Evan R. Larson

Tree rings detect earthworm invasions and their effects in northern hardwood forests

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.

Changing climate response in near-treeline bristlecone pine with elevation and aspect

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 Northfacing slopes. High growth rates in the treeline South-facing trees have declined since the mid1990s. 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.

An Assessment Of The Dendroclimatic Potential Of Three Conifer Species In Northern Minnesota

Abstract 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 growth-climate 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.

Variability in Fire Regimes of High-Elevation Whitebark Pine Communities, Western Montana, USA

Abstract: We investigated the stand history of whitebark pine forests on 3 mountains in the Lolo National Forest, Montana, USA to characterize the fire regimes and other disturbance agents that historically operated at these sites and to explore the potential influences of modern fire suppression on these forests. We used hLarsonistorical fire atlas data and dendroecological data to reconstruct the distinct stand. The fire regimes of each site fit within the general definition of mixed-severity fire regimes, but distinct differences in fire frequency anween them. All 3 stands contained at least 1 post-disturbance cohort and had experienced at least 1 widespread fire over their histories. We found no consistent fire—climate relationship at these sites. Mountain pine beetles were the primary mortality agent in the current stands at all 3 sites. Subalpine fir began establishing at each site within 2 decades of the most recent widespread fire and well before fire suppression was effective in this region. Fire suppression may have reduced the occurrence of fire during the late 20th century at all 3 sites, but only the forest on Point Six has exceeded the mean interval between widespread fires. The differences in fire activity and effects of fire suppression that we observed at these sites are likely the result of different biophysical site characteristics and disturbance legacies and hold important implications for the development of site-specific management strategies for whitebark pine restoration. Nomenclature: ITIS, 2008.

Tree Establishment During Dry Spells At An Oak Savanna In Minnesota

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 (1809–1939), 24 of 42 trees with pith dates (62%) grew to 30-cm height during dry years (Palmer Drought Severity Index < −1), 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 (χ2  =  10.738, df  =  1, p-value  =  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.

The Need and Means To Update Chronologies In A Dynamic Environment

Abstract The International Tree-Ring Data Bank (ITRDB) provides public access to over 3000 tree-ring data sets collected over the past century, yet 809 of these sites have end dates between AD 1950 and 1980. These data cannot be calibrated with at least the past 30–40 years of instrumental data when used in climate reconstructions. We developed new tree-ring data sets at five sites in Maine, USA, to update earlier collections. Four of the five collections were successfully updated, with environmental changes at the fifth site limiting our success. Our results highlight the limits to tree longevity in a dynamic world and the need to increase and formalize efforts toward updating chronologies. We initiate a discussion to set forth explicit guidelines that help create consistent efforts to updating chronologies and provide a guide to beginning dendrochronologists who are particularly well suited to contribute to this area of work. The research was carried out through an introductory dendrochronology course taught at the University of Wisconsin—Platteville and offers a model to help direct the increasing availability of human resources to the rapidly growing field of dendrochronology.

A Multi-Proxy Environmental Narrative of Oregon White Oak (Quercus garryana) Habitat in the Willamette Valley, Oregon

Abstract Oregon white oak (Quercus garryana) habitat is a priority for conservation in the Pacific Northwest, with process-oriented management considered fundamental for the restoration of these ecosystems. Given that natural and anthropogenic processes operate across a wide spatial and temporal range, an historical perspective provides a more holistic understanding of the complex dynamics of these systems. To this end, we employed a multi-proxy approach to develop an environmental narrative of the Willamette University at Zena Forest (WUZ), Willamette Valley, Oregon between AD 1800 and 2014. We used historic written records, aerial photographs, ecological community characteristics, climate data, and dendrochronological data to investigate forest patterns and processes at the plot, stand, site, and regional scales. Our results illustrate temporal shifts in the importance of factors, including land use and ecological succession, in driving landscape change and forest development at WUZ. Critically, this narrative indicates that modern, patchwork vegetation patterns at WUZ are largely a consequence of anthropogenic practices that became increasingly targeted and intensive over the period of interest. This use of multiple proxy records provides a more comprehensive understanding of the dynamic ecological legacies and human-environment relationship necessary to guide restoration. Setting current management into a broader historical context enhances the efficacy of restoration efforts focused on improving the ecological health and function of oak habitat at WUZ, while contributing to the network of key sites maintaining these priority oak habitats in the Pacific Northwest.

Fire History at the Confluence of the Driftless Area and Central Sand Plains of Wisconsin: A Case Study from Castle Mound Pine Forest State Natural Area

ABSTRACT: 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.