Age structure of young- and old-growth Quercus pyrenaica stands in Spain

Our objective was to examine the age structure of successionally young and mature old-growth stands dominated by Quercus pyrenaica, widely distributed on silicious substrates at 400-1800 m elevation in the Iberian Peninsula, France, and Morocco. Many hectares of young forest are recovering from intensive grazing and wood cutting as a result of land abandonment during the past several decades. Eight early-successional undisturbed stands, one intermediate-aged stand, and four old-growth stands were sampled. Age was determined by coring a subset of trees of various diameters and applying a regression formula to the dbh of 1316 trees among all 13 stands, so that every diameter could be expressed in years of age. Linear regression formulas accounted for 69-81 % of all diameter-age variation. For young stands, almost 2/3 of all trees were 30-39 yr of age, a dip in regeneration characterized the most recent 30 yr, and the oldest tree encountered was 90 yr old. For old stands, more than half the trees were 56-125 yr of age, a dip in regeneration characterized the most recent 75 yr, and the oldest tree encountered was 358 yr old. Thus, both young and mature stands showed depressed regeneration for at least the past three decades, a phenomenon shared by ecologically related oaks in California, where such age structures are interpreted as evidence of long-term population imbalance and instability.