Silviculture alters the genetic structure of an eastern hemlock forest in Maine, USA

We evaluated the influence of long-term silvicultural selection on the genetic structure of an eastern hemlock (Tsuga canadensis (L.) Carr.) forest at the Penobscot Experimental Forest, in Maine, USA. Plots in this forest received one of the following three treatments: (1) selection cuts in which small and poorly formed trees were preferentially re- moved in 1957 and 1977; (2) diameter-limit cuts in which trees at least 24 cm in diameter were removed in 1952, 1973, and 1994; or (3) no harvesting (an unmanaged control). Because of an association between the occurrence of rare alleles and tree phenotypes, phenotypically based tree removals were associated with a shift in allelic frequency. Where smaller trees with inferior phenotypes were preferentially removed (selection cut), the number of rare alleles and estimates of future genetic potential were lower than in the control group. Because of the theoretical long-term evolutionary benefit of unique gene forms, the loss of rare alleles could diminish the potential of populations to adapt to and survive ongo- ing environmental change. In contrast, alleles that were rare in the control group existed at a higher frequency in the diameter-limit cut. However, productivity was low in this stand, where the frequency of characteristically rare alleles was artificially amplified. Resume : Les auteurs ont evalue les effets de la selection sylvicole pratiquee pendant une longue periode sur la structure genetique d'un peuplement de pruche du Canada (Tsuga canadensis (L.) Carr.) a la foret experimentale de Penobscot dans le Maine, aux Etats-Unis. Les parcelles experimentales de cette foret ont recu l'un des trois traitements suivants : (1) des coupes de jardinage ou les arbres mal formes ou de faible dimension ont ete preferentiellement recoltes en 1957 et 1977; (2) des coupes a diametre limite ou les arbres de 24 cm de diametre et plus ont ete recoltes en 1952, 1973 et 1994; ou (3) aucune recolte (parcelles temoins non amenagees). En raison d'une association entre la presence d'alleles rares et le phenotype des arbres, la recolte des arbres effectuee a partir d'une evaluation du phenotype etait associee a un changement des frequences d'alleles. La ou les petits arbres au phenotype inferieur etaient preferentielle- ment recoltes (coupe de jardinage), le nombre d'alleles rares et les estimations du potentiel genetique futur etaient plus faibles comparativement au traitement temoin. Etant donne le benefice theorique a long terme des formes alleliques uniques du point de vue evolutif, la perte des alleles rares pourrait reduire la capacite des populations a s'adapter et a survivre aux changements environnementaux actuels. A l'oppose, les alleles qui etaient rares au sein du traitement te- moin avaient une frequence plus elevee apres la coupe a diametre limite. Toutefois, la productivite etait faible dans ce peuplement ou la frequence des alleles typiquement rares avait ete artificiellement augmentee.