Landscape Changes in the Southwestern United States: Techniques, Long-term Data Sets, and Trends
by
Craig D. Allen
U.S. Geological Survey
Midcontinent Ecological Science Center
Jemez Mountains Field Station
Los Alamos, New Mexico 87544
505/672-3861 ext. 541
craig_allen@usgs.gov
Julio L. Betancourt
U.S. Geological Survey
Desert Laboratory
Tucson, Arizona 85745
520/670-6821 ext. 112
jlbetanc@usgs.gov
Thomas W. Swetnam
Laboratory of Tree-Ring Research
University of Arizona
Tucson, Arizona 85721
520/621-2112
tswetnam@ltrr.arizona.edu
Abstract. The great ecological diversity of landscapes
in the American Southwest results from combinations of the underlying patterns
of topographic complexity, climatic variability, and environmental histories.
This chapter illustrates some high-resolution and long-term data sets and
approaches for reconstructing landscape change in the Southwest, including the
paleobotanical record, repeat photography, and fire-scar histories from tree
rings. We explore the effectiveness of collecting historical data at multiple
locations to build networks that allow analyses to be scaled up from
localities to regions and the use of historical data to discriminate between
natural and cultural causes of environmental change.
Introduction
The American Southwest is a region where great ecological diversity is
maintained by topographic complexity and extreme variability in climate.
Despite the pervasive influence of livestock grazing and other human land
uses in the Southwest, natural vegetation predominates over vast tracts of
public land. Because natural processes are still very much in play, human
impacts in this region are seldom "clearly" evident. In fact, the greatest
challenge in assembling and interpreting a land-use history of the Southwest
is disentangling cultural from natural causes of environmental change. We
have employed a variety of tools, techniques, and data types to address this
challenge.
Here we illustrate some historical and paleoecological perspectives on
environmental change in the Southwest. Among the themes we explore are:
- The importance of climatic variability in driving ecological
processes, as well as in modulating human land uses and their effects
on southwestern landscapes.
- The use of historical and paleoecological data to detect and
explain trends in ecological patterns and processes across southwestern
landscapes.
- The effectiveness of network approaches in the development of
historical data sets. By aggregating data spatially, observations and
inferences can be scaled up from localities to landscapes and regions.
- The use of historical data to discriminate between natural and
cultural causes of environmental change.
- The use of historical data to define and constrain natural ranges
of variability and, in some cases, to set targets or determine templates
for restoration and sustainable use of ecosystems.
This chapter illustrates some approaches for reconstructing landscape
change in the Southwest from high-resolution and long-term data sets
including the paleobotanical record, repeat photography, and fire-scar
histories from tree rings.
Methods
The Paleobotanical Record
An important backdrop for evaluating human impacts on southwestern
landscapes is the long-term dynamics of vegetation change. Glacial
climatic and vegetation patterns have characterized most of the
Pleistocene (the past 1.2 million years). Just 12,000 years ago, the
Earth underwent major environmental changes in the transition to the
current interglacial period, the Holocene. Dramatic swings in atmospheric
chemistry and climate, as well as global ice volumes and sea level,
caused massive shifts in biotic distributions. Vegetation change in
humid areas has been reconstructed from analysis of pollen grains
preserved in lake sediments, but opportunities for pollen analysis are
limited in arid regions due to scarcity of persistent water bodies, low
proportion of wind-pollinated plant species, and poor pollen preservation
in alkaline sediments. In the arid interior of North America, a novel
way of reconstructing vegetation change has been the analysis of plant
and animal remains preserved in fossil packrat (Neotoma spp.) middens,
deposits that are ubiquitous in rocky environments. About the size of a
laboratory rat, packrats gather nearby plant materials (within 100 m at
most) and accumulate them in dry caves and crevices; there, the plant
and other debris (including arthropod and vertebrate remains) are
cemented into large masses of crystallized urine (referred to as
amberat), which can persevere for tens of thousands of years. About
2,500 of these deposits have been dated within the limit of the
radiocarbon method (the last 50,000 years) and analyzed for plant
and animal remains (Betancourt et al. 1990). The preservation of plant
remains in packrat middens is excellent, allowing identification of
species and diverse morphological, geochemical, and genetic analyses
(e.g., Van de Water et al. 1994; Smith et al. 1995). The extensive
archive of sorted, identified, and dated material represents the
richest and best-documented source of plant remains in the world,
with hundreds of species identified and available for corollary studies.
Maps of modern versus Pleistocene vegetation in the Southwest imply
remarkable changes during the last 12,000 years
(Fig. 9-1); plant migrations initiated
during the Holocene may still be ongoing and hence complicate
simple cultural versus natural explanations of vegetation
change (Figs. 9-2 and 9-3).
Fig. 9-1. As this comparison of modern and Pleistocene
vegetation shows, southwestern landscapes have changed dramatically since
the end of the last ice age, 12,000 years ago. During the last
ice age, desert vegetation was restricted to the lower elevations
(<300 m) in Death Valley and the mouth of the Colorado River. Hallmarks
of the Sonoran Desert, such as the giant saguaro cactus
(Carnegiea gigantea) and the palo verde (Cercidium sp.), were
displaced far south into Mexico. Creosotebush (Larrea tridentata),
the dominant shrub of the Chihuahuan, Sonoran, and Mojave deserts,
had its northernmost populations along the Arizona-Sonora border.
Extensive pinyon-juniper-oak woodlands, now restricted to the highlands,
covered what are now desert elevations (300-1700 m). The extensiveness
of spruce-fir, mixed-conifer, or subalpine forests and woodlands during
glacial times is evident in their coverage over much the same territory
as modern pinyon-juniper woodlands, currently the third largest
vegetation type in the United States (20 million ha). The biggest
surprise from the packrat midden record is the virtual absence of
ponderosa pine (Pinus ponderosa), a tree that today extends from
central Mexico along the axis of the Rockies into Canada. In the
United States, much of that range developed through migration
during the last 10,000 years. Populations of this commercial species
in the northern Rockies and western High Plains may represent
arrivals within the last few millennia and perhaps the last few
centuries.
Fig. 9-2.
Diagram showing fossil packrat midden records with papershell
pinyon (Pinus remota, south of the Hueco Mountains near El Paso), and
Colorado pinyon (Pinus edulis, north of Hueco Mountains) during
radiocarbon time for the last 40,000 years. The tickmarks on each
vertical line represent over 350 radiocarbon-dated middens that show
the presence or absence of pinyon pines along a 15 latitude
(ca. 1,00 km) transect from Bermejillo, Mexico (Durango Province), to
Fort Collins, Colorado. The diagram depicts the local extinction of
pinyon populations growing in the Chihuahuan Desert during the last
deglaciation (around 11,000 radiocarbon years ago) and the sequential
migration to higher elevations and more northerly latitudes during the
Holocene (the last 11,000 years). Note that Colorado pinyon's
distribution in the state of Colorado may be just a few hundred years
old and probably is not yet in equilibrium with modern climate. In
Colorado and northern New Mexico, this recent migration makes it
difficult to discriminate the last phases of Holocene migration from
historical tree expansion due to fire suppression and overgrazing.
Fig. 9-3.
An isolated stand of Colorado pinyon (Pinus edulis) at Owl Canyon,
north of Fort Collins, Colorado, represents the endpoint of its
northward migration since the end of the last ice age
(Betancourt et al. 1991). This 5 km 2 stand was colonized by pinyon pine
less than 500 years ago, possibly from accidental plantings by Cheyenne
and Arapaho, who carried pinyon nuts in their "trail mix" on treks
along the Front Range. The nearest potential source populations are
250 km to the south near Colorado Springs. It is unclear what role
humans played in movement of seeds and plant migration during the
Holocene. Note the rapid increase in canopy cover from 1950 to
1989, characteristic of an expanding population.
(Photos: 1950, J.D. Wright; 1989, R.M. Turner.)
Repeat Photography
Ground-Based Photography
Historical photographs of key landscapes, from hillslopes to wetlands,
are available for practically any area of the western United States
(Rogers et al. 1984). As a first approximation, past environmental
change can be measured by finding the site of a historical photograph,
reoccupying the original camera position, and making a new photograph
of the same scene. Differences between then and now provide a basis for
identifying and even quantifying changes, while the new photograph
establishes a benchmark for future evaluation. Repeat photography is a
simple, inexpensive, and elegant tool for reconstructing past
environmental changes and monitoring future ones; it is particularly
well suited for the relatively open landscapes of the western
United States (Hastings and Turner 1965; Rogers 1982;
Veblen and Lorenz 1991; Webb 1996). Repeat photography in the
Southwest has focused on key ecological concerns relevant to
management of public lands, including shrub and tree encroachment
upon grasslands (Figs. 9-4 and 9-5), climatic effects on demographic
trends in woodlands, postdisturbance histories, and geomorphic,
hydrologic, and vegetation changes in riparian areas (Figs. 9-6 and 9-7).
Fig. 9-4.
Creosotebush (Larrea tridentata) arrived at the Sevilleta
Long-Term Ecological Research site south of Albuquerque about
2,500 years ago and expanded into what was once open grassland
during the twentieth century, as shown in photographs from July
1915 and August 1989. The site of the photograph was open
grassland when the Spanish began grazing sheep and cattle in the 1700's,
but this grassland was invaded by snakeweed (Gutierrezia sarothrae)
and later by creosotebush (Larrea tridentata). Note the lone
creosotebush at the right foreground and the beachball-sized
snakeweed throughout the foreground of the 1915 photograph. This
photograph was taken after one of the wettest years in New Mexico
history. By the time of the 1989 photograph, creosotebush had
expanded throughout this former grassland, with the invasion
accelerated during an extended drought between 1942 and 1972
(Betancourt et al. 1993). Also note the increase in one-seed
juniper (Juniperus monosperma) on the slopes in the right background.
(Photos: 1915, N.H. Darton; 1989, R.M. Turner and J.L. Betancourt.)
Fig. 9-5.
Views from Acoma Pueblo to Enchanted Mesa, 100 km west of
Albuquerque, in 1899 and 1977. Note expansion of junipers
(Juniperus monosperma) between 1899 and 1977. In many parts of
the west, juniper expansion has been blamed on fire suppression
and livestock grazing, justifying an aggressive program of
chaining and burning pinyon-juniper woodlands in the 1960's and 1970's to
improve forage and water yield. Several authors have suggested that
pinyon-juniper expansion may instead represent recovery from
prehistoric fuel harvesting, at least in those areas that were
heavily populated within the last 1,000 years (Samuels and Betancourt
1982; Kohler 1988). One such place could be Acoma Pueblo.
(Photos: 1899, W.H. Jackson; 1977, H.E. Malde.)
In the Southwest, the process of desertification has involved expansion
of desert shrubs and trees into former grasslands (Figs. 9-4 and 9-5).
Shrub encroachment is difficult to reverse because nutrients and other
resources quickly begin to accumulate underneath shrubs, creating
resource islands that discourage grassland recovery
(Schlesinger et al. 1990). Explanations for shrub encroachment have
ranged from fire suppression and livestock grazing
(Grover and Musick 1990) to interdecadal climatic variability
(Neilson 1986) and, most recently, to CO2 enrichment shifting the
balance from warm-season grasses to cool-season shrubs (Idso 1992).
The debate is confounded by the fact that progressive range
deterioration since 1870 has been inferred from historical data
(Bahre and Shelton 1993), while long-term monitoring indicates
substantial range improvement with wetter conditions following the
drought of the 1950's (McCormick and Galt 1994).
One of the most remarkable changes in southwestern landscapes involved
late nineteenth and early twentieth century channel entrenchment
(Fig. 9-6). Between 1865 and 1915, arroyos developed in alluvial
valleys of the southwestern United States across a wide variety
of hydrological, ecological, and cultural settings. That they
developed more or less simultaneously has encouraged the search
for a common cause, some phenomenon that was equally widespread and
synchronous. As with most recent environmental changes, whether global
or local, efforts to understand arroyo genesis have been hindered by
the inability to discriminate between natural and cultural factors.
Much debate has focused on the regional and local causes for historic
arroyo-cutting (Bull 1997). Range managers have been quick to point
to the removal of plant cover by livestock, whereas climatologists have
naturally looked to the skies for an explanation. The geologist,
accustomed to studying the products of erosion over long periods of
time, sees arroyos as symptomatic of inherent instability in arid
landscapes, while acknowledging that geomorphic thresholds may be
exceeded with changes in climate and vegetation. Following arroyo
initiation, two of the more pervasive impacts on southwestern
watersheds have been deterioration of wetlands and degradation of
streamside vegetation, caused by groundwater withdrawal and
urbanization (Fig. 9-7).
Fig. 9-6.
In July and August of 1890, heavy flooding cut a deep
channel in the Santa Cruz River at Tucson. As arroyo cutting
progressed, it ultimately destroyed nearby Silver Lake, an
impoundment on the Santa Cruz River which powered the waterwheels of
local flour mills and provided irrigation water for agricultural
lands downstream. Compare these photographs of Silver Lake in 1891 and
1982 (Betancourt and Turner 1988). (Photos: 1891, unknown; 1982,
R.M. Turner and J.L. Betancourt.)
Fig.9-7.
Downstream view of the confluence of the west branch of
the Santa Cruz River in Tucson, looking northeast from the lower
slope of Sentinel Peak. Between 1904 and 1981 deterioration of
the riparian vegetation is evident due to groundwater depletion and
urbanization, along with arroyo cutting. Many other southwestern
floodplains have undergone similar changes, including reaches of the
Rio Grande, the Salt River, and the Gila River. (Photos: 1904,
unknown; 1981, R.M. Turner and J.L. Betancourt.)
Aerial Photography
Aerial photography and other remote sensing approaches (e.g.,
satellite imagery) provide powerful means of determining
widespread changes in landscape patterns through time, especially
when used in concert with geographic information systems
(Sample 1994). Aerial photography was performed across most of the
Southwest in the mid-1930's, providing a baseline from which modern
landscape changes can be assessed (Allen and Breshears 1998).
Fig. 9-8.
Map of changes in montane grassland area between 1935 and 1981
in the southeastern Jemez Mountains, New Mexico. Area of open grassland
(with less than 10% tree canopy cover) was determined from aerial
photographs.
Groundbased evidence, such as tree ages and soil patterns, indicate
that conifer trees have widely invaded ancient montane grasslands
in the Jemez Mountains of northern New Mexico during this century
(Allen 1989). Aerial photographs confirm these observations and
reveal the extensiveness of the tree encroachment (Fig. 9-8), which
reduced the area of open montane grasslands by 55% between 1935
and 1981 across the 100,000 ha mapped area. The tree invasion has
been tied to changes in land-use history, primarily livestock grazing
and fire suppression (Allen 1989).
Changes in road networks through time reflect and determine land use
histories, as illustrated in this Jemez Mountains example. Total road
density in 1935 (Fig. 9-9) was greatest on the homesteaded lands just
north of Bandelier National Monument, where dirt and primitive roads
provided access to agricultural fields, dwellings, and timber and
fuelwood resources. West of Bandelier National Monument, roads provided
access to ranches, mines, and some timber operations. Large portions
of the Jemez area remained roadless.
Fig. 9-9.
Map of all roads visible in 1935 and 1981 aerial photographs
across 187,858 ha around Bandelier National Monument, in the Jemez
Mountains, New Mexico. The current national monument boundaries are
shown. "Dirt" roads have a bulldozed surface, while "primitive" is a
variable category that includes logging skid trails, informal
woodcutting tracks, some powerline corridors, and off-road
vehicle paths.
In 1935 the Denver and Rio Grande Railroad was still in operation
through the eastern edge of the map area. Completed between 1880
and 1886, this important connection between the Jemez Mountains and
the outside world markedly altered land use patterns in this area
(Rothman 1992). The improved linkages to outside markets provided
by railroads throughout the Southwest in the late 1800's allowed
the concurrent, region-wide buildup of extreme numbers of livestock
(Wooton 1908), which precipitated key landscape changes such as
vegetation transformations and altered fire regimes.
By 1981 (Fig. 9-9) the length of mapped roads
increased nearly twelvefold, from 719 km in 1935 to 8,433 km
(Allen 1989). The pattern of paved roads north of Bandelier reflects
intensive human development activities, as the agricultural homesteads
turned into the industrialized technical areas of Los Alamos National
Laboratory, with its associated townsites of Los Alamos and White Rock.
The dense networks of dirt and primitive roads to the west of Bandelier
were created by a variety of logging activities on public and private
lands during the 1960's and 1970's (e.g., the striking spiral patterns
of dirt roads observed in the northwest quadrant of
Fig. 9-9). The largest remaining roadless
tract was the designated wilderness areas in and adjoining Bandelier
National Monument. Estimated total area of road surfaces grew from
0.13% of the map area in 1935 (247 ha) to 1.67% in 1981 (3,132 ha).
These estimates of road surface areas do not include shoulders, cut
and fill slopes, or ditches, and thus are conservative estimates of
landscape area directly altered by roads.
The great increase in road networks observed since 1935 in the Jemez
Mountains suggests the possibility of significant, landscape-wide
ecological impacts (Allen 1989). The U.S. Forest Service has recently
recognized the existence of over 690,000 km of national-forest roads
on its lands across the United States (see details at
http://www.fs.fed.us/news/roads), highlighting the magnitude of
wildland road networks in this country. Roads can have many
ecological effects, ranging from habitat fragmentation and reduced
landscape productivity to the direct conversion of roadways into
compacted and sparsely vegetated surfaces. They can also provide
routes for the spread of nonnative weeds, accelerate erosion rates,
and increase stream sediment loads. Roads act as fire breaks and
facilitate extensive access to formerly remote areas for fire
suppression. Roads also allow increased human access for
recreational and consumptive purposes, resulting in widespread
habitat modifications (e.g., cutting of snags for fuelwood) and
disturbances to wildlife (e.g., through vehicle traffic and hunting)
that alter biotic communities. Overall, road networks often provide
distinctive landscape signatures of the histories and ecological
effects of human land uses.
Fire-Scar Histories
Well-dated fire-scar chronologies aggregated over space and time
provide powerful, multiscale perspectives of the variability of
past fire regimes (Figs. 9-10 and 9-11). These fire-scar chronologies
document a history of frequent, widespread surface fires in many
southwestern forest types (Swetnam and Baisan 1996; Fig. 9-12). Fire
is a "keystone process" (see Holling 1992) in the Southwest, and
patterns of change in the fire-scar record are interpretable in the
context of climatic variation and changes in land use and forest
stand structures (including fuel conditions). Thus, fire histories
record the ecological "pulse" of southwestern forests, integrating
both natural and cultural histories.
Fig. 9-10.
Repeated surface fires cause a sequence of overlapping wounds.
The heat-killed wood tissues extend into the annual rings, which can be
dated to the calendar year.
Fig. 9-11.
Map of fire history study sites in the southwestern United
States. The red dots show locations of tree-ring and fire-scar
collections in 27 mountain ranges. Most collections are in
ponderosa pine and mixed conifer forests.
Fig.9-12.
Fifty-five fire-scar chronologies for different forest
sites in 27 mountain ranges of the southwestern United States. The
yellow and red tick marks on each time series are fire dates recorded
by fire-scarred trees. At least 10 fire-scarred trees were sampled in
each site, and the tree rings and fire scars were dated by
dendrochronology methods. Each time series is a composite of the
fires recorded by at least two trees in each site. The red tick
marks show regional fire years defined by 10 or more of the 55
chronologies (sites) recording the fire date. The yellow tick marks
show the other fire dates recorded in nine or fewer sites. The
step-line graph at the bottom is a summation of the number of sites
recording the fire dates; the regional fire years are in red and are
labeled.
Regional fire years (Fig. 9-12) were an
episodic phenomena in southwestern forests, and the synchronized
nature of these events demonstrates the importance of interannual
climate in controlling local to regional-scale fire occurrence. The
El Niño-Southern Oscillation (ENSO), a global climatic pattern,
is associated with these fire patterns, both in the past and in
current southwestern fire regimes (Swetnam and Betancourt 1990, 1998).
Regional fire years tend to occur during La Niña events and droughts,
while reduced fire activity corresponds to El Niños and wet years.
Moreover, regional fire years tend to occur during average or dry
years that follow one to three wet years, indicating the important
role of fine fuel production (i.e., grasses and tree needles) in fire
dynamics, especially in ponderosa pine forests and lower elevation
forests. Hence, when the ENSO has high variation and amplitude, with
extreme dry years following extreme wet years, fire activity is
entrained across regional scales.
Long-term changes in fire frequency over the past four centuries
(Fig. 9-12) were related to both climate and
human activities. Native Americans probably set many of the fires
recorded by fire scars before 1900, but lightning was (and is) so
frequent in the Southwest that, in most places and times, fire
frequencies were probably controlled primarily by climate and fuel
dynamics, rather than by ignition source. The decrease in fire
frequency after the late 1800's (Fig. 9-12)
was due mainly to the rise of intensive livestock grazing,
when fine fuels (e.g., grasses) that carried surface fires were
consumed by millions of sheep, goats, cattle, and horses (Wooton 1908;
Swetnam and Baisan 1996). Disruption of fuel continuity by trailing
and herding large numbers of animals was probably also involved.
Disentangling climatic factors (regional scale) from cultural factors
(local scale) as causes of observed variations can proceed from
comparative analyses within a regional network of paleoecological
study sites. Interpretations can be based on the degree of synchronism
among events across spatial scales and the degree of correspondence among
multiple, independently derived time series of disturbance, climate,
and land-use chronologies. For example, the importance of intense
livestock grazing as a cause of the disruption of natural fire regimes
is confirmed by the comparison of different case studies. A few sites
in northern New Mexico and Arizona that were grazed by sheep and
goats owned by Spanish colonists and Navajos (DinJ) show fire
frequencies declining in the early nineteenth century, or earlier,
and corresponding to the documented timing of pastoral activities in
these areas (Savage and Swetnam 1990; Touchan et al. 1996; Baisan and
Swetnam 1997). In contrast, remote sites with no evidence of early,
intensive grazing sustained some surface fires into the middle of the
twentieth century, when aerial firefighting resources began to be
most effective in suppressing fires (Grissino-Mayer 1995). Finally,
a remote mountain in northern Sonora, Mexico (lowermost fire-scar
chronology in Fig. 9-12), where neither
intensive livestock grazing nor effective fire suppression has
occurred, shows episodic surface fires burning throughout the
twentieth century.
One of the strengths of spatial networks of well-dated fire
chronologies (or other disturbance chronologies) is that
they can be aggregated across spatial scales, providing
multiscale spatial and temporal perspectives. Analyzing
patterns in such spatio-temporal data networks may reveal
scaling rules and underlying mechanisms and controls of
disturbance processes (e.g., see Holling 1992). The 1748
fire year in the Southwest (Fig. 9-13) was an example of a
cross-scale disturbance event; extensive fires burned at
all spatial scales within the region. This extensiveness is
indicated by the high synchrony of fires for this date
recorded in most sampled trees within stands, in most sampled
stands within watersheds, in most watersheds within mountain
ranges, and in most mountain ranges within the region.
The importance of extreme interannual climate changes in
triggering this regional event is indicated by
dendroclimatic and Spanish archival sources confirming
that 1748 was an extreme drought year following an
extremely wet year (1747).
Fig. 9-13.
A cross-scale comparison of the largest regional fire year
in the Southwest during the past 400 years: 1748. The synchrony
of the 1748 fire year among fire-scarred trees at the smallest
spatial scale (a forest stand) is shown in the bottom panel. Patterns of
synchrony, which are a measure of relative areal extent, are then
illustrated at higher levels of aggregation (larger scales, coarser grain
size) up through the watershed, mountain range, and finally the regional
level (uppermost panel).
Ecological changes are often best evaluated by comparing
multiple lines of historical evidence. Twentieth-century
changes in southwestern ponderosa pine forests have been
well documented by several generations of ecologists and
foresters, ranging from Aldo Leopold (1924) and
Gus Pearson (1933) to Weaver (1951) and Covington and Moore
(1994). Numerous comparisons of early versus recent
photographs and forest stand descriptions have demonstrated
that stand densities have increased while grass cover has
decreased. These changes were caused by a combination
of intensive livestock grazing and, subsequently,
organized fire suppression by government agencies. Tree-ring
reconstructions of forest age structure and fire history,
however, can identify new elements in this story. For example,
while many pine forests today are dominated by the
post-grazing/fire suppression "tree irruption" of the early 1900's,
another pulse of tree recruitment apparently took place during
the early 1800's. This pulse is evident in the Monument
Canyon Research Natural Area (Fig. 9-14) and other
southwestern sites. This pulse corresponds to the longest
intervals between widespread fires in numerous sites in
the Southwest, changes in fire frequencies and seasonality,
and shifts in climate (Grissino-Mayer 1995; Swetnam and
Betancourt 1998).
Fig. 9-14.
Ponderosa pine forest changes from repeat photography, tree
demographic data, and fire history. The upper left photograph
is of an open ponderosa pine stand around 1930 with a few clumps
of 10- to 20-year-old saplings and the upper right a recent photo of a
typical ponderosa pine stand today in Monument Canyon Research Natural
Area (RNA), Jemez Mountains. The current stand is choked with dense
"dog-hair thickets." The bar graph below the photographs shows the age
structure (tree-recruitment dates) of more than 400 trees sampled in
the Monument Canyon RNA. The horizontal line with vertical tick marks
below the bar graph shows the fire dates recorded by widespread fires
within the same stand.
These patterns may indicate that the historical variability
1in age structures of southwestern ponderosa pine were
characterized by pulses of heavy tree recruitment in
particularly favorable years embedded in a background of a more
continuous but lower level of tree recruitment. Recent
studies have confirmed the importance of the famous "1919 seed
year" first identified by Pearson (1933) in the Southwest
and have demonstrated the role of warm, wet summers in
good ponderosa pine seed germination and seedling survival
(Savage et al. 1996). Hence, ponderosa generational groups
were a contingent product of climatic variability and fire
regime responses in both the presettlement and postsettlement
eras. An implication for new forest restoration initiatives
in the Southwest is that current ponderosa forests, characterized
by trees that germinated in the 1919 seed year, may not be entirely
an artifact of grazing and fire suppression, and therefore thinning
programs should not necessarily seek to eliminate this cohort as a
distinct demographic pulse.
While climate is often a key driver of plant regeneration in
the semiarid Southwest, ultimately it is the linked
influences of climate, fire regimes (and other disturbances),
and land-use histories that determine the demography
of plant populations and southwestern vegetation patterns.
These interactive effects are demonstrated by the extensive
mortality of ponderosa pines and pinyon during the 1950's
drought in the Southwest (Betancourt et al. 1993; Allen
and Breshears 1998), as the drought effects (climate)
were likely exacerbated by competition for scarce water among
unusually dense stands of woody plants (a result of modern
changes in land use and fire regimes). Also, while a pulse
of tree seedlings has established since about 1976 in
southwestern forests and woodlands (Swetnam and Betancourt
1998) in conjunction with a recent wet period (associated
with an unusual string of El Niño events), the survivorship
and ultimate recruitment of these trees partly depends upon
patterns of land use and fire. Monitoring of these current
demographic processes and reconstruction of past patterns
are needed to fully understand ongoing changes and their
historical context.
Summary
Several important themes emerge from the illustrations of
southwestern environmental change discussed here.
- High-resolution, long-term, (and in some cases unique)
historical and paleoecological data sets, coupled with
diverse, specialized approaches for reconstructing landscape
change, are available to detect and explain trends in
ecological patterns and processes across southwestern landscapes.
- Network approaches are very useful in the development of
regional historical data sets which can be utilized to
construct land use histories. By aggregating data spatially,
observations and inferences can be scaled up from
localities to landscapes and entire regions. Development of
regional time-series networks provides opportunities
to quantify both spatial and temporal variability as a
function of scale.
- Climatic variability is a key driver of ecological
processes; it also modulates human land uses and their effects
on southwestern landscapes. Regional climatic signals must
be extracted before landscape changes can be attributed to
other causes, such as human activities.
- All landscapes are historically contingent systems whose
structure and dynamics reflect continuous modification of
preexisting systems (Brown 1995). Historical data can be
used to discriminate between natural and cultural causes of
environmental change. Environmental variability and trends
have regional and local components. One effective approach
to determining causation is to identify synchronous regional
responses of biotic systems (which are often climate-driven)
and asynchronous, disparate responses observed at local scales
(which are often attributable to human land uses and other
local disturbances). Additionally, comparison of multiple
lines of evidence from different types of ecological
reconstructions (e.g., photographs, tree ages, fire scars,
cultural histories, climate records) can be the key to
identifying causal factors.
- Historical data can be used to define and constrain natural
ranges of variability, providing important information for
management of ecosystems and landscapes (Allen 1994). In the
case of wilderness areas and parks, these perspectives may be
directly relevant to setting management goals or targets. In
other cases, improved knowledge of the origin of existing
ecosystem conditions will be the primary value of historical
data. Ecosystem management efforts to sustain valued wildland
resources (from endangered species to surface water) will
benefit from improved knowledge of the patterns and causes of
past environmental change.
Much unrealized potential exists to develop detailed land use
histories and associated causal narratives in the Southwest.
Valuable initiatives would include further regionalization of
localized paleoecological data sets (e.g., tree-ring collections),
systematic programs to assemble and use repeat photographs
(including the extensive aerial photography of the mid-1930's),
regional-scale applications of the extraordinary wealth of
archeological data present in the Southwest to environmental
histories, and the development of regional approaches to
monitoring ongoing changes in landscape patterns.
Additional Figures
These figures where created for the original version of this paper, however they
were not used in the official publication "Perspectives on the Land
Use History of North America: A Context for Understanding Our Changing Environment".
They are included here as additional references.
Acknowledgments
We appreciate the assistance and support of Chris Baisan,
Kay Beeley, Hal Malde, Will Moir, Esteban Muldavin,
Steve Tharnstrom, Tom Van Devender, the Global Change Program
and Biological Resources Division of the U.S. Geological Survey,
Bandelier National Monument, and the U.S. Forest Service
(Rocky Mountain Forest and Range Experiment Station,
Southwest Regional Office, and Santa Fe National Forest).
This chapter benefited from review comments by
R. Scott Anderson, Tom Sisk, and two anonymous reviewers.
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Next Chapter:
Biodiversity and Land-use History of the Palouse Bioregion:
Pre-European to Present
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