United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical

Report

PNW-GTR-421

April 1998

The Effects of Wind Disturbance on

Temperate Rain Forest Structure

and Dynamics of Southeast Alaska

Gregory J. Nowacki and Marc G. Kramer
Authors GREGORY J. NOWACKI is the regional ecologist, U.S. Department of Agriculture,

Forest Service, Alaska Region, 709 West 9th Street, Juneau, AK 99801; and

MARC G. KRAMER is a graduate research assistant, Oregon State University,

Corvallis, OR 97331.
Conservation and Resource

Assessments for the Tongass Land

Management Plan Revision

Charles G. Shaw III, Technical Coordinator

Kent R. Julin, Editor

The Effects of Wind Disturbance on Temperate

Rain Forest Structure and Dynamics of

Southeast Alaska

Gregory J. Nowacki

Marc G. Kramer

Published by:

U.S. Department of Agriculture

Forest Service

Pacific Northwest Research Station

Portland, Oregon

General Technical Report PNW-GTR-421

April 1998
Nowacki, Gregory J.; Kramer, Marc G. 1998. The effects of wind disturbance on

temperate rain forest structure and dynamics of southeast Alaska. Gen. Tech.

Rep. PNW-GTR-421. Portland, OR: U.S. Department of Agriculture, Forest

Service, Pacific Northwest Research Station. 25 p. (Shaw, Charles G., III, tech.

coord.; Julin, Kent R., ed.; Conservation and resource assessments for the

Tongass land management plan revision).

Wind disturbance plays a fundamental role in shaping forest dynamics in southeast

Alaska. Recent studies have increased our appreciation for the effects of wind at

both large and small scales. Current thinking is that wind disturbance characteristics

change over a continuum dependent on landscape features (e.g., exposure, landscape position, topography). Data modeling has revealed the existence of distinct

wind disturbance regimes, grading from exposed landscapes where recurrent,

large-scale wind events prevail to wind-protected landscapes where small-scale

canopy gaps predominate. Emulating natural disturbances offers a way to design

future management plans and silvicultural prescriptions consistent with prevailing

ecological conditions.

Keywords: Tongass National Forest, old growth, forest development, small-scale

canopy gaps, large-scale catastrophic blowdown, predictive windthrow model,

silviculture.

Abstract
The role of natural disturbance in shaping forest structure and function is recognized

globally (Attiwill 1994, Perry and Amaranthus 1997, Pickett and White 1985). Management activities tailored to maintain natural disturbance processes provide a basis for

ecosystem management (Alverson and others 1994, Campbell and Liegel 1996); for

instance, by examining disturbance regimes under which forest systems have evolved,

land managers may be able to predict forest response and recovery following human

disturbance. With this information, silvilculturists could match harvest methods to the

prevailing ecological conditions governing tree regeneration and growth (Franklin and

others 1997). Although a variety of disturbance agents exist in southeast Alaska, wind

is the most pervasive force shaping forest composition and structure (Harris and Farr

1974). Its ubiquity and varying intensity over the land (compared to the localized nature

of landslides, avalanches, and flooding) make wind an environmental factor with

considerable relevance to management. As such, mapping wind disturbance patterns

across the landscape may provide a useful template for developing and evaluating land

management practices. This paper synthesizes results from wind disturbance studies

and unpublished data sets in southeast Alaska and discusses their possible implications for forest management.

The vast expanses of relatively unaltered temperate rain forest in southeast Alaska

present an exceptional opportunity to study the effects of disturbance on forest development and succession (Alaback 1988). An understanding of these interrelations can

be used to evaluate and, if considered desirable, design silvicultural and land management practices to promote ecosystem function and vigor, biodiversity, and wildlife

habitat availability. For example, if management activities were tailored to emulate

disturbances common to an area, then natural processes may be maintained and

thresholds above which adverse environmental impacts are likely can be identified

and avoided (Hansen and others 1991, Swanson and Franklin 1992). This approach

also provides an opportunity to restore ecosystem function in highly modified landscapes (Kimball and others 1995).

A variety of natural disturbances occurs in forests, including wind and ice storms,

droughts, fires, landslides, avalanches, floods, insect and disease outbreaks, and

animal browsing (Harris and Farr 1974, Rogers 1996). Disturbances affect forested

ecosystems through tree mortality and the reallocation of resources such as light,

water, nutrients, and growing space (Franklin and others 1987). In contrast to most

North American ecosystems where fire is the predominant disturbance agent (Agee

1993, Pyne 1982, Wright and Bailey 1982), the cool, maritime climate of southeast

Alaska greatly suppresses fire (Noste 1969); here, wind is the most widespread and

ecologically important agent of large-scale natural disturbance (Harris 1989).

The impact that a wind event can have on a forested landscape depends on a combination of biotic factors (stand composition, canopy structure, size, age, and vigor) and

abiotic factors (wind severity and direction, soil and site properties, and orographic

effects on wind flow) (Harris 1989, Ott 1997). The interaction among these factors is

complex, making wind disturbance particularly difficult to characterize and predict

(Attiwill 1994, Everham 1996, Fosburg and others 1976, Harris 1989). Effects

of wind disturbance often change over a continuum, grading from areas where chronic,

single- or small multiple-tree openings prevail in forest canopies to areas exposed to

recurrent, large-scale blowdowns (Attiwill 1994, Foster and Boose 1992).

Introduction

Background

1
To evaluate the effects of disturbance on forest pattern and process, it is imperative to

understand the basic structural changes undergone by forests over time. Four developmental stages are conceptually recognized in temperate forests (Alaback 1982, Bormann

and Likens 1979, Oliver and Larson 1996) and widely cited in ecological literature.

Oliver and Larson (1996) refer to these stages as “stand initiation,” “stem exclusion,”

“understory reinitiation,” and “old growth.” From this model, a conceptual developmental

timeline has been constructed that is specific to southeast Alaska (fig. 1); it is based on

empirical information (stand age-understory relations; fig. 2) and field observations

of Kissinger,1 Garvey,2 Barkhau,3 and the authors. Differing rates of forest development

due to site conditions (i.e., forest development unfolds more rapidly on high-productive

sites as compared to low-productive sites) account for the temporal overlap among

stages shown in figure 1.

_____________________

1 Personal communication. 1997. E. Kissinger, soil scientist,

Stikine Supervisor’s Office, Petersburg, AK 99833.

2 Personal communication. 1997. T. Garvey, GIS coordinator, Chatham Supervisor’s Office, Sitka, AK 99835.

3 Personal communication. 1997. K. Barkhau, forester, Sitka

Ranger District, Sitka, AK 99835.

Figure 1—A conceptual timeline portraying developmental stages for temperate rain forests of

southeast Alaska. Shaded bars represent temporal overlap among developmental stages.

2

0 100 200 300 400 500

Time since catastrophic disturbance (years)

Understory

reinitiation

Old

growth

Stem

exclusion

Stand

initiation
Figure 2—A timeline of forest stands categorized by developmental stage based on data from northeast Chichagof Island (see footnote 4).

3

The stand-initiation stage begins after catastrophic disturbance eliminates the former

overstory and progresses until a complete tree canopy forms (≈25-35 years; Alaback

1982). The stem-exclusion stage is characterized by high tree mortality and a precipitous drop in understory biomass due to resource limitations (light, growing space). Site

monopolization declines after about 100 years of self-thinning as interstitial space

arises among canopy trees. Increased availability of resources (light, growing space)

allows the revival of understory plants and transition to the understory-reinitiation stage.
Within the understory-reinitiation stage, two substages have been observed in southeast Alaska: an early “shrub” substage, where primarily herbs and shrubs recolonize,

and a later “conifer” substage where tree regeneration occurs (fig. 2). The latter substage seems to correspond with the emergence of larger canopy gaps, which are not

subject to closure by lateral extension of the overstory. The enduring nature of these

gaps allows enough light and growing space for trees to establish and grow (i.e., gapphase replacement). A two-aged, two-layered stand initially takes shape during this

substage, and eventually succeeds to a multiaged, multilayered stand with continuing

gap-phase replacement. The old-growth stage appears when the tree component is

principally derived from gap-phase replacement (Oliver and Larson 1996). At this time,

age and size distributions approximate inverse J-shapes and mortality generally balances growth. Stand-structure characteristics traditionally associated with old growth

exist, including large and decadent trees with heavy and craggy limbs, standing snags,

multiple canopy layers interspersed with overhead gaps and regeneration patches, and

coarse woody accumulation on the forest floor and in streams (Franklin and others

1981). Maximum lichen diversity and biomass also are reached during the old-growth

stage (Neitlich 1993). The absolute age when forests become old growth differs with

site and forest type. It seems reasonable, however, that about 350 years is required to

achieve the old-growth conditions characterized above.

To characterize wind disturbance regimes within the Tongass, we focused on studies

conducted within the “perhumid” rain forest zone of North America (Lawford and others

1996), which spans a rugged coastline from Yakutat Bay, Alaska, south to the northern

tip of Vancouver Island, British Columbia. This zone is typified by cool summers with

abundant precipitation and mild winters where snow is usually transient at lower elevations. Conifer domination of these rain forests is truly impressive, with western hemlock

(Tsuga heterophylla (Raf.) Sarg.) and Sitka spruce (Picea sitchensis (Bong.) Carr.)

dominating uplands and mixing with shore pine (Pinus contorta Dougl. ex Loud. var.

contorta), mountain hemlock (Tsuga mertensiana (Bong.) Carr.), western redcedar

(Thuja plicata Donn ex D. Don), and Alaska-cedar (Chamaecyparis nootkatensis (D.

Don) Spach) on wetlands (Pojar and MacKinnon 1994). Mountain hemlock representation generally increases with elevation.

Small-scale canopy disturbances are typical in older forests displaying high structural

complexity (understory reinitiation and old-growth stages) (Lertzman and others 1996,

Spies and others 1990). Although wind, gravity, and winter snow loading are the forces

that physically bring down trees, often it is other agents, such as decay, that weaken

and predispose trees in old-growth forests to fall. Hennon (1995) found heart-rot fungi

to be a primary factor contributing to small-scale disturbance in southeast Alaska. Heart

rot increases as trees and stands age, reaching high levels in all old-growth forests

where they lead to frequent but scattered tree mortality by stem snap. The process may

be cyclical as falling trees wound adjacent trees, leading to fungal infection, heart rot,

and further stem snap. This link between fungi and treefall (structural failure causing

trees to fall) was recognized by early researchers in the Pacific Northwest (Hubert 1918).

Although the dynamics of small-scale disturbance have received considerable attention

in North America (e.g., mixed mesophytic and northern hardwood forests: Barden 1980,

1981; Canham 1988; Fox 1977; Runkle 1981, 1982), disturbance processes that cause

canopy gaps in perhumid rain forests have only recently been studied. Hocker (1990)

studied small-scale gap dynamics in a 300- to 500-year-old (old-growth) forest (Lemon

Creek site, Juneau) and a 169-year-old (mature) forest of blowdown origin (Heintzelman

4

Small-Scale Canopy

Gap Dynamics

Studies of Wind

Disturbance
Table 1—Canopy gap and gap-maker attributes of mature and old-growth stands of perhumid rain forests,

southeast Alaska

Canopy gap characteristics Gap makers

 

Size-class Trees/ Trees/ Other/

Source and Total distribution Size Mean Median gap, gap, Dead Stem- Up- unStage Forest type site name area shape range size size range mean standing snap rooted known

Percent – – – – – – – – Feet – – – – – – – – – – – – – – – Percent – – – – – –

Mature (169 yr) Hocker 1990 3.8 Bell 269-2,625 1,162 1,076 1-5 2.4 NA 87 13 NA

Tsuga-Picea (Heintzelman)

OGa Tsuga Hocker 1990 12.6 Skewed or 495-16,22 3,788 2,238 1-6 2.3 NA 99 1 NA

 (Lemon Creek) neg. exp.

OG Tsuga Ott 1997 5.8 Skewed or 140-2,841 710 516 1-5 2.0 0 95 5 0

(Lemon Creek) neg. exp.

OG Tsuga Ott 1997 7.8 Skewed or 247-1,840 753 570 1-7 2.7 15 64 19 2

(Outer Point) neg. exp.

OG Tsuga Ott 1997 12.6 Skewed or 65-2,281 560 409 1-6 2.6 4 69 25 2

(Sitka) neg. exp.

OG Tsuga Ott 1997 8.7 Skewed or 65-2,841 678 495 1-7 2.4 6 76 17 1

(3-site mean) neg. exp.

OG Picea Ott 1997 NA NA NA NA NA NA NA 25 59 16 0

(Fish Creek;

Douglas Is.)

OG Tsuga- Ott 1997 23.1 NA NA NA NA NA NA NA NA NA NA

Chamaecyparis (NW Baranof Is.)

OG Tsuga-Picea Ott 1997 7.4 NA NA NA NA NA NA NA NA NA NA

complex (Eaglecrest;

Douglas Is.)

OG Tsuga-Picea- Ott 1997 33.7 NA NA NA NA NA NA NA NA NA NA

Chamaecyparis (NW Baranof Is.)

complex

NA = Data not available or reported.

a OG = Old growth.

site, Juneau). Results from this study are summarized in table 1. Gap-size range and

distributional shape differed notably between the two stands. Although the mean number

of trees per gap were virtually the same, the mean gap size was more than three times

larger in the old-growth forest compared to the mature forest. Overall, a greater percentage of canopy gaps were in old growth than in the mature forest. Based on radial growth

trends of subcanopy trees, intervals between episodes leading to canopy gaps were

shorter in old growth than in the mature forest. The mode of treefall (and the creation of

gaps) was principally stem snap in old-growth and mature forest. Thirty-five percent of

the gap makers in the old-growth forest had heart rot or fungal infection whereas only

5 percent did in the mature forest. The general orientation of gap makers in a westsouthwest direction parallels katabatic winds (Taku winds) blowing off the Coast Mountains. Wind and fungal heart rot were considered primary and proximal disturbance

agents, respectively, in causing canopy gaps to form.

5
Ott (1997:chapter 2) describes canopy gap characteristics for three old-growth western

hemlock stands in the northern portion of the Tongass. These stands represent a range

of small-scale disturbance levels from relatively protected areas (Lemon Creek site,

Juneau) to areas with some exposure to prevailing southeast gales (Outer Point site,

Juneau and Sitka site, Sitka). The proportion of forest in canopy gaps ranged from 6

to 13 percent among sites (9 percent average; table 1) and seemed to increase with

exposure to prevailing winds. Gap processes in old-growth forests can vary considerably within the same plant association, but canopy gaps tended to be small with 52

percent being less than 540 square feet and 85 percent less than 1,080 square feet.

Figure 3—Size-frequency distribution of canopy gaps for western hemlock forests in the Chatham Area, Tongass National Forest, Alaska

(slightly modified from Ott 1997:chapter 2).

6

0

5

10

15

20

25

30

35

0-260

270-530

540-800

810-1070

1080-1330

1340-1600

1610-1870

1880-2140

2150-2410

2420-2680

2690-2950

Canopy gap area (ft 2)

Frequency (percent)

Average of the three sites

0

5

10

15

20

25

30

35

0-260

270-530

540-800

810-1070

1080-1330

1340-1600

1610-1870

1880-2140

2150-2410

2420-2680

2690-2950

Canopy gap area (ft2)

Frequency (percent)

Lemon Creek

0

5

10

15

20

25

30

35

0-260

270-530

540-800

810-1070

1080-1330

1340-1600

1610-1870

1880-2140

2150-2410

2420-2680

2690-2950

Canopy gap area (ft 2)

Frequency (percent)

Outer Point

0

5

10

15

20

25

30

35

0-260

270-530

540-800

810-1070

1080-1330

1340-1600

1610-1870

1880-2140

2150-2410

2420-2680

2690-2950

Canopy gap area (ft2)

Frequency (percent)

Sitka
Gap-size distributions were rightward skewed or negative exponential in shape (fig. 3)

with medians of 409 to 570 square feet. Most canopy gaps (about 82 percent) are

created by the loss of three or fewer overstory trees (gap makers). On average, the

majority of gap makers were snapped stems (76 percent), about 17 percent were

uprootings, and the remainder were standing dead or leaning trees. Based on the high

degree of stem-snap, firmly rooted trees with significant heart rot probably abound in

these old-growth stands. Stem-snap and uprooting were inversely related, with increased representation of uprooting on windier sites (up to 26 percent of total gap

makers). Overall, the mode of treefall seems to be linked to soil drainage and depth

(determines rooting depth and anchorage), size and structural integrity of component

trees, and wind exposure.

Ott (1997:chapter 3) also studied relations between prevailing winds and treefall in

two old-growth stands (Lemon Creek and Outer Point sites). Tree crown “flagging”

(asymmetrical crowns pointing leeward) and orientation of terminal leaders in overstory

western hemlocks were used to determine prevailing wind direction. At Outer Point, the

mean direction of gap-maker treefalls (291o) was significantly aligned with the direction

of prevailing winds, thereby indicating that canopy gaps were formed by southeast

gales. Prevailing Taku winds originating over the Juneau Icefield was confirmed at the

Lemon Creek site. This site was relatively steep (slopes up to 34o) and faced southeast.

The mean direction of gap-maker treefalls (190o) and its 95-percent confidence interval

was equidistant between the prevailing wind direction (blowing southwest at 225o) and

aspect (112o). Ott (1997) concludes that both prevailing wind direction and slope

(gravity) contributed to the orientation of treefalls at the Lemon Creek site.

The studies summarized above suggest that small-scale disturbances in mature and

old-growth stands result in minimal soil churning, because stem-snap is the principal

mode of treefall and gap formation. As such, disruptions to the forest floor may be

largely restricted to periods of intense, large-scale disturbance when uprooting is more

probable. Canopy gaps in the mature forest were smaller, were of shorter duration, and

appeared less frequently than in old-growth stands. This makes intuitive sense given

that younger forests are comprised of small-canopied trees which, upon death, result

in no gaps (subordinate trees) or gaps of limited size and persistence (see Spies and

others 1990). As forests age, heart rot increasely contributes to gap formation by

weakening or killing trees (Hennon 1995, Hocker 1990). However, it is wind, snowloading, and gravity, alone or in combination, that ultimately brings trees down and

initiates secondary treefall via tree-to-tree collisions.

Catastrophic winds cause large-scale blowdown commonly throughout southeast

Alaska (Deal and others 1991, Harris 1989, Lawford and others 1996). Depending on

intensity, wind can create single-generation stands with uniform canopies or multigeneration stands with diverse canopy and size structures. These catastrophic winds

can affect site productivity through tree uprooting and subsequent soil churning, which

increase soil permeability and nutrient cycling (Bormann and others 1995). Also,

exposure of mineral soil caused by uprooting has direct effects on forest composition,

particularly in facilitating Sitka spruce regeneration (Bormann and others 1995, Deal

and others 1991).

7

Large-Scale Wind

Disturbance
Intensive studies of large-scale wind disturbance have only recently been conducted

within southeast Alaska. Through aerial photointerpretation, wind-generated stands of 2

acres or larger (unless stated otherwise) were identified and described in four locations:

northeast Chichagof Island,4 southeast Chichagof Island5 (1-acre minimum size), Kuiu

Island,6 and Prince of Wales and associated islands (Harris 1989).

Garvey (see footnote 4) examined partial and complete blowdowns on 275,000

acres that included both National Forest System and Huna Totem lands on northeast Chichagof Island. Aerial photos taken before extensive clearcutting were used

to delineate patches of blowdown. Blowdown polygons were described in terms of

age, structure, and remnant cover. Similar protocols were used to collect blowdown

data for 260,000 acres on southeast Chichagof Island (see footnote 5). Kissinger

(see footnote 6) delineated stands originating from partial and complete blowdown as

far back as 500 years. This data set was analyzed and described by Kramer (1997).

Harris (1989) studied partial and complete blowdown patches that originated mostly

from the Thanksgiving Day storm of 1968. Due to inherent differences in objectives,

photointerpretation methods, timing, location, and areas sampled, data from these

four studies are not necessarily equivalent in type or quality; hence, one should view

any summaries and comparisons of these studies with these differences in mind.

These studies indicate that blowdowns in southeast Alaska range widely in size

(1 to 1,000 acres; table 2) and disproportionally occur as smaller patches (typically

<50 acres) as depicted by size distributions on Chichagof Island (fig. 4).

Harris (1989) working on Prince of Wales and associated islands and Kramer (1997)

on Kuiu Island found blowdowns to be concentrated on certain topographic locations

(table 2). Areas prone to blowdown often were south-facing slopes directly exposed to

prevailing winds. Blowdown patches frequently extend onto east- and west-facing slopes

where winds tend to accelerate as they round mountain flanks (Harris 1989). These

areas often support multiple-age stands indicative of repeated disturbance by mediumintensity winds (Kramer 1997). In general, trees growing on topographic features impeding airflow are quite susceptible to blowdown (Harris 1989). In particular, high-intensity

blowdowns frequently occur on exposed areas, often on hilltops and along ridge noses

(Harris 1989). South-southeast gales during fall and winter probably are the primary

cause of blowdown in southeast Alaska (Harris 1989; Ott 1997:chapter 3). Harris (1989)

concluded that areas least prone to large-scale blowdown occur within wind shadows

along north- and northwest-facing slopes or in the lee (northward, in this case) of high

mountains that deflect wind. High-velocity katabatic winds coming off the Coast Mountains during winter also can cause local, large-scale blowdowns along the entire mainland coast (Harris 1989; Ott 1997:chapter 3). At some locations, southeasterly cyclonic

storms and strong katabatic winds from the opposing direction may contribute to

blowdown, such as on northeast Chichagof Island (Ott 1997:chapter 3).

8

_____________________

4 Garvey, T. 1996. Unpublished data and map from Chichagof

Island blowdown study. On file with: Chatham Area Office,

204 Siginaka Way, Sitka, AK 99835.

5 Thomas, J.M.; Shephard, M.; Winn, L.A. [and others]. 1997.

Southeast Chichagof Island landscape analysis (draft). Sitka, AK:

USDA Forest Service Technical Report R10-TP-68.

6 Kissinger, E. 1995. Kuiu Island/Rocky Pass blowdown stands

[GIS polygon map]. Petersburg, AK: Stikine Area, Tongass

National Forest.
Table 2—Characteristics of large-scale wind disturbance from 4 study areas in southeast Alaska

Characteristics Southeast Northeast Prince of Wales and

Chichagof Islanda Chichagof Islandb Kuiu Islandc associated islandsd

Blowdown statistics:

 Number studied 1,118 444 1,886 1,010

 Size range (acres) 1-250 2-770 2-1,000 2-175

 Mean size (acres) 10 35 39 18

 Median size (acres) 5 19 14 NA

Blowdown prone areas:

 Prevailing wind direction (degrees) NA 45 + 135 135-225 135-180

 Topographic position NA NA Ridge noses Valley bottoms, side

and flanks slopes, and rolling terrain

 Aspect (degrees) 45-225 45-225 135-225 90-270

 Elevation (feet) 0-500 0-500 0-600 0-600

 Slope (percent) 20-60 0-40 ≈10-40 60-100

 Soils NA Nonhydric Nonhydric Shallow or highly

productive

 Vegetation type NA Western hemlock Western hemlock Western hemlock /

forests forests Sitka spruce forests

 Net board feet/acre NA 8,000-30,000 21,000-30,000 21,000-30,000

NA = Data not available or reported.

a See footnote 5 in text.

b See footnote 4 in text and Ott 1997.

c Kramer 1997.

d Harris 1989.

9

Forest susceptibility to blowdown may be highest during wet, stormy periods, when

gusty conditions are prevalent and soil stability is reduced by saturation (Foster 1988,

Harris 1989). Other site-specific edaphic and physiological factors that can influence

blowdown susceptibility include rooting characteristics and tree height. In southeast

Alaska, productive western hemlock and western hemlock-Sitka spruce forests are the

least windfirm. These productive forests are particularly susceptible to blowdown

because of large, top-heavy canopies and tall tree heights (Foster 1988, Foster and

Boose 1992, Lohmander and Helles 1987). In contrast, cedar-dominated forests

(western redcedar and Alaska-cedar) growing on wet sites are most windfirm (Harris

1989). The windfirmness of wetland forests, where canopies are rather open and trees

are short and tapered, has been recognized elsewhere (Foster and Boose 1992).
10

Figure 4—Comparative size distributions for harvest and blowdown patches on southeast Chichagof

Island (see footnote 5) and northeast Chichagof Island (see footnote 4).
In contrast to treefall caused by stems snapping, which was found to cause the majority of gaps in the old-growth forests studied (Hocker 1990; Ott 1997:chapter 2), uprooting may be the most prevalent form of treefall in large blowdown patches (Peterson

and Pickett 1991). This concept is consistent with tree-wind mechanics and soil saturation during high-velocity storms (Foster 1988), although specific data are lacking

for perhumid temperate rain forests.

To test the hypothesis that a long-term storm damage gradient exists on Kuiu Island,

Kramer (1997) developed a spatially explicit predictive windthrow model based on four

abiotic factors: topographic exposure to prevailing storm direction, soil stability, slope,

and elevation. From Kissinger’s delineations of large-scale blowdown (see footnote 6),

every 2-acre cell on the forested landscape was labeled as being either windthrown

or non-windthrown. Each 2-acre forested cell was then attributed with ordinal values

generated for each of the four abiotic factors (table 3). Topographic exposure to prevailing storm direction was determined from a modification of the model EXPOSE

developed by Boose and others (1994). The final, best candidate model included all

four abiotic factors, interactions, and second-order terms. The model was validated by

extrapolating the Kuiu model coefficients to nearby Zarembo Island and constructing

an independent large-scale windthrow data set, based on the same methods as used

by Kissinger for Kuiu Island (see footnote 6). The abiotic factors accounted for 72

percent of the variation in large-scale blowdown patches on Zarembo Island.

11

Table 3—Ordinal categories (from lowest to highest) for slope, elevation, soil,

and exposure, used for the predictive windthrow model, southeast Alaska

Ordinal category Soil stability Storm exposure

(increasing value) Slope Elevation (stability class) (by inflection angle)

Percent Feet

1 0 0 0 never exposed

2 1.00 200 1 14

3 2.15 400 2 12

4 4.60 600 3 10

5 10.00 800 4 (highest) 8

6 21.50 1,000 — 6

7 46.40 1,200 — 4

8 100.00 1,400 — 2

9 1,000.00 3,650 — 0

(steepest) (highest) (most exposed)

Source: Kramer 1997.

Long-Term Gradient of

Storm Damage Patterns

on Kuiu Island
12

Figure 5—Topography, vegetation, wind exposure, and windthrow probability for each forested acre on Kuiu

Island, southeast Alaska. Light orange areas have the lowest probability of large-scale windthrow, whereas

dark orange areas have the highest probability of large-scale windthrow. Source: Hunter (in press).
13

A color map showing gradations of windthrow probability was generated from the Kuiu

Island model (fig. 5). Darker colored areas represent those forested sites most likely to

be affected by high-intensity wind. Sites least susceptible to destructive winds were

theorized to be places where small-scale disturbance processes were likely to prevail.

Data from 122 plots in storm-prone and storm-protected landscapes were collected in

1996 to verify the model in the field. Exploratory regressions between the age and size

characteristics of forest plots and the probability of windthrow occurrence suggest that

stands show increasing homogeneity as a function of increasing probability of windthrow.

Forests that completely blow down cycle through stand initiation, stem exclusion, and

understory reinitiation stages sequentially before attaining an old-growth state (ca. 350

years; fig. 1). Kramer (1997) found that on wind-exposed landscapes, storm intervals

seem to be frequent enough to restrict forests to the first three stages of development

(fig. 6). Depending on intensity and prevailing forest conditions, wind events can cause

complete or partial blowdown that give rise to single- or multiple-aged forests, respectively. It is often difficult to distinguish the latter forests from old growth: They differ in

having distinct age groups of trees (cohorts) directly linked to periodic exogenous

disturbance, whereas old-growth forests have continual tree recruitment associated

with endogenous, gap-phase processes (see Oliver and Larson 1996). Many stands

previously designated as “old growth” in southeast Alaska may in fact be multicohort

or very old, single-cohort stands (Deal and others 1991, Harris 1989). On Kuiu Island,

for example, wind-prone landscapes with mainly single-cohort and multicohort stands

comprise about 15 percent of the productive forest land (Kramer 1997).

Figure 6—Disturbance and recovery cycles in temperate rain forests of southeast Alaska that occur on windexposed landscapes where large-scale processes predominate.

Partial

Complete

Disturbance

Recovery

Understory reinitiation

(multicohort structure)

Stem exclusion

Stand initiation
14

Figure 7—Disturbance and recovery cycles in temperate rain forests of southeast Alaska that occur on windprotected landscapes where small-scale gap processes predominate.

For landscapes least prone to large-scale blowdown, Kramer (1997) concluded that the

storm interval is sufficiently long to allow forests to achieve old-growth status (ca. 350+

years) (fig. 7). These areas are akin to the northern hardwoods ecosystem described

by Bormann and Likens (1979) where forests undergo long periods free of catastrophic

disturbance and eventually attain old-growth states where carbon gain and loss are

balanced. On wind-protected landscapes in southeast Alaska, frequent canopy gaps

created by autogenic processes described by Hocker (1990) and Ott (1997:chapter 2)

prevail over extended periods, until interrupted by intense, exogenous disturbances

such as wind storms. As a result, forest canopies are partially or wholly destroyed.

Eventually, these stands cycle back to old-growth conditions, unless preempted by

another large-scale natural disturbance or by timber harvesting. About 35 percent of

the productive forest land on Kuiu Island occurs within landscapes least prone to largescale blowdown (Kramer 1997).

Kramer (1997) concluded that the remaining 50 percent of the forested landscape

(150,000 acres) on Kuiu Island is a mix of both small- and large-scale disturbances

where a variety of forest structures and successional pathways probably exist. Forest

structures representative of wind-protected, intermediate, and wind-exposed landscapes are best characterized by the “Rose,” “Hawk,” and “Yankee Basin” stands of

Deal and others (1991).

Partial

Complete

Disturbance

Recovery

Understory reinitiation

Stem exclusion

Stand initiation

Old growth
Traditional ecological theory, based on stability and equilibrium, has dominated ecological thinking over the last century (Glenn-Lewin and others 1992, Pahl-Wostl 1995, Reice

1994) and seems to have profoundly influenced early research in the temperate rain

forests of southeast Alaska. With fire and human disturbance (prior to European arrival)

relatively minor, the idea that climax, old-growth conditions prevailed over the forested

landscapes of southeast Alaska went undisputed. So pervasive was the stability and

equilibrium mindset that even recent vegetation classification adopted the plant association approach in southeast Alaska (Martin 1989), whereby succession is considered

orderly and deterministic and proceeds to stable, self-perpetuating communities

(Clements 1936). Under this model, disturbance is simply an element that “resets”

succession to an earlier state. Contrary to this view, there is evidence that disturbance

plays an important, multifaceted role in forest dynamics and succession (Christensen

1989, Pickett and White 1985), a role that varies across the landscape (Foster and

Boose 1992). A disturbance continuum driving forest processes exists over southeast

Alaska (Kramer 1997), spanning landscapes ranging from those buffeted by recurrent

large-scale wind disturbance (fig. 6) to those protected from catastrophic wind where

autogenic, gap-phase processes operate (fig. 7).

This new paradigm raises questions concerning data interpretations of previous ecological studies; for instance, how representative were previous studies in collecting data

that actually captured steady-state “climax” conditions for plant association classification

and old-growth modeling? Were multicohort stands often used unknowingly to characterize successional endpoints or to quantify old-growth conditions? In the future, sites

should be evaluated for principal modes of disturbance (large vs. small scale) through

field reconnaissance and tree-age data before full-scale research or data analysis is

conducted. Indeed, tree-age data are essential for understanding forest dynamics and

developing robust, process-based, old-growth definitions in the future (Blozan 1994,

Hayward 1991, Nowacki and Abrams 1994, Roovers and Rebertus 1993).

Current timber practices in southeast Alaska favor large-scale clearcutting at ca. 100-

year rotations. Important ecological implications arise when this cutting regime occurs

on wind-protected landscapes where small-scale gap processes prevail. Under this

type of management, tree canopies are completely removed in contiguous blocks

(averaging 40 to 60 acres) and forest development is restricted to the two earliest

stages: stand initiation and stem exclusion (fig. 1). These practices are in marked

contrast to the natural disturbance regime under which these forests evolved and

attendant species adapted. Clearcutting greatly reduces structural diversity and eliminates large-tree habitat for wildlife species (Hanley 1991, Parker and others 1996).

Although some intense blowdowns result in complete overstory destruction, most

cause only a partial loss of overstory trees and considerable forest legacy is retained

in standing snags and down woody material. Large, simply configured clearcuts reduce

landscape diversity, sever forested wildlife corridors, and curtail gene flow as well as

fragmenting, isolating, and reducing old-growth patches (Franklin and Forman 1987).

These effects may be most pronounced in areas already consisting of highly fragmented forest mosaics (Kramer 1997). The short cycling of forest development through

short-rotation clearcutting prevents structural characteristics associated with older trees

Management

Considerations

15

A New Paradigm of

Forest Dynamics
16

from occurring and may eliminate certain understory species (Alaback 1982, Halpern

and Spies 1995). Many understory plants invest considerable energy in their root

systems, adapting vegetative strategies for reproduction rather than seeding (Alaback

1982). Due to intense shading and resource monopolization by trees during the stem

exclusion stage, understory species often drop out of stands under short-rotation

management. They may be extirpated from these sites when development of stands

into understory reinitiation and old-growth stages does not proceed. To date, thinning

of young-growth stands has not effectively maintained understory shrubs and herbs

important to various wildlife species (Alaback and Tappeiner 1991, Deal and Farr 1994).

Clearcuts contrast most sharply with the natural disturbance regime when they occur on

wind-protected landscapes where large-scale disturbance is infrequent. Large clearcuts

and the resulting single-cohort structure greatly reduce two types of disease—heart rot

and hemlock dwarf mistletoe (Arceuthobium tsugense (Rosendahl) G.N. Jones)—both

of which appear to play key roles in the small-scale gap processes in old-growth systems.7 On wind-prone landscapes, clearcut harvest differs physically from postblowdown

conditions through woody biomass removal (both standing and down debris) and lack of

deep soil churning. On certain sites, the latter may lead to possible paludification

(waterlogging) through the formation of soil layers impermeable to water (Bormann and

others 1995).

Partial forest harvests, where the more valuable timber trees were usually removed,

predominated early logging activities in southeast Alaska (Harris and others 1974).

These operations shifted abruptly in the 1950s, when emphasis was placed on largescale timber removal using clearcutting. The ecological effects of this silvicultural

method can be evaluated by comparing it to large-scale wind disturbances, its closest

natural analogue. Incongruities between clearcut harvests and wind disturbance patterns are given below.

Landscape disturbance patterns—Clearcutting practices rarely emulate prevailing

wind disturbance patterns and processes in southeast Alaska. For example, on Kuiu

Island more than 50 percent of the timber harvest units are in areas where small-scale

gap processes are likely to predominate (Kramer 1997). Timber harvest in these areas

has an additive effect; that is, old-growth stands are converted to young-growth stands

that may not develop beyond the stem exclusion stage under planned cutting schedules.

These early developmental stages are historically infrequent on such landscapes. In

contrast, only 20 percent (5,000 acres) of the timber harvest units on Kuiu Island occurred in areas where large-scale blowdowns are likely to predominate.

Patch size and configuration—The size, distribution, and shape of past clearcut

units differ considerably from blowdowns. Clearcut size contrasts with that of natural

blowdowns by having a normal distribution that peaks around 45 to 50 acres (fig. 4).

Most blowdowns are small (≤ 50 acres) and scattered. In general, clearcut patches

were normally larger and more geometric relative to their natural wind-generated

counterparts (Kramer 1997).

Harvesting and the

Natural Disturbance

Regime

_____________________

7 Personal communication. 1997. P. Hennon, research

forest pathologist, Forest Sciences Laboratory, Juneau,

AK 99801.
Residual biomass—Clearcutting leaves less remnant tree cover than does most

blowdown. Clearcutting removes most trees from a site, whereas a wide range of structural conditions exist following blowdown events. Only occasionally are most standing

trees eliminated from a site by wind disturbance (fig. 8). Immediately after disturbance

and during the stem exclusion stage of stand development, remnant trees may serve

several important functions, such as seed sources for regeneration, habitat structures

for animals, and shade for light-sensitive understory plants (Halpern and Spies 1995).

Large-scale wind disturbance creates considerable large woody material, whereas

clearcutting and yarding removes most of it from the site.

Soil mixing—Uprooting associated with wind disturbance may have important ecological benefits for maintaining forest productivity (Bormann and others 1995). In contrast

to wind-generated disturbances, tree yarding associated with clearcuts removes large

woody material and normally results in limited soil disturbance confined primarily to the

surface. With most blowdowns, uprooting causes considerable soil disturbance at lower

depths. Bormann and others (1995) surmised that soil churning caused by uprooted

trees during intense wind storms may be critical for maintaining site productivity. Soil

churning breaks apart impermeable horizons that may otherwise lead to paludification

and can enrich soil through mechanical mixing of organic and mineral horizons. Possible

disruptions to disturbance-productivity processes by clearcutting are most probable on

landscapes where periodic, catastrophic wind disturbance and uprooting prevail.

17

Figure 8—Number of blow downs by remnant-cover class on southeast Chichagof Island

(see footnote 5) and northeast Chichagof Island (see footnote 4).

0

50

100

150

200

1-10 11-30 31-50 51+

Remnant tree cover (percent)

Number

Northeast Chichagof Island

N = 394

Midpoint mean = 17%

Midpoint median = 15%

Midpoint mode = 5%

0

100

200

300

400

500

600

0-10 11-25 26-50 51+

Remnant tree cover (percent)

Southeast Chichagof Island

N = 1047

Midpoint mean = 15%

Midpoint median = 5%

Midpoint mode = 5%

Number
Disturbance frequency rate comparisons—Analysis of blowdown patches on northeast Chichagof Island (see footnote 4) indicates that tree regeneration related to wind

disturbance has been relatively constant since the 1700s (fig. 9). One notable spike

occurred in the 1880s, which affected about 3 percent of the productive forest land.

A definite shift away from this regime of limited stand-replacement occurred with the

onset of widespread clearcutting (1960s), with up to 9 percent of the productive forest

land being harvested between 1980 and 1990. The area clearcut has exceeded the

area that blew over by some 4 to 17 times on Chichagof Island (table 4). Similar trends

are evident on Prince of Wales and Kuiu Islands, where clearcutting has proceeded at

a rate four and nine times, respectively, of that recorded for high-intensity or complete

blowdown (table 4). The disparity between blowdown and clearcut acres is of particular

significance on Prince of Wales Island considering that the timeframe used to calculate

blowdown acreage spans a time of major large-scale disturbance in 1968. Calculating

actual blowdown acreages is difficult because logging may have masked the true extent

of blowdown (Harris 1989). In addition, reoccurring windthrow events may have affected

some sites by obscuring earlier events (Kramer 1997). Owing to the magnitude of rate

differences between wind and clearcutting, however, the general trends indicated by the

data are considered reasonable.

18

Figure 9—A frequency distribution of wind- and harvest-regenerated areas by decade across all land ownerships on northeast Chichagof Island

(see footnote 4).
Table 4—Comparative harvest and blowdown statistics for productive forest land from 4 studies in

southeast Alaska

Ratio of

Analysis period Area affected clearcutting to

Disturbance type by island or length Productive forest land per century blowdown

Acres Percent Acres

Southeast Chichagof Island: a 150,897 100.0

Clearcut ~100 yr 21,569 14.3 21,569

Clearcut ~30 yr 19,918 13.2 66,393

Blowdown ~300 yr 11,844 7.8 3,948 5:1 and 17:1

Northeast Chichagof Island: b 166,625 100.0

Clearcut 1900 to 1996 23,577 14.1 24,559

Blowdown 1700 to 1996 15,492 9.3 5,234 5:1

Blowdown 1800 to 1996 11,230 6.7 5,730 4:1

Blowdown 1900 to 1996 2,064 1.2 2,150 11:1

Kuiu Island: c 320,000 100.0

Clearcut ~last 100 yr 26,000 8.1 26,000

High-intensity blowdown ~last 500 yr 14,000 4.4 2,800 9:1

Medium- and high-intensity blowdown ~last 500 yr 63,000 19.7 12,600 2:1

Prince of Wales and associated islands: d 1,062,462 e 100.0

Clearcut 1980-1995 63,164 5.9 421,093

 1,295,697 f 100.0

Complete blowdown 1961-1972 9,418 0.7 94,180 4:1

Partial and complete blowdown 1961-1972 18,537 1.4 185,370 2:1

a See footnote 5 in text.

b See footnote 4 in text.

c See footnote 6 in text and Kramer 1997.

d Productive forest land acreage differs because separate analyses were conducted; total acreages for blowdown statistics from

Harris (1989) and clearcut statistics from Tongass land management plan database.

e Tongass land management plan database.

f

 Harris 1989.

19

To adequately assess the effects of past and current harvest practices on ecological

systems, the forest should be evaluated within the context of the natural disturbance

regime (Walker and others 1996). Theoretically, the response and resiliency of constituent species have been at least somewhat shaped by the prevailing disturbance regime

(Leibowitz and others 1992). For example, systems that have evolved with periodic,

intense disturbance should possess inherent qualities that allow harvesting of similar

frequency, intensity, and magnitude (Walker and others 1996). Long-term management

practices that are consistent with past environmental conditions thus may help to

maintain the ecological integrity of the land and provide for future management options.

Effects on animal and plant communities may increase as spatial and temporal disparity

between natural and human disturbance regimes widen.

Emulating Natural

Disturbance Regimes
20

The amount of forest in the later stages of stand development will be noticeably reduced if current harvest trends continue in southeast Alaska. Placing greater emphasis

on single-tree or small-group selection harvesting in areas where gap-phase forests

occur would serve to more closely emulate natural processes. One tradeoff, however,

is that more frequent entries over larger areas and additional roads would be needed

to maintain a set harvest volume.

The scale of events contrasts markedly between current harvesting and the natural

disturbance regime in southeast Alaska (fig. 9; table 4). Natural disturbance frequency

decreases sharply as patch size increases. Larger blowdown patches on Kuiu Island

often resulted from multiple wind events occurring at different times (Kramer 1997). In

this context, intense, large-scale canopy disturbances are relatively infrequent events

that occur periodically. The relatively frequent harvest of large-sized patches over a

continuous period departs substantially from the natural disturbance regime. On windprone landscapes, repetitive harvest entries may convert the disturbance regime from

a phased to an unphased condition, and thus may have unexpected impacts on species

diversity (Abugov 1982). Relatively short harvest rotations, moreover, effectively eliminate windthrow from occurring where it otherwise might, which may reduce site productivity through increased surface-organic accretion and potential waterlogging (Bormann

and others 1995).

If managers wish to mimic natural disturbance regimes, then they could apply the

following guidelines:

• Practice limited uneven-aged management through single-tree or small-group

selections in areas where gap-phase processes predominate (wind-protected

landscapes). This approach requires preharvest survey data to accurately identify

such sites.

• Leave remnant trees within harvest units in densities representing a range of

conditions consistent with natural blowdown (fig. 8).

• Decrease the size of clearcut units and the frequency of harvest entries, and locate

clearcut units in wind-exposed landscapes where natural stand replacement events

are common.

• Extend rotation lengths of clearcut units to allow the later forest stages to develop;

avoid restricting managed stands to only the stand-initiation and stem-exclusion stages.

• Give consideration to topographic locations most susceptible to recurrent standreplacement episodes when prescribing clearcuts. These locations are generally

south-facing slopes, hilltops, and ridge noses. In a harvesting system designed to

emulate natural processes, north-facing slopes and other wind-protected locales

seem optimal for selection or partial harvests. Because emphasis is placed on oldgrowth-forest conditions for maintaining species viability, wind-protected landscapes

harbor the best prospects for locating concentrations of old-growth patches for longterm protection (i.e., old-growth reserves).
The authors thank Robert Ott (University of Alaska-Fairbanks), Everett Kissinger (soil

scientist, Tongass National Forest, Stikine Area), Tim Garvey (GIS coordinator, Tongass

National Forest, Chatham Area), and Lisa Winn and Kent Barkhau (foresters, Tongass

National Forest, Sitka Ranger District) for allowing access to data sets, preliminary

findings, and maps and for their support through stimulating conversations and critiques

of early drafts. We also thank Al Harris, Bernard Bormann, Bill Pawuk, Kent Julin, Terry

Shaw, Paul Hennon, and Kim Hastings for reviewing early drafts.

Abugov, R. 1992. Species diversity and phasing of disturbance. Ecology. 63: 289-293.

Agee, J.K. 1993. Fire ecology of Pacific Northwest forests. Washington, DC: Island

Press. 493 p.

Alaback, P.B. 1982. Dynamics of understory biomass in Sitka spruce-western hemlock

forests of southeast Alaska. Ecology. 63: 1932-1948.

Alaback, P.B. 1988. Endless battles, verdant survivors. Natural History 8: 45-48.

Alaback, P.B.; Tappeiner, J.C., II. 1991. Response of western hemlock (Tsuga

heterophylla) and early huckleberry (Vaccinium ovalifolium) seedlings to forest

windthrow. Canadian Journal of Forest Research. 21: 534-539.

Alverson, W.S.; Kuhlmann, W.; Waller, D.M. 1994. Wild forests: conservation biology

and public policy. Washington, DC: Island Press. 300 p.

Attiwill, P.M. 1994. The disturbance of forest ecosystems: the ecological basis for

conservative management. Forest Ecology and Management. 63: 247-300.

Barden, L.S. 1980. Tree replacement in a cove hardwood forest of the southern Appalachians. Oikos. 35: 16-19.

Barden, L.S. 1981. Forest development in canopy gaps of a diverse hardwood forest of

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Blozan, W. 1994. The importance of increment core samples and disturbance history in

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Boose, E.R.; Foster, D.R.; Fluet, M. 1994. Hurricanes and landscape-level disturbance

in tropical and temperate forests. Ecological Monographs. 64: 369-400.

Bormann, B.T.; Spaltenstein, H.; McClellan, M.H. [and others]. 1995. Rapid soil

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21

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Pacific Northwest Research Station

333 S.W. First Avenue

P.O. Box 3890

Portland, Oregon 97208-3890

Nowacki, Gregory J.; Kramer, Marc G. 1998. The effects of wind disturbance on

temperate rain forest structure and dynamics of southeast Alaska. Gen. Tech. Rep.

PNW-GTR-421. Portland, OR: U.S. Department of Agriculture, Forest Service,

Pacific Northwest Research Station. 25 p. (Shaw, Charles G., III, tech. coord.;

Julin, Kent R., ed.; Conservation and resource assessments for the Tongass land

management plan revision).

Wind disturbance plays a fundamental role in shaping forest dynamics in southeast

Alaska. Recent studies have increased our appreciation for the effects of wind at

both large and small scales. Current thinking is that wind disturbance characteristics

change over a continuum dependent on landscape features (e.g., exposure, landscape

position, topography). Data modeling has revealed the existence of distinct wind

disturbance regimes, grading from exposed landscapes where recurrent, large-scale

wind events prevail to wind-protected landscapes where small-scale canopy gaps

predominate. Emulating natural disturbances offers a way to design future management

plans and silvicultural prescriptions consistent with prevailing ecological conditions.

Keywords: Tongass National Forest, old growth, forest development, small-scale canopy

gaps, large-scale catastrophic blowdown, predictive windthrow model, silviculture.
U.S. Department of Agriculture

Pacific Northwest Research Station

333 S.W. First Avenue

P.O. Box 3890

Portland, OR 97208

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