James M. Dyer

The Legacy of Episodic Climatic Events in Shaping Temperate, Broadleaf Forests

In humid, broadleaf-dominated forests where gap dynamics and partial canopy mortality appears to dominate the disturbance regime at local scales, paleoecological evidence shows alteration at regional-scales associated with climatic change. Yet, little evidence of these broad-scale events exists in extant forests. To evaluate the potential for the occurrence of large-scale disturbance, we used 76 tree-ring collections spanning ∼840 000 km2 and 5327 tree recruitment dates spanning ∼1.4 million km2 across the humid eastern United States. Rotated principal component analysis indicated a common growth pattern of a simultaneous reduction in competition in 22 populations across 61 000 km2. Growth-release analysis of these populations reveals an intense and coherent canopy disturbance from 1775 to 1780, peaking in 1776. The resulting time series of canopy disturbance is so poorly described by a Gaussian distribution that it can be described as “heavy tailed,” with most of the years from 1775 to 1780 comprising th...

Assessment of climatic warming using a model of forest species migration

Abstract Significant shifts in plant species ranges are anticipated next century if climate warms due to greenhouse gas emissions. The magnitude of the projected warming is considerable; the rate at which it is predicted to occur is unprecedented. There is genuine reason for concern that the extent of the range shifts will exceed the dispersal abilities of many plant species, especially in the context of extensive habitat fragmentation. This study attempts to assess explicitly the influence of two factors — mechanism of dispersal and land use configuration — on the ability of plant species to migrate in response to climatic warming. Computer models were developed to simulate dispersal at the time interval of a generation for wind-dispersed and bird-dispersed tree species. These models were applied to three study areas in the eastern United States, each consisting of two 1 : 250 000 USGS land use land cover quadrangles, which had been reclassified according to probabilities of successful colonization. The study areas reflected the continuum of human impact on the landscape, from areas in intensive agriculture to heavily forested areas. The fastest modeled migration rate observed was 81 m/yr for the wind-dispersed species and 136 m/yr for the bird-dispersed species. Average migration rates were significantly lower. The wind-dispersed species was especially sensitive to habitat isolation and fragmentation. Significant variations in average bird-dispersed migration rates occurred with modest differences in the land use pattern within a landscape; no single predictor of bird-dispersed migration success emerged. Model results indicate that many species may be unable to migrate as range limits shift with a climatic warming, resulting in long-term climatic disequilibrium.

Revisiting the Deciduous Forests of Eastern North America

Abstract Deciduous Forests of Eastern North America, written by E. Lucy Braun and published in 1950, included a map depicting “original” (virgin) forest pattern. Her classification of forest regions remains an influential reference, though it was shaped by ecological assumptions that researchers consider outdated today. In this article, I present a new map of forest regions, using a data set from an extensive network of contemporary forest plots. Although there are differences between the two maps, including the homogenization of forests in the central section of the deciduous forest formation, the geography of Brauns forest regions is largely maintained. The similarities between the maps are noteworthy, considering the methodological differences in their creation and the intensive land use changes, fire suppression, introduction of exotic species, and changes in atmospheric chemistry that have occurred since Brauns work.

Wind disturbance in remnant forest stands along the prairie-forest ecotone, Minnesota, USA

Strong winds are an important disturbance agent in northern Minnesota forests. On June 19, 1994, strong winds (>160 km h(-1)) associated with a tornado damaged forested areas within the Rydell National Wildlife Refuge, situated in Polk County Minnesota along the prairie-forest boundary. Field sampling was conducted immediately following the storm to quantify the type and extent of damage in four different community types, and to project future composition based on the nature of the storm damage and current understory characteristics, including the impact of overbrowsing by deer. Basal area in six sampled remnant forest stands was reduced by 33.5%, although the damage was heterogeneous; basal area in one stand was reduced by 68.1%. The overall effect of the storm was the removal of early- successional species (primarily Populus tremuloides) in larger size classes. Trees situated at stand edges were not more susceptible to snapping or uprooting than interior trees. Projections of future stand composition indicate that wind disturbance, unlike other agents of disturbance such as fire, may accelerate succession on the Refuge, such that early-successional stands will assume a later-successional character, while Acer-Tilia stands should maintain their late-successional character. Overbrowsing and preferential foraging by deer may significantly alter stand recovery patterns.

Assessing topographic patterns in moisture use and stress using a water balance approach

Through its control on soil moisture patterns, topography’s role in influencing forest composition is widely recognized. This study addresses shortcomings in traditional moisture indices by employing a water balance approach, incorporating topographic and edaphic variability to assess fine-scale moisture demand and moisture availability. Using GIS and readily available data, evapotranspiration and moisture stress are modeled at a fine spatial scale at two study areas in the US (Ohio and North Carolina). Model results are compared to field-based soil moisture measurements throughout the growing season. A strong topographic pattern of moisture utilization and demand is uncovered, with highest rates of evapotranspiration found on south-facing slopes, followed by ridges, valleys, and north-facing slopes. South-facing slopes and ridges also experience highest moisture deficit. Overall higher rates of evapotranspiration are observed at the Ohio site, though deficit is slightly lower. Based on a comparison between modeled and measured soil moisture, utilization and recharge trends were captured well in terms of both magnitude and timing. Topographically controlled drainage patterns appear to have little influence on soil moisture patterns during the growing season. In addition to its ability to accurately capture patterns of soil moisture in both high-relief and moderate-relief environments, a water balance approach offers numerous advantages over traditional moisture indices. It assesses moisture availability and utilization in absolute terms, using readily available data and widely used GIS software. Results are directly comparable across sites, and although output is created at a fine-scale, the method is applicable for larger geographic areas. Since it incorporates topography, available water capacity, and climatic variables, the model is able to directly assess the potential response of vegetation to climate change.

Climate remains an important driver of post‐European vegetation change in the eastern United States

Department of Geography, University of Wisconsin-Madison, 550 North ParkStreet, Madison, WI 53706, USAThe influence of climate on forest change during thepast century in the eastern United States was evalu-ated in a recent paper (Nowacki & Abrams, 2014)that centers on an increase in ‘highly competitivemesophytic hardwoods’ (Nowacki & Abrams, 2008)and a concomitant decrease in the more xerophyticQuercus species. Nowacki & Abrams (2014) con-cluded that climate change has not contributed sig-nificantly to observed changes in forest composition.However, the authors restrict their focus to a singleelement of climate: increasing temperature since theend of the Little Ice Age ca. 150 years ago. In theirstudy, species were binned into four classifications(e.g., Acer saccharum – ‘cool-adapted’, Acer rubrum –‘warm-adapted’) based on average annual tempera-ture within each species range in the United States,reducing the multifaceted character of climate into asingle, categorical measure. The broad temperatureclasses not only veil the many biologically relevantaspects of temperature (e.g., seasonal and extremetemperatures) but they may also mask other influ-ences, both climatic (e.g., moisture sensitivity) andnonclimatic (e.g., competition).Understanding the primary drivers of forest changeis critically important. However, using annual tem-perature reduces the broad spectrum of climaticinfluence on forests (e.g., Jackson & Overpeck, 2000;Jackson et al., 2009) to a single variable. Tsuga canad-ensis illustrates one example of the complex interac-tion between trees and temperature. In the southernpart of its range, Tsuga canadensis growth is weakly,but positively correlated with early growing-seasontemperature. However, this relationship becomesstronger and shifts to later in the season toward thenorthern part of its range (Cook & Cole, 1991). More-over, Tsuga canadensis growth is significantly andnegatively correlated with just May temperaturesduring the current growing season in the northeast-ern United States (Cook, 1991; Cook & Cole, 1991;Vaganov et al., 2011), while in the southeastern Uni-ted States it is strongly and negatively correlatedwith summer (June–August) temperatures (Hart et al.,2010). Trees can also be sensitive to diverse and ofteninteracting climate variables at various stages of theirlife cycles (Jackson et al., 2009). Interactions betweenprecipitation and temperature are clearly important(Harsch & Hille Ris Lambers, 2014; Martin-Benito &Pederson, accepted), and often lead to counterintui-tive responses. For example, some plant species thatwould have been expected to move north and ups-

Vegetation Development in a Modified Riparian Environment: Human Imprints on an Allegheny River Wilderness

Pristine floodplain forests are virtually nonexistent in the eastern United States, requiring that preservation efforts focus on relatively intact representatives of these unique ecosystems, many situated where hydrologic modifications are the norm. This article examines the vegetation dynamics for one such natural area, a wilderness island in northwestern Pennsylvania, to assess how the ecological processes of a riparian preserve are affected by changes to the surrounding environment. Ordination of a vegetation sample identifies several landscape patches on the island; the structure and historical development of these communities are analyzed using tree ring patterns, aerial photography, and the flood regime characteristics preceding and following construction of a large dam upstream. Research on natural riparian sites has emphasized the role of floods as a disturbance that generates early successional habitat. Here, however, moderation of the hydrologic regime has shifted the impact of floods from disturbance to stressor. Peak flows are no longer sufficient to open sites for colonization, while the duration of flooding has increased. Without flood disturbance, later stages of succession become more widely represented, and species regeneration occurs in the context of competitive—rather than open—sites. The altered disturbance regime thus favors species with life history characteristics atypical of the pre-dam environment, including nonnative species, resulting in altered composition and vegetation dynamics. Managerial expectations that natural successional processes will eventually restore degraded riparian habitats in these modified settings are therefore unlikely to be fulfilled.

Land use pattern, forest migration, and global warming

Abstract Range limits of many plant species are expected to shift dramatically if climatic warming, driven by the release of greenhouse gases, occurs in the next century. The ability of species to migrate in response to the range shifts has been questioned, especially in the context of extensive habitat fragmentation which occurs in modern-day landscapes. Simulation models are presented which incorporate two factors, land use pattern and means of dispersal, to assess potential responses of forest species to climatic warming. Study areas displayed a range of human influence on the landscape, from heavily forested areas to areas dominated by urbanization and agriculture. The effect of establishing corridors (greenways) through fragmented landscapes is also assessed. Results indicate that many species may be unable to track shifts in climatically-controlled range limits, resulting in widespread disequilibrium between vegetation and climate. A variety of mitigating options likely will be necessary to offset the negative consequences of climatic warming on biological diversity. Land use planners and managers are encouraged to incorporate climate warming into long-term planning.

A Comparison of Moisture Scalars and Water Budget Methods to Assess Vegetation-Site Relationships

Quantification of vegetation-site relationships is often required in biogeographic research. Methods linking species to particular sites typically assess evaporative demand and soil moisture availability at the site, though methods differ in how these factors are assessed. This study compares three approaches—a water-budget approach, and field-based and map-based moisture scalars—in their ability to predict the occurrence of a single species, American beech (Fagus grandifolia), observed in 102, 0.04 ha plots in SE Ohio. Actual evapotranspiration and deficit provided results superior to field-based and map-based scalars. Map-based techniques are potentially limited at fine spatial scales, due to the large discrepancy between observed topographic variables, and those modeled with 7.5-minute elevation grids. The study concludes that a water budget approach is applicable to a wide range of studies exploring vegetation-site linkages. It has advantages of being objective in its computation, and applicable at a wide range of spatial scales. Perhaps most importantly with regard to global change research is the dynamic nature of the method: a sites classification will change concurrently with changes in climate.

REMNANT FOREST STANDS AT A PRAIRIE ECOTONE SITE: PRESETTLEMENT HISTORY AND COMPARISON WITH OTHER MAPLE-BASSWOOD STANDS

The maple-basswood community type has long been associated with the “Big Woods” of Minnesota and adjacent Wisconsin, although this community type also exists in discontinuous phases within surrounding forest types. This study looks at the apparent most northwestern outlier of the maple-basswood community type, Rydell National Wildlife Refuge (NWR) in Minnesota. Specific goals are (1) to determine if the maple-basswood stands at Rydell NWR existed historically, and (2) to compare the composition of the Rydell NWR stands to maple-basswood “core” sites, as well as other outlier sites. Public Land Survey records for the period between 1872 and 1876 for a 16-township block encompassing Rydell NWR indicate that, although this area was dominated by an oak-aspen forest type, a distinct maple-basswood region existed prior to European settlement and this had escaped disturbance from fire and wind. Present-day stand composition data from six Rydell stands are compared with published data from other maple-basswood st...