IMPROVING ESTIMATES OF TOTAL TREE AGES BASED ON INCREMENT CORE SAMPLES

Accurate (i.e., annual resolution) determination of total tree ages wi th increment core samples is difficult because of the improbability of intercepting the pith at the root collar for most trees. For the xeri c conifer Austrocedrus chilensis (D. Don) Florin and Boutelje in north ern Patagonia (Argentina), we developed and tested a three-stage proce dure for improving estimates of total tree ages. i) For increment core samples not reaching the pith, a graphical technique is used to estim ate the missing length of the tree radius. ii) Cumulative radial growt h curves are used to estimate the numbers of rings in the missing leng ths at variable heights above the root collar. And, iii) number of yea rs required to reach coring height are derived from height-growth curv es for seedlings growing under different site conditions. Approximatel y 500 seedlings (< 100 cm tall) were uprooted and sectioned for determ ination of height-growth curves and radial-growth curves at different stem heights. From these curves, total ages were estimated for trees s ampled with increment borers. These procedures may reduce errors resul ting from assumptions of circular ring symmetry or constant radial gro wth rate. However, where age determination requires estimation of the location of the missed pith, complete accuracy is not likely in all ca ses. For example, for slow-growing Austrocedrus trees, a missing core length of just 1 cm may result in errors of 10 to 20 years. Difference s in rates of tree growth among stands of Austrocedrus indicate that t he most accurate estimates of total tree ages require determination of seedling growth rates for each homogeneous site sampled for age struc ture. Best estimates must also take into account the differential grow th rates of subpopulations of seedlings within each stand due to micro -site variation and competitive influences.