No abstract

A novel dendroecological procedure was developed to elucidate canopy disturbances spanning a >300‐yr period for oak (Quercus) forests of central Pennsylvania. Running comparisons of sequential 10‐yr ring‐width averages may effectively neutralize both short‐term (i.e., drought) and long‐term growth trends associated with climate while enhancing detection of abrupt and sustained radial‐growth increases characteristic of canopy disturbance. Thinning‐response studies revealed the conservative tendencies of overstory oak, with substantial basal area reductions (>1/3) required to attain moderate and consistently detectable growth increases. Based on empirical evidence, a minimum growth‐response threshold of 25% was established to depict canopy disturbances. This is in contrast to the 50–100% sustained radial‐growth release often used to detect disturbance using understory trees in closed forests. Our default threshold was adjusted higher as necessary for those trees highly correlated to climatic trends (as represented by the Palmer drought severity index). Canopy disturbances detected with this dendroecological approach were further substantiated using tree‐recruitment data (age cohorting). By coupling these data sets, we estimated return intervals of standwide disturbance from 21 yr in presettlement times (prior to 1775) and during heavy Euro‐American exploitation (1775–1900) to 31 yr in modern times (after 1900). Although disturbance periodicity remained stable between presettlement and early post‐settlement (exploitation) eras, the mode of disturbance shifted from mainly natural (wind and fire) to anthropogenic forces (intense harvesting for charcoal production), based on the historical record. In the process, presettlement oak–pine (Pinus)–chestnut (Castanea) forests on ridges were rapidly converted to young coppice stands of oak and chestnut. The reduction of harvesting and fire events coupled with the eradication of chestnut by blight this century have allowed these coppice stands to mature into oak‐dominated forests that exist today. This analytical technique for ascertaining disturbance histories holds much potential and should be considered for use with mature, overstory trees in other forest types with appropriate modifications.

Many ecological phenomena combine to direct vegetation trends over time, with climate and disturbance playing prominent roles. To help decipher their relative importance during Euro-American times, we employed a unique approach whereby tree species/genera were partitioned into temperature, shade tolerance, and pyrogenicity classes and applied to comparative tree-census data. Our megadata analysis of 190 datasets determined the relative impacts of climate vs. altered disturbance regimes for various biomes across the eastern United States. As the Euro-American period (ca. 1500 to today) spans two major climatic periods, from Little Ice Age to the Anthropocene, vegetation changes consistent with warming were expected. In most cases, however, European disturbance overrode regional climate, but in a manner that varied across the Tension Zone Line. To the north, intensive and expansive early European disturbance resulted in the ubiquitous loss of conifers and large increases of Acer, Populus, and Quercus in northern hardwoods, whereas to the south, these disturbances perpetuated the dominance of Quercus in central hardwoods. Acer increases and associated mesophication in Quercus-Pinus systems were delayed until mid 20th century fire suppression. This led to significant warm to cool shifts in temperature class where cool-adapted Acer saccharum increased and temperature neutral changes where warm-adapted Acer rubrum increased. In both cases, these shifts were attributed to fire suppression rather than climate change. Because mesophication is ongoing, eastern US forests formed during the catastrophic disturbance era followed by fire suppression will remain in climate disequilibrium into the foreseeable future. Overall, the results of our study suggest that altered disturbance regimes rather than climate had the greatest influence on vegetation composition and dynamics in the eastern United States over multiple centuries. Land-use change often trumped or negated the impacts of warming climate, and needs greater recognition in climate change discussions, scenarios, and model interpretations.

We reviewed literature in the fields of anthropology, archaeology, ethnobotany, palynology and ecology to try to determine the impacts of Native Americans as active and passive promoters of mast (nuts and acorns) and fruit trees prior to European settlement. Mast was a critical resource for carbohydrates and fat calories and at least 30 tree species and genera were used in the diet of Native Americans, the most important being oak ( Quercus), hickory ( Carya) and chestnut ( Castanea), which dominated much of the eastern forest, and walnut ( Juglans) to a lesser extent. Fleshy tree fruits were most accessible in human-disturbed landscapes, and at least 20 fruit- and berry-producing trees were commonly utilized by Native Americans. They regularly used fire and tree girdling as management tools for a multitude of purposes, including land clearing, promotion of favoured mast and fruit trees, vegetation control and pasturage for big-game animals. This latter point also applies to the vast fire-maintained prairie region further west. Native Americans were a much more important ignition source than lightning throughout the eastern USA, except for the extreme Southeast. First-hand accounts often mention mast and fruit trees or orchards in the immediate vicinity of Native American villages and suggest that these trees existed as a direct result of Indian management, including cultivation and planting. We conclude that Native American land-use practices not only had a profound effect on promoting mast and fruit trees but also on the entire historical development of the eastern oak and pine forests, savannas and tall-grass prairies. Although significant climatic change occurred during the Holocene, including the `Mediaeval Warming Period' and the `Little Ice Age', we attribute the multimillennia domination of the eastern biome by prairie grasses, berry-producing shrubs and/or mast trees primarily to regular burning and other forms of management by Indians to meet their gastronomic needs. Otherwise, drier prairie and open woodlands would have converted to closed-canopy forests and more mesic mast trees would have succeeded to more shade-tolerant, fire-sensitive trees that are a significantly inferior dietary resource.