Dendroecology and climatic impacts for a relict, old-growth, bog forest in the Ridge and Valley Province of central Pennsylvania, U.S.A.

Abstract

We report on the 440-year dendrochronological history of a relict, bog forest in the Ridge and Valley Province of central Pennsylvania that contains extreme southern, disjunct populations of Picea mariana (Mill.) BSP and Abies balsamea (L.) Mill. The forest is dominated by Tsuga canadensis (L.) Carr. (49% relative importance value), Picea mariana (16%), and Acer rubrum L. (15%). The few remaining Abies balsamea trees are in a advanced state of decline. Many Nyssa sylvatica Marsh. and T. canadensis trees recruited from 1560 and 1700, respectively, until 1890. However, the majority of the other tree species recruited during a 40-year period following selective logging of the forest in the 1890s and fires in about 1900 and 1914. We found a scarcity of tree saplings and no evidence of recruitment into the tree-size class for any species after 1950. The master tree-ring chronology for both N. sylvatica and T. canadensis exhibits a marked increase after the 1890s logging and a decrease after a 1900 fire. In addition, a large number of releases in individual tree chronologies occurred over the last 400 years, indicating the frequent occurrence of small-scale disturbances. Tree-ring growth during the 20th century was reduced by droughts and cool temperatures in the 1920s and in the early to middle 1960s. Abies balsamea cores exhibit a marked growth decline in 1986. Tsuga canadensis growth was very low between 1970 and 1998, despite a generally warm and wet climate during that time. Picea mariana had a dramatic increase in growth during very warm and wet climate between 1995 and 1998. Most Abies balsamea trees have reached their pathological age of 50-85 years and have active Armillaria root rot, insect infestations, and very poorly developed crowns. These symptoms or severe growth declines are not present in Picea mariana. It appears that the 10 000 year history of Abies balsamea presence at Bear Meadows will end soon, with no opportunity to reestablish itself because of the lack of a local seed source. The results of this study suggest that relict tree populations in the eastern United States may be particularly sensitive to direct and indirect anthropogenic impacts and climatic variations, and represent important benchmarks for comparisons with future studies.Key words: tree rings, disturbance, climate, global change, central Pennsylvania.