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In the White Mountains of California, eight bristlecone pine (Pinus longaeva) tree-ring width chronologies were developed from trees at upper treeline and just below upper treeline along North- and South-facing elevational transects from treeline to ~90 m below. There is evidence for a climate-response threshold between approximately 60-80 vertical m below treeline, above which trees have shown a positive growth-response to temperature and below which they do not. Chronologies from 80 m or more below treeline show a change in climate response and do not correlate strongly with temperature-sensitive chronologies developed from trees growing at upper treeline. Rather, they more closely resemble lower elevation precipitation-sensitive chronologies. At the highest sites, trees on South-facing slopes grow faster than trees on North-facing slopes. High growth rates in the treeline South-facing trees have declined since the mid-1990s. This suggests the possibility that the climate-response of the highest South-facing trees may have changed and that temperature may no longer be the main limiting factor for growth on the South aspect. These results indicate that increasing warmth may lead to a divergence between tree growth and temperature at previously temperature-limited sites.

The Border Lakes Region of Minnesota and Ontario has long been viewed as a fire‐dependent ecosystem. High‐severity fire in the region's near‐boreal forests has been a focus of ecological research and public fascination. However, the surface fire history within this transnational wilderness landscape has received more limited attention. We used an interdisciplinary, dendroecological approach to characterize the surface fire history of the region, assess potential drivers of historical surface fires, and document the ecological legacies of frequent fires within the red pine forests of the Boundary Waters Canoe Area Wilderness (BWCAW) in northern Minnesota. We used tree‐ring and fire atlas data to reconstruct multi‐century surface fire records for 101 sites and document age structure and composition at 32 sites across the BWCAW. Stratification of these sites relative to their proximity to a primary travel and trade corridor used first by Indigenous groups and later by Euro‐American fur traders through the late 1800s provided strong evidence of human augmentation of fires. The patterns of fire activity, fire–climate relationships, and forest development indicate that traditional landuse by Anishinaabeg (Ojibwe) increased rates of local surface fire and played an important role in shaping the landscape. The decline of traditional subsistence practices by the Border Lakes Anishinaabeg coincided with a sharp decline in surface fires and a period of abundant tree establishment. In the absence of repeat surface fires, many red pine sites have shifted in composition, increased in stem density, and grown vulnerable to forest‐type conversion through future high‐severity fire. These results highlight the need for active fire reintroduction to red pine forests of the Great Lakes Region and underscore the importance of collaboration and guidance from Indigenous Knowledge Keepers in this process. A blended knowledge approach to fire restoration that directly engages with Indigenous perspectives and cultural practices can perpetuate the distinctive character of the largest remaining tracts of long‐lived pine forest in the Great Lakes Region. Carefully developed fire restoration practices would enhance the visitor use experience within one of the most frequently visited wilderness areas in the United States while re‐engaging directly with Indigenous knowledge and traditional cultural practices.

We examined a set of five proxy reconstructions of the Pacific Decadal Oscillation (PDO) to test whether the choice of reconstruction affected the association between the PDO and widespread forest fires in the western United States. Exact binomial tests suggest the PDO has little direct impact on wildfires, with a statistically‐significant association between the phase of the PDO and regional fire activity observed with only one reconstruction. Region‐wide fires were not consistently associated with specific phase combinations of ENSO and the PDO. Any conclusion that extensive wildfires are more or less common when the PDO is in one phase or the other depends entirely on the choice of PDO reconstruction. Without a better understanding of low‐frequency behavior in the north Pacific prior to 1900, efforts using proxy data to determine whether or not the PDO affects wildfire activity in the western United States will produce ambiguous results. Key Points The PDO is believed to influence wildfire activity over one to several decades The choice of reconstruction determines the inferred PDO/fire relationship Without guidance from paleoclimatology, PDO‐fire tests will remain uncertain

Aim: To understand how the biophysical environment influences patterns of infection by non-native blister rust (caused by Cronartium ribicola) and mortality caused by native mountain pine beetles (Dendroctonus ponderosae) in whitebark pine (Pinus albicaulis) communities, to determine how these disturbances interact, and to gain insight into how climate change may influence these patterns in the future. Location: High-elevation forests in south-west Montana, central Idaho, eastern and western Oregon, USA. Methods: Stand inventory and dendroecological methods were used to assess stand structure and composition and to reconstruct forest history at sixty 0.1-ha plots. Patterns of blister rust infection and mountain pine beetle-caused mortality in whitebark pine trees were examined using nonparametric Kruskal-Wallis ANOVA, Mann-Whitney 17-tests, and Kolmogorov-Smirnov two-sample tests. Stepwise regression was used to build models of blister rust infection and mountain pine beetle-related mortality rates based on a suite of biophysical site variables. Results: Occurrence of blister rust infections was significantly different among the mountain ranges, with a general gradient of decreasing blister rust occurrence from east to west. Evidence of mountain pine beetle-caused mortality was identified on 83% of all dead whitebark pine trees and was relatively homogenous across the study area. Blister rust infected trees of all ages and sizes uniformly, while mountain pine beetles infested older, larger trees at all sites. Stepwise regressions explained 64% and 58% of the variance in blister rust infection and beetle-caused mortality, respectively, indicating that these processes are strongly influenced by the biophysical environment. More open stand structures produced by beetle outbreaks may increase the exposure of surviving whitebark pine trees to blister rust infection. Main conclusions: Variability in the patterns of blister rust infection and mountain pine beetle-caused mortality elucidated the fundamental dynamics of these disturbance agents and suggests that the effects of climate change will be complex in whitebark pine communities and vary across the species' range. Interactions between blister rust and beetle outbreaks may accelerate declines or facilitate the rise of rust resistance in whitebark pine depending on forest conditions at the time of the outbreak.

New dates from culturally modified red pine rediscovered in the Boundary Waters Canoe Area Wilderness in northern Minnesota provide an opportunity to merge tree-ring records of human land use with archaeological records, historical travel accounts, and traditional knowledge to enhance understanding of Anishinaabeg land tenure in the Wilderness. Records from 244 culturally modified trees (CMTs) demonstrate varying intensities of human use along historical water routes, notably the Border Route that connected Grand Portage to Rainy Lake and Lake of the Woods during the North American fur trade. Crossdated modification years from 119 CMTs provide direct evidence of human-landscape interaction along historical travel routes utilized by Anishinaabeg and Euro-American traders from the mid-1700s to the early 1900s. This CMT network preserves a fading biological record of fur-trade-era cultural history that contributes to a growing cross-cultural conversation on the storied traditional use of a cultural landscape that is now the most visited federal wilderness area in the United States.

Castle Mound Pine Forest State Natural Area (CMPF) is a 48-ha reserve at the confluence of the Driftless Area and Central Sand Plains of Wisconsin. Here, we report the first tree-ring—based fire history study for central Wisconsin and examine the relationships among fire, forest structure and composition, and historical land use at the site. We crossdated 12 fire-scar samples from Pinus resinosa stumps and inventoried and cored 83 trees along four transects to quantify the fire history, forest composition, and forest age structure at the site. The fire history data span the years 1788–2006 and include 15 years when trees were scarred by a fire on the site. Most fire scars were recorded in the earlywood of the recording growth ring, suggesting spring or early summer fires. The mean fire return interval for the site was six years when calculated between the first fire in 1841 and the last fire in 1923. The time-since-fire at the time of our study was 91 years. The canopy of the site was dominated by P. resinosa, individuals of which represent the oldest trees on the site. The subcanopy and recent age structure was dominated by mesic species, indicating that the forest at CMPF is transitioning away from the historical dominance by P. resinosa to a more closed forest following the cessation of fire at the site. Our data highlight an opportunity to establish ecological baseline data for this area to inform fire management and restoration activities.

Drought and mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreaks have affected millions of hectares of high-elevation conifer forests in the Northern Rocky Mountains during the past century. Little research has examined the distinction between mountain pine beetle outbreaks and climatic influence on radial growth in endangered whitebark pine (Pinus albicaulis Engelm.) ecosystems. We used a new method to explore divergent periods in whitebark pine radial growth after mountain pine beetle outbreaks across six sites in western Montana. We examined a 100-year history of mountain pine beetle outbreaks and climate relationships in whitebark pine radial growth to distinguish whether monthly climate variables or mountain pine outbreaks were the dominant influence on whitebark pine growth during the 20th century. High mortality of whitebark pines was caused by the overlapping effects of previous and current mountain pine beetle outbreaks and white pine blister rust infection. Wet conditions from precipitation and snowpack melt in the previous summer, current spring, and current summer benefit whitebark pine radial growth during the following growing season. Whitebark pine radial growth and climate relationships were strongest in sites less affected by the mountain pine beetle outbreaks or anthropogenic disturbances. Whitebark pine population resiliency should continue to be monitored as more common periods of drought will make whitebark pines more susceptible to mountain pine beetle attack and to white pine blister rust infection.

Culturally modified red pine (Pinus resinosa) can be added to the long list of natural and artificial features that comprise the cultural landscape of the Boundary Waters Canoe Area Wilderness of Minnesota, U.S.A. This study provides historical context for the first cultural-modification dates derived from the dendrochronological evaluation of primary-growth, fire-killed red pine in the Border Lakes region. The treering-based data are interpreted in the context of archaeological records, fur-trade era travel accounts, and ethnographic information to shed new light on the economy of the Border Lakes Ojibwe from the late 1700s to the early 1900s. Multiple lines of evidence from Lake Saganaga suggest that these culturally modified red pine were intentionally wounded in order to extract pitch, which was used to make the gum critical for constructing and repairing birch-bark canoes. Culturally modified red pine are a tangible artifact class that may serve to reframe modern perspectives on the critical role of the birch-bark canoe for transport in the historical Border Lakes landscape. These trees are living symbols of historical land-use patterns and are indicative of the strategic geography of the fur trade. Se puede añadir el pino rojo americano modificado culturalmente (Pinus resinosa) a la larga lista de elementos naturales y artificiales que forman el paisaje cultural del área salvaje de Boundary Waters Canoe en Minnesota (Estados Unidos). Este estudio ofrece el contexto histórico de las primeras fechas de modificación cultural a partir del análisis dendrocronológico del pino rojo americano, de crecimiento primario y extinguido por el fuego en la región de Border Lakes. Los datos basados en los anillos del árbol se interpretan en el contexto de archivos arqueológicos, relatos de viaje de la época del comercio de pieles o información etnográfica para arrojar nueva luz sobre la economía de Border Lakes Ojibwe entre finales del siglo XVIII y principios del XX. Múltiples pruebas del lago Saganaga sugieren que el pino rojo culturalmente modificado fue dañado de forma intencionada para extraer brea, que se utilizaba para fabricar pegamento, fundamental en la construcción y reparación de canoas de corteza de abedul. Los pinos rojos culturalmente modificados constituyen una clase de artefacto tangible que puede servir para redefinir las perspectivas modernas sobre el papel esencial de estas canoas en el transporte en los paisajes históricos de Border Lakes. Estos árboles son símbolos vivientes de los patrones históricos del uso del suelo e indicativos de la geografía estratégica del comercio de pieles. Les pins résineux (Pinus resinosa) culturellement modifiés peuvent être inscrits sur la longue liste des caractéristiques naturelles et artificielles qui composent le paysage culturel de la Boundary Waters Canoe Area Wilderness du Minnesota aux É.-U. La présente étude dresse un contexte historique pour les premières dates de modification culturelle dérivées de l’évaluation dendrochronologique des pins résineux tués par le feu depuis la croissance primaire. Ces données dendrochronologiques sont interprétées dans le contexte de données archéologiques, de récits de voyage datant de la traite des fourrures et de renseignements ethnographiques pour réexaminer l’économie du peuple Ojibwé de Border Lakes des années 1700 au début du 20e siècle. Plusieurs preuves récoltées à Lake Saganaga suggèrent que les pins résineux culturellement modifiés furent volontairement entaillés pour en extraire de la résine utilisée pour fabriquer la gomme essentielle à la construction et la réparation des canots en écorce de bouleau. Les pins résineux culturellement modifiés appartiennent à une catégorie d’artefacts concrets pouvant permettre de restructurer les points de vue modernes sur le rôle essentiel des canots en écorce de bouleau dans le transport du paysage historique de Border Lakes. Ces arbres sont les symboles vivants des modèles d’utilisation historique des terres et indicatifs de la géographie stratégique de la traite des fourrures.

Invasions of European earthworms into the forests of northern North America are causing dramatic changes in forest floor structure, vegetation communities, biogeochemical cycling, and site hydrology. However, long-term studies on the effects of invasive earthworms are limited because little data exist on the timing and rate of earthworm invasion at specific sites. We successfully used tree rings to identify the timing of earthworm invasions and the effects of earthworm activity on the Acer saccharum overstory of two recently invaded sites in northern Minnesota, thereby establishing a method to date earthworm invasions at other sites. In addition to identifying a tree-ring signature related to earthworm invasion, we found trees growing in invaded conditions were more sensitive to drought than trees growing in earthworm-free conditions. Increased drought sensitivity by A. saccharum has important implications for possible range shifts under climate change scenarios that include increasing drought frequency and severity.

Non-native, invasive earthworms are altering soils throughout the world. Ecological cascades emanating from these invasions stem from rapid consumption of leaf litter by earthworms. This occurs at a midpoint in the trophic pyramid, unlike the more familiar bottom-up or top-down cascades. These cascades cause fundamental changes (“microcascade effects”) in soil morphology, bulk density, and nutrient leaching, and a shift to warmer, drier soil surfaces with a loss of leaf litter. In North American temperate and boreal forests, microcascade effects can affect carbon sequestration, disturbance regimes, soil and water quality, forest productivity, plant communities, and wildlife habitat, and can facilitate other invasive species. These broader-scale changes (“macrocascade effects”) are of greater concern to society. Interactions among these fundamental changes and broader-scale effects create “cascade complexes” that interact with climate change and other environmental processes. The diversity of cascade effects, combined with the vast area invaded by earthworms, leads to regionally important changes in ecological functioning.