James Dyer

Professor of Geography

Publications & Abstracts

Climate models project warmer summer temperatures will increase the frequency and heat severity of droughts in temperate forests of Eastern North America. Hotter droughts are increasingly documented to affect tree growth and forest dynamics, with critical impacts on tree mortality, carbon sequestration, and timber provision. The growing acknowledgement of the dominant role of drought timing on tree vulnerability to water deficit raises the issue of our limited understanding of radial growth phenology for most temperate tree species. Here, we use well-replicated dendrometer band data sampled frequently during the growing season to assess the growth phenology of 610 trees from 15 temperate species over six years. Patterns of diameter growth follow a typical logistic shape, with growth rates reaching a maximum in June, and then decreasing until process termination. On average, we find that diffuse-porous species take 16–18 days less than other wood-structure types to put on 50% of their annual diameter growth. However, their peak growth rate occurs almost a full month later than ring-porous and conifer species (ca. 24 ± 4 days; mean ± 95% credible interval). Unlike other species, the growth phenology of diffuse-porous species in our dataset is highly correlated with their spring foliar phenology. We also find that the later window of growth in diffuse-porous species, coinciding with peak evapotranspiration and lower water availability, exposes them to a higher water deficit of 88 ± 19 mm (mean ± SE) during their peak growth than ring-porous and coniferous species (15 ± 35 mm and 30 ± 30 mm, respectively). Given the high climatic sensitivity of wood formation, our findings highlight the importance of wood porosity as one predictor of species climatic sensitivity to the projected intensification of the drought regime in the coming decades.

Topography exerts strong control on microclimate, resulting in distinctive vegetation patterns in areas of moderate to high relief. Using the Thornthwaite approach to account for hydrologic cycle components, a GIS-based Water Balance Toolset is presented as a means to address fine-scale species–site relationships. For each pixel within a study area, the toolset assesses inter-annual variations in moisture demand (governed by temperature and radiation) and availability (precipitation, soil storage). These in turn enable computation of climatic water deficit, the amount by which available moisture fails to meet demand. Summer deficit computed by the model correlates highly with the Standardized Precipitation–Evapotranspiration Index (SPEI) for drought at several sites across the eastern U.S. Yet the strength of the approach is its ability to model fine-scale patterns. For a 25-ha study site in central Indiana, individual tree locations were linked to summer deficit under different historical conditions: using average monthly climatic variables for 1998–2017, and for the drought year of 2012. In addition, future baseline and drought-year projections were modeled based on downscaled GCM data for 2071–2100. Although small deficits are observed under average conditions (historical or future), strong patterns linked to topography emerge during drought years. The modeled moisture patterns capture vegetation distributions described for the region, with beech and maple preferentially occurring in low-deficit settings, and oak and hickory dominating more xeric positions. End-of-century projections suggest severe deficit, which should favor oak and hickory over more mesic species. Pockets of smaller deficit persist on the landscape, but only when a fine-resolution Light Detection and Ranging (LiDAR)-derived Digital Elevation Model (DEM) is used; a coarse-resolution DEM masks fine-scale variability and compresses the range of observed values. Identification of mesic habitat microrefugia has important implications for retreating species under altered climate. Using readily available data to evaluate fine-scale patterns of moisture demand and availability, the Water Balance Toolset provides a useful approach to explore species–environment linkages. 

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.   

When public lands were surveyed in the U.S., “witness trees” were often recorded to facilitate the relocation of property boundaries, and these records provide a snapshot of forest conditions prior to Euro-American settlement and land clearing.  This study utilizes witness trees and present-day plot data to explore long-term vegetation changes at a regional scale.  Landscape classes for a 5000 km2 study area in Appalachian Ohio were defined by slope, aspect, topographic position, soil pH, and available water capacity.  Cluster analysis and ordination revealed topo-edaphic patterns in the presettlement (c. 1800) and present-day forests, based on 5765 witness trees and 3249 contemporary trees occurring on 547 Forest Inventory and Analysis (FIA) subplots.  Mesophication is evident, as the oak-hickory presettlement forest is now dominated by maple-poplar.  Size-class analysis suggests the presettlement forest also experienced mesophication following xeric conditions of the preceding centuries.   In both presettlement and contemporary forests, ridges form distinctive communities compared to slopes and valleys, although topographic distinctiveness is now more prevalent.  Several taxa revealed changes in site affinities over the past two centuries; shifts in their realized niches suggest mesophication acts through diverse individualistic responses to a multiple set of interacting drivers.  Specifically, regionally documented changes in land use, drought, N deposition, and fire at the time of the original surveys lead to altered competitive relationships.  

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 ca 840,000 km2 and 5,327 tree recruitment dates spanning ca 1.4 million km2 across the humid eastern US. 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 1775-1780 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 1775-1780 comprising the heavy-tail portion of the distribution. Historical documents provide no evidence that hurricanes or ice storms triggered the 1775-1780 event. Instead, we identify a significant relationship between prior drought and years with elevated rates of disturbance with an intense drought occurring from 1772-1775. We further find that years with high rates of canopy disturbance have a propensity to create larger canopy gaps indicating repeated opportunities for rapid change in species composition beyond the landscape scale. Evidence of elevated, regional-scale disturbance reveals how rare events can potentially alter system trajectory – a substantial portion of old-growth forests examined here originated or were substantially altered more than two centuries ago following events lasting just a few years. Our recruitment data, comprised of at least 21 species and several shade-intolerant species, document a pulse of tree recruitment at the subcontinental scale during the late-1600s suggesting that this event was severe enough to open large canopy gaps. These disturbances and their climatic drivers support the hypothesis that punctuated, episodic climatic events impart a legacy in broadleaf-dominated forests centuries after their occurrence. Given projections of future drought, these results also reveal the potential for abrupt, meso- to large-scale forest change in broadleaf-dominated forests over future decades. 

This study examined the effect of the soil moisture gradient on tree growth response to prescribed fire and thinning in oak-dominated forests of Ohio. Six hundred and ninety-six increment cores (348 trees, five species) were collected from eighty 0.1 ha plots distributed across four treatments (control, thin, burn, thin + burn) in two sites. Ring widths were converted to basal area increments (BAIs).  A water balance approach based on geographic information systems (GIS) was used to assess the potential evapotranspiration (PET) and moisture deficit for each tree, along with a long-term integrated moisture index, also based on GIS. The moisture gradients varied considerably across the landscape, with the highest PET and moisture deficit on ridges and south-facing slopes. This variation influenced the BAI of the studied species, but more strongly in the control stands than in the managed stands, where treatment effects became the main drivers of growth. Oaks exhibited greater BAI on sites with intermediate moisture demand or stress, whereas the non-oaks had greater BAI on more mesic sites.  Moisture deficit and PET also interacted to influence BAI of yellow-poplar (Liriodendron tulipifera L.) and white oak (Quercus alba L.), particularly in the control.  These results demonstrate the strong regulatory effect of the topographically controlled soil moisture gradient on tree growth in mixed-oak (Quercus spp.) forests, which can be explored to better understand community response to prescribed fire and thinning treatments. 

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.

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. 

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.