of vegetation across landscapes depend on climate, perhaps best illustrated
by the wholesale movement of plant species across geographic and topographic
gradients during the last deglaciation (1).
The responses of vegetation to variations in climate are expected to be most
rapid and extreme at ecotones, the boundaries between ecosystems (2–5), with semiarid ecotones considered to be among the most sensitive (6).
Persistent vegetation shifts are most clearly detected in the distributions
and abundances of long-lived woody plants, namely, trees and shrubs (7).
Previous studies of woody ecotones document landscape-scale shifts in vegetation
as occurring only over relatively long (decades to millennia) periods (8–12).
Moreover, most field studies and model-based assessments of vegetation responses
to climate have focused on changes associated with natality and growth, which
are inherently slow processes for woody plants—even though the most rapid
changes in vegetation are caused by mortality rather than natality (13).
In coming decades, climate changes are expected to produce major shifts in
vegetation distributions at unprecedented rates, in large part due to mortality
(14); however, largely because of
the lack of field data on vegetation mortality, current models do not represent
adequately such rapid effects (14, 15). Furthermore, as woody vegetation contains 80% of the world's terrestrial carbon (16),
an improved understanding of mortality-induced responses of woody vegetation
to climate is essential for addressing the environmental and policy implications
of climate variability and global change (17, 18).
wish to highlight how rapidly shifts in vegetation can take place in response
to climate. Here we demonstrate that the ecotone between semiarid ponderosa
pine forest and piñon–juniper woodland shifted extensively (2 km or more)
and rapidly (<5 years) through mortality in response to a severe drought.
Remnant forest patches became much more fragmented, and high soil erosion
rates were initiated in the ecotone shift zone. Moreover, the shift has been
a persistent one. The rapidity and the complex dynamics of this persistent
shift specifically point to the need to represent more accurately the mortality
dynamics of woody vegetation in assessments of the effects of global climate
The focus of our study was an ecotone between ponderosa pine forest (Pinus ponderosa) and piñon–juniper woodland (Pinus edulis and Juniperus monosperma). The site we selected, a 2,378-ha (1 ha = 104 m2)
portion of Frijolito Mesa, Bandelier Wilderness (35° 51′ N, 106° 19′ E),
in the Jemez Mountains of northern New Mexico, offered strong evidence of
a recent ecotone shift in response to climate. The site ranges in elevation
from 1,800 to 2,200 m and spans a corresponding climatic gradient; precipitation
varies around a mean of 41 cm/yr at 2,010 m (19).
Other factors in our selection of this site as a study area were its upland
topography, with only one major elevation gradient; its designated wilderness
status, with minimal history of human disturbance; and an extensively documented
fire history (20). These conditions allowed us to isolate and focus more accurately on the effects of climate variations on vegetation.
quantified changes in the ecotone over a 40-yr period on the basis of Geographic
Information System (GIS) analyses of a sequence of aerial photographs taken
between 1935 and 1975—a period encompassing a severe regional drought in
the mid-1950s (21, 22).
Full-coverage photographs of the area, which existed for 1935, 1954, 1963,
and 1975, and partial coverage photographs from 1951 were used to map vegetation
patches in terms of ponderosa pine cover: areas with more than 10% cover
were classified as forest, and the remainder of the area as piñon–juniper
woodland. Higher resolution (scale = 1:5,000), partial-coverage photographs
from 1958 allowed us to sharpen our estimate of the timing of ecotone changes;
written documents on file at Bandelier National Monument further confirmed
this estimate. We verified our mapping results with field observations of
the persistence and mortality of ponderosa pines, which included documenting
the presence of live ponderosa trees and the remains of dead trees as a function
of elevation and topographic position. In addition, we confirmed the relationship
between the local elevation/moisture gradient and water stress for ponderosa
pine, as measured by the changes in stem diameter of 30 trees monitored continuously
since 1991 (10 trees at each of three sites along this gradient).
Most striking is how extensively (Fig. 1) and how rapidly (Fig. 2
the ecotone shifted in conjunction with the 1950s drought. The extensive
mortality of ponderosa pine at drier, lower elevations caused a landscape-scale
shift in <5 yr (1954–1958; Fig. 2
A). The shift coincided with the culmination of the drought (Fig. 2
B), which was one of the most severe of the past 500 yr (refs. 23–25; Fig. 2
That the mortality of the ponderosa pines was primarily a result of this
brief but extreme variation in climate is supported by data from the GIS
analyses (Fig. 3
A) and from ground-based field observations (Fig. 3
which show that lower-elevation sites, which are drier, suffered higher loss
of trees. Further, recent dendrometer measurements of ponderosa pines along
an elevational gradient in the same landscape reveal that the trees at lower
elevations exhibit greater water stress and slower growth (Fig. 3
factors amplified the climate-induced ecotone shift. Human suppression of
fires since the 1880s had allowed piñon and juniper to become more densely
established beneath or close to low-elevation ponderosa pines by the time
of the 1950s drought (22). The novel
association between species thus created—which had been precluded in the
past by frequent fires—meant that during this drought many piñon and juniper
trees were competing with ponderosa for available water. Because both piñon
and juniper are very efficient at using shallow water (27), water stress on the ponderosa pines was probably exacerbated. In addition, infestations of bark beetles (Dendroctonus sp. and Ips sp.) accompanied the drought, as indicated by historic documents (22).
These infestations certainly contributed to the mortality of the ponderosa
pines, as drought-stressed ponderosa pines are commonly unable to defend
themselves from beetle attacks (28).
Although we cannot segregate the intertwined effects of the beetles and of
the drought, it is clear that the drought was the underlying cause of the
mortality. As shown in Fig. 3,
the spatial pattern of ponderosa pine mortality corresponds directly to elevation/moisture
gradients. Furthermore, mortality of ponderosa pine in the 1950s was apparently
widespread on drier, low-elevation sites across the drought-affected regions
of the southwestern United States (29, 30). It is noteworthy that piñon mortality also occurred at drier, lower-elevation sites in the Bandelier area (22), as well as elsewhere in New Mexico (25, 31), even though piñon are not attacked by the same species of bark beetles that affect ponderosa pine (28). Also, when wet conditions beginning in 1957 broke the 1950s drought (Fig. 2
B), widespread ponderosa pine mortality reportedly ceased within about a year (22).
Collectively, these observations indicate that the timing and spatial pattern
of ponderosa pine mortality was driven by the drought rather than by bark
beetles. Hence, we assert that the mortality of the ponderosa pines was caused
by the drought and was amplified by historic fire suppression, increased
density of piñon and juniper, and drought-triggered infestations of bark
As a result of the ecotone shift, the patches of ponderosa
pine forest became more fragmented, as theory suggests should occur with
climate-induced ecotone shifts (32, 33).
Between 1954 and 1963, the number of forest patches in our study area increased
from 20 to 42, and their perimeter-to-area ratio increased from 0.012 m−1 to 0.020 m−1. This fragmentation is important because many ecosystem properties are a function of patch size and pattern (34).
Moreover, the effects of the drought in the ecotone shift zone have been persistent (Fig. 2
There is still little evidence of ponderosa pine reestablishment in spite
of favorable climatic conditions in recent decades, and hence the shift in
the ponderosa pine forest has endured for 40 yr. Furthermore, the 1950s drought
apparently initiated other persistent changes in ecosystem properties, particularly
soil erosion (35). Large reductions in herbaceous cover were widely observed elsewhere in the southwestern United States during the 1950s (36, 37).
At our site, herbaceous cover was not quantified in the 1950s, but low values
of herbaceous groundcover and high soil-erosion rates have been documented
since at least the early 1970s (22). Currently herbaceous cover remains very sparse (2%) and erosion rates remain extremely high [4 mm/yr (38)].
Reductions in herbaceous cover, such as those caused by drought, can trigger
a shift across a threshold to high erosion rates (35)
like those we currently observe. Thus, a short-duration climatic event not
only brought about persistent changes in the ecotone but might also have
altered ecosystem properties.
Mortality-induced vegetation shifts take
place more rapidly than do natality-induced shifts associated with plant
establishment and migration (13, 39).
We suggest that mortality-induced shifts as rapid as the one we report (i.e.,
<5 yr) have occurred frequently and extensively in the past, but have
not been documented previously at high temporal or spatial resolution because
of constraints inherent in most paleoecological methods [whereas shifts over
periods of decades to millenia certainly have been well documented (8–12, 40)].
It is now becoming increasingly possible to delineate such landscape-scale
ecotone shifts given the development of ever-lengthening time series of remote
sensing data (e.g., satellite imagery and aerial photography) that explicitly
record patterns of landscape change.
Most of the previously developed
models that predict how vegetation distributions may shift in response to
future climate have focused on the slower changes associated with natality
and growth rather than the more rapid changes resulting from mortality (15, 41).
Although assessments based on current models do include the expectation that
global warming will accelerate the mortality of woody plants and thereby
produce changes in vegetation distributions worldwide (14),
the models generally assume an equilibrium between climate and vegetation.
In contrast, our findings show that even brief climatic events can have profound
and persistent ecosystem effects, reinforcing the importance of more accurately
incorporating vegetation mortality and the complexity of associated ecosystem
responses (e.g., increased forest fragmentation, soil erosion, insect outbreaks,
and fire) into models that predict vegetation dynamics (17, 42–44).
vegetation changes we report have some site-specific characteristics that
could limit the application of our findings to other ecosystems: (i) the ecotone shift occurred in conjunction with the mortality of a single dominant species (ponderosa pine); (ii) piñon and juniper had already become established before the drought; and (iii)
bark beetles amplified the mortality effects of the drought. Nonetheless,
we propose that the unprecedentedly rapid climate changes expected in coming
will produce rapid and extensive contractions in the geographic distributions
of long-lived woody species and shifts in associated ecotones such as the
one we document. These shifts are very likely to occur globally because semiarid
forests and woodlands and their associated ecotones are widespread and considered
to be among the most sensitive to changes in climate (6).
Because regional droughts of even greater magnitude and longer duration than
the 1950s drought are expected as global warming progresses (45, 48),
the ecological effects of droughts associated with global climate change
are likely to be even greater than those documented here.