Biological Conservation 58 (1991) 1-18
Disturbance, Equilibrium, and Environmental Variability:
What is 'Natural' Vegetation in a Changing Environment?
D o u g l a s G, S p r u g e l
College of Forest Resources, AR-10, University of Washington,
Seattle, Washington 98195, USA
(Received 16 May 1990; revised version received 29 November 1990;
accepted 6 December 1990)
ABSTRACT
To most early ecologists, the 'natural' ecosystem was the community that
would be reached after a long period without large-scale disturbance (fire,
windstorm, etc.). More recently, it has been realized that in most areas some
type of large-scale disturbance is indigenous, and must be included in any
realistic definition of 'naturalness'. In some areas an equilibrium may exist in
which patchy disturbance is balanced by regrowth, but in others equilibrium
may be impossible because (1) individual disturbances are too large or
infrequent; (2) ephemeral events have long-lasting disruptive effects; and~or
(3) climate changes interrupt any movement toward equilibrium that does
occur. Examples of non-equilibrium ecosystems include the African savannas,
the Big Woods of Minnesota, the lodgepole pine forests of Yellowstone
National Park, and possibly the old-growth Douglas-fir forests of the Pacific
Northwest.
Where an equilibrium does not exist, defining the 'natural' vegetation
becomes much more challenging, because the vegetation in any given area
would not be stable over long periods of time even without man's influence. In
many areas it may be unrealistic to try to define the natural vegetation for a
site; one must recognize that there are often several communities that could be
the 'natural' vegetation for any given site at any given time.
INTRODUCTION
Much attention has recently been paid to attempts to maintain 'natural'
conditions in wilderness or natural areas, especially in the presence of
1
Biol. Conserv. 0006-32707/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England.
Printed in Great Britain
2
D.G. Sprugel
frequent disturbance such as fire and windstorms. This has led, not
surprisingly, to controversy (Bonnicksen & Stone, 1985; Parsons et al., 1986;
Pyne, 1989) and even acrimony (Bonnicksen, 1989) over exactly what is
meant by the 'natural' conditions for any given area--and indeed, over the
very definition of 'natural'. Is the 'natural' vegetation what the first white
explorers saw? the first settlers? writers? photographers? or the first plant
ecologists? And does the specific date when the vegetation was first described
make a difference? Would the first European explorers have seen the same
thing if they had reached the eastern US in the 1300s instead of the 1500s?
What benchmark--if any--can we use to define the 'natural' condition of
the landscape?
A BRIEF HISTORY OF N O R T H A M E R I C A N ECOLOGICAL IDEAS
ABOUT N A T U R A L DISTURBANCE
Much of the controversy about 'natural' conditions relates to the role of
natural disturbance in natural ecosystems, which has been the subject of
much interest and wide swings of opinion throughout the history of
American plant ecology. One of the first American ecologists to discuss
disturbance as a normal component of some natural ecosystems was W. S.
Cooper, who described the forest vegetation of Isle Royale as 'a complex of
windfall areas of differing ages... [that] changes continually in a manner
that may almost be called kaleidoscopic when long periods of time are
considered' (Cooper, 1913).
Although Cooper considered recurrent natural disturbance as a normal
and inevitable component of community structure and function, his
appreciation of disturbance as a natural ecosystem component was not
shared by many of his early colleagues. Most ecologists in the first half of the
20th century believed that ecosystems typically progressed steadily and
predictably along well-defined successional pathways until they reached a
stable, self-sustaining state ('climax'), which was the 'normal' condition for
communities in that geographic region. While it was impossible to ignore the
fact that the vastlmajority of the inhabited earth was covered by successional
ecosystems, most ecologists believed that this was due solely to man's
pervasive burning, clearing, and plowing. F. E. Clements and J. E. Weaver,
who dominated ecological thinking in America in the 1920s and 1930s
(Tobey, 1981), wrote in their classic textbook: 'As a consequence of almost
universal use and misuse by man, subseres [successional communities
developing after disturbance] in every possible stage of succession constitute
the most abundant of all communities... In regions long settled, subseres
form practically the entire cover of vegetation' (Weaver & Clements, 1938).
Environmental variability and 'natural' vegetation
3
But: 'Before the advent of civilized man, nearly the whole area of each climax
was occupied by the [climax] dominant species.' (Weaver & Clements, 1938).
There were objections to this well-ordered view of the world even in the
heyday of Clementsian ecology (Sernander, 1936; Graham, 1941), and these
objections became louder and more persistent in the 1950s and 1960s (Watt,
1947; Drury, 1956; Lutz, 1956; Biswell, 1961; Rowe, 1961). But the orderly
succession-to-climax paradigm remained dominant in most ecological
thinking and writing (and especially in writing for the lay public) well into
the final third of the 20th century.
The pendulum of ecological opinion finally swung toward a recognition
of the importance of natural disturbance in the early 1970s. At that time, a
flood of papers more-or-less simultaneously proclaimed the importance of
natural disturbance in chaparral (Hanes, 1971; Biswell, 1974), northern
boreal forests (Heinselman, 1971, 1973; Rowe & Scotter, 1973), western
conifer forests (Habeck & Mutch, 1973; Kilgore, 1973; Loope & Gruell,
1973), eastern conifer forests (Sprugel, 1976; Reiners & Lang, 1979; Fig. 1),
deciduous forests (Loucks, 1970; Bormann & Likens, 1979), tropical forests
(Whitmore, 1974, 1975), rocky intertidal zones (Dayton, 1971; Levin &
Paine, 1974) and a variety of other ecosystems (White, 1979). In each of these
ecosystems, the authors said, natural disturbance is so common that it keeps
the system from ever reaching a stable state, so it is unrealistic to assume that
climax is the 'normal' condition for ecosystems to be in. Most of the systems,
it turned out, were akin to Cooper's Isle Royale forests--a 'continually
Fig. 1.
Wave-regenerated forest on Whiteface Mt, New York.
4
D.G. Sprugel
changing mosaic or patchwork' of patches of different ages, with disturbance
normally intervening before any patch reaches a stable condition.
However, although the pendulum swung away from the orderly 'climax'
paradigm toward one featuring recurrent but unpredictable disturbance, the
notion of ecosystem stability was retained at a higher level. Several authors
(e.g. Heinselman, 1973; Wright, 1974; Sprugel, 1976; Van Wagner, 1978;
Bormann & Likens, 1979; Shugart & West, 1981) emphasized that even
ecosystems with a high disturbance frequency could be in a 'steady-state' or
'equilibrium' if the creation of new patches was balanced by the maturation
of old ones. A small patch might give the impression of constant change, but
on a larger spatial scale (soon to be called a landscape) a perceptive observer
would observe a balance between disturbance and succession. Thus the basic
notion of a stable ecosystem still prevailed, but stability was expected to
occur at a larger spatial and temporal scale. Cooper would doubtless have
been pleased, since the new ideas about 'equilibrium landscapes' were really
just restatements of his contention that 'the balsam-birch-white spruce
forest, in spite of appearances to the contrary, is, taken as a whole, in
equilibrium.., the changes in various parts balancing each other' (Cooper,
1913).
E Q U I L I B R I U M A N D N O N - E Q U I L I B R I U M LANDSCAPES
The idea of an area maintained in a dynamic equilibrium by a balance
between disturbance and recovery is psychologically attractive, because it
provides some sense of stability even in the presence of constant change.
Although it is difficult to define an equilibrium landscape precisely, most
discussions regard it as one in which some parameter of interest (species
composition, biomass, net primary productivity, etc.) is roughly constant
from year to year when averaged over the whole landscape, or in which
opposing processes (e.g. gross primary production vs total respiration, or
nutrient input vs nutrient loss) are approximately balanced on a landscape
scale (Bormann & Likens, 1979; Shugart, 1984). In any real ecosystem there
will be some year-to-year variation in all these parameters, but most authors
have considered these small-scale variations unimportant. What is more
critical is whether there are long-term trends, which in turn depends on
whether or not the proportion of the landscape in major developmental
stages varies substantially over time periods on the order of a decade or a
century. If that occurs, then populations of species that depend on different
developmental stages, or processes that are accelerated or diminished there,
will not be even approximately constant on a landscape scale.
This general definition of an equilibrium landscape may well be met
Environmental variability and 'natural' vegetation
5
where natural disturbances are frequent and small compared with natural
landscape units. One well-documented example of this situation is the
deciduous forest of eastern North America, where the predominant type of
natural disturbance seems to be windthrow of individual trees or small
groups (Bormann & Likens, 1979). Since a windthrow typically affects a few
hundred to l o o 0 m 2 (Runkle, 1982), equilibrium could theoretically be
reached in an area of less than 1 km 2. Bormann and Likens (1979) have
described the resultant patchwork of gaps of various ages as the 'ShiftingMosaic Steady State'. Another example is the wave-regenerated balsam fir
Abies balsamea forests of the northeastern US, where natural 'mortality
waves' move through the forest once every 50-70 years (Sprugel, 1976;
Sprugel & Bormann, 1981). Since the waves move at a fairly uniform rate,
there are always freshly killed areas, and since consecutive waves are only
about 100m or so apart, an area of a few dozen hectares will normally
contain all phases of the disturbance cycle (Figs 1 and 2). By almost any
criterion either of these ecosystem types is in an equilibrium or quasiequilibrium state; disturbance is sufficiently frequent and small-scale
compared to the landscape area that most populations and processes must
be fairly constant over the whole area, and there is little reason to expect
long-term trends under the current climatic regime.
Non-equilibrium due to spatial scale of disturbances
Although the notion of equilibrium landscapes is attractive and satisfies
human longing for order in natural processes, it is becoming increasingly
clear that not every landscape is in or can reach an equilibrium state
(Delcourt et aL, 1982). The most obvious example of non-equilibrium is
where the typical spatial scale of disturbance approaches or exceeds that of
I
MATURE
DYINGREGENERMING
I YOUNGt INTERMEDIATE
MMURE
Fig. 2. Diagrammatic view of a fir wave.
6
D.G. Sprugel
landscape units as generally understood (Franklin & Hemstrom, 1981;
Shugart & West, 1981; Shugart, 1984). Wherever individual disturbances are
so large that a single disturbance event can affect a relatively large
proportion of the landscape, achievement of an equilibrium state is unlikely;
there will inevitably be wide swings from one decade or century to the next in
the proportion of the landscape in different developmental stages. An
example might be the low-elevation forests of the Pacific Northwest, where
in presettlement times individual fires may have covered hundreds of
thousands of hectares (Franklin & Hemstrom, 1981; Pyne, 1982). Eastern
forests along major hurricane tracks (Smith, 1946) are another example; in
both cases, even if one considers a landscape the size of an entire state, it is
difficult to see how any kind of equilibrium could become established where
disturbances are so large. Even before the advent of settlement and logging,
there must have been large changes from decade to decade or century to
century in species populations and rates of ecological processes as the
relative proportion of different-aged patches varied over time.
Non-equilibrium due to unique events in the past
One-time events, both natural and anthropogenic, may also have longlasting impacts on vegetation. A well-documented example is the
appearance of a novel pathogen of eastern hemlock Tsuga canadensis about
4800 years ago, which caused a sharp decline in hemlock populations
throughout the species' range (Davis, 1981). It took almost two millenia
before hemlock populations returned to their pre-decline levels. A more
recent example whose importance has yet to be fully evaluated is the
devastation wrought by European diseases (especially smallpox) on
American Indian populations in the 16th and 17th centuries (Crosby, 1986).
Since Indians interacted strongly with local plant communities (e.g. by
setting fires; Grimm, 1984), a decline in Indian populations must have had a
sharp impact on local vegetation. It is quite likely that in many areas the
vegetation was still re-equilibrating to changed Indian impacts when the
first white settlers arrived.
Non-equilibrium due to climatic variability
Finally, a landscape may remain out of equilibrium because of climate
changes. Humans, as relatively short-lived organisms (compared to trees),
usually think of climate and other environmental characteristics as fairly
constant. We recognize succession as a dynamic play, but regard the stage on
which it is played out as static and unchanging. When we do think about past
climate change, we usually think in terms of the 'big' climate shifts such as
Environmental variability and "natural" vegetation
7
the Pleistocene glaciation, and assume that changes that occur on a shorter
time scale have little effect on vegetation structure and function. But this
presumption of environmental stability is not necessarily accurate.
Substantial natural climate changes, large enough to cause significant
changes in vegetation distribution and prevent the achievement of
equilibrium, occur on a time scale that is well within the lifespans of forest
trees (Delcourt et al., 1982; Davis, 1984). For example, 800-1000 years ago
the climate was so warm that Vikings were able to colonize Greenland, and
vineyards were planted in England (Lamb, 1982). While a thousand years
may seem a long time in human terms, it is less than one-third of the lifetime
of a giant sequoia.
The examples that follow will illustrate the kind of changes that can and
have occurred in ecosystems which appear to be stable, and which have a
direct effect on the kind of vegetation we see today. (While these examples,
like those above, are drawn primarily from North American temperate
forests, the principles they illustrate apply to a wide variety of ecosystems
around the world.) These examples will also illustrate just how incorrect our
perceptions of 'stable' vegetation can be.
Example 1: The African savannas
One of the most dramatic images in popular biology is the African savanna,
with its expansive vistas, impressive large grazers, and peculiar umbrellashaped trees. To many people this represents unspoiled nature at its finest.
However, there is good evidence that the trees, at least, are not 'natural';
despite their conspicuousness now, they were not there (at least not so
widespread) 100 years ago and (if present trends continue) will not be there
100 years from now. The widespread presence of trees (mostly Acacia tortilis)
over large areas of the savannas seems to be a direct result of the
introduction into Africa of the cattle disease rinderpest, in about 1895
(Sinclair, 1979). Rinderpest caused catastrophic mortality among native and
introduced ungulates; its effects on the ecosystem were complex and will not
be detailed here, but the ultimate effect was that trees that had previously
been restricted to protected areas spread rapidly into areas where tree
establishment had been prevented by a combination of grazing and burning
(Fig. 3). Later, as resistance to rinderpest developed in native ungulates and
vaccines were developed to protect domestic livestock, tree establishment
was again suppressed, so few new trees have become established in the last
fifty years. As a result, there are now few sapling-sized trees to take the place
of the 60-80-year-old cohort that is now dying. This has created
extraordinary problems for the park and reserve managers of the region;
visitors to East Africa (who provide the money that justifies the parks'
existence) expect to see big, umbrella-shaped trees, and are disappointed if
8
D.G. Sprugel
RINDERPEST
(introd. 1985)
t
95 % mortality of wild
and domestic
ungulate~
S
reduction in grazing
and brow~g
/ -~ man-eating~llons----.,~_reduction in
hungry lions
7an
populations
fewer ignitions;
lower fire frequency
in tree recruitment
I.ove.opmen, o..eo e woo.,.n, learl 'O0'sl]
(ca. 1920)
Resistance develops
Woodland
among wild u n g u l a t e ~
yanoples
close
Tree seedling
recruitment declines
Idevelopment of open woodland I
Fig. 3. Effect of the introduction of rinderpest on tree populations in East Africa. Based
on Sinclair (1979) and Norton-Griffiths (1979).
they do not. Regardless of the fact that the plains without trees may be more
'natural' than plains with trees, our image of the savannas has become fixed
in a static early 20th century mold.
The fundamental problem for park managers, then, is not that the trees of
the African savannas are dying, but that visitor expectations have become
fixed around an apparently stable vegetation type that was actually
ephemeral. To quote Pellew (1983): 'The woodland structure desired by park
management is an unstable transition stage.., which has been adopted by
management as the status quo because it happened to be present in the good
old days of ten years ago'.
Example 2: The Big Woods of Mhmesota
When early French trappers arrived in southeastern Minnesota, they were
so impressed by the grandeur of the forests that they called them 'Bois Fort'
or 'Bois Grand'. Early American plant ecologists were equally impressed;
the noted ecologist R. P. Daubenmire wrote his doctoral dissertation and a
now-classic paper on the few fragments of Big Woods that remained in the
1930s, and determined that the maple-elm-basswood forests that occurred
there represent the climax formation for the region--that is, the vegetation
Environmental variability and 'natural" vegetation
9
toward which all areas in the region would tend given enough time
(Daubenmire, 1936).
One would assume that a mesic, climax forest covering several thousand
square kilometers and impressive enough to be called the 'Big Woods' must
be an old and well-established vegetation feature. Such an assumption
would be wrong. Daubenmire noted that much of the Big Woods was
underlain by prairie soils, and pollen analysis (Grimm, 1983, 1984) has
demonstrated that the Big Woods actually developed only about 300 years
ago--hardly long enough for any kind of equilibrium involving long-lived
trees to be established. Before about 1650 the area was covered by oak
woodlands, in which frequent fires apparently restricted elm Ulmus
americana, sugar maple Acer saccharum, basswood Tilia americana and
other fire-intolerant mesic species to unusually moist and protected sites. It
appears that during the 17th century a climatic change--possibly associated
with the onset of the coldest part of the 'Little Ice Age'--tipped the balance
toward mesic forests by reducing fire frequencies enough to permit mesic
species to invade (Grimm, 1983; Clark, .1988). Once established, elm-maplebasswood forests do not burn easily (Grimm, 1984), so they were apparently
able to maintain themselves even after climates warmed up again. (We will
never know how much longer the Big Woods species would have been able
to retain their hold on these sites, because they have now been almost
completely converted to farms and housing developments.) Thus, like the
umbrella-trees of the Serengeti, the solid, stately 'Big Woods' were a much
more recent vegetative feature than one might think at first glance, and may
have owed their existence to a relatively transient environmental event.
Example 3: The lodgepole pine forests of Yellowstone Park
The lodgepole pine Pinus contorta var. latifolia forests of Yellowstone Park
are superb examples of the natural forests of the northern Rocky Mountains
and have been relatively little altered by the activities of man. Since the
forests were rarely used by Indians and have never been logged, one might
expect that if an equilibrium landscape could develop anywhere in the
western US, Yellowstone would be a likely place to look for it. (Note that, as
was suggested earlier, an equilibrium landscape is not one in which the
individual stands are stable--in a lodgepole pine forest this would be
unlikely--but rather one in which the changes in different parts of the
landscape balance each other so that the overall effect is one of no net
change.) However, Yellowstone Park is not in anything like an equilibrium,
and probably never has been. Romme, and Despain (1989) mapped the
disturbance history of a large part of the park and found that in the 1980s it
was heavily dominated by 250- to 300-year-old stands, with forests less than
250 years old much less common. In 1988 the park was struck by massive
10
D.G. Sprugel
fires, so now instead of old forests, young and regenerating forests cover
wide areas of the P a r k - - b u t the lack of an equilibrium persists.
The proximal explanation for this chronically unbalanced age distribution seems to be that much of what was to become the Park was swept by
fires in the 18th century, and that regenerating lodgepole pine forests do not
burn readily until they are at least 200-250 years old (Romme, 1982). (For
this reason, Romme and others doubt that human fire exclusion policies
have actually had much effect on fire frequency in Yellowstone, since so
much of the area was in a relatively non-flammable state until the last few
decades.) It then remains to ask why such a large area burned in the mid-18th
century. Possibly it is another case where spatial scale prevents equilibrium;
the natural size of fires in high-altitude lodgepole pine forests may be so large
that the entire burned area represents a handful of large fires that
coincidentally occurred within a few years of each other. However, another
possibility is that fire intensity is closely tied to weather conditions, so that in
exceptionally dry years (such as 1988), fires start throughout the park and the
surrounding region and burn over wide areas. If this is true, then a few
drought years in the early 1700s may have synchronized the forests so that
they all reached a flammable state at about the same time. The massive fires
of 1988 may not have been due to climate alone, but rather to an interplay of
climate, topography and forest development patterns that locks the highaltitude forest into a long cycle of synchronized development alternating
with wide-ranging fires, so that the system never approaches equilibrium.
Example 4: The old-growthforests of the Pacific Northwest
The old-growth forests of the Pacific Northwest are some of the most aweinspiring forests in the world. The immense size of the trees and the cool
quiet mustiness of the forest floor evoke a sense of the Forest Primeval that
few ecosystems anywhere can match. And it is certainly true that many
stands are very old; stands dating back to the 13th and 14th centuries are
c o m m o n in the remaining old-growth areas. But since many northwestern
trees can live over 1000 years, the fact that the forests are old is no guarantee
that they have reached an equilibrium. So it is still reasonable to ask: were
the old-growth forests of the Pacific Northwest in an equilibrium state
before logging began to take its toll?
The answers to these questions are less clear than in the previous
examples, because the forests are so large and difficult to study, and because
they have been so fragmented by logging and other development. However,
it is well known that most of the 'old-growth' forests are dominated by firefollowing successional species (mainly Douglas-fir Pseudotsuga menziesii)
rather than the more shade-tolerant species that would dominate after a
prolonged period without fire or other large-scale disturbance. But the
Environmental variability and 'natural" vegetation
11
limited evidence available suggests that in recent centuries the fire frequency
has been very low; Hemstrom and Franklin (1982) calculated a natural fire
rotation of about 450 years for the presettlement forests of Mt Rainier
National Park. With such a long fire interval, one would expect the forests
to include a substantial proportion of very old stands, but this does not
seem to be the case. In Mt Rainier National Park, for example, less than
2% of the forests are older than go 1230 (Hemstrom & Franklin, 1982). If
the presettlement forests had been in equilibrium, then according to the
negative exponential model that is normally used to predict equilibrium
stand-age distributions in disturbance-prone ecosystems (Van Wagner,
1978) a 450-year natural fire rotation would mean that about 17% of the
forests should have been more than 800 years old. In another example,
Yamaguchi (1986) aged 55 very old trees spread over a 600km 2 area
northwest of Mt St Helens and found that 50 of them had established
between 1300 and 1350, but only five before 1300--again, a somewhat
surprising lack of the very old trees and stands one would have expected
scattered through an equilibrium forest. This does not simply reflect
difficulty in dating very old forests due to death of the original colonizers,
since Douglas-fir is quite capable of living 1000-1200 years (Franklin &
Waring, 1980; Hemstrom & Franklin, 1982). Rather, it seems to reflect a
real anomaly; while 600-700-year-old stands were comparatively c o m m o n
before logging, there were simply not as many pre-13th century forests as
one might expect in an equilibrium situation.
As with the establishment of the Big Woods, there is a reasonable climatic
explanation for this pattern. As was noted earlier, during the 'Medieval
Optimum' (AD1000-1300), climates in many parts of the Northern
Hemisphere were substantially warmer than they are now. T. W. Swetnam
(1989, pers. comm.; Swetnam et al., 1990) has found that the fire frequency in
Sequoia-Kings Canyon National Park (California) was much higher in this
period than it was in the following centuries. While there is no proof that the
Pacific Northwest was also unusually warm and dry during this period, it
may have been, and if so one would expect that fire frequency in Northwestern forests would also have increased. For example, Hemstrom and
Franklin (1982) deduced that about 47% of the forested area in what is now
Mt Rainier National Park burned in 1230. Thus there may be few pre-13th
century stands simply because almost no stands survived the Medieval
Optimum unburned. The cooling trend in the early 1300s that ended the
Viking venture in Greenland may have brought a drop in fire frequency
which permitted the establishment of the giant forest we see today.
The apparent disruption of the old-growth stands 800 years ago was not a
unique event interrupting a long period of stability. Indeed, the pollen record
suggests that old-growth Douglas-fir forests as we know them probably did
12
D. G. Sprugel
not exist until about 6000 years ago; before that, stands with the structure
and species composition of today's old-growth stands either did not exist or
occurred only in small protected areas (Brubaker, 1991). Thus the majestic
old-growth vegetation type itself has been in existence for only about 5-10
Douglas-fir life-spans.
It seems likely, then, that even before logging began the old-growth forests
were not in equilibrium with their disturbance regime, and probably never
would have been. In the late 1800s they were still showing the effects of a
warmer, drier climate almost a thousand years ago, and before the effects of
that perturbation could be damped out another climate change would
probably have altered fire frequencies again.
DISCUSSION
These examples of vegetational instability suggest at least four basic points
about vegetational equilibria:
(1) Many types of natural vegetation are far less stable than they appear to be
Because trees live so much longer than humans, there is a natural tendency
to think of any tree-dominated ecosystem as long-lasting and stable. The
four examples cited above illustrate that even solid, firmly established
vegetation types are often less permanent than one might think. The
umbrella trees that are so distinctive in the African savannas have been
there less than 100 years, and are the result of human introduction of a
devastating exotic pathogen. The Big Woods are only a little older, and may
result from a relatively minor climatic change in the 17th century. The
Yellowstone forests undergo wide swings in average maturity; John Colter,
the first white man to visit Yellowstone, saw a much different ecosystem in
1807 than the tourists in the mid-20th century. And although the old-growth
forests of the Pacific Northwest create the impression of great age and
stability, they seem to be the products of relatively recent disturbance; many
are apparently still recovering from a period of higher fire frequency around
the time of the N o r m a n Conquest, and the vegetation type itself has only
been around for a few millenia. In all of these cases, then, the current
ecosystems are more variable or more ephemeral than they might seem at
first glance.
(2) Small or transient environmental changes can cause large and long-lasting
vegetation changes
In the long run, vegetation distribution is remarkably sensitive to climate,
and a comparatively small change in mean temperature or precipitation can
cause a large change in vegetation. For example, the coldest part of the Little
Environmental variability and 'natural' vegetation
13
Ice Age, which apparently permitted the development of Big Woods
vegetation in southern Minnesota, was only about 1-1-5°C cooler than
the preceding period (Lamb, 1982). Indeed, even during the peak of the
Pleistocene glaciations, mean air temperatures in the Northern Hemisphere
were only about 5-6°C lower than those at the beginning of the 20th century
(COHMAP, 1988). Long-lived vegetation may be slow to respond to a climate
change, but given enough time even small climate variations can lead to
large vegetation changes.
One of the most significant results of small (1-2°C) climatic changes
may be their effect on disturbance regimes (Grimm, 1984; Clark, 1988;
Swetnam et al., 1990). In particular, cool conditions reduce fire frequency,
and warm conditions increase it. Thus Clark (1988) found that fire in northern Minnesota recurred about once every 44 years before the Little Ice Age
began, but only once every 88 years after it had started. These changes in
turn can affect vegetation structure and distribution, and indeed may be the
main way in which small climate changes alter vegetation. This was
probably the case in the transition from oak woodland to Big Woods in
southern Minnesota; lower temperatures and higher rainfall would
probably not have been sufficient by themselves to give maple and basswood
a competitive advantage over oaks, but they apparently reduced fire
frequency enough so that fire-intolerant mesic species were able to gain a
foothold in areas where only fire-tolerant species were formerly able to
survive (cf. Grimm, 1984).
(3) Every point in time is special
The 1963 report of the Advisory Committee on Wildlife Management in the
National Parks, usually known as the Leopold Report, recommended that
in Park management 'above all other policies, the maintenance of
naturalness should prevail', and more specifically, that 'the biotic association
within each park should be maintained, or where necessary recreated, as
nearly as possible in the condition that prevailed when the area was first
visited by the white man'. As we learn more about vegetation history it
becomes increasingly obvious that, while the general idea of trying to
preserve vegetation in a 'natural' state might be desirable, identifying a
specific point in time as epitomizing the 'natural' state is ill-advised,
particularly for non-equilibrium systems (Johnson & Agee, 1988;
Christensen, 1988). The Yellowstone stiuation provides an excellent
example: since the majority of the Park burned over in the 18th century,
perpetuating the condition--including the wildlife and fish populations-that prevailed when the area was seen by the first white man (1807) would
require a massive program of prescribed burning to keep the forest
dominated by young, regenerating stands. This is surely not what the Leopold
14
D.G. Sprugel
Committee had in mind, and it points out a generic problem: at any specific
point in time, the vegetation of any area has some special characteristics
that make it different from other times that might equally well have been
chosen.
(4) Because of vegetational instability, it may be impossible to define the
natural vegetation or the natural disturbance regime in many areas
When it comes to defining 'natural' vegetation, the present is little help;
nearly all ecosystems have been altered by fire or predator control and
exotic species introductions. And the past provides at best an equivocal
guide to what the present might have been, since climatic variability, change
events, and the simple passage of time might have brought dramatic changes
even in the absence of man. For example, the Big Woods might have reverted
to oak savanna during the drought of the 1930s (if it had not been converted
to farmland and housing developments first), and even without man's efforts
Yellowstone would have changed dramatically in the 19th and 20th
centuries. Thus even if we knew exactly what the vegetation and disturbance
regime were like two or five hundred years ago, there is no guarantee--or
even any particular reason to expect--that without man's influence they
would have been the same today.
The problem is not just one of an inability to predict what might have
happened in man's absence; rather, it is that in a non-equilibrium
environment the whole notion of 'the' (unique) natural vegetation or
disturbance regime is flawed. Because chance factors and small climatic
variation can apparently cause very substantial changes in vegetation, the
biota and associated ecosystem processes for any given landscape will vary
substantially over any significant time period--and no one variant is more
'natural' than the others. Park and reserve managers and other ecologists
concerned with 'natural' area management must recognize this, and
understand that the range of 'natural' vegetation and processes is probably
much broader than is commonly imagined. If a given vegetation type or
disturbance regime occurred in an area at some time during the past 1000
years, how can one say it is not a (if not necessarily the) 'natural' type today
(Christensen, 1988)?
Worster (1977) claimed that 'where the climax is ignored or distorted as an
ideal, the only criterion left is the marketplace'. This view is not only shortsighted, ignoring as it does the role of disturbance as an important natural
process, but also simply wrong. Clearly, not all possible biotic assemblages
are 'natural'; a planted, fertilized, pesticide-saturated cornfield, or a forest
freckled with clean, square clearcuts, is not a natural ecosystem by any
reasonable definition. Paleoecological research can often determine the
range of vegetation types and disturbance regimes that have occurred within
Environmental variability and 'natural' vegetation
15
the recoverable past, and such research can guide management activities
aimed at keeping the vegetation within the 'natural' range. The notion of
'natural' vegetation or ecosystem processes need not be abandoned as a goal
for park or reserve management, even though it must be revised to recognize
that there is a range of ecosystems that can legitimately be considered
'natural'.
A C K N O W L E D G E M ENTS
Dr L. B. Brubaker and N. L. Christensen stimulated me to think about the
role of climatic variation in long-term vegetation dynamics, and the
management implications of non-equilibrium natural ecosystems. Drs
Brubaker and J. K. Agee read drafts of this manuscript and provided useful
comments. Salary support during the development of the ideas and the
preparation of the manuscript was provided by grants from the National
Science Foundation, the US Department of Agriculture Competititive
Grants program, and the US Environmental Protection Agency.
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