James Dyer

Professor of Geography

Publications & Abstracts

The classification, mapping, and description of ecosystems are fundamental components of land management. The National Hierarchical Framework of Ecological Units (Cleland et al. 1997) provides the basis for the USDA Forest Service to accomplish these tasks. This framework allows the recognition of ecosystems or ecological units at eight spatial scales within a nested hierarchy. The highest levels of ecological units have been delineated nationally from domains through subsections. Our project, described here, is an extension of this national effort and concentrates on the local mapping and description of ecological landtypes, the seventh level of the hierarchy. It is envisioned that the results of this project will serve the Wayne National Forest in Ohio and its partners in many ways. This work provides an ecological basis by which future land management plans can be developed and executed. It helps give direction to silvicultural activities designed to maintain ecological sustainability, biological diversity, and local economies. It also informs research by distinguishing different ecosystems for experimental design and spatial extrapolation of findings.   

Many forests of eastern North American are undergoing a species composition shift in which maples ( Acer spp.) are increasingly important while oak ( Quercus spp.) regeneration and recruitment has become increasingly scarce. This dynamic in species composition occurs across a large and geographically complex region. The elimination of fire has been postulated as the driver of this dynamic; however, some assumptions underlying this postulate have not been completely examined, and alternative hypotheses remain underexplored. Through literature review, and a series of new analyses, we examined underlying assumptions of the “oak and fire” hypothesis and explored a series of alternative hypotheses based on well-known ecosystem drivers: climate change, land-use change, the loss of foundation and keystone species, and dynamics in herbivore populations. We found that the oak maple dynamic began during a shift in climate regime – from a time of frequent, severe, multi-year droughts to a period of increased moisture availability. Anthropogenic disturbance on the landscape changed markedly during this same time, from an era of Native American utilization, to a time characterized by low population densities, to Euro-American settlement and subsequent land transmogrification. During the initiation of the oak-maple dynamic, a foundation species, the American chestnut, was lost as a canopy tree across a broad range. Several important browsers and acorn predators had substantial population dynamics during this period, e.g., white-tailed deer populations grew substantially concurrent with increasing oak recruitment failure. In conclusion, our analyses suggest that oak forests are reacting to marked changes in a suite of interlocking factors. We propose a “multiple interacting ecosystem drivers hypothesis,” which provides a more encompassing framework for understanding oak forest dynamics.

Question :  Are contemporary herb and tree patterns explained by historic land use practices?  If so, are observed vegetation patterns associated with life-history characteristics, soil properties, or other environmental variables?

Location :  Southeastern Ohio, USA.

Methods :  Using archival records,currently forested sites were identified with distinct land use histories: cultivated, pastured (but not plowed), and reference sites which appear to have never been cleared.  Trees were recorded by size and species on twenty 20 m x 20 m plots; percent cover was estimated for each herb species in nested 10 m x 10 m plots.  Environmental characteristics were noted, and soil samples analyzed for nutrient availability and organic matter.  Nonmetric multidimensional scaling ordination was performed separately on both tree and herb datasets to graphically characterize community composition among plots.  Life-history traits were investigated to explain observed compositional differences.

Results :  Vegetation patterns were explained by current environmental gradients, and especially by land use history.   Cultivated and pastured sites had similar tree composition, distinct from reference sites.  Herb composition of pastured and reference sites was similar and distinct from cultivated sites, suggesting the “tenacity” of some forest herbs on formerly cleared sites. Tilling removes rhizomatous species, and disfavors species with unassisted dispersal.  These life-history traits were underrepresented on cultivated sites, though ant-dispersed species were not.

Conclusions :  Historic land use practices accounted for as much variation in species composition as environmental gradients.  Furthermore, trees and herbs responded differently to past land use practices.  Life-history traits of individual species interact with the nature of disturbance to influence community composition. 

Research into exotic species invasions has focused both on life-history traits of the invaders, as well as on the site qualities that may make ecosystems prone to invasion.Successful invasion of an ecosystem is typically a result of conducive combinations of both species and site characteristics.Crull’s illustrates this interaction well, having been aggressively colonized by two invasive species following the construction of an upstream dam.The island will be used as a case study to highlight the dynamics of a highly “invasible” system.The process of invasion will then be discussed within the context of resiliency theory. Originally posited for ecological systems, resiliency theory has been extended to address linked ecological and social systems. Although a few studies have applied the framework of resiliency theory to investigations of exotic species, little has been done regarding invasive plants.Our examination of highlights the many dilemmas associated with natural area management in the eastern , where extensive human modification of landscape processes has combined with introduction of non-native species to fundamentally alter the functioning of the region’s ecosystems.

In 1950, E. Lucy Braun published Deciduous Forests of Eastern North America , which included a map depicting “original” forest pattern.Her classification of forest regions remains an influential reference, though it was shaped by ecological assumptions that we consider outdated today.Using a data set from an extensive network of contemporary forest plots, a new map of forest regions is presented.Although differences between the maps are noted, including the “homogenization” of forests in the central section of the deciduous forest formation, the “geography” of Braun’s forest regions is largely maintained.Similarities between the maps are noteworthy considering methodological differences in their creation, and despite intensive land use changes, fire suppression, introduction of exotic species, and changes in atmospheric chemistry that have occurred since Braun’s work.

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 southeastern 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 site’s classification will change concurrently with changes in climate.

In 1787, the U.S. Congress authorized the sale of the “Ohio Company Purchase,” c. 5000 km2 in Appalachian Ohio.  The land was surveyed using a Township and Range system shortly thereafter.  Data on >5600 witness trees were transcribed from the survey records, and witness tree locations were plotted on a digital map.  This information was used to evaluate presettlement forest composition and structure, and to investigate vegetation-site relationships before widespread alteration of the forests had taken place.  Presettlement conditions were compared to present conditions using Forest Inventory and Analysis (FIA) data.

Two hundred years ago, the forests of southeastern Ohio were dominated by large individuals of Quercus alba L., Carya Nutt. spp., Quercus velutina Lam., and Fagus grandifolia Ehrh.  These four taxa comprised 74% of all witness trees.  Although almost 70% of the region is forested today, the second-growth forest has witnessed a decrease in Quercus and Carya , and an increase in Acer saccharum Marsh., A. rubrum L., and many early successional species in smaller size classes.  Despite the significant shift in forest composition and structure, species in general seem to be occupying similar positions in the present-day landscape compared to the presettlement forest; topographic variables most strongly control species occurrence in this landscape.

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 to (1) determine if the maple-basswood stands at Rydell NWR existed historically, and to (2) compare the composition of the Rydell NWR stands to maple-basswood “core” sites, as well as other outlier sites.

Public Land Survey records 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, that 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 stands, using detrended correspondence analysis.  A strong geographic pattern is evidenced in the ordination diagram, which is attributed to differences in September precipitation and actual evapotranspiration between the sites. Local-scale environmental gradients act to modify the dominant climatic trend on composition, however, as some Rydell stands plotted closely to the core region in ordination space, whereas other Rydell stands demonstrated greater similarity to oak and aspen stands in North Dakota.

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, to assess the impact of overbrowsing by deer on stand recovery, and to project future composition based on the nature of the storm damage and current understory characteristics.

Basal area in 6 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 Michx.) in larger size classes.  Trees situated at stand edges were not more susceptible to snapping or uprooting.  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 maple-basswood stands should maintain their late successional character.  Overbrowsing and preferential foraging by deer may significantly alter stand recovery patterns.

S ignificant 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.

Range limits of many plant species are expected to shift dramatically if climatic warming, driven by the release of greenhouse gases, occurs 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.