Forestry
An International Journal of Forest Research
Forestry 2013; 86, 147 – 157, doi:10.1093/forestry/cps058
Advance Access publication 4 September 2012
Review
Wind as a natural disturbance agent in forests: a synthesis
S.J. Mitchell*
University of British Columbia, 3041 –2424 Main Mall, Vancouver, BC V6T 1Z4, Canada
*Corresponding author [email protected]
Windthrow is all too often looked at as an exceptional, catastrophic phenomenon rather than a recurrent natural
disturbance that falls within the spectrum of chronic and acute effects of wind on forests, and that drives ecosystem patterns and processes. This paper provides an integrative overview of the nature, contributing factors
and impacts of wind-caused disturbance in forests, including its effects on trees, stands, landscapes and soils.
Windthrow is examined through the integrating concepts of: the capacity of trees for acclimative growth, the limitation of acclimative growth under inter-tree competition, the recurrent nature of severe weather, how terrain
and soil conditions affect local stand vulnerability and the effect of recurrent windthrow on stand dynamics
and soils. Windthrow management should take place within a framework of general risk management, with
evaluation of the likelihood, severity and potential impacts of wind damage considered – with reference to
the broad and specific aims of management. There is much to be gained from interdisciplinary communication
about the nature and consequences of recurrent wind damage. There are opportunities for climatologists,
engineers, ecologists, geomorphologists and others to develop integrative process models at the tree, stand
and landscape scales that will improve our collective understanding, and inform management decision-making.
Introduction
This paper provides an integrative overview of the nature, contributing factors and impacts of wind-caused disturbance in
forests, including its effects on trees, stands, landscapes and
soils. There is a great deal of literature on each of these topics,
including reviews on wind effects on trees,1,2 on factors contributing to stand-level damage and recovery,3 on the mechanics of
windthrow,4 – 6 on windthrow modeling7 and on the ecological
impacts of windthrow in boreal, temperate and tropical
forests.8 – 11 Schaetzl12 and Šamonil et al. 13 review windthrow
effects on soils, and Bouget and Duelli14 review windthrow
effects on insect communities.
Windthrow is all too often looked at as an exceptional, catastrophic phenomenon rather than a recurrent driver of ecosystem patterns and processes that falls within the spectrum of
chronic and acute effects of wind on forests. Windthrow disturbance patterns are complex and it is often difficult to see order in
the aftermath of an individual storm. While individual windthrow
events have immediate impacts, the long-term and cumulative
impacts of recurrent windthrow are often overlooked by scientists and managers. Furthermore, the study of windthrow and
the management of windthrow-prone landscapes encompasses
many scientific and professional disciplines. One of my goals in
this paper is to bridge some interdisciplinary gaps. A second
goal is to reduce the apparent complexity of windthrow by presenting a set of integrating concepts, knowledge of which
helps in the interpretation of the apparent inconsistencies in
windthrow patterns within and between events. In this paper,
the focus is on windthrow as a recurrent natural disturbance
process in forests. Windthrow is examined through the integrating concepts of: the capacity of trees for acclimative growth, the
limitation of acclimative growth under inter-tree competition,
the recurrent nature of severe weather, how terrain and soil conditions affect local stand vulnerability and the effect of recurrent
windthrow on stand dynamics and soils. The interactions
between tree, weather and site conditions are framed in terms
of diagnostic considerations that should promote greater
clarity in predicting and interpreting windthrow patterns and
consequences. The implications of recurrent windthrow for
stand and landscape management are briefly discussed.
Integrating concepts
Natural disturbance definition
Grime15 defines disturbance as external factors that limit plant
biomass by causing partial or total destruction. Pickett and
White16 place this in the context of ecosystems by defining disturbance as discrete events that disrupt ecosystem, community
or population structure and change resource or substrate availability, or the physical environment. Forman and Godron17 consider these disruptions as departures from normal patterns or
ecosystem functioning. In establishing what is normal, it is
# Institute of Chartered Foresters, 2012. All rights reserved. For permissions, please e-mail: [email protected]
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Forestry
useful to think of the range of natural variability, and the temporal and spatial scales at which this variability occurs.18 There
is typically an inverse relationship between the disturbance intensity (energy released by the physical process of disturbance,
or in the case of wind, the peak wind speeds during the storm)
and periodicity (interval between disturbances), and a positive
relationship between the disturbance intensity and severity
(amount of mortality among tree and other plant populations)
in the affected area.10
develops in trees, and the result of this process.30,31 Acclimation
to wind loads includes thigmotropism, deformation and sublethal mechanical impacts (abrasion, failure) and their collective
effects on the form and biomechanical properties of roots, stems
and branches. Since deformation and loss of tissues via impact
do not constitute growth, it would be more accurate to use
the broader term ‘acclimative responses’ to describe the full
spectrum of effects.
Acute effects of wind on trees and stands
Chronic effects of wind on trees: acclimative responses
The effects of wind on trees range from chronic to acute, incorporating a spectrum of sub-lethal to lethal effects (see reviews by
Grace1 and Telewski2,19). Airflow across the surface of leaves
facilitates transpiration and the exchange of carbon dioxide
and oxygen between the leaf and the atmosphere. Shortduration displacements of branches and leaders by wind leads
to thigmomorphogenetic responses such as formation of
flexure wood, increased secondary thickening of stems and
structural roots, and reduced shoot extension.2,20 These growth
responses produce shorter, thicker and, therefore less slender,
trunks and branches which better resist deflection,5 and
improve root anchorage.21,22
Trees are self-designing structures that experience increasing
loads as they grow larger. They also compete for light and soil
resources with neighbouring plants, and must allocate photosynthates efficiently.23 This optimization leads to the conformation of trees with some general design principles.24 Bending
of a tree under its own weight, or because of lateral loading by
wind, produces stress in the outer fibres and the cambium. The
portion of photosynthate allocated to structural increment
would be most efficiently used if the cambium produced new
wood in a manner that equalizes the distribution of stress
along the outer surface of the stem, and this is known as the
‘constant’ or ‘uniform stress hypothesis’.25 While there is observational and experimental evidence in support of this hypothesis,
there is ongoing debate on the universality of the theory. Niklas
and Spatz26 note that while it is logical to assume that natural
selection has lead to biomass partitioning patterns that
achieve a balance between bending and counter-resisting
moments, leaf, stem and root biomass partitioning patterns
must also accommodate the simultaneous performance of
other functions.
In combination, the growth responses in roots, stems and
branches to deflection and bending enable tree crowns to
maintain what Sinnott27 refers to as a genetically determined
equilibrium shape – where wind exposure is not excessive.
Under higher, directional, sustained winds, the spectrum of
responses extends to the formation of reaction wood, asymmetric growth of stems and root systems and flagged (windswept)
crowns.22,28,29 In examining the literature concerning windswept
crowns, Telewski19 distinguishes between redirection of plant
form under constant unidirectional wind (a biophysical response)
and mechanoperception coupled with thigmotropism (a growth
response). He concludes that redirection of branches is the
more probable mechanism for the windswept crown form.
The terms ‘acclimative growth’ and ‘adaptive growth’ are
used by windthrow researchers in referring to the process by
which acclimation to wind and other mechanical loading
148
If wind loads exceed the resistance of stem or root/soil systems,
trees break (stem snap, windbreak) or uproot. These modes of
failure are often lethal, but trees may survive, particularly
where they are capable of resprouting.32 In either case, this disturbance affects stand and soil conditions. Uprooting and stem
breakage are incorporated in the term ‘windthrow’ or more colloquially ‘blowdown’ and ‘windfall’.4,6 In the case of hurricanes,
there are gradients of wind speed across the hurricane track,
with resulting gradients of damage to trees and stands.33
These gradients provide insights into the hierarchy of tree and
stand-level responses to increasing acute wind speeds. Stem or
root system failure may be preceded during extreme wind
events by partial defoliation or branch failure.11 The loss of
foliage and branch shedding may be an adaptive strategy in
storm-exposed landscapes, since it reduces drag and the potential for stem breakage and uprooting.34,35 Partial or complete
failure of branches also occurs as crowns of neighbouring trees
collide during more routine wind events. Periodic pruning of
branches or branch tips via collision can lead to gaps between
adjacent tree crowns, termed crown shyness, and has implications for tree growth and understory development.29,36,37 At
higher wind speeds, as trees within stands begin to uproot or experience stem breakage, they may knock off branches, break or
uproot adjacent trees as they fall through the canopy. The
extreme winds and flying debris during hurricanes and tornadoes
can strip the bark from stems.11
Stand-level damage ranges from the creation of small canopy
gaps via failure of individual or small groups of trees, through
intermediate levels of failure of overstory trees, to complete
and extensive failure of the overstory canopy. Damage patterns
within stands can be variable or uniform. The propagation of
damage through a stand during a high wind event, and the uniformity of this damage, depends in part on the uniformity of
trees within the stand, along with uniformity in terrain and soil
conditions that affect wind exposure and root anchorage.6,38
Windthrow mechanics, stand-level instability
Windthrow results when the wind-induced drag force on the tree
crown, multiplied over the lever-arm of the stem, results in a
turning moment that exceeds the bending resistance of the
stem, or the root anchorage. Tree design appears to balance
core rigidity with peripheral flexibility.39 The bending and reconfiguration of branches, and consequently tree crowns, during
high winds streamlines foliage and reduces the frontal area.
Over the range of wind speeds at which this reconfiguration
occurs, drag on tree crowns is directly proportional to wind
speed rather than to the square of wind speed, as is the case
for solid bodies.35,40 The resistance of the stem to failure is
Review
proportional to the cube of the stem radius, so small increases in
radial growth add substantially to stem strength.5,41 Byrne and
Mitchell42 found that the critical moments for trees that fail via
stem breakage were similar to the critical moments for trees
that failed via uprooting for trees of similar size and form. Root
anchorage is a function of root architecture and soil properties,
and is empirically very tightly correlated with stem mass.43 The
effects of acclimative growth on tree stability in Sitka spruce
plantations in Britain were examined by Nicoll et al. 30 who
found that anchorage increased as wind exposure increased.
The stability of trees is affected by precipitation and temperature.
Soil moisture affects root anchorage, and uprooting often occurs
when high winds coincide with saturated soils.44 Peltola et al. 45
found that all of their winched trees failed via stem breakage
when soils were frozen, and with higher critical moments than
for similar sized trees winched during non-frozen conditions.
Above-canopy wind speeds attenuate rapidly within dense
stand canopies.6 While open-grown trees are exposed to and acclimate to routine wind events, stand-grown trees are partially
sheltered by neighbouring trees. Stand grown trees are also in
competition with these neighbours for growing space, and
photosynthate is allocated preferentially to shoot growth and
fine root production, and in diminishing priority to storage,
radial increment and protective chemicals.23,46 Shade-grown
trees allocate fewer resources to radial increment than trees
grown in full sunlight,47 even when mechanical loads are artificially increased.48 In stands grown at high densities, stems
become increasingly slender, crown centre of gravities shift
higher and higher and individual trees become increasingly unstable.41,47,49 Crop planning tools such as stand growth models
or stand density management diagrams enable projection of
height and diameter growth, slenderness, and in some cases
crown length. These can be coupled with windthrow models to
enable prediction of stand stability for different planting
density or thinning regimes.7 At some critical stand height,
there is the potential for uniform stands to become mechanically
unstable. This likely reflects both the increased leverage for tall,
high crowned trees, but also the potential for trees to domino
fall during damage events rather than simply falling into and
being supported by their neighbours, as happens in short,
dense stands. The mechanics of this are demonstrated in mechanistic windthrow models,7,50 albeit without representation of
tree collisions. The phenomenon of critical stand height has
been observed in planted Sitka spruce (Picea sitchensis (Bong.)
Carr.) in the UK,51 and can be observed in natural stands of
black spruce (Picea mariana (Mill.) BSP) in eastern Canada,52
mountain beech (Nothofagus solandri var cliffortioides (Hook.) f.)
in New Zealand,53 and likely underlies the recurrent standreplacing windthrow observed in even-aged stands of southern
beech (Nothofagus pumilio Poeppig and Endl.) in Tierra del
Fuego54 and western hemlock (Tsuga heterophylla (Raf.) Sarg.)
and associated species in coastal Alaska.55,56
Factors contributing to windthrow
Recurrent extreme winds
The severity of damage to forests during extreme weather events
depends on the duration of the event, the maximum sustained
wind speed and precipitation immediately prior to and during
the event – in other words, on the intensity of the event.3 The
role of wind as a driver of ecosystem processes also depends
on how frequently intense weather events recur within a
region. Extreme winds are traditionally ranked according to increasing intensity as gales (62 –88 km h21), storms (89 –
117 km h21) and hurricanes (≥118 km h21) according to the
Beaufort scale.57 Hurricanes/typhoons and tornadoes are associated with specific synoptic conditions and are ranked according
to the Saffir–Simpson and Fujita scales, respectively.58,59 Galeforce and storm-force winds can be produced by a variety of synoptic conditions. In mid-latitude temperate zones, extra-tropical
cyclones are a major source of recurrent gale and storm force
winds, and consequently forest disturbance. Extra-tropical
cyclones develop over the Pacific and Atlantic oceans in response
to temperature gradients between tropical and polar climates.
These are very large-scale systems that are most intense
during the winter months, and are often accompanied by high
rainfall. These systems rotate counter-clockwise in the northern
hemisphere, and clockwise in the southern hemisphere. They
are embedded in mid-latitude airflow and typically track from
west to east, in both hemispheres. These systems routinely
produce winds up to 120 km h21 as they make landfall and
cause a substantial damage to forests over wide areas in
coastal regions in the mid-latitudes. These systems occasionally
bring winds in excess of 120 km h21 well into continental land
masses, as occurred in Europe during storms Vivian in 1990
and Lothar in 1999.60 Tropical cyclones develop in response to
temperature differentials between the sea-surface and the atmosphere, and are most intense during the late summer and
fall months. These relatively compact systems are accompanied
by high rainfall and by definition produce sustained winds in
excess of 117 km h21. Tropical cyclones deteriorate rapidly over
land and cause the greatest damage in coastal regions, but
can bring storm force winds well inland. These tropical systems
are initially embedded in tropical airflow, tracking from east
to west in both hemispheres, but often veer into the midlatitudes.58 Convective storms develop over land in tropical and
temperate regions on days with high surface heating. Localized
storm cells produce vertical down drafts and track with regional
airflow.59 Derechos, or straight-line windstorms can develop in
association with fast moving bands of intense thunderstorms
and can cause bands of damage that extend for long distances.61,62 Under specific atmospheric conditions, convective
supercells produce tornadoes, with very localized winds that
can exceed 300 km h21.59,63 Localized temperature and pressure
gradients that develop daily or seasonally in coastal, mountainous or glaciated regions under specific atmospheric conditions
can result in high winds, such as bora winds. These often recur
with sufficient regularity to attract local names,64 and may
produce chronic or acute effects on trees and stands. Regional recurrence of damaging storms can be reconstructed from weather
records and forest damage reports,65 stand history studies and
dendrochronology,66 and analysis of remote-sensing images.56,67
Diagnostic considerations – In evaluating regional climate,
consider the return period of high wind events, their coincidence
with high rainfall or wet soils, dominant wind directions and the
differential between severe weather conditions and routine wind
conditions.
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Forestry
Topographic exposure
Stand conditions
Movement of large-scale air masses generated by extra-tropical
and tropical cyclones is modified by regional and local topography.
Wind speeds generally increase with altitude, and winds can increase rapidly in higher terrain, particularly in coastal regions.
Winds funnel through and over terrain constrictions, leading to
areas of higher or lower mean flow and turbulence. Ridgelines,
valley gaps and shoulders where large valleys change direction experience higher than average wind speeds.68 Numerical airflow
modelling is becoming increasingly sophisticated at capturing
the effects of terrain, but is computationally intensive and
depends on accurate characterization of surface and atmospheric
conditions.69,70 In the absence of airflow models, topographic
indices such as topex, distance-limited topex68,71 and expose72
have been found to be useful predictors of wind exposure.
The differing topographic exposure of terrain units can lead to
distinctive local disturbance regimes within a landscape.56,72
Trees and stands in windward locations acclimate via crown
shaping and height growth suppression,19,28 and local tree-lines
may form.53,73 Conversely, shelter from routine winds may lead
to increased vulnerability of lee-slope stands to storm winds.73
Diagnostic considerations – In evaluating wind exposure in
complex terrain, consider whether a given terrain unit is exposed
or sheltered from routine winds and how this will affect the acclimation of trees and stands to wind. Then consider how exposure
may change during extreme wind conditions. Finally, consider the
effects of slope angle and ground concavity on drainage, since this
will affect soil moisture during extreme weather events.
Given the variation in climate, soil, terrain and stand conditions
within and between forested regions, it is not surprising that
observations on the relationship between tree and stand factors
and vulnerability to wind damage vary between studies. Everham
and Brokaw3 reviewed 119 reports on wind storm damage from
around the world. Although results varied from report to report,
they came to the following general conclusions regarding tree
size and vulnerability to wind damage: most studies found a unimodal distribution of damage relative to stem size – suggesting
that the smallest trees are sheltered from wind while the tallest
trees are better acclimated; the proportion of broken (vs uprooted)
trees is higher in the smallest and largest tree size classes – smaller
trees are often broken when large trees fall on them, whereas large
trees may be relatively well anchored and fail via crown or stem
break. Everham and Brokaw3 found a considerable variation in
the effects of stand structure, composition and age. They found
that even-aged stands were generally reported to have greater
damage than uneven-aged stands, but point out that the
uneven-aged stands are often older, and comprised of species
mixes, and often of natural rather than planted origin.
Examining trends within forest types, and differentiating
natural from managed stands, may provide greater clarity. In
European managed forest landscapes, dominated by even-aged
stands, storm damage typically increases with stand height and
stand age, and with percentage of conifers, particularly of
Norway spruce (Picea abies (L.) Karst), and recent thinning or
edge exposure by harvesting is an additional risk factor in
these forests.75,84
Additional insights come from patterns of wind-driven disturbance in moist, windy landscapes dominated by natural forests.
Rebertus et al. 54 in Tierra del Fuego and Nowacki and Kramer55
in coastal Alaska identify a pattern of recurrent windthrow of
even-aged stands, within landscapes that include older stands
with complex structure. While the even-aged, windthrow-origin
stands are more common in wind-exposed positions, the presence of uneven-aged stands in wind-exposed locations indicates
that they can persist once established. Although trees in dense,
uniform canopied stands may experience relatively less wind
loading while the canopy is intact, the high degree of uniformity
in crown size and stem form can lead to a substantial propagation
of damage from newly exposed stand edges during extreme wind
events. Conversely, the heterogeneity in tree form in irregular
structured or mixed species stands leads to a wider range of stability and the potential for partial stand survival at all but the
highest wind speeds.
Reports on the relative vulnerability of different tree species
vary widely. Everham and Brokaw3 arrive at the following conclusions: differences in morphology, wood properties and disease
resistance do result in some differences in species vulnerability
within and between stands; these differences can be obscured
by differences in site and stand conditions, and silvicultural
history for different populations of the same species. Vulnerability to different modes of damage (crown, stem, roots) also varies
between species, and this along with the post-damage response
of damaged individuals is a key determinant in subsequent successional dynamics.
Diagnostic considerations – In evaluating stand-level windfirmness, or conducting storm post-mortems, consider whether
Soil
Soil conditions affect site fertility and the capacity of trees to
develop effective anchorage. Windthrow is often observed
within stands in localized areas of shallow or poorly drained
soil,75 and in their analysis of UK tree-winching experiments,
Nicoll et al. 43 found anchorage to be stronger on deeper soils,
for trees of equivalent mass. However, sites with deep soil are
often found to be more windthrow-prone than sites with
shallow soil.54,55,67,75 This apparent inconsistency is likely due
to the effect of soil fertility on tree growth and inter-tree competition, and the limiting effects of inter-tree competition and
mutual shelter on acclimative growth.23,47,48 Mitchell et al. 75
and Bouchard et al. 67 observed that windthrow occurred more
frequently on fertile sites than on low-fertility sites, and in
dense stands rather than open-grown stands. Deep soils can
support tall, dense stands. Sites with fertile but shallow soils,
such as seepage sites, would be expected to produce the least
stable stands, since height growth and canopy density is promoted, without concomitant root anchorage.
Recent nitrogen fertilization is associated with higher windthrow rates in Scandinavia.77 – 79 The mechanism for this is not
clear. Fertilization can affect stem allometry and above-ground
biomass distribution,80,81 however it may not affect coarse root
to above-ground biomass ratios.82,83
Diagnostic considerations – In evaluating soil effects on stand
stability, consider – is the soil fertile enough to support a closed
canopied stand in which trees grow tall and compete for
growing space? Does the soil restrict anchorage, or become saturated during severe weather events?
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Review
individual trees are well acclimated to above-canopy winds, and
whether if destabilized during a storm they would fall and destabilize neighbouring trees, enabling propagation of damage
through the stand during a storm.
Consequences of windthrow
Windthrow affects vegetation and soil components of forest ecosystems at the landscape, stand and microsite scale (see reviews
by Webb,8 Ulanova9 and Lugo11). Individual windthrow events
affect stand and soil developmental trajectories and leave legacies that last for centuries or millenia. At the landscape level,
wind affects stand condition, patch size and distribution of
stand types across the landscape. At the stand-level, windthrow
damages the overstory and makes light and soil resources available to subcanopy and understory plants, deposits pulses of
foliage, branch and stem material on or near the ground, and
inverts and exposes soil profiles. At the microsite scale,
downed woody material and upturned rootwads create distinctive substrates, and microtopographic features that persist long
after the woody material have decayed. This combination of
effects makes windthrow different from other natural disturbance processes in forests. Where windthrow recurrence intervals
are shorter than the potential lifespan of local tree species, windthrow becomes a major driver of ecosystem dynamics, particularly in ecosystems where fire is rare.85
Effects on stand dynamics and composition
Windthrow patches vary in size depending on storm intensity
and duration, and heterogeneity in site and stand conditions.
In natural forest landscapes, individual windthrow gaps can
exceed 100 ha, but most are smaller than 8 ha and many are
very small, on the scale of a few trees.54,55 Similarly, overstory
loss rates within gaps vary, to the point that it is difficult to distinguish between a large gap with a high proportion of surviving
trees, and a mosaic of small gaps. The recovery of forest ecosystems following extreme weather events depends on the severity
and extent of the initial damage, subsequent damage by other
disturbance agents, the growth responses of surviving overstory
trees, release of understory trees and colonization of available
growing space by new regeneration, or expansion of understory
shrubs or lianas at the expense of trees. These alternate
mechanisms of recovery from wind damage are described by
Everham and Brokaw3 as regrowth, release, recruitment and repression. If windthrow preferentially removes shade-intolerant
early seral species from the overstory, and these trees are
replaced by regrowth or release of more shade-tolerant
late-seral species, then windthrow can be viewed as accelerating
succession.86,87
Stand-level disturbances can be broadly categorized as resulting in whole-stand, cohort- and gap replacement (gap-phase) of
canopy trees, and these become disturbance regimes when they
recur over multiple stand developmental cycles. Over very long
time frames, the three regimes may interact, and of course
wind is just one of many natural disturbance agents in forest
landscapes.10 In landscapes with wind-driven disturbance dynamics, mixed stand/cohort/gap-replacement regimes occur
where disturbances such as hurricanes produce a gradient of
damage across their trackways,33 or where convective storms
produce swathes of damage in landscapes that are also affected
by lower intensity extra-tropical cyclones.67,88 Local terrain and
soil conditions, combined with the response of local tree
species, may make a given regime more likely within particular
landscape units, especially in topographically complex terrain
where severe weather is associated with large-scale but moderate intensity weather systems such as extra-tropical
cyclones.54,56
In whole-stand replacement, virtually all of the overstory
trees present on a site are damaged during a storm event and
new dense cohort of trees is released or rapidly establishes by
infill seed. This leads to highly uniform stands with properties
that make them vulnerable to recurrent whole-stand replacement as they age.54,55 The dense regeneration that establishes
in wind-damaged areas and subsequent stand uniformity predisposes these stands to future instability leading to recurrent episodes of whole-stand replacement. In areas with whole-stand
replacement, the potential exists for early seral species to
occupy large gaps. However, recruitment of pioneer species
depends on their presence within the landscape and their capacity to regenerate on the substrates available within windthrown areas. In coastal Alaska, stand-replacement patches
are dominated by western hemlock, a shade-tolerant species
that regenerates from seedling banks and infill seed, and by
Sitka spruce, a semi-tolerant species that establishes primarily
by infill seeding on mineral soil associated with uprooted
trees.55,89 In Tierra del Fuego, stand-replacement patches are
dominated by southern beech which regenerates en-masse following windthrow events from a bank of short-lived understory
seedlings and infill seed.54
In cohort replacement, a substantial proportion of main
canopy trees are damaged, often those that have developed
large crowns or defective stems with age. For example, in the
forests of coastal Alaska, hemlock dwarf mistletoe (Arceuthobium tsugense (Rosendahl) G.N. Jones) along with heart rotting
fungi, weaken the crown and bole of older hemlock trees. Recurrent storm damage leads to multi-cohort stands dominated by
this shade-tolerant species, with distinct age cohorts.55
In gap-replacement, disturbances lead to canopy gaps from
one to several mature tree crowns wide. This is a common
mode of wind disturbance in forests and is particularly well
studied in the hardwood and mixed forests of the eastern
US.10,33,90 For example, the evergreen-deciduous forests of the
Great Lakes Region of North America experience recurrent
damage from thunderstorm activity, and extra-tropical
cyclones.85,88 Partial loss of canopy trees during these storms
leads to a mixed regime of cohort replacement and small gap
creation. Whole-stand replacement is rare in the Great Lakes
Region, leading to forests dominated by shade-tolerant species
that regenerate primarily from seedling banks and resprouting
of damaged stems. In contrast, Ulanova,9 working in the
Russian Boreal forest, reported that light-demanding birch
(Betula pendula Roth.) was able to establish on mineral soil seedbeds alongside more shade-tolerant Norway spruce, leading to
multi-aged mixed species stands in forests with recurrent winddriven small gap formation. Gap models such as SORTIE have
been used to simulate successional trajectories in mixed
species stands for gradients of storm damage.91
In cold climates, stresses induced by low temperatures are
compounded by the effects of wind on dessication, wind
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Forestry
loading and abrasion by wind-driven snow, freezing rain or ice
crystals. In areas with high wind exposure, these combined influences affect the ability of trees to colonize or persist as an erect
growth form, and create and modify local tree-lines.73,92 The
interaction between wind-induced stresses and tree age, and
the consequences for stand development are elegantly illustrated in wave forests. Wave forests are typically mono-specific
forests of relatively short-lived trees growing in windy landscapes
with strong prevailing wind directions, and have been reported in
northeastern US,93 in Newfoundland,29 Japan94 and Tierra del
Fuego.95 An exposed front of ageing trees develops perpendicular to the wind direction, with synchronous mortality as trees
along this front die from the combined effects of age, exposure
and mechanical damage.96 Dying trees are replaced by a new
cohort of regeneration of the same species. This front of senescing trees and regeneration gradually moves across the landscape in the direction of the prevailing wind, like a wave, and
parallel waves of these bands of progressively old to young
trees occur across the landscape. The width of a band reflects
the lifespan of the dominant tree species – the time taken for
the trees to grow into a condition of fatal wind vulnerability.
Diagnostic considerations – In evaluating the effect of the local
severe weather regime on stand dynamics consider whether
severe weather recurs at intervals shorter than the potential lifespan of local tree species, where the local disturbance regime
falls on the spectrum of whole-stand replacement to small gap
creation, or is a mix of regimes, and the manner in which terrain
features influence this pattern. Consider whether stands recover
via recruitment of shade-intolerant species, release of shade intolerants or regrowth of surviving trees. Consider also the woody
debris and soil legacies left by windthrow and the interaction of
the windthrow regime with other disturbance processes.
Effects on large woody debris dynamics, insects
and fuel loading
Like other forest disturbances such as wildfire, insect or disease,
windthrow converts living trees to dead broken or downed large
woody debris, and this material persists as a legacy of the previous stand, and the damage event. Dead standing and downed
trees are an important component of forest ecosystems, providing habitat and substrate for a succession of microbial, insect,
plant and animal communities through progressive stages of decomposition.14,97 Log decay rates depend on local climate, log
size, species and whether logs are initially above or in contact
with the ground, or saturated.98 Large wood that becomes incorporated in stream channels contributes complexity to channel
morphology and in-stream habitat.99 Bahuguna et al.,100
however, noted that it may take decades for spanning logs to
decay to the point that they drop into the stream channel, and
the in-stream longevity of this already decayed material is
likely shorter than that of the material entrained in alluvial sediments while still sound.
The weakened trees and debris created by wind storms can
also pre-dispose stands to secondary disturbance. Freshly
damaged trees attract a succession of insects, including bark
beetles – populations of which can build up in downed and
damaged material following major windstorms and spread to
live-standing trees.14,101 The branch and other fine fuel deposited
on or near the ground following storm events adds to the fuel
152
loading in forests, and storm events may be followed by wildfire,
even in forest types where live forests are not typically flammable.10 Large-scale windthrow associated with major storms
has the potential to influence atmospheric carbon dynamics.
Chambers et al. 102 estimated that the forest biomass loss associated with Hurricane Katrina was equivalent to 50 –140% of the
net annual US carbon sink in forest trees.
Diagnostic considerations – When evaluating the role of windthrow in local large woody debris dynamics, consider the periodicity of windthrow events, the species and sizes of trees affected
and the proportion of broken vs uprooted trees. Consider how
the arrangement of downed material affects decomposition
rates, and the contribution of downed and damaged material to
bark beetle population dynamics, wildfire fuel loads, terrestrial
habitat and stream morphology.
Effects on soil
Windthrow has profound and long-term effects on soil properties. Unlike wildfire, insect or disease-mediated tree mortality,
wind-uprooted trees produce upturned root systems and
expose and invert volumes of mineral soil and forest floor. This
process has been referred to as an example of bioturbation, floralturbation or floral pedoturbation and appears to be a standard
feature of forests around the world, even in fire-dominated landscapes.13,103,104 As the woody components decay, a pit-mound
complex remains with the mound forming on the leeward side
of the pit from soil that was entrained in the upturned portion
of the root system. The dimensions of these pit-mound complexes vary depending on whether the leeward root system
acts as a hinge (hinge fall), or the leeward portion of the root
system slips back into the pit during failure (rotational fall), rootsystem volume and dimensions, soil depth and cohesion to the
root system during failure.103 Pit-mounds may persist for hundreds to potentially thousands of years after the windthrow
event.13 Pit-mounds introduce microtopographic and microclimatic complexity – mounds are warmer and drier than pits
and adjacent undisturbed soils during the growing season. This
microsite-level heterogeneity promotes diversity in understory
flora105 and influences successional pathways in storm prone
landscapes.9,63
The distinctive microsites and repeated soil turnover caused
by recurrent windthrow affect the nature and rate of pedogenesis, particularly in climates where podzolization and paludification occur. At the microsite scale, pits accumulate organic
matter, and this combined with the cooler and moister conditions leads to enhanced illuviation of sesquioxides compared
with adjacent mounds.13 At the site level, the pulse of downed
branch and stem material, inversion of the soil during recurrent
windthrow events, reverses podzolization and affects soil
carbon and nutrient dynamics,106 site fertility and microbial
communities.107,108 In the absence of periodic soil inversion in
cool humid climates, the accumulation of organic matter at
the soil surface and the illuviation of iron and aluminium
oxides and organic matter impairs soil drainage.104,106 The
resulting accumulation of organic matter, decline in forest productivity and ultimately bog formation on initially well-drained
sites have been described by Simard et al. 109 as successional
paludification. Bormann et al. 104 suggest that windthrow, or disturbances that mimic windthrow, may be necessary at intervals
Review
of 200 –400 years in order to maintain soil productive capacity in
wet cool temperate ecosystems.
Windthrow of trees in areas of steep terrain can also lead to
gradual down slope transport of sediment. The potential for
gradual down slope movement results when soil entrained in
the upturned rootwad deposits on the downslope side of a developing mound, rather than in the upslope pit from which it
came.110,111 In areas with thin soil mantles, root penetration of
fractured bedrock, and the entrainment and exposure of
bedrock in upturned rootwads, facilitates weathering and soil
production.112 The role of windthrow in the production and
down slope movement of surface soil mantles has been explicitly
included in geomorphological models for hillslope processes in
forested landscapes.113
Diagnostic considerations – When evaluating the role of windthrow in local soil processes and site fertility, consider the abundance, size and longevity of pit-mounds and the effects of
periodic soil inversion on organic matter accumulation and soil
drainage. Consider the interaction between soil fertility and
stand stability.
Management in windy landscapes
Forests are managed for a variety of purposes under widely
varying management paradigms. The aims of forest management, even within a given region, may span conservation or restoration of natural areas, extensive forest management where
production of goods and services is balanced with maintenance
of native ecosystems and biological diversity, or intensive production of timber or other forest crops via plantation or other
forms of forest cultivation. In forested regions with recurrent
severe weather, managers can embrace windthrow as a
natural disturbance process in natural or semi-natural ecosystems while attempting to reduce its impacts in managed
forests and built-up areas – but they should not overlook or
ignore it. Windthrow management should take place within a
framework of general risk management, with the likelihood, severity and potential impacts considered prior to deciding on acceptance, mitigation or cost-sharing responses.114 The impacts
of windthrow, and preemptive or reactive management
responses, should be considered with reference to the broad
and specific aims of stand and landscape-level management.
Recognition of the historical natural disturbance regime and
how it affects stand and landscape dynamics is identified as a
guiding principle in ecologically sustainable management of
native forests.115 The differences between stand-replacing and
stand-maintaining fire regimes are reasonably well understood
and form the basis for stand- and landscape-level planning
and prescriptions in fire-prone landscapes.116 A similar appreciation for the role of wind is developing, and this has been incorporated into the concept of natural disturbance types which
forms a framework for ecosystem management in British Columbia.117,118 The challenge is to link local disturbance regimes to
the climatic and topographic conditions which affect the recurrence, extent and severity of damaging events, and to relate
this to the resulting patterns in vegetation and soil conditions.
The link between climate, landscape unit conditions and local
disturbance regimes is well developed in the field of geomorphology, and Montgomery119 provides a potentially useful
integrating framework in his concept of geomorphic ‘process
domains’. Geomorphic process domains are spatially identifiable
areas in which the local geology, topography, climate and biota
result in distinctive suites of recurring geomorphic disturbances
which in turn affect soil and vegetation community development. Montgomery119 focused on the geomorphic consequences
of natural disturbances, but the process domain concept incorporates fire, wind and other recurrent natural disturbance processes that are similarly affected by the interaction of
topography, soils, climate and biotic communities. Re-examining
landscapes through the lens of natural disturbance process
domains would improve our understanding of the processes
underlying soil fertility and successional dynamics of vegetation
communities, and enable managers to make more informed
decisions about conservation or land-use activities. One such application would be in the design of salvage operations following
major storms. As noted by Lindenmayer and Noss,120 logging in
the wake of natural disturbances can remove biological legacies
and modify rare post-disturbance habitats. These may be distinctive features in landscapes with intermittent, but recurrent
wind damage.
In landscapes where management is focused on cultivation
of forests for goods and services via plantation and other
forms of intensive forestry, and where tree failure impacts
human-built infrastructure, managers need to recognize and
consider the implications of recurrent severe weather and windthrow disturbance regimes. At the tree and stand level, silviculture regimes can be designed to promote tree-level
acclimation and avoid abrupt changes in exposure of retained
trees.121 Schelhaas et al. 65 reviewed the literature concerning
trends in natural disturbances in European forests from 1850
to 2000. They concluded that while storm intensity does not
appear to have increased, the forest damage caused by storms
has substantially increased since the 1950s. They attribute this
to a number of factors, including the expansion of the forest
estate and the establishment and maturation of coniferdominated forests in storm prone locations that had not supported forests for many human generations.
There are various approaches to predicting windthrow in
natural or managed forests including diagnostic techniques,
and statistical and mechanistic models. The phenomenon of
stand failure at critical height in UK Sitka spruce plantations
motivated the development initially of a windthrow hazard classification system51 and then the ForestGALES model,122 to assist
with the siting and harvest scheduling of plantations in this windy
landscape. Hybrid-mechanistic models such as ForestGALES can
be refined and extended to apply to a variety of forest types,
disturbance regimes and management scenarios,7 and when
coupled with stand growth and regional climate models, can be
used to explore implications of changing climates.123 However,
there remains a significant amount of work to develop processbased models that integrate disturbance mechanics and their
short- and long-term effects on ecosystem dynamics, and this
is a fertile area for interdisciplinary collaboration.124
Conclusion
Recurrent windthrow is a feature of many forested landscapes
and falls within the spectrum of wind effects on forests. It is a
153
Forestry
complex disturbance process that results from the interaction of
regional weather systems with terrain, stand and soil conditions,
and influences stand dynamics and soil processes. The variability
in windthrow patterns can be better understood if the nature of
the regional wind climate and interaction with terrain is considered, if stand stability is evaluated in terms of acclimative
growth, and if soils are examined through their effects on
stand productivity and anchorage. There is much to be gained
in interdisciplinary communication about the nature and consequences of recurrent wind damage. There are opportunities for
climatologists, engineers, ecologists, geomorphologists and
others to develop integrative process models at the tree, stand
and landscape scales that will improve our collective understanding, and inform management decision-making.
Conflict of interest statement
None declared.
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