CONSEQUENCES OF RAPID EXPANSION OF LATE DEVONIAN FORESTS

Mid Devonian landscapes were mosaics of complex horizontally stratified patches of low diversity dominated by shrubby aneurophyte progymnosperms and cladoxylalean ferns. Earliest Late Devonian landscapes resemble these with the addition of Archaeopteris, a large progymnosperm tree. By mid Frasnian low diversity forests of Archaeopteris dominated most riparian landscapes. Diversity of late Frasnian to mid Famennian landscapes increased by addition of specialist niche exploiters. Archaeopteris went extinct by terminal Devonian as seed plants and ferns increased abundance and diversity. Rapidity of ecological domination by Archaeopteris forests is seen in a basal Frasnian (Oneonta Fm) delta margin exposed at Ashland, NY. Census of the lowest quarry level marine deposits shows that the regional flora was dominated by aneurophytes (69%). Lesser components include Archaeopteris (21%) and cladoxylalean trees and vines (8%). Vegetated floodplains with sandy paleosols occur upsection. A complete vertical section through one paleosol is seen next to a horizontal exposure of its lower part. The sequence coarsens upwards as a migrating floodplain channel cut into, scoured, and buried the remaining paleosol with thick, cross-bedded channel sandstones. The upper paleosol is sandier and extensively root bioturbated. The lower, muddier, less bioturbated paleosol is exposed vertically at the base of the highwall and horizontally on the quarry bench floor. Larger stump bottoms/root clumps are more widely spaced (0.5-1.2m) while smaller ones are closer (0.3-0.9m) and clustered farther from the larger stumps, which suggest variable sized crowns competing for light. Large/small stump ratios compare to our census data and suggest that this floodplain community is representative of the regional vegetation of its time. Except for Archaeopteris, these were short-lived taxa, which implies considerable successional turnover. Nearby quarries of slightly younger strata show greater domination by Archaeopteris, which reaches 95% of biomass by mid Frasnian. This mixed community was rapidly re-organized by the world-wide spread of Archaeopteris, a large, deeply rooted, long-lived tree that stabilized stream banks and had a much branched, leafy shade producing crown that shed detritus into fluvial ecosystems, thus increasing aquatic trophic richness.