Dendroecological studies in New Zealand 1. An evaluation of tree age estimates based on increment cores

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Abstract
Tree-ring counts from increment cores are widely used in ecological studies for determining tree ages. In New Zealand many canopy trees are slow-growing and long-lived, and have extremely narrow rings. Single rings or groups of rings may be absent on some radii. Such narrow and absent rings cause difficulties in ring counting and necessitate careful sample preparation. The errors associated with age estimates derived from cores which do not reach the chronological centre of the tree (partial cores) are discussed. Four partial core lengths and three methods of estimation were used on cross-sections of known age from Agathis australis, Libocedrus bidwillu, Nothofagus solandri, and Prumnopitys taxifolia. It is concluded that mean errors may be less than ± 10% where the core length represents 90% of the geometric radius (half the measured diameter) increase with shorter cores. However, much greater errors may apply to individual estimates, up to ± 78% in one case. Moreover, the direction of the error (above or below the true value) is largely dependent upon the early growth conditions and is unpredictable. These errors appear to occur for three main reasons: eccentric growth, age dependent growth variations, and other growth variations.