Cross-scale interactions and the migration of trees

Dr Stephen Jackson1

1Southwest Climate Adaptation Science Center, USGS, Tucson, United States, 2Department of Geosciences, University of Arizona, Tucson, United States

Paleoecological studies reveal how abundance and distribution of tree species are governed by interactions of slow and fast ecological processes in a changing environment.  Holocene migrations of several North American species (Juniperus osteosperma, Pinus edulis, Pinus ponderosa, Betula allegheniensis, Fagus grandifolia, Larix laricina) show episodic patterns and spatial patchiness.  Spatial patchiness derived both from mosaics of differentially suitable habitat and from long-distance dispersal.  Expansion episodes, paced by climate variability at multidecadal to millennial timescales, were sometimes accelerated by widespread disturbance episodes.  Although long-distance dispersal events (tens to hundreds of kilometers) occurred, in some cases their low frequency limited rate of expansion.  Growth of colonizing populations (e.g., Pinus ponderosa) was slowed in some cases by Allee effects.  Some tree species underwent local range contractions (Pinus resinosa, Pinus banksiana, Betula papyrifera, Juniperus communis, Picea pungens, Pseudotsuga menziesii). In many cases, local extirpations were highly attenuated; populations persisted for hundreds to thousands of years after onset of decline.  Others (Tsuga canadensis, Fagus grandifolia) experienced widespread population declines in the mid- to late Holocene, driven by interactions among climate events and disturbance.  Future states will be shaped by interactions among climate change, climate variability, disturbance episodes, regeneration processes, and, to some extent, interventions by resource managers.


Stephen T. Jackson is Director of the Department of the Interior Southwest Climate Adaptation Science Center, a partnership between the US Geological Survey and a multi-university consortium led by the University of Arizona. In this position, he works to foster effective engagement between researchers and resource-management decision-makers. He is also Adjunct Professor of Geosciences and of Natural Resources & Environment at the University of Arizona. Before assuming his current position in 2012, he was at the University of Wyoming, where he was founding Director of the Program in Ecology and is now Professor Emeritus of Botany. His research utilizes the past 25,000 years of earth history as a source of natural experiments to explore ecological responses to environmental changes of various kinds, rates, and magnitudes.

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