the denuded site is colonized by a 
sequence of species that gradually al- 
ters the habitat in ways favoring a 
return to the structure and content 
of the original climax composition. 
Clements viewed the successional 
changes initiated by external distur- 
bance as part of a progressive, inter- 
nally controlled process of develop- 
ment, analogous to the transforma- 
tions in the ontogeny of an organism. 
The natural state of vegetation, then, 
is mature climax: permanent and per- 
petually stable, although marred oc- 
casionally by transient patches of suc- 
cessional plant communities, the kinds 
of herbs and shrubs that colonize 
abandoned farmland before the ma- 
ture forest develops. 
This view of disturbance as a violent 
force external to stable vegetation had 
some direct effects on wilderness-pres- 
ervation policies. About the turn of 
the century, when large expanses of 
forest were set aside as public land, 
a major priority was the establishment 
of a system of fire suppression. Iso- 
lated fire guards in mountain watch- 
towers, highly trained platoons of 
firefighters, and complex systems of 
weather forecasting all proliferated in 
national parks and forests, accompa- 
nied by public exhortations proclaim- 
ing the evil of fire caused by the care- 
less hand of man. Economic and safety 
considerations also shaped the policy 
of fire suppression, but conservation- 
ists and plant ecologists alike, wishing 
to preserve some virgin wilderness, ac- 
cepted unquestioningly the beneficial 
effects of such policies on forest pres- 
ervation. The destructive effects of 
fires set by humans were seen as a 
tremendous threat. 
Just how frequent were catastrophic 
fires prior to the arrival of settlers? 
To answer this question, Miron Hein- 
selman, a forest ecologist at the Uni- 
versity of Minnesota, spent six years 
carefully reconstructing, back to a.d. 
1595, the fire history of virgin co- 
niferous forests in the Boundary Wa- 
ters Canoe Area of northern Minne- 
sota. By probing the historical ac- 
counts of explorers and trappers and 
the records of geologic surveys and 
the Forest .Service, Heinselman was 
able to reconstruct a history of re- 
current fire. He then mapped the dis- 
tribution of forest communities, par- 
ticularly noting fire-adapted species, 
such as jack pine, whose seedlings ger- 
minate and establish primarily after 
burning. Heinselman supplemented 
this analysis with age records of in- 
dividual trees, determined by count- 
ing the annual rings in a core of wood 
sampled from the trunk. By analyzing 
the history of close to a million acres, 
he has identified large stretches of 
landscape that have experienced one 
or more burns and can precisely es- 
tablish the time of each fire. 
Since 1900, when the Forest Service 
began its policy of fire suppression, 
relatively few major burns have oc- 
curred in the Boundary Waters Canoe 
Area, with an average interval of 
twenty-six years between significant 
fires anywhere in the forest. During 
the fifty years of settlement prior to 
the adoption of fire-suppression poli- 
cies, a serious fire destroyed some part 
of the forest every two years. More 
surprising, Heinselman discovered 
that fires were occurring nearly as fre- 
quently— at intervals of only four 
years — for the three hundreo years 
before the first European settlements. 
On average, a given stand of forest 
will burn every hundred years, indi- 
cating that the climax condition is 
barely achieved before it is destroyed. 
Thus, the virgin spruce-fir climax was 
normally subject to frequent cata- 
strophic destruction, which contin- 
ually maintained some fraction of the 
vegetation in the process of succession. 
Albert Swain, a colleague of Hein- 
selman’s, has extended this fire history 
an additional 1,000 years into the past 
by analyzing sediment cores from Fake 
of the Clouds, situated in the center 
of the virgin forest. Each year, this 
lake lays down a distinguishable layer 
of bottom mud consisting of sediments 
and pollen carried in from the surround- 
ing drainage basin. By identifying char- 
coal bands and jack pine pollen, Swain 
dated the major fires in the vicinity. 
For any forest stand, he found the aver- 
age frequency of burning was sixty to 
seventy years. The cores also contain 
charcoal deposits and a high abundance 
of jack pine pollen throughout the his- 
torical record. 
Fire, and the consequent destruc- 
tion and succession of plant commu- 
nities, appears to be a recurrent and 
natural event in these forests. Histori- 
cally, the major burnings coincide 
with years of summer drought and 
are probably ignited by lightning. In 
addition to maintaining species diver- 
sity, frequent fires recycle mineral nu- 
trients tied up in plant tissue and pre- 
vent the accumulation of highly flam- 
mable brushwood and dead timber, 
which support massive, uncontrollable 
conflagrations. A policy of fire sup- 
pression could be causing permanent 
and unpredictable changes in the nat- 
ural vegetation of wilderness areas. 
Fire is not the only kind of dis- 
turbance. John Henry and Mark 
Swann, working at the Harvard For- 
est, have published an equally me- 
ticulous historical reconstruction of 
virgin Pisgah Forest in southwestern 
New Hampshire back to a.d. 1665. 
By coring living and dead trees and 
carefully dissecting soil layers for evi- 
dence of dead trees and fire, these 
workers found a recurrent pattern of 
violent disturbance that prevented any 
orderly succession. In 1665, the vege- 
tation was completely destroyed by 
a catastrophic fire, which the authors 
speculate may have been fueled by 
the many dead, dry trees blown down 
by a hurricane that passed through 
the area in 1635. During the following 
270 years, three violent windstorms, 
in 1898, 1909, and 1921, interrupted 
the development of a forest of hemlock 
and white pine and brought down 
nearly half the trees. Finally, the 
Great Hurricane of 1938 swept 
through central New England and de- 
stroyed the entire forest, leaving a 
crisscross lattice of uprooted trunks. 
Today these ghosts of former trees, 
each with a jagged web of roots ra- 
diating from its base, lie in decay be- 
neath layers of moss and fallen litter. 
In the wake of the hurricane’s de- 
struction, a new forest completely dif- 
ferent in composition began to develop 
from saplings that had survived the 
winds and from seedlings able to ger- 
minate and grow in the open pits of 
raw soil. Henry and Swann found little 
evidence for any process of orderly 
succession and concluded that the 
composition of the forest was largely 
determined by the history of distur- 
bance. 
The behavior of each species in Pis- 
gah Forest is illuminating. White pine 
seedlings, for instance, colonized the 
ash layer only after the severe fire 
of 1665 and only until 1687. Hemlock 
also invaded after the 1665 burning 
but continued to establish plants until 
1780. No new hemlock seedlings ap- 
peared until the windstorms about the 
beginning of the twentieth century. 
Black birch and red maple were absent 
from the forest until the major tree 
blowdowns of 1921 and 1938. Black 
birch appears particularly adapted to 
the raw soil clinging to the upturned 
roots of trees, while red maple prefers 
the damper pits of accumulating litter 
beneath. The establishment of these 
24 
