Frontiersin Ecology
and the
Environment
The forgotten stage of forest
succession: early-successional
ecosystems on forest sites
MMaarrkk EE SSwwaannssoonn,, JJeerrrryy FF FFrraannkklliinn,, RRoobbeerrtt LL BBeesscchhttaa,, CChhaarrlleess MM CCrriissaaffuullllii,, DDoommiinniicckk AA DDeellllaaSSaallaa,,
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haarrdd LL HHuuttttoo,, DDaavviidd BB LLiinnddeennmmaayyeerr,, aanndd FFrreeddeerriicckk JJ SSwwaannssoonn
Front Ecol Environ 2010; doi:10.1890/090157
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© The Ecological Society of America
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S
evere natural disturbances – such as wildfires, wind-
storms, and insect epidemics – are characteristic of
many forest ecosystems and can produce a “stand-replace-
ment” event, by killing all or most of the dominant trees
therein (Figure 1). Typically, limited biomass is actually
consumed or removed in such events, but many trees and
other organisms experience mortality, leaving behind
important biological legacies (structures inherited from the
pre-disturbance ecosystem; Franklin et al. 2000), including
standing dead trees and downed boles (tree trunks;
Franklin et al. 2000). Such legacies provide diverse physi-
cal/biological properties and suitable microclimatic condi-
tions for many species. Thereafter, species-diverse plant
communities develop because substantial amounts of pre-
viously limited resources (light, moisture, and nutrients)
become available. These emerging plant communities cre-
ate additional habitat complexity and provide various
energetic resources for terrestrial and aquatic organisms.
The ecological importance of early-successional forest
ecosystems (ESFEs) has received little attention, except as a
transitional phase, before resumption of tree dominance. In
forestry, this period is often called the “cohort re-establish-
ment” or “stand initiation” stage, with attention obviously
focused on tree regeneration and the re-establishment of
closed forest canopies (Franklin et al. 2002). Ecological
studies have focused primarily on plant-community devel-
opment and the needs of selected animal (mostly game)
species, and not on the diverse ecological roles of ESFEs.
Here, we highlight important features of ESFEs, includ-
ing their role in sustaining ecosystem processes and biodi-
versity, so that they may be appropriately considered by
resource managers and scientists, and included within
management/research programs dedicated to maintaining
these functions, particularly at larger spatio-temporal
scales. Most published examples focus on sites in western
North America, but ESFEs are important elsewhere
(Angelstam 1998; DeGraaf et al. 2003). We also discuss
how traditional forestry practices, such as clearcutting,
tree planting, and post-disturbance logging, can affect
early-successional communities.
REVIEWS REVIEWS REVIEWS
The forgotten stage of forest succession:
early-successional ecosystems on forest sites
MMaarrkk EE SSwwaannssoonn
11**
,, JJeerrrryy FF FFrraannkklliinn
22
,, RRoobbeerrtt LL BBeesscchhttaa
33
,, CChhaarrlleess MM CCrriissaaffuullllii
44
,, DDoommiinniicckk AA DDeellllaaSSaallaa
55
,,
RRiicchhaarrdd LL HHuuttttoo
66
,, DDaavviidd BB LLiinnddeennmmaayyeerr
77
,, aanndd FFrreeddeerriicckk JJ SSwwaannssoonn
88
Early-successional forest ecosystems that develop after stand-replacing or partial disturbances are diverse in
species, processes, and structure. Post-disturbance ecosystems are also often rich in biological legacies, includ-
ing surviving organisms and organically derived structures, such as woody debris. These legacies and post-dis-
turbance plant communities provide resources that attract and sustain high species diversity, including
numerous early-successional obligates, such as certain woodpeckers and arthropods. Early succession is the
only period when tree canopies do not dominate the forest site, and so this stage can be characterized by high
productivity of plant species (including herbs and shrubs), complex food webs, large nutrient fluxes, and high
structural and spatial complexity. Different disturbances contrast markedly in terms of biological legacies, and
this will influence the resultant physical and biological conditions, thus affecting successional pathways.
Management activities, such as post-disturbance logging and dense tree planting, can reduce the richness
within and the duration of early-successional ecosystems. Where maintenance of biodiversity is an objective,
the importance and value of these natural early-successional ecosystems are underappreciated.
Front Ecol Environ 2010; doi:10.1890/090157
IInn aa nnuuttsshheellll::
• Naturally occurring, early-successional ecosystems on forest
sites have distinctive characteristics, including high species
diversity, as well as complex food webs and ecosystem
processes
• This high species diversity is made up of survivors, oppor-
tunists, and habitat specialists that require the distinctive
conditions present there
• Organic structures, such as live and dead trees, create habitat
for surviving and colonizing organisms on many types of
recently disturbed sites
• Traditional forestry activities (eg clearcutting or post-distur-
bance logging) reduce the species richness and key ecological
processes associated with early-successional ecosystems; other
activities, such as tree planting, can limit the duration (eg by
plantation establishment) of this important successional stage
1
Washington State University, Pullman, WA
*
(markswanson@wsu.
edu);
2
University of Washington, Seattle, WA;
3
Oregon State
University, Corvallis, OR;
4
USFS Pacific Northwest Research
Station, Amboy, WA;
5
National Center for Conservation Science
and Policy, Ashland, OR;
6
University of Montana, Missoula, MT;
7
Australian National University, Canberra, Australia;
8
USDA
Forest Service, Corvallis, OR
Early-successional forest ecosystems ME Swanson et al.
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© The Ecological Society of America
Early-successional ecosystems on forest sites
Initial conditions after stand-replacing forest disturbances
vary generically, depending on the type of disturbance; this
includes the types of physical and biological legacies avail-
able. For example, aboveground vegetation may be limited
immediately after the disturbance, as in the case of severe
wildfires or volcanic eruptions. Conversely, intact under-
story communities may persist where forests have been
blown down by severe windstorms. Spatial heterogeneity
in conditions is characteristic, given that disturbances vary
greatly in the amount of damage they cause (Turner et al.
1998). For instance, severe wildfires frequently include
substantial areas of unburned as well as low to medium lev-
els of mortality, creating variability in shade, litterfall, soil
moisture, seed distribution, and other factors.
We define ESFEs as those ecosystems that occupy
potentially forested sites in time and space between a
stand-replacement disturbance and re-establishment of a
closed forest canopy. These ecosystems undergo composi-
tional and structural changes (succession) during their
occupancy of a site. Changes begin immediately post-
disturbance, as a result of the activities of surviving organ-
isms (eg plants, animals, and fungi), including plant
growth and seed production. Developmental processes are
enriched by colonization of flora and fauna from outside
the disturbed area. Successional change is often character-
ized by progressive dominance of annual and perennial
herbs, shrubs, and trees, although all of these species are
typically represented throughout the entire sequence of
forest stand development (or sere; Halpern 1988).
The ESFE developmental stage ends with re-establish-
ment of tree cover that is sufficiently dense to suppress
and often eliminate many smaller shade-intolerant plants
(Franklin et al. 2002). Consequently, the
duration of ESFEs varies inversely with
rapidity of tree regeneration and growth,
which, in turn, depend on such variables
as tree propagule availability, conditions
affecting seedling or sprout establish-
ment, and site productivity. ESFE
longevity after natural disturbances is
therefore highly variable.
Development of a closed forest canopy
may require a century or more in areas
with limited seed sources, harsh environ-
mental conditions, severe shrub compe-
tition (in some instances), or combina-
tions thereof (Hemstrom and Franklin
1982). For example, tree canopy closure
after wildfire in the Douglas fir region of
western North America often requires
several decades (Poage et al. 2009), but
can occur much more rapidly when
canopy seedbanks are abundant (eg
Larson and Franklin 2005). Closed forest
canopies may develop quickly in forests
dominated by trees with strong sprouting ability (eg many
angiosperms) or when windstorms “release” understories
of shade-tolerant tree seedling banks by removing all or
most of the overstory (Foster et al. 1997).
Attributes of early-successional ecosystems
After severe disturbances, forest sites are characterized by
open, non-tree-dominated environments, but have high
levels of structural complexity and spatial heterogeneity
and retain legacy materials.
Environmental conditions
Removal of the overstory forest canopy during distur-
bances dramatically alters the site’s microclimate, includ-
ing light regimes. These changes lead to increased expo-
sure to sunlight, more extreme temperatures (ground and
air), higher wind velocities, and lower levels of relative
humidity and moisture in litter and surface soil. Shifts in
these environmental metrics favor some species, while
creating suboptimal or intolerable conditions for others.
For example, post-disturbance plant community composi-
tion, cover, and physiognomy are altered as shade-tolerant
understory herbs are largely displaced by shade-intolerant
and drought-tolerant species. New substrates deposited by
floods or volcanic eruptions may lack nutrients, provide
additional water-holding capacity, or have high albedo, all
of which favor shifts in plant communities.
Survivors
Organisms (in a variety of forms) that survive severe dis-
turbances are extremely important for repopulating and
FFiigguurree 11..
Stand-replacement disturbance events in forests create large areas free of
tree dominance and rich in physical and biological resources, including legacies of the
pre-disturbance ecosystem.
ME Swanson et al. Early-successional forest ecosystems
© The Ecological Society of America
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restoring ecosystem functions in the
post-disturbance landscape. Even in
severely disturbed areas, organisms may
survive as individuals (mature or imma-
ture) or as reproductive structures (eg
spores, seeds, rootstocks, and eggs), which
become in situ propagule sources. For
example, after the 1980 volcanic eruption
of Mount St Helens (Washington State),
most pre-eruption flora and many fauna
(especially aquatic and burrowing terres-
trial species) survived within the blast
zone through several different mecha-
nisms (Dale et al. 2005).
Surviving organisms are also often vital
for the prompt re-establishment of impor-
tant ecosystem functions, such as conser-
vation of nutrients and stabilization of
substrates. For instance, the important
role of resprouting vegetation in curbing
massive losses of nitrogen was demon-
strated by experimentally clearcutting
and applying herbicides in a watershed at
Hubbard Brook Experimental Forest
(Bormann and Likens 1979).
Structural complexity
The structural complexity of ESFEs depends initially on
legacies, the general nature of which varies with the type of
disturbance (Table 1; Figure 2); for example, snags and
shrubs originating from belowground perennating (ie
resprouting) parts or seeds are dominant legacies after wild-
fires, whereas downed boles and largely intact understories
are typical post-disturbance characteristics of windstorms.
Woody legacies, such as snags and downed boles, play
numerous roles in structuring and facilitating the devel-
opment of the recovering ecosystem – providing habitat
for survivors and colonists, moderating the physical envi-
ronment, enriching aquatic systems in the disturbed area
(Jones and Daniels 2008), and providing long-term
sources of energy and nutrients (Harmon et al. 1986).
Although subject to decomposition, these legacies can
persist for many decades and sometimes even centuries.
Table 1. Different types of intense disturbances generate different types of biological legacies
Disturbance
Biological legacies Wildfire Wind Insect Volcano Clearcut
Live trees Infrequent Variable Variable (depends Infrequent – Infrequent or
on stand composition) confined to absent
margins
Snags Abundant Variable Abundant Abundant Infrequent or
(spatially variable) absent
Downed woody debris Variable, but Abundant Variable, but Abundant Infrequent
typically abundant eventually abundant (spatially variable)
Undisturbed understory Infrequent Abundant Abundant Infrequent – confined Infrequent
to disturbance margins
Spatial heterogeneity of High Variable High High Variable –
recovery usually low
Time in early-successional Variable Variable Long Variable – Variable –
condition usually long usually short
FFiigguurree 22..
Different types of disturbances produce different types of biological legacies,
including living organisms and structures: (a) standing dead trees (snags) are dominant
structural legacies after severe wildfires; (b) downed tree trunks and nearly intact
understory communities are characteristic legacies after major windstorms; (c) standing
dead trees are also dominant structural legacies after heavy insect infestations; and (d)
clearcuts typically eliminate most aboveground structural legacies. Values for each
metric are shown in Table 1 and are described in detail in the text.
(a) (b)
(c)
(d)
Early-successional forest ecosystems ME Swanson et al.
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© The Ecological Society of America
Structural complexity is further enhanced by the estab-
lishment and development of a variety of plant species,
which often include perennial herbs and shrubs charac-
teristic of open environments, as well as individual trees
(Figure 3). The diversity of plant morphologies (maxi-
mum height, crown width, etc) increases structural rich-
ness, so that this associated flora contributes to both hor-
izontal and vertical heterogeneity.
Spatial heterogeneity
Spatial heterogeneity is evident in early-successional
ecosystems and has multiple causes: (1) natural variabil-
ity in the geophysical template (topography and lithol-
ogy) of the affected landscape; (2) variability in condi-
tions in the pre-disturbance forest ecosystem; (3)
variability in the intensity of the disturbance event; and
(4) variability in rates and patterns of subsequent devel-
opmental processes in the ESFE. The first two sources
relate to existing geophysical and biological patterns
within the disturbed area. Land formations and patterns
of geomorphic processes are certainly key geophysical ele-
ments (Swanson et al. 1988). The presence of surface
water, such as streams and ponds, can be particularly
influential in facilitating survival and re-establishment of
biota.
Natural disturbances create heterogeneous environ-
ments at multiple spatial scales (Heinselmann 1973),
because disturbances do not cause damage uniformly.
Disturbances such as wildfires and windstorms are vari-
able in intensity (eg “spotting”, or initiation of new flame
fronts by wind-thrown firebrands, during fire events).
Alternatively, geographic variation in en-
vironmental conditions and topography
(Swanson et al. 1988) influences the intensity
of the disturbance and results in heterogene-
ity at multiple scales. Variability in the struc-
ture and composition of the pre-disturbance
forest also creates spatial and temporal vari-
ability (Wardell-Johnson and Horowitz
1996). Some of these patterns may be tran-
sient, such as residual snowbanks protecting
tree regeneration after the aforementioned
Mount St Helens eruption (Dale et al. 2005).
Post-disturbance developmental processes
also lead to spatial heterogeneity. For exam-
ple, varying distances to sources of tree seed
result in different rates and densities of tree
re-establishment (Turner et al. 1998).
Structural legacies can greatly influence the
rates at which wind- or waterborne organic
(including propagules) and inorganic materi-
als are deposited. Finally, animal activity can
strongly influence patterns of revegetation, as
illustrated by the multiple effects that
gophers (Thomomys spp) can have on post-
disturbance landscapes (Crisafulli et al.
2005b) or the way ungulate browsing may impede tree
regeneration (Hessl and Graumlich 2002).
Biological diversity
ESFEs in temperate forest seres show great diversity in the
abundance of plant and animal species (Fontaine et al.
2009). Species composition may consist of a mix of forest
survivors, opportunists, or ruderals (plants that grow on
disturbed or poor-quality lands), and habitat specialists
that co-exist in the resource-rich ESFE environment
(Figure 3). Most forest understory flora can survive distur-
bances as established plants, perennating rootstocks, or
seeds. In one study, in western North America, over 95%
of understory species survived the combined disturbance
of logging and burning of an old-growth Douglas-
fir–western hemlock stand (Halpern 1988). Some impor-
tant early-successional species (eg Rubus spp [blackberry;
raspberry], Ribes spp [gooseberry], and Ceanothus spp
[buckbrush]) may persist as long-lived seedbanks.
Opportunistic herbaceous species are often conspicuous
dominants early in the development of ESFEs (Figure 4).
Many of these weedy species (particularly annuals)
decline quickly, although other opportunists will persist
as part of the plant community until overtopped by
slower growing shrubs or trees. Consequently, diverse
plant communities of herbs, shrubs, and young trees
emerge in ESFEs; this, combined with the structural lega-
cies from the pre-disturbance ecosystem, often results in
high levels of structural richness (Figure 3).
Many animals, including habitat specialists and species
typically absent from the eventual tree-dominated com-
FFiigguurree 33..
Plant communities with well-developed shrub and perennial herb
species are characteristic of early-successional communities on forest sites and
provide diverse food resources. Twenty-five years after the Mount St Helens
eruption in 1980, this community, which was within the blast zone, includes
well-developed shrubs (eg Sorbus and Vaccinium spp), trees, and perennial
herbs (eg Epilobium angustifolium).
