Improving estimates of total tree ages based on increment core samples

Accurate (i.e., annual resolution) determination of total tree ages with increment core samples is difficult because of the improbability of intercepting the pith at the root collar for most trees. For the xeric conifer Austrocedrus chilensis (D. Don) Florin and Boutelje in northern Patagonia (Argentina), we developed and tested a three-stage procedure for improving estimates of total tree ages. i) For increment core samples not reaching the pith, a graphical technique is used to estimate the missing length of the tree radius. ii) Cumulative radial growth curves are used to estimate the numbers of rings in the missing lengths at variable heights above the root collar. And, iii) number of years required to reach coring height are derived from height-growth curves for seedlings growing under different site conditions. Approximately 500 seedlings (< 100 cm tall) were uprooted and sectioned for determination of height-growth curves and radial-growth curves at different stem heights. From these curves, total ages were estimated for trees sampled with increment borers. These procedures may reduce errors resulting from assumptions of circular ring symmetry or constant radial growth rate. However, where age determination requires estimation of the location of the missed pith, complete accuracy is not likely in all cases. For example, for slow-growing Austrocedrus trees, a missing core length of just 1 cm may result in errors of 10 to 20 years. Differences in rates of tree growth among stands of Austrocedrus indicate that the most accurate estimates of total tree ages require determination of seedling growth rates for each homogeneous site sampled for age structure. Best estimates must also take into account the differential growth rates of subpopulations of seedlings within each stand due to microsite variation and competitive influences.