A 300-year successional sequence in an eastern United States riparian hardwood forest1

Hardwood-dominated riparian forest was surveyed in 83 quadrats on 3 floodplains, 11 lower terraces, and 8 upper terraces along 11 km of Zoar Valley Canyon, a minimally disturbed segment of 5ᵗʰ–6ᵗʰ order Cattaraugus Creek in western New York State, USA. Increment core-based stand ages from 8 to >300 years constituted the longest middle-order riparian sequence yet studied in the East. Comparisons of aerial images starting in 1929 with present-day increment cores indicated trees have reached breast height and persisted on recent lower terraces within at most 6–27 years after deposition, suggesting landform age and type associated primary succession rather than flood-regime driven secondary succession or maintenance of early seral stages. In 2009, the second largest flood since 1940 caused only modest damage even to floodplain stands, supporting this premise. Regressions of stand characteristics on age across the 300-year sequence revealed logarithmic increases in tree diameter and height, stand basal area, and shade tolerance as succession has progressed. Non-metric multidimensional scaling (NMDS) ordination of stand characteristics and species distributions further revealed coherent successional gradients, including affinity vs. divergence among species of varying life history traits; e.g., an association between late-seral Acer saccharum and Fagus grandifolia and their divergence from pioneering Populus deltoides and Salix nigra. It also revealed a convincing association of biotic variables with a directional and strongly discriminated ordination distribution of landform types of increasing maturity; i.e., geomorphic patterns. Successional trends eventually leveled off beyond ∼150–200 years' stand age, suggesting a dynamic equilibrium (perhaps now reflecting a gap-phase and no longer true primary succession) that bears structural, compositional, and aesthetic resemblance to scattered old-growth in the surrounding uplands. It has been questioned, especially in the East, whether riparian succession often proceeds so far, or if hydrologic processes more likely interrupt the progression. In Zoar Valley, where vegetational succession appears to have been generally protected from both natural (flood damage, inundation) and anthropogenic disruption (logging, flow regulation), multi-aged old-growth stages occur on upper terraces throughout the riparian zone.