Ensemble species distribution models combine the strengths of several species environmental matching models, while minimizing the weakness of any one model. Ensemble models may be particularly useful in risk analysis of recently arrived, harmful invasive species because species may not yet have spread to all suitable habitats, leaving species-environment relationships difficult to determine. We tested five individual models (logistic regression, boosted regression trees, random forest, multivariate adaptive regression splines (MARS), and maximum entropy model or Maxent) and ensemble modeling for selected nonnative plant species in Yellowstone and Grand Teton National Parks, Wyoming; Sequoia and Kings Canyon National Parks, California, and areas of interior Alaska. The models are based on field data provided by the park staffs, combined with topographic, climatic, and vegetation predictors derived from satellite data. For the four invasive plant species tested, ensemble models were the only models that ranked in the top three models for both field validation and test data. Ensemble models may be more robust than individual species-environment matching models for risk analysis.

A mountain pine beetle (Dendroctonus ponderosae Hopkins) epidemic has caused widespread mortality of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) trees across western North America,. We characterized the initial effects of beetle-induced mortality on forest structure and composition in Rocky Mountain National Park, Colorado. In 2008, we surveyed stand structure and tree species composition across lodgepole pine dominated forests in the western portion of the Park. We defined five lodgepole pine forest types to describe variability in pre-epidemic forest conditions. This forested landscape appears to be resilient to the effects of the beetle. Surviving trees, including both canopy trees and saplings, were plentiful in most of the post-epidemic forests, even after accounting for anticipated future mortality. Subalpine fir (Abies lasiocarpa (Hook.) Nutt.), Engelmann spruce (Picea engelmannii Parry ex Engelm.), and aspen (Populus tremuloides Michx.) had modestly higher relative abundances after the epidemic. Lodgepole pine remained the dominant species on approximately 85% of the landscape. The impact of the outbreak on forest structure and composition varied considerably among the five forest types, suggesting that post-epidemic forest developmental trajectories will vary according to preoutbreak stand characteristics. Active management efforts to regenerate lodgepole pine forests, e.g., tree planting, will likely not be necessary on this landscape.Résumé : Une épidémie de dendroctone du pin ponderosa (Dendroctonus ponderosae Hopkins) a causé beaucoup de mortalité chez le pin tordu latifolié (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) partout dans l'ouest de l'Amérique du nord. Nous avons caractérisé les premiers effets de la mortalité causée par le dendroctone sur la structure et la composition de la forêt dans le parc national des Montagnes Rocheuses, Colorado. En 2008, nous avons inventorié la structure et la composition en espèces d'arbres des peuplements parmi les forêts dominées par le pin tordu dans la partie ouest du parc. Nous avons défini cinq types de forêts de pin tordu pour décrire la variabilité de la forêt qui existait avant l'épidémie. Ce paysage forestier semble résilient aux effets du dendroctone. Les arbres qui ont survécu, incluant des arbres dominants et des gaules, étaient nombreux dans la plupart des forêts après l'épidémie, même en tenant compte de la mortalité anticipée. Le sapin subalpin (Abies lasiocarpa (Hook.) Nutt.), l'épinette d'Engelmann (Picea engelmannii Parry ex Engelm.) et le peuplier faux-tremble (Populus tremuloides Michx.) avaient une abondance relative légèrement plus élevée après l'épidémie. Le pin tordu demeurait l'espèce dominante dans environ 85 % du paysage. L'impact de l'épidémie sur la composition et la structure de la forêt variait considérablement parmi les cinq types forestiers, ce qui indique que la trajectoire du développement de la forêt après une épidémie variera en fonction des caractéristiques du peuplement avant une épidém...

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Aim Range shifts associated with 20th-century warming have been documented for a wide range of taxa, but many species are not migrating fast enough to keep pace with the rapidly changing climate. Tree species can experience particularly long time lags in their migration response, resulting in altered forest composition and potentially delaying the migration of other obligate species. Here we review potential causes of these time lags and develop a conceptual framework for understanding how migration timing affects the observed rate of change.Location Global forest ecosystems. MethodsWe synthesize evidence from present-day tree species migrations to determine how different migration constraints can delay tree species range shifts. ResultsThe rate of present-day tree migrations is frequently slower than expected, and many factors may contribute to observed migration lags. Migration constraints can be overcome given the right combination of circumstances, resulting in episodic range shifts that create temporal variability in migration rates. Given projected increases in forest disturbances and extreme climatic events, episodic range shifts are likely.Main conclusions Recent efforts to explain the slow rate of tree migration have primarily focused on dispersal limitation and niche-based constraints such as competition and other biotic interactions. We argue that these constraints cannot be fully understood without considering the temporal context of tree migration. Attempts to forecast and manage future distribution shifts must consequently consider how migration timing may affect observed patterns of change.

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