top breaks had occurred at the seething stage, within-tree cross-dating gave better results. Cross-dating was achieved between short stem sections from the base of the tree with the aid of conspicuous rings that were followed down through the lower stem, section by section. This enabled more accurate dating of the pith in the lower portions of the stem where many rings are missing in the outer parts due to early bending and formation of adventitious roots which allocate stern growth above the point of origin. The results have wide implications for studies of Norway spruce stand age structures. When analysing tree age in Norway spruce stands and stands of other species that are capable to extremely slow juvenile growth (leading to initiation of adventitious roots and missing rings), ages may be systematically underestimated, and short periods of successful regeneration may “disappear” in age–class diagrams when age data are derived from counted tree rings in cores extracted at root collar level. The within-tree cross-dating method and pith node counting are two methods that (1) provide a precise age determination for small suppressed Norway spruce and (2) can give a better age estimation of the dating error for bigger trees where the oldest part (juvenile wood) of the tree is not possible to date accurately with normal coring and ring counting.