On the general failure of coppice conversion into high forest in Quercus pyrenaica stands: a genetic and physiological approach

Genetic, dendrochronological and physiological studies are reviewed to investigate causes of tree stagnation of abandoned Quercus pyrenaica coppices, Tree decline–displayed by slow stem growth, branch dieback and scarce acorn production–has led to advice on the conversion of abandoned oak coppices into high forests by thinning. However, this practice has often failed. The shortage of genetic diversity and disequilibrium between above- and below-ground organs in large, over-aged stools (clones) are adduced consequences of centenary coppicing driving stand stagnation in Q. pyrenaica. Our results evidence that historical coppicing has not enhanced genetic diversity losses; on the contrary, it has allowed for the recruitment of new genotypes and the existence of uneven-aged stands of heterogenic clonal structure. Tree excavations have revealed widely spread root systems. High carbon expenditures in root respiration in large and centennial clones may constrain their aboveground development, and thinning practices aiming at converting Q. pyrenaica coppices into high forests might not succeed or even enhance a physiological root-to-shoot imbalance.