No abstract

Interactions among multiple causes of ecological perturbation, such as climate change and disturbance, can produce "ecological surprises." Here, we examine whether climate-fire-vegetation interactions can produce ecological changes that differ in direction from those expected from the effects of climate change alone. To do so, we focus on the "Big Woods" of central Minnesota, USA, which was shaped both by climate and fire. The deciduous Big Woods forest replaced regional woodlands and savannas after the severity of regional fire regimes declined at ca. AD 1300. A trend toward wet conditions has long been assumed to explain the forest expansion, but we show that water levels at two lakes within the region (Wolsfeld Lake and Bufflehead Pond) were low when open woodlands were transformed into the Big Woods. Water levels were high instead at ca. 2240-795 BC when regional fire regimes were most severe. Based on the correlation between water levels and fire-regime severity, we infer that prolonged or repeated droughts after ca. AD 1265 reduced the biomass and connectivity of fine fuels (grasses) within the woodlands. As a result, regional fire severity declined and allowed tree populations to expand. Tree-ring data from the region show a peak in the recruitment of key Big Woods tree species during the AD 1930s drought and suggest that low regional moisture balance need not have been a limiting factor for forest expansion. The regional history, thus, demonstrates the types of counterintuitive ecosystem changes that may arise as climate changes in the future.

No abstract

Early summer snowmelt from mountains in northern Colorado and southeastern Wyoming supplies the North Platte River, supporting nationally important agriculture, energy production, and urban development. Repeated decisions from the U.S. Supreme Court have fully apportioned Platte River waters among Colorado, Wyoming, and Nebraska, underscoring societal strains on this system. Now, climate change threatens the regional allocation of water. Tree-ring records indicate that past centuries contained multidecadal "megadroughts" far more severe than those of the historic period. However, the potential for even more persistent droughts, as the result of climate change, is poorly known. We document and evaluate the severity of recent and prehistoric droughts via a combination of data sources: modern temperature, precipitation, and stream gauge data; evidence of low lake-level stands; and related estimates of past hydroclimate change. Modern climate and stream data show an increase in spring temperatures of 2.21 • C since 1916, an increase in the frequency of peak spring runoff before 1 May, and a reduction in winter precipitation. Lakes, however, that have only experienced minor hydrologic changes historically were desiccated during prehistoric dry periods during the past 12,000 years. Prehistoric lake shorelines indicate that water supplies were substantially reduced over centuries and millennia, such as from >8,000 to <5,000 years before present. The magnitude of these droughts likely also resulted in ephemeral river flows and thus indicates the potential for persistent shifts in regional hydrology. Such shifts should, therefore, be considered as part of long-term economic and legal planning. Key Words: climate change, drought, lake levels, North Platte River, water supply.El derretimiento de la nieve al comienzo del verano en las montañas del norte de Colorado y sudeste de Wyoming alimenta el Río North Platte, dando así soporte a una agricultura de importancia nacional, la producción de energía y desarrollo urbano. Las aguas del Río Platte han sido repartidas enteramente entre los estados de Colorado, Wyoming y Nebraska por medio de repetidas decisiones de la Corte Suprema de los EE.UU., lo cual subraya las tensiones sociales que afectan este sistema fluvial. Esta asignación regional de agua está ahora amenazada por el cambio climático. Los registros de los anillos de los troncos deárboles indican que en siglos pasados se presentaban allí "megasequías" multidecadales mucho más severas que las registradas en tiempos históricos. Sin embargo, muy poco se conoce del potencial para que puedan ocurrir sequías de persistencia aun mayor como resultado del cambio climático. Documentamos y evaluamos la severidad de sequías prehistóricas recientes por medio de una combinación de fuentes de información: datos modernos de temperatura, precipitación y caudales de las corrientes; evidencia de menor nivel de la superficie