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James Dyer
Professor of Geography

Publications & Abstracts


McEwan, R.W., J.M. Dyer, and N. Pederson. 2011. Multiple interacting ecosystem drivers: toward an encompassing hypothesis of oak forest dynamics across eastern North America. Ecography 34: 244-256.
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.


Dyer, J.M. 2010. Land use legacies in a central Appalachian forest: Differential response of trees and herbs to historic agricultural practices. Applied Vegetation Science 13: 195-206.

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. 


Dyer, J.M. 2009. Assessing topographic patterns in moisture use and stress using a water balance approach. Landscape Ecology, 24: 391-403.
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 U.S. (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. 


Dyer, J.M., and C.M. Cowell, 2008. Invasive Species and the Resiliency of a Riparian Environment.
Pages 87-103 in R.K. Kohli and S. Jose (eds.) Invasive Plants and Forest Ecosystems. CRC Press, Boca Raton, FL.
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 Island, Pennsylvania 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 Crulls Island highlights the many dilemmas associated with natural area management in the eastern United States, 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.


Dyer, J.M. 2006. Revisiting the Deciduous Forests of Eastern No
rth America.  BioScience 56: 341-352.
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.


Dyer. J.M. 2004. A water budget approach to predicting tree species growth and abundance, utilizing paleoclimatology sources. Climate Research 28: 1-10. 
Biogeographers and ecologists often need to quantify species-environment relationships to understand the distribution of vegetation, and to assess changes in pattern resulting from environmental change.  A water budget approach, which incorporates evaporative demand and moisture availability, is compared to traditional climatic variables in their ability to predict species growth and abundance in eastern North America.  Tree growth is first examined using tree-ring chronologies obtained from the International Tree-Ring Data Bank, correlated with climatic data from the nearest site in the U.S. Historical Climatology Network.  Secondly, logistic regression is used to model the range of American beech (Fagus grandifolia) using pollen records from the World Data Center for Paleoclimatology, and climatic data from NCAR’s CCM1 General Circulation Model for the control, 6 ka, and 11 ka runs.   Tree growth, especially for oaks and other deciduous trees, correlates more strongly with early growing-season deficit than with precipitation.  Water budget variables (actual evapotranspiration and deficit) also are more effective than traditional climatic variables in modeling the range of beech.  A water budget approach is attractive for modeling vegetation dynamics because it transcends geographic scale; it is able to model both local and continental-scale phenomena.


Dyer, J.M.  2002. A comparison of moisture scalars and water budget methods to assess vegetation-site relationships.  Physical Geography, 23: 245-258.
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.


Cowell, C.M., and J.M. Dyer. 2002. Vegetation development in a modified riparian environment: Human imprints on an Allegheny River Wilderness. Annals of the Association of American Geographers 92: 189-202. 
Pristine floodplain forests are virtually nonexistent in the eastern United States, necessitating that preservation efforts focus on relatively intact representatives of these unique ecosystems -- many situated where hydrologic modifications are the norm. This paper 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. Whereas research on natural riparian sites has emphasized the role of floods as a disturbance that generates early successional habitat, moderation of the hydrologic regime here 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 non-native 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.


Dyer, J.M. 2001. Using witness trees to assess forest change in southeastern Ohio. Canadian Journal of Forest Research 31: 1708-1
718.
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.


Dyer, J.M., and K.E. Moffett. 1999. Meadow invasion from high-elevation spruce-fir forest in south-central New Mexico. The Southwestern Naturalist 44: 445-457.
Stands of corkbark fir - Englemann spruce were sampled on Buck Mountain (elev. 3,282 m) within the White Mountains of south-central New Mexico.  A time series of air photos suggests that adjacent meadows have been invaded by these high-elevation stands continuously on the SW slope since the 1930s, although the forest-meadow boundary on the NE slope has been relatively stable for decades.  In order to obtain baseline stand information, and to assess patterns of meadow encroachment, quadrats were established in intact forest on both the SW and NE slope, and contiguous quadrats extended into the adjacent meadow.  Increment cores were extracted from the apparent oldest trees within each quadrat (n=53) to estimate establishment dates.  Based on field data and historical records, we conclude that climatic change is a more likely explanation for the encroachment of trees into the adjacent meadow, rather than fire suppression or changes in grazing intensities at this site.
 

Dyer, J.M., and P.R. Baird. 1997. Remnant forest stands at a prairie ecotone site: presettlement history and comparison with other maple-basswood stands. Physical Geography 18: 146-159.
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.


Dyer, J.M., and P.R. Baird. 1997. Wind disturbance in remnant forest stands along the prairie-forest ecotone, Minnesota, U.S.A. Plant Ecology 129: 121-137.
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.

 

Dyer, J.M. 1995. Assessment of climatic warming using a model of forest species migration. Ecological Modelling 79: 199-219.
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.
 

Dyer, J.M. 1994. Implications of habitat fragmentation on climate change-induced forest migration. Professional Geographer 46: 449-459.
A rapid warming caused by the release of greenhouse gases could result in the displacement of climatic controls limiting the current ranges of many species.  Projected northward displacement for loblolly pine is over 400 km, with only a narrow region of overlap between the current and projected future range limits.  A model of dispersal developed for loblolly pine is presented.  The model utilizes a GIS to assess the critical influence of land use pattern on climate change-induced migration through modern landscapes.  Results from two relatively large (150 x 150 km) study areas in the eastern U.S. suggest that potential migration rates may fall short by at least an order of magnitude of that necessary to track projected range shifts.  Management options of species transplanting and the establishment of greenways are explored with the model.  Species that are unable to keep pace with changing range limits may experience a reduction in population size and exist in climatic disequilibrium.

Dyer, J.M. 1994. Land use pattern, forest migration, and global warming. Landscape and Urban Planning 29: 77-83.
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.

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.


Dyer, J.M., and G.A. Brook. 1991. Spatial and temporal variations in temperate forest soil carbon dioxide during the non-growing season. Earth Surface Processes and Landforms 16: 411-426.
In the Whitehall Forest of Georgia during the 1985-1986 non-growing season soil CO2 varied with soil depth, varied spatially at constant depth, and varied temporally with changing environmental conditions.  Variations with depth in the upper 1.4 m of the soil were of greater magnitude than temporal variations and spatial differences at 30 cm depth were of lesser magnitude.  Mean soil CO2 in evergreen forest was higher (0.207%) than in deciduous and mixed forest (0.157%).  There were no trends in soil CO2 along hillslopes or with changes in soil texture, bulk density, moisture content, or temperature.  Soil CO2 did increase near trees possibly due to increased root densities and/or more numerous pockets of microbial activity.  For CO2 at 30 cm depth, two variables -- the mean daily temperature range in the month before measurement and actual evapotranspiration in the week before measurement (AET7) -- explained 76% of the variation in mean soil CO2.  At the profile site, where soil CO2 was measured at five depths, 66% of the variability in CO2 was explained by soil depth, AET7, and the average daily temperature range in the two months before measurement.
 


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