Professor of Geography
Dyer, and N. Pederson. 2011. Multiple interacting ecosystem
drivers: toward an encompassing hypothesis of oak forest dynamics
across eastern North America. Ecography
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
Location: Southeastern Ohio,
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
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
from cultivated sites, suggesting the
“tenacity” of some forest
herbs on formerly cleared sites. Tilling removes rhizomatous species, and
disfavors species with unassisted
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
Dyer, J.M. 2009.
Assessing topographic patterns in moisture use and stress
using a water balance approach. Landscape Ecology, 24: 391-403.
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
demand and moisture availability. Using
GIS and readily-available data, evapotranspiration and moisture stress
modeled at a fine spatial scale at two study areas in the U.S. (Ohio
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
found on south-facing slopes, followed by ridges, valleys, and
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
and timing. Topographically-controlled
drainage patterns appear to have little influence on soil moisture
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
offers numerous advantages over traditional moisture indices. It
assesses moisture availability and
utilization in absolute terms, using readily-available data and
software. Results are directly
comparable across sites, and although output is created at a fine
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
of vegetation to climate change.
Dyer, J.M., and
C.M. Cowell, 2008.
Riparian Environment. Pages 87-103 in R.K.
and S. Jose (eds.) Invasive
and Forest Ecosystems. CRC
Press, Boca Raton, FL.
into exotic species invasions has focused both on life-history traits
invaders, as well as on the site qualities that may make ecosystems
invasion. Successful invasion of an
ecosystem is typically a result of conducive combinations of both
Crull’s Island, Pennsylvania illustrates this
interaction well, having been aggressively colonized by two invasive
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
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
fundamentally alter the functioning of the region’s ecosystems.
2006. Revisiting the
Deciduous Forests of Eastern North
BioScience 56: 341-352.
In 1950, E. Lucy
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
we consider outdated today. Using a data
set from an extensive network of contemporary forest plots, a new map
regions is presented. Although differences
between the maps are noted, including the “homogenization” of forests
central section of the deciduous forest formation, the “geography” of
forest regions is largely maintained.
Similarities between the maps are noteworthy considering
differences in their creation, and despite intensive land use changes,
suppression, introduction of exotic species, and changes in atmospheric
chemistry that have occurred since Braun’s work.
J.M. 2004. A water
budget approach to predicting tree species
growth and abundance, utilizing paleoclimatology sources.
ecologists often need to quantify species-environment
relationships to understand the distribution of vegetation, and to
changes in pattern resulting from environmental change. A water
approach, which incorporates evaporative demand and moisture
is compared to traditional climatic variables in their ability to
species growth and abundance in eastern North America. Tree
is first examined using tree-ring chronologies obtained from the
Tree-Ring Data Bank, correlated with climatic data from the nearest
in the U.S. Historical Climatology Network. Secondly, logistic
is used to model the range of American beech (Fagus grandifolia) using
pollen records from the World Data Center for Paleoclimatology, and
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
than with precipitation. Water budget variables (actual
and deficit) also are more effective than traditional climatic
in modeling the range of beech. A water budget approach is
for modeling vegetation dynamics because it transcends geographic
it is able to model both local and continental-scale phenomena.
J.M. 2002. A
comparison of moisture scalars and water
budget methods to assess vegetation-site relationships. Physical
vegetation-site relationships is often required in
biogeographic research. Methods linking species to particular
typically assess evaporative demand and soil moisture availability at
site, though methods differ in how these factors are assessed.
study compares three approaches – a water-budget approach, and
and map-based moisture scalars – in their ability to predict the
of a single species, American beech (Fagus grandifolia),
in 102, 0.04 ha plots in southeastern Ohio. Actual
and deficit provided results superior to field-based and map-based
Map-based techniques are potentially limited at fine spatial scales,
to the large discrepancy between observed topographic variables, and
modeled with 7.5-minute elevation grids. The study concludes that
a water budget approach is applicable to a wide range of studies
vegetation-site linkages. It has advantages of being objective in
its computation, and applicable at a wide range of spatial
Perhaps most importantly with regard to global change research is the
nature of the method: a site’s classification will change concurrently
with changes in climate.
and J.M. Dyer. 2002.
in a modified riparian environment: Human imprints on an Allegheny
forests are virtually nonexistent in the eastern
United States, necessitating that preservation efforts focus on
intact representatives of these unique ecosystems -- many situated
hydrologic modifications are the norm. This paper examines the
dynamics for one such natural area, a wilderness island in northwestern
Pennsylvania, to assess how the ecological processes of a riparian
are affected by changes to the surrounding environment.
of a vegetation sample identifies several landscape patches on the
the structure and historical development of these communities are
using tree ring patterns, aerial photography, and the flood regime
preceding and following construction of a large dam upstream. Whereas
on natural riparian sites has emphasized the role of floods as a
that generates early successional habitat, moderation of the hydrologic
regime here has shifted the impact of floods from disturbance to
Peak flows are no longer sufficient to open sites for colonization,
the duration of flooding has increased. Without flood
later stages of succession become more widely represented, and species
regeneration occurs in the context of competitive, rather than open,
The altered disturbance regime thus favors species with life-history
atypical of the pre-dam environment, including non-native species,
in altered composition and vegetation dynamics. Managerial expectations
that natural successional processes will eventually restore degraded
habitats in these modified settings are therefore unlikely to be
to assess forest change
in southeastern Ohio. Canadian
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
witness trees were transcribed from the survey records, and witness
locations were plotted on a digital map. This information was
to evaluate presettlement forest composition and structure, and to
vegetation-site relationships before widespread alteration of the
had taken place. Presettlement conditions were compared to
conditions using Forest Inventory and Analysis (FIA) data.
Two hundred years
the forests of southeastern Ohio were dominated
by large individuals of Quercus alba L., Carya Nutt.
velutina Lam., and Fagus grandifolia Ehrh. These
taxa comprised 74% of all witness trees. Although almost 70% of
region is forested today, the second-growth forest has witnessed a
in Quercus and Carya, and an increase in Acer
Marsh., A. rubrum L., and many early successional species in
size classes. Despite the significant shift in forest composition
and structure, species in general seem to be occupying similar
in the present-day landscape compared to the presettlement forest;
variables most strongly control species occurrence in this landscape.
Dyer, J.M., and
1999. Meadow invasion from high-elevation
spruce-fir forest in south-central New Mexico. The Southwestern
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
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
within each quadrat (n=53) to estimate establishment dates. Based
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.
stands at a prairie ecotone site: presettlement history and comparison
other maple-basswood stands. Physical Geography 18:
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
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.
Dyer, J.M., and P.R.
in remnant forest stands along the prairie-forest ecotone, Minnesota,
Plant Ecology 129: 121-137.
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
stands are compared with published data from other maple-basswood
stands, using detrended correspondence analysis. A strong
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
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.
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
may significantly alter stand recovery patterns.
warming using a model
of forest species migration.
Significant shifts in
plant species ranges are anticipated next century
if climate warms due to greenhouse gas emissions. The magnitude
the projected warming is considerable; the rate at which it is
to occur is unprecedented. There is genuine reason for concern
the extent of the range shifts will exceed the dispersal abilities of
plant species, especially in the context of extensive habitat
This study attempts to assess explicitly the influence of two factors -
mechanism of dispersal and land use configuration - on the ability of
species to migrate in response to climatic warming. Computer
were developed to simulate dispersal at the time interval of a
for wind-dispersed and bird-dispersed tree species. These models
were applied to three study areas in the eastern United States, each
of two 1:250,000 USGS land use land cover quadrangles, which had been
according to probabilities of successful colonization. The study
areas reflected the continuum of human impact on the landscape, from
in intensive agriculture to heavily forested areas. The fastest
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
sensitive to habitat isolation and fragmentation. Significant
in average bird-dispersed migration rates occurred with modest
in the land use pattern within a landscape; no single predictor of
migration success emerged. Model results indicate that many
may be unable to migrate as range limits shift with a climatic warming,
resulting in long-term climatic disequilibrium.
Dyer, J.M. 1994.
on climate change-induced forest migration. Professional Geographer
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
over 400 km, with only a narrow region of overlap between the current
and projected future range limits. A model of dispersal developed
loblolly pine is presented. The model utilizes a GIS to assess
critical influence of land use pattern on climate change-induced
migration through modern landscapes. Results from two relatively
(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
species transplanting and the establishment of greenways are explored
with the model. Species that are unable to keep pace with
range limits may experience a reduction in population size and exist in
Dyer, J.M. 1994.
Land use pattern,
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
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
a range of human influence on the landscape, from heavily forested
areas to areas dominated by urbanization and agriculture. The
of establishing corridors (greenways) through fragmented landscapes is
that many species may be unable
to track shifts in climatically-controlled range limits, resulting in
widespread disequilibrium between vegetation and climate. A
mitigating options likely will be necessary to offset the negative
consequences of climatic warming on biological diversity. Land
planners and managers are encouraged to incorporate climate warming
into long-term planning.
Dyer, J.M., and
G.A. Brook. 1991.
temporal variations in temperate forest soil carbon dioxide during the
non-growing season. Earth
Surface Processes and Landforms
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
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
trends in soil CO2 along hillslopes or with changes in soil texture,
bulk density, moisture content, or temperature. Soil CO2 did
near trees possibly due to increased root densities and/or more
numerous pockets of microbial activity. For CO2 at 30 cm depth,
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
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.
Back to Main Page