Geography

Recent research has challenged the long-standing hypothesis that forests in the Upper Midwest of the United States developed during wetter periods and retreated during dry periods. We explored this debate by examining patterns of tree establishment on an oak savanna in east-central Minnesota within the context of variable moisture availability and fire suppression. We used superposed epoch analyses (SEA) to evaluate the mean moisture conditions for a 21-year window surrounding tree establishment dates. Before effective fire suppression , 24 of 42 trees with pith dates (62%) grew to 30-cm height during dry years (Palmer Drought Severity Index , 21), versus only 5 of 42 (12%) that established in wet years (PDSI . 1). Significantly more trees established during dry periods (negative PDSI values) than would be expected with the proportion of wet-to-dry years (x 2 5 10.738, df 5 1, p-value 5 0.001). Twenty of the complete sample of 74 trees with pith dates (27%) established during drought in the 1930s. We hypothesize that dry conditions limited plant productivity, which in turn decreased competition between grasses and tree seedlings and reduced rates of accumulation of fine fuels, enabling seedlings to grow tall enough to resist subsequent fires. We recommend SEA as a methodological approach to compare historical climate conditions with the timing of regeneration success in other regions of forest expansion.

Concern exists regarding the decline of Eastern white pine (Pinus strobus L .) in the pine-mixed hardwood forests of the Great Lakes region . Using dendroecological techniques, we reconstructed the disturbance history and successional dynamics of an isolated old-growth white pine-mixed hardwood forest in northeastern Minnesota . The oldest trees in the stand were white pine, but a transition to late-successional species began in the mid-1800s prior to extensive anthropogenic influences in the region . This was facilitated by a change in the dominant disturbance type from fire to a combination of wind throw and white-tailed deer browsing . This transition appears to have occurred under natural conditions and will continue to favor both the establishment and continued dominance of late-successional species . This study highlights the importance of studying the natural range of variability of white pine forests in the Great Lakes region of Minnesota .

To accommodate human needs in the future, we are required to understand what disturbance processes and climatic conditions lead to the modern environment. Dendrochronology is a high-resolution application used to examine processes that sustain or modify both natural and human environments over shortand long-term periods (Fritts, 1971; Hughes et al., 1982; Cook and Kairiukstis, 1990). Dendrochronology provides a means to better understand environmental processes that operate over large spatial scales such as climate and wildfires. Climate ...

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.

Ring-width chronologies from Pinus resinosa Ait., Pinus strobus L., and Thuja occidentalis L. were developed in two areas of the Boundary Waters Canoe Area Wilderness to assess their growthclimate response and their potential for developing reconstructions of climate. New red pine chronologies were combined with existing chronologies to extend the ring-width record both into the past and into the present. Ring-width response to climate, assessed using correlation analysis and response functions, was broadly similar among all three species with relatively significant positive relationships with June-July precipitation and significant negative (but less consistent) associations with June-July temperatures (p , 0.05). White-cedar appeared to have a broader phenological window of response with a stronger spring influence when compared to other species included in this study. Comparisons with other nearby proxies showed relatively strong coherence overall but with some important regional differences. Overall, these species may be useful for placing current climatic patterns in the Boundary Waters within a longer term perspective but care should be taken with respect to identifying appropriate climatic records for calibration.

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.