United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical

Report

PNW-GTR-767

November 2008

A Key for Predicting Postfire

Successional Trajectories in

Black Spruce Stands of Interior

Alaska

Jill F. Johnstone, Teresa N. Hollingsworth, and F. Stuart Chapin III
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Authors

Jill F. Johnstone is a plant ecologist and assistant professor, Department of Biology, University of Saskatchewan, Saskatoon, SK, S7N 5E2 Canada, and a senior

investigator for the Bonanza Creek Long Term Ecological Research Program (BNZ

LTER). Teresa N. Hollingsworth is a vegetation and community research ecologist, and a senior investigator, BNZ LTER, Boreal Ecology Cooperative Research

Unit, Pacific Northwest Research Station, P.O. Box 756780, University of Alaska

Fairbanks, Fairbanks AK 99775. F. Stuart Chapin III is a professor of ecology and

the principal investigator with the BNZ LTER, Department of Biology and Wildlife

and Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK

99775.

Produced as part of the JFSP Project, Managing Fire with Fire in Alaskan Black

Spruce Forests
Abstract

Johnstone, Jill F.; Hollingsworth, Teresa N.; Chapin, F. Stuart, III. 2008. A key

for predicting postfire successional trajectories in black spruce stands of interior

Alaska. Gen. Tech. Rep. PNW-GTR-767. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research Station. 37 p.

Black spruce (Picea mariana (Mill) B.S.P) is the dominant forest cover type in

interior Alaska and is prone to frequent, stand-replacing wildfires. Through impacts on tree recruitment, the degree of fire consumption of soil organic layers can

act as an important determinant of whether black spruce forests regenerate to a forest composition similar to the prefire forest, or to a new forest composition dominated by deciduous hardwoods. Here we present a simple, rule-based framework for

predicting fire-initiated changes in forest cover within Alaska’s black spruce forests.

Four components are presented: (1) a key to classifying potential site moisture, (2) a

summary of conditions that favor black spruce self-replacement, (3) a key to predicting postfire forest recovery in recently burned stands, and (4) an appendix of

photos to be used as a visual reference tool. This report should be useful to managers in designing fire management actions and predicting the effects of recent and

future fires on postfire forest cover in black spruce forests of interior Alaska.

Keywords: Succession, fire, disturbance, boreal, regeneration.
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Introduction

Background

Black spruce (Picea mariana (Mill) B.S.P.) is the dominant forest cover type in

interior Alaska, and stand-replacing wildfires are the main form of disturbance in

these forests. Fires create important subsistence and economic forest uses and

generate crucial habitat for many wildlife species (Chapin et al. 2003) but can also

pose a danger to human populations and values. Land managers in Alaska are faced

with the challenge of managing fire in a way that preserves human life and property while at the same time conserving the key ecological processes driven by fire.

Because managers have little capacity to alter fire behavior by direct fuel manipulations across the large extent of Alaskan boreal forest, the ability to use and interpret

natural variations in fire processes provides an important contribution to long-term

fuel management in the region.

Fire and successional processes in black spruce forests have traditionally been

viewed as a fairly simple cycle of spruce self-replacement, with fire causing few

changes in the pre- versus postfire vegetation composition (e.g., Van Cleve et al.

1991). However, a recognition of current changes in fire regime in Alaska, and

expected future changes with climate warming, have led to a reevaluation of the

sensitivity of black spruce forests to changes in fire conditions. Research on postfire

recruitment across many parts of the boreal forest indicate that the degree of fire

consumption of soil organic layers has important impacts on patterns of postfire

tree recruitment (e.g., Charron and Greene 2002, Chrosciewicz 1974, Greene et al.

2005, Johnstone and Chapin 2006, Zasada et al. 1983). In Alaska, postfire organic

layer depth acts as an important determinant of whether black spruce forests

regenerate to a forest composition similar to the prefire forest or regenerate to a

new forest composition dominated in early succession by deciduous hardwoods

(Johnstone and Chapin 2006, Johnstone and Kasischke 2005). It is this variation

of residual organic depths, along with other prefire environmental site characteristics, that determines the landscape mosaic of stand types postfire. Under current

and future fire regimes, it is possible that increased burn severity and combustion

of soil organic layers could lead to altered successional pathways in black spruce

forests of interior Alaska.

Increases in deciduous tree cover on sites previously dominated by black spruce

are important because deciduous- and spruce-dominated forests differ dramatically

in their fuel types and potential fire behavior. Black spruce stands represent a
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GENERAL TECHNICAL REPORT PNW-GTR-767

highly flammable configuration of fuels because of the large amounts of fine twigs

and needles, high resin content, low moisture content, and large quantities of fine

ground fuels that are connected to the canopy by low-lying tree branches. Stands

with increasing components of hardwoods are generally less flammable because

of the increased moisture content of the fine fuels and reduced ground and ladder

fuels (Hély et al. 2000, Johnson 1992, NIFC 1992). The potential for deciduous

stands to act as fire breaks makes transitions from conifer- to deciduous-dominated

stands one of the main goals for fire hazard reduction in boreal forests. Thus,

understanding the factors that can shift black spruce forests to alternative successional trajectories, such as mixed deciduous forests, is a key element in developing

adaptive fire management in Alaska and in predicting how changing fire regimes

may shape future forest cover in this region.

To address these issues, we have developed a simple, rule-based key that uses

our current knowledge of successional processes in black spruce forests to predict

future forest composition as a function of prefire vegetation, site moisture, and fire

conditions. Although this key builds off decades of research on fire and succession

in boreal forests, it is particularly informed by recent research on early postfire

recovery across a range of black spruce sites burned by the widespread fires of

2004 in Alaska. This research, funded by the Joint Fire Science Program, has

focused specifically on the effects of fire severity and site conditions in driving

successional trajectories after fire in black spruce forests. The key that follows

distills our research results into a predictive framework that captures what appear

to be the major factors controlling postfire tree recruitment and subsequent stand

development. Further details on the research in the 2004 burns can be found in the

project final report (project 05-1-2-06) available on the Joint Fire Science Program

Web site (http://www.firescience.gov/).

Details of the Report

The aim of this report is to provide a rule-based, predictive framework for estimating fire-initiated changes in forest cover within Alaska’s black spruce forests. This

framework can be used to (a) design fire management actions that promote a desired forest cover type, (b) rapidly assess the effects of recent fires on postfire forest recovery, and (c) provide a general set of rules to estimate forest responses to

future changes in fire regime. The framework is specifically intended for use in

black spruce forests of interior Alaska, but may be generally applicable to other

regions of the boreal forest.
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

The report is arranged as a four-part workbook. First, we present a field-based

system for classifying potential site moisture. Site moisture is used to represent

variations in moisture availability that influence successional patterns. Second,

we provide a summary of the general conditions that favor a typical spruce selfreplacement trajectory versus alternative successional trajectories. This summary

can be used to develop guidelines for fuel or fire management actions aimed at

influencing patterns of postfire recovery. Third, we present a key to predicting

postfire forest recovery in recently burned stands. This key is intended to be used

for ground-based assessments during the initial 1 to 2 years after a fire to predict

future successional trajectories. It can be used as a tool for predicting the impacts

of wildfires or prescribed burns on future forest composition. Finally, an appendix

of photographs (app. B) provides a visual reference to different fire conditions and

successional trajectories that are referred to in the text.

The summaries and keys in this report are focused on processes acting at the

scale of the forest stand, which we intuitively define as a patch of forest that can be

easily viewed from a central point (about 0.1 to 1.0 ha). Larger, landscape-level

patterns may be at least partially inferred from the aggregation of multiple forest

stands, although effects of other large-scale processes should also be considered in

the scaling process.

Materials required for making the field assessments described here include (a) a

copy of the report, (b) a calibrated steel rod for measuring depth of thaw, (c) a soil

corer or spade for obtaining mineral soil samples, and (d) a camera for documenting canopy and surface conditions (recommended).

Defining Site Moisture

For the purpose of this key, site moisture can be defined as the potential moisture

available for plant growth. We are particularly interested in soil moisture near the

surface, because this is the soil layer that most strongly influences seedling establishment. Our classification of site moisture is based primarily on the topographic

drainage at a given site. We then modify our initial classification based on factors

that control drainage within the soil (permafrost and soil texture). Therefore, a

sloping site on fine-textured soils with shallow permafrost might have a similar

site moisture as a flat site with coarse, sandy soils or no permafrost. Our classification system is based on the topographic system of Komárková (1983), and on work

by J. Harden and K. Manies at the U.S. Geological Survey (unpublished) describing site drainage classification in interior Alaska. The steps required to classify site

moisture are described in text, below, and as a pictograph in figure 1.
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GENERAL TECHNICAL REPORT PNW-GTR-767

STEP 1: Determine the site moisture based on topography (modified from

Komárková 1983).

Dry sites:

• Xeric—little surface moisture: soil surface is perpetually dry except

immediately after precipitation. Typical sites are stabilized sand dunes and

convex ridgetops.

• Subxeric—some noticeable surface moisture: the soil surface is generally

dry but can remain moist for a few days after a precipitation event. Typical

sites are well-drained slopes or flat ridges.

• Subxeric to mesic—very noticeable surface moisture: the soil surface remains moist for many days after precipitation before drying out and has

some recharge of moisture from lateral water flow. Typical sites are level

to gently sloping, flat terrain.

Figure 1—Pictographic key to classifying the potential site moisture of black spruce stands in interior

Alaska.
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Moist sites:

• Mesic—moderate surface moisture: the soil surface is often moist (not dry

and crumbly), with reliable moisture recharge from lateral waterflow.

Typical sites are flat or shallow depressions, including toeslopes.

• Mesic to subhygric—considerable surface moisture: the soil surface is

almost always moist, with persistent moisture recharge from surrounding

areas. Typical sites are depressions or concave toeslopes.

• Subhygric—very considerable surface moisture: surface soil is typically

saturated with free water, often with small (< 5 percent of area) patches of

standing water. Lateral drainage is generally impeded. Typical sites are

lowland depressions with the water table near the surface, often adjacent to

hygric wetlands.

STEP 2: Does the site have extensive shallow permafrost?

If there is shallow permafrost, then the site becomes one classification wetter. For

example, a site classified as subxeric to mesic based on topography, would be reclassified as mesic if it is underlain by shallow (near-surface) permafrost. Our

operational definition of shallow permafrost is a site where the maximum depth

of thaw is less than 75 cm below the soil or moss surface. The presence of nearsurface permafrost inhibits soil drainage by creating a water-impermeable layer

near the soil surface.

To assess the importance of shallow permafrost at a site, it is usually necessary

to visit a site late in the growing season (August and September) to assess depth of

thaw. The depth of thaw, or distance to frozen ground, can be measured by inserting a calibrated steel rod into the ground as far as possible. The presence of frozen

soil or rocks will block further penetration of the rod. Carefully differentiate rocks

from permafrost when making this assessment.

STEP 3: What is the soil texture of the site?

If the soil is very coarse in texture, then the site becomes one classification drier.

For example, a site classified as subxeric to mesic based on topography would be

reclassified as subxeric if the predominant soil texture is coarse. We define coarsetextured soils as those dominated by sand or gravel in at least the upper 50 cm of

the soil profile, starting at the ground surface (but excluding any green moss).

Coarse soil texture tends to increase soil drainage and decrease site moisture by

allowing surface moisture to rapidly drain to deeper soil layers (fig. 1).
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GENERAL TECHNICAL REPORT PNW-GTR-767

Factors Predicting Shift in Successional Trajectory

Background

In most situations, we expect black spruce forests (app. photo 1) to regenerate

after fire to a stand composition and density that is similar to the prefire community (Van Cleve et al. 1991, Van Cleve and Viereck 1981, Viereck 1983). There are

two factors that are particularly important in supporting stand self-replacement as

the dominant successional pattern in Alaskan black spruce forests:

• The formation of a moss- or lichen-dominated understory (app. photos 13

and 14) leads to the accumulation of thick (>10 cm) organic layers at the

soil surface; the poor combustion of those layers during a fire leads to a

postfire seedbed that is dominated by organic substrates.

• The development of aerial seedbanks that are held in the semiserotinous

cones of black spruce (app. photo 2), leading to a large supply of onsite

seed for black spruce after fire.

Thick, postfire organic seedbeds (>5 cm) are usually porous and subject to

rapid drying and often form poor-quality seedbeds for the establishment of tree

seedlings (Johnstone and Chapin 2006). However, black spruce can partially overcome this limitation by producing abundant seed after fire from an aerial seedbank held in semiserotinous cones. Thus, even though the success rate of seedling

establishment is low, black spruce trees are able to compensate by having a strong

local seed source that provides lots of seed. This large number of seeds increases

the likelihood that some seeds will land in microsites suitable for germination and

growth. Consequently, the combination of organic seedbeds and an aerial seedbank

gives black spruce a strong advantage in regenerating in sites after fire.

Some potentially co-habitating species (app. photo 5), such as birch (Betula

neoalaskana Sarg; formerly B. papyifera Marsh) (app. photo 6) or aspen (Populus

tremuloides Michx.) (app. photo 7), have well-distributed seeds but also very small

seed sizes. These small seeds have fewer reserves to support early seedling root

growth and consequently have a particularly poor ability to successfully establish

on porous organic seedbeds (Greene et al. 2007, Johnstone and Chapin 2006).

Other Alaskan conifers, such as white spruce (Picea glauca (Moench)Voss) (app.

photo 3), have seeds that are similar in size to those of black spruce, but lack the

serotinous seed habit that provides a large supply of onsite seeds (app. photo 4).
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Nevertheless, exceptions to the general pattern of black spruce self-replacement

after fire do occur, and these may be widespread under certain conditions. The

potential to initiate stands with alternative successional trajectories is largely driven

by interactions between the prefire vegetation composition, site conditions, and fire

effects on the reproductive potential of alternate dominant species. However, species interactions such as herbivory and plant competition with understory shrubs

may also alter the direction and timing of successional transitions. Below, we list

the scenarios likely to result in black spruce stands shifting to an alternative successional trajectory following fire. This is not an exhaustive list, but represents the

most likely scenarios of change based on our current understanding of successional

processes in black spruce forests of interior Alaska.

Key to conditions that favor the formation of alternative successional

trajectories in black spruce forests after fire:

1. Substantial combustion of the surface organic layers leads to exposed

mineral soil or deep humic layers. These seedbeds provide a more stable

moisture supply to support seedling recruitment. Deciduous trees can invade

by seed, even at sites many kilometers distant from live seed sources.

a. Deciduous invasion on postfire humic layers or moist organic

soil is expected to lead to deciduous trees that grow relatively

slowly and do not strongly impact the recruitment of understory

spruce. Postfire succession will have an increased deciduous

component, but the deciduous phase may functionally resemble

the willow-dominated phases of spruce self-replacement.

b. Deciduous tree growth and productivity is expected to be highest

at sites with widespread exposure of mineral soil (>30 percent)

or very shallow organics (<3 cm). Where deciduous species

invade and grow rapidly, successful recruitment of black spruce

may be reduced. Postfire succession is likely to lead to a

prolonged deciduous-dominated phase (lasting on the order of 50

to 80 years postfire) that succeeds to low-density, mature black

spruce in later stages. High rates of herbivory may change the

likelihood or timing of transitions between deciduous and conifer

phases by reducing the productivity of deciduous trees (browsing

by moose, Alces alces) or young conifers in the understory

(browsing by snowshoe hares, Lepus americanus).
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GENERAL TECHNICAL REPORT PNW-GTR-767

2. Deciduous tree species are present in the prefire stand. When present and

abundant (>50 stems/ha for aspen, >200 stems/ha for birch), aspen and birch

are able to asexually regenerate after fire and give rise to a rapidly growing,

early deciduous phase. Asexual regeneration of aspen and birch is likely to be

favored by moderate- to low-severity fires that are less likely to damage the

sprouting potential of roots and tree bases. Rapid growth and abundant litter

production of resprouting deciduous trees may reduce the potential for spruce

establishment at well-drained sites. Postfire succession is likely to lead to a

prolonged deciduous-dominated phase (lasting on the order of 60 to 90 years

postfire) with succession to low-density, mature black spruce in later stages. As

stated above, high rates of herbivory may change the likelihood or timing of

transitions between deciduous and conifer phases, by reducing the productivity

of deciduous trees (moose), or young conifers in the understory (hares).

3. Adjacent stands of unburned white spruce provide a strong source of white

spruce seed into the burned black spruce stand. Where white spruce seed is

abundant, it can recruit on organic surfaces at a level similar to black spruce.

White spruce seed availability will be highest where burned stands are adjacent

to live white spruce stands, or when a late-season fire allows for the dispersal

of viable seed from burned white spruce. In both cases, spatial proximity to a

white spruce seed source is a major factor determining whether white spruce

can become co-dominant with black spruce in the postfire stand.

4. Local seed for black spruce is reduced by fire effects (trees fallen, cones

burned, cones not open, or young trees burned after a short fire interval).

Because all of these factors reduce the availability or dispersal of black spruce

seeds from an aerial seedbank, they reduce the potential for black spruce regeneration. In these cases, postfire succession will have reduced black spruce

density.

a. If postfire seedbeds are dominated by organic soils, succession

will proceed to a prolonged shrub- or graminoid-dominated phase

with the eventual development of a very open canopy of black

spruce.

b. If postfire seedbeds are dominated by mineral soils, deciduous

species are likely to successfully establish and lead to the development of deciduous stands with only a sparse or absent

component of understory spruce. Return to black spruce may

not occur before the stand burns again.
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Dichotomous Key for Predicting Successional

Trajectory (0 to 2 Years Postfire)

Initial key: Site factors driving recruitment potential

1. Site is moist (mesic to subhygric) with the upper duff layer largely intact and

low exposure (<5 percent) of humic or mineral soils (app. photos 24 to 27).

2. Site is moist (mesic to subhygric) and the organic layer has been severely

burned, leading to shallow (<5 cm average depth) residual organic layers and

at least moderate (> 5 percent) ex-posure of humic or mineral soil (app. photos

28 to 31).

3. Site is dry (mesic/subxeric to xeric) with an intact organic layer and low (<15

percent) mineral soil exposure (app. photos 32 to 34).

4. Site is dry (mesic/subxeric to xeric) with extensive (>15 percent) mineral soil

exposure (app. photos 35 to 37).

1. Moist sites (mesic to subhygric) with postfire organic layers intact. These

sites may experience a range of conifer recruitment, depending on the availability of local conifer seed. Where seed is abundant, wetter sites will have

higher densities of conifer seedlings. For deciduous species, the potential for

seedling recruitment is generally low on thick organic layers. If deciduous

trees were present in the prefire stand, some recruitment may occur through

resprouting. However, the cool and moist soils are likely to keep the biomass

or cover of deciduous trees low in all stages of succession. Consequently, most

sites of this type will rapidly return to dominance by black spruce.

IA. There are multiple standing black spruce trees with open cones at

the site providing a strong local seed source.

I. The residual organic layer is thick (>20 cm), with abundant

scorched tussocks (app. photo 8) or sphagnum moss (app.

photo 10) . Thick moss layers may prevent the penetration of

seedling roots to a stable moisture supply at sites without

freely available moisture. Thick moss also favors cold soil

temperatures that are likely to limit seedling growth. The

expected successional tra-jectory is open-canopy black

spruce (app. photo 15).
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GENERAL TECHNICAL REPORT PNW-GTR-767

II. The residual organic layer is less than 20 cm thick, and is

composed primarily of burned or scorched feathermoss

(app. photo 11) rather than sphagnum. Thinner moss layers

combined with moist site conditions allow higher rates of

black spruce establishment. The expected successional

trajectory is intermediate- to closed-canopy black spruce

(app. photo 16).

IB. The local seed source for black spruce is limited by most trees

having fallen over, trees being too young to bear many cones, or

the cones being either deeply charred (severe crown fire) or not

open (fire did not crown). Consequently, seedling establishment is

likely to be very low, leading to the formation of open-canopy

black spruce, or spruce-lichen woodland (app. photo 17).

2. Moist sites (mesic to subhygric) with shallow surface organic layers or

exposed mineral soil. These sites generally have a high recruitment potential

for both conifer and deciduous species. The exposed mineral or humic soils

have high bulk densities and are able to wick moisture to the soil surface, increasing the germination and survival of conifer and deciduous seedlings.

Where there is extensive mineral soil exposure, mosses may rapidly colonize

and cover the substrate surface, limiting successful establishment to mostly

those seeds that arrive immediately after the fire (often deciduous seed). These

sites have a range of possible successional trajectories.

2A. There are multiple standing black spruce trees with open cones at

the site that provide a strong onsite seed source for black spruce.

 I. Prefire aspen or birch trees are present at the site, with intact

roots or stumps, or there is a stand of live deciduous trees in

the area (within 1 to 2 km) that can provide a local seed

source for deciduous trees.

 a. There is extensive exposure of mineral soil (>30

percent of surface).

 1. Mineral soils are very moist and support

rapid colonization by broad-leaved liverwort

(Marchantia polymorpha L.) (app. photo 12).

Deciduous trees can establish rapidly from seed

or resprouting, but seedbed quality rapidly declines
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

and limits black spruce establishment. The expected successional trajectory is closed deciduous

forest (app. photo 20) with a sparse understory of

black spruce.

 2. Mineral soils are less moist, and support slower

colonization by small, single-stemmed mosses such

as firemoss (Ceratodon purpureus (Hedw.) Brid.)

(app. photo 9). Both deciduous trees and black

spruce have high potential rates of establishment.

The expected successional trajectory is mixed

deciduous-black spruce forest (app. photo 19).

b. Mineral soil exposure is intermediate (5 to 30 percent)

or limited to only exposure of humic soil. Deciduous

trees are likely to have variable establishment success

on the mixed seedbed, and black spruce is expected to

be able to establish on all substrate types. The expected

successional trajectory is mixed deciduous-black

spruce forest.

II. There are few deciduous trees in prefire stand or surrounding

unburned forest. Deciduous trees have few opportunities to

establish from seed or by resprouting. The expected

successional trajectory is intermediate- to closed-canopy

black spruce forest.

2B. The local seed source for black spruce is limited by most trees

having fallen over, trees being too young to bear many cones, or

the cones being either deeply charred (severe crown fire) or not

open (fire did not crown).

I. There is a stand of live deciduous trees in the area (within 1

to 2 km) that can provide a local seed source for deciduous

trees, and/or prefire deciduous stems are common at the site

with roots or stumps intact. Deciduous recruitment is likely

to be abundant, but low seed availability will limit black

spruce establishment. The expected successional trajectory is

open deciduous forest with sparse or absent understory

of black spruce (app. photo 18).
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GENERAL TECHNICAL REPORT PNW-GTR-767

II. There are few live deciduous trees in the area and no prefire

birch or aspen at the site. Recruitment of both deciduous

trees and black spruce is likely to be strongly limited by low

seed availability. The expected successional trajectory is

nonforested shrubland/grassland (app. photo 23), where

grasses rapidly colonize, or open spruce-lichen woodland

where the understory is dominated by low shrubs.

3. Drier sites (mesic/subxeric to xeric) with postfire organic layers largely

intact. Sites may have moderate recruitment potential for conifers, but deciduous recruitment from seed will be limited by dry conditions at the surface of

the intact organic layers. Establishment of deciduous species will be largely

limited to asexual regeneration (resprouting).

3A. There are multiple standing black spruce trees with open cones at

the site that provide a strong onsite seed source for black spruce.

I. Prefire aspen or birch trees are present at the site, with intact

roots or stumps to support asexual regeneration. The

expected successional trajectory is mixed deciduous-black

spruce forest.

II. Prefire aspen or birch trees are rare or absent from the site.

Intact organic layers restrict deciduous colonization from

seed. The expected successional trajectory is intermediateto closed-canopy black spruce forest.

3B. The local seed source for black spruce is limited by most trees

having fallen over, trees being too young to bear many cones, or

the cones being deeply charred (severe crown fire).

I. Prefire aspen or birch trees are present at the site, with intact

roots or stumps to support asexual regeneration. The

expected successional trajectory is open deciduous forest

with sparse or absent understory of black spruce.

II. Prefire aspen or birch trees are rare or absent from the site.

Intact organic layers restrict deciduous colonization from

seed. The expected successional trajectory is nonforested

shrubland/grassland, or spruce-lichen woodland.
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

4. Drier sites (mesic/subxeric to xeric) with organic layers largely consumed,

leading to exposed mineral soil. These sites have a moderate to high recruitment potential for both conifers and deciduous trees. Conifer recruitment

density is likely to be moderate at the driest sites owing to dry surface conditions even where mineral soil is exposed. Recruitment of deciduous species will

be affected by the potential for asexual regeneration. These sites also have the

potential to support establishment of white spruce if there is a live stand of

white spruce close to the site (<1 km). Sites in this category have the highest

potential to shift from black spruce to alternate successional trajectories.

4A. There are multiple standing black spruce trees with open cones at

the site that provide a strong onsite seed source for black spruce.

I. Prefire aspen or birch trees are present at the site, with intact

roots or stumps. Resprouting of deciduous species is likely to

be abundant, and high litter production in the early postfire

years may reduce levels of black spruce recruitment. The

expected successional trajectory is closed-canopy deciduous

forest with an open understory of black spruce (app.

photo 21).

II. Prefire aspen or birch trees are rare or absent from the site.

a. There is at least one stand of live deciduous trees in the

area (within 1 to 2 km) that can provide a local seed

source for deciduous trees, and there is good seed production in the year after the fire. The expected successional trajectory is mixed deciduous-black spruce

forest.

b. There are few live deciduous trees in the area or bad

seed years after fire, resulting in low deciduous

recruitment. The stand is likely to develop into

intermediate- to closed-canopy black spruce forest.

4B. The local seed source for black spruce is limited by most trees

having fallen over, trees being too young to bear many cones, or

the cones being deeply charred (severe crown fire).
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GENERAL TECHNICAL REPORT PNW-GTR-767

I. Prefire aspen or birch trees are present at the site, with intact

roots or stumps. Resprouting of deciduous species is likely to

be abundant, with little recruitment of black spruce. The

expected successional trajectory is open deciduous forest

with sparse or absent understory of black spruce.

II. Prefire aspen or birch trees are rare or absent from the site.

a. There is at least one stand of live deciduous trees in the

area (within 1 to 2 km) that can provide a local seed

source for deciduous trees, and there is good seed

production in the year after the fire. The expected

successional trajectory is open deciduous forest with

sparse or absent understory of black spruce.

b. There are few live deciduous trees in the area or bad

seed years after fire. Both conifer and deciduous

recruitment is low, leading to open aspen parkland

(app. photo 22) or nonforested shrubland/grassland.

Acknowledgments

Several people aided in the production of this report. We would particularly like to

thank the managers and researchers who have provided comments and feedback

throughout the development of the report, including Randi Jandt, Karen Murphy,

John Lawrence, Thomas Paragi, Roger Ruess, and Patricia Wurtz. Julianne Benioff,

Emily Tissier, and Chrissy Apodaca provided valuable technical assistance that was

instrumental in producing the final report. Funding for this project was provided by

the U.S. Joint Fire Science Program (project 05-1-2-06), with additional support

from the U.S. Forest Service and the Bonanza Creek Long-Term Ecological Research program.

English Equivalents

When you know: Multiply by: To find:

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Kilometers (km) .621 Miles

Hectares (ha) 2.47 Acres
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Cited References

Chapin, F.S., III; Rupp ,T.S.; Starfield, A.M.; DeWilde, L.; Zavaleta, E.S.;

Fresco, N.; McGuire, A.D. 2003. Planning for resilience: modeling change in

human-fire interactions in the Alaskan boreal forest. Frontiers in Ecology and

the Environment. 1: 255–261.

Charron, I.; Greene, D.F. 2002. Post-wildfire seedbeds and tree establishment in

the southern mixedwood boreal forest. Canadian Journal of Forest Research.

32: 1607–1615.

Chrosciewicz, Z. 1974. Evaluation of fire-produced seedbeds for jack pine regeneration in central Ontario. Canadian Journal of Forest Research. 4: 455–457.

Greene, D.F.; Macdonald, S.E.; Cumming, S.; Swift, L. 2005. Seedbed variation from the interior through the edge of a large wildfire in Alberta. Canadian

Journal of Forest Research. 35: 1640–1647.

Greene, D.F.; Macdonald, S.E.; Haeussler, S.; Domenicano, S.; Noel, J.;

Jayen, K.; Charron, I.; Gauthier, S.; Hunt, S.; Gielau, E.T.; Bergeron, Y.;

Swift, L. 2007. The reduction of organic-layer depth by wildfire in the North

American boreal forest and its effect on tree recruitment by seed. Canadian

Journal of Forest Research. 37: 1012–1023.

Hély, C.; Bergeron, Y.; Flannigan, M.D. 2000. Effects of stand composition on

fire hazard in mixed-wood Canadian boreal forest. Journal of Vegetation

Science. 11: 813–824.

Johnson, E.A. 1992. Fire and vegetation dynamics. Studies from the North

American boreal forest. Cambridge, United Kingdom: Cambridge University

Press: 1–129.

Johnstone, J.F.; Chapin, F.S., III. 2006. Effects of soil burn severity on postfire

tree recruitment in boreal forests. Ecosystems. 9: 14–31.

Johnstone, J.F.; Kasischke, E.S. 2005. Stand-level effects of soil burn severity on

postfire regeneration in a recently-burned black spruce forest. Canadian Journal

of Forest Research. 35: 2151–2163.

Komárková, V. 1983. Environmental data for 235 plots in the Gunnison and

Uncompahgre National Forests. Contract number 28-K2-234. On file with:

Rocky Mountain Research Station, 2150 Centre Ave., Bldg. A, Suite 376,

Fort Collins, CO 80526-1891.
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GENERAL TECHNICAL REPORT PNW-GTR-767

National Interagency Fire Center [NIFC]. 1992. Fire behavior field reference

guide. Boise, ID: National Wildfire Coordinating Group.

Van Cleve, K.; Chapin, F.S., III; Dyrness, C.T; Viereck, L.A. 1991. Element

cycling in taiga forest: state-factor control. BioScience. 41: 78–88.

Van Cleve, K.; Viereck, L.A. 1981. Forest succession in relation to nutrient

cycling in the boreal forest of Alaska. In: West; D.C.; Shugart; H.H.; Botkin,

D.B., eds. Forest succession, concepts and application. New York: SpringerVerlag: 184–211.

Viereck, L.A. 1983. The effects of fire in black spruce ecosystems of Alaska

and northern Canada. In: Wein, R.W.; MacLean, D.A., eds. The role of fire

in northern circumpolar ecosystems. Chichester, United Kingdom: Wiley:

201–220.

Zasada, J.; Norum, R.A.; Van Veldhuizen, R.M.; Teutsch, C.E. 1983. Artificial

regeneration of trees and tall shrubs in experimentally burned upland black

spruce/feather moss stands in Alaska. Canadian Journal of Forest Research. 13:

903–913.
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Appendix A: Operational Definitions

Spatial Scale of Assessment

The key has been designed to assess potential successional trajectories at the scale

of individual forest stands. Forest stands can be intuitively defined as a patch of

forest that can be visually assessed from a central point (approximately 20- to 30-m

radius around the observer). On an areal basis, the scale of assessment is expected

to be on the order of 0.25 ha, with a reasonable range of 0.1 to 1.0 ha.

Classifications of Postfire Seedbeds

Sites are classified with respect to the availability of different postfire seedbeds,

largely on the basis of the depth and coverage of any remaining surface organic

layers. Assays with a shovel or soil corer should be used at a site to estimate the

characteristics of the soil organic layer. Stated thresholds in seedbed classifications

are based on empirical observations of 2004 burns in Alaska studied as part of the

Joint Fire Science Program.

1. Mineral-soil dominated: seedbeds dominated by more than 30-percent exposure

of mineral soil or shallow organic layers (dense, humic layers <3 cm deep)

2. Organic-soil dominated: seedbeds dominated by cover of low-density organic

layers, particularly soils with >5 cm organic layer depth and less than 15

percent mineral soil exposed.

3. Intermediate soils: seedbeds have moderate cover (15 to 30 percent) of exposed

mineral soil or shallow (<3 cm deep) organic layers. Local conditions will

determine whether these behave as “mineral-” or “organic”-dominated soils

with respect to tree regeneration and successional trajectories.
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GENERAL TECHNICAL REPORT PNW-GTR-767
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Appendix B: Reference Photo Gallery

Site Identifier Species

Black spruce (Picea mariana (Mill) B.S.P.)

Photo 1—Black spruce stand.

Photo 2—Black spruce needles and cones. T. Hollingsworth T. Hollingsworth
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GENERAL TECHNICAL REPORT PNW-GTR-767

White spruce (Picea glauca (Moench)Voss)

Photo 3—Lone white spruce.

Photo 4—White spruce needles and cones. T. Hollingsworth J. Johnstone
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Decidious trees (Betula neoalaskana Sarg. and Populus tremuloides Michx.)

Photo 5—Mixed birch and aspen stand.

Photo 6—Closeup of birch bark. Photo 7—Closeup of aspen bark.T. Hollingsworth T. Hollingsworth

T. Hollingsworth
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GENERAL TECHNICAL REPORT PNW-GTR-767

Tussocks: Sedges that form tight clumps; predominantly cotton

grass (Eriophorum vaginatum L.)

Photo 8—Cotton grass (Eriophorum vaginatum).

J. Johnstone
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Firemoss (Ceratodon purpureus (Hedw.) Brid.): One of the

most common types of small, unbranched mosses that colonizes

organic and mineral soil after fire.

Photo 9—Firemoss (Ceratodon purpureus).

T. Hollingsworth
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GENERAL TECHNICAL REPORT PNW-GTR-767

Sphagnum moss (Sphagnum spp.): mosses in the genus

Sphagnum.

Photo 10—Sphagnum moss (Sphagnum spp.).

T. Hollingsworth
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Feathermoss: Pleurocarpous mosses commonly found in later

successional stages of black spruce forest, such as Hylocomium

splendens (Hedw.) Schimp. and Pleurozium schreberi (Brid.)

Mitt.)

Photo 11—Feathermoss (Hylocomium splendens and Pleurozium schreberi).

T. Hollingsworth
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GENERAL TECHNICAL REPORT PNW-GTR-767

Marchantia: A broad-leaved liverwort (Marchantia polymorpha L.)

that is an abundant early colonist after fire of moist to wet mineral

soils and exposed humic layers.

Photo 12—Broad-leaved liverwort (Marchantia polymorpha).

T. Hollingsworth
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Lichen: An association of fungus and phytosynethic symbiont

resulting in vegetation body with a recognizable structure.

Photo 13—Fruticose lichens especially Cladonia spp. and Cladina spp. but also including

Stereocaulon spp. and Cetraria spp.

Photo 14—Broad-lobed lichens including Lobaria linita, Nephroma arcticum, and Peltigera

spp.

T. Hollingsworth T. Hollingsworth
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GENERAL TECHNICAL REPORT PNW-GTR-767

Examples of successional trajectories

Photo 15—Open-canopy black spruce.

Emily Tissier

Photo 16—Intermediate- to closed-canopy black spruce.

T. Hollingsworth
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Photo 17—Spruce-lichen woodland.

Photo 18—Open deciduous forest with sparse or absent understory of black

spruce.

J. Johnstone

J. Johnstone
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GENERAL TECHNICAL REPORT PNW-GTR-767

Photo 19—Mixed deciduous-black spruce forest.

Brian Charlton

Photo 20—Closed deciduous forest. Bonanza Long–Term Ecosystem Research Program
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Photo 21—Closed-canopy deciduous forest with an open understory of black spruce.

J. Johnstone
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GENERAL TECHNICAL REPORT PNW-GTR-767

Photo 22—Open aspen parkland.

J. Johnstone
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Photo 23—Nonforested shrubland/grassland.

Postfire site assessment

These photos were taken in late May in the year after the stands

had burned. All sites were dominated by black spruce prior to

burning. Sites are taken as examples as part of a larger study of

postfire succession in black spruce forests.

J. Johnstone
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GENERAL TECHNICAL REPORT PNW-GTR-767

Photos 24-27—Moist site with the organic layer largely intact.

T. Hollingsworth

J. JohnstoneJ. Johnstone T. Hollingsworth
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Photos 28-31—Moist sites with shallow residual organic layers and

frequent patches of exposed humic or mineral soil.

J. Johnstone T. Hollingsworth

T. Hollingsworth J. Johnstone
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GENERAL TECHNICAL REPORT PNW-GTR-767

Photos 32-34—Dry sites with the organic layer largely intact

after the fire.

J. Johnstone

J. Johnstone T. Hollingsworth
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A Key for Predicting Postfire Successional Trajectories in Black Spruce Stands of Interior Alaska

Photos 35-37—Dry site where much of the organic layer has

been consumed by the fire, exposing frequent patches of

mineral soil. T. Hollingsworth all photos
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