United States Department of Agriculture

DEPAR TMENT OF AGRICULT URE

Forest 

Service

Pacific Northwest 

Research Station

Research Paper

PNW-RP-609

June

2017

Understory Plant Development 

in Artificial Canopy Gaps in an 

81-Year-Old Forest Stand on 

Chichagof Island, Southeast Alaska

Scott H. Harris and Jeffrey C. Barnard
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Authors

Scott Harris is a graduate student, Oregon State University, Department of Forest 

Ecosystems and Society, 321 Richardson Hall, Corvallis, OR97331; Jeffrey Barnard is a fisheries biologist, U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Research Station, Forestry Sciences Laboratory, 11175 Auke Lake Way, 

Juneau, AK 99801.

Cover: An artificial canopy gap created in an approximately 74-year-old stand at 

the Todd study site with untreated forest in the background in 2006. Photo by Mike 

McClellan. 
Abstract

Harris, S.H, Barnard, J.C. 2017. Understory plant development in artificial 

canopy gaps in an 81-year-old forest stand on Chichagof Island, southeast Alaska. 

Res. Pap. PNW-RP-609. Portland, OR. U.S. Department of Agriculture, Forest 

Service, Pacific Northwest Research Station. 26 p.

This study assesses the understory plant response and associated effects on forage 

resources available to Sitka black-tailed deer (Odocoileus hemionus sitkensis), to 

the creation of artificial canopy gaps in a young-growth forest stand in the coastal 

temperate rain forest of southeast Alaska. The forest stand was approximately 58 

years old when gaps were created and approximately 81 years old when the most 

recent set of data was collected in 2013. Plant canopy cover estimates were collected 

five times over the 23-year period following the creation of nine artificial canopy 

gaps. We estimated the biomass of individual plant species and plant groups in the 

gaps and adjacent untreated forest using canopy cover to biomass regression equations to convert the data into estimates of kilograms of plant material per hectare. 

We estimated “deer days” per hectare for several summer and winter scenarios, 

in the gaps and untreated forest, by using the Forage Resource Evaluation System 

for Habitat (FRESH) deer model. Results were analyzed with a Wilcoxian signed 

rank test (P<0.05). Results indicated that plant biomass and deer days per hectare 

were significantly higher in the gaps than the adjacent untreated forest at 23 years 

following creation of the gaps, increasing over the 23 years with no apparent peak. 

Compared to understory plant response for three thinning studies from southeast 

Alaska, results from this study indicate that deer days per hectare for the summer 

scenarios were greater for the thinning treatments, but lower for the winter scenario 

where low-lying forbs would likely be buried by snow. However, data from the 

thinning studies are available for only a few years following the treatments, so the 

comparisons are not precise. Although the results from our gap study should not 

be considered representative for southeast Alaska because they are from only one 

stand, they do suggest that creating artificial canopy gaps in 58-year-old younggrowth stands is a viable option for increasing forage resources for deer.

Keywords: Southeast Alaska, Sitka black-tailed deer, canopy gaps, forest ecology, FRESH deer. 
Summary 

In southeast Alaska, maintaining and enhancing forage for Sitka black-tailed 

deer in young growth stands is a priority for forest managers on the Tongass 

National Forest. Creating artificial canopy gaps is one technique to accomplish 

this. We assess the understory plant response in nine artificial canopy gaps that 

were created in an approximately 58-year-old stand over the following 23 years, a 

project known as the Todd gap study. We estimated the biomass of individual plant 

species and plant groups in the gaps and adjacent untreated forest as kilograms of 

plant material per hectare. Vegetation response was evaluated in terms of forage 

resources using the Forage Resource Evaluation System for Habitat (FRESH) 

deer model to quantify habitat value in terms of deer days per hectare, where one 

deer-day is the food required to maintain an adult female deer for one day at userspecified metabolic requirements.

Results indicate the following: In the gaps, the total mean understory vascular 

plant biomass increased by a factor of 35.5 within 23 years following the creation 

of the canopy gaps. Forbs (forest herbs) are a key forage group for deer. Mean forb 

biomass increased by 8.3 times over the 23-year monitoring period. Vaccinium

species are key winter forage items. Their biomass increased by 17 times over the 

monitoring period. 

In young growth treated with thinning or with artificial gaps, conifers can 

compete with and shade out understory forage, so monitoring their growth is an 

important consideration in understory vegetation studies. In this study, western 

hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings were absent in the gaps at one 

year after treatment. They did subsequently increase in biomass, but at a lesser rate 

than the shrubs. 

FRESH deer analysis showed that at 23 years after gap creation, estimates of 

deer days per hectare were 8.1 times higher in the gaps than the untreated forest for 

the summer scenario with one adult female, 3.7 times in winter without snow, and 

3.4 times in winter with snow.

Compared with recent studies of precommercially and commercially thinned 

stands, the thinning studies had more deer days per hectare than the Todd gaps for 

the two summer scenarios and the winter no-snow scenario, but fewer deer days per 

hectare than the Todd gaps for the winter with-snow scenario. The thinned and gap 

treatments all show a delayed forb response compared to the shrubs and conifers. 

Estimates of both understory plant biomass and deer days per hectare in the 

gaps increased throughout the 23-year monitoring period without reaching an 

apparent peak. The results from our study suggest that canopy gaps can be a viable 

treatment option to improve forage in young-growth stands. 

Contents

1 Introduction

3 Methods

3 Study Site

5 Field Data Collection

7 Analysis

8 Results

15 Discussion

20 Conclusions

21 Acknowledgments

21 English Equivalents

22 Literature Cited

25 Appendix: Scientific and Common Names of Plant 

Species in This Publicationa
1

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Introduction

Creating artificial canopy gaps in young-growth forests to stimulate understory 

plant development is a common wildlife habitat restoration practice on the Tongass 

National Forest in southeast Alaska, yet the response of plants to this treatment 

is not well understood. Here, we report findings from a 23-year monitoring effort 

that examined the understory plant response to artificial canopy gaps at one study 

site. The results shed light on the persistence and magnitude of understory plant 

responses to this treatment.

Past timber harvest practices have had an impact on wildlife habitat in the 

coastal temperate rain forests of southeast Alaska. These practices have changed 

forest structure and plant species distribution, and hence have changed habitat 

conditions for many forest species (Orians et al. 2013). Investigations throughout 

southeast Alaska demonstrate a pattern of habitat change following harvest, 

particularly for Sitka black-tailed deer (Odocoileus hemionus sitkensis). Forest 

stand and understory plant development following clearcut harvest typically 

follows a pattern of natural tree regeneration and initially high understory plant 

biomass. Within about 30 years, however, the regenerated cohort of trees creates 

a closed canopy that shades the understory, resulting in a reduction in the biomass 

and diversity of plants. Without management intervention or other disturbance, 

this stem exclusion phase (Oliver and Larson 1996), with a forest floor relatively 

devoid of herbaceous and woody plants, can last 100 years or more (Alaback 1982, 

Nowacki and Kramer 1998).

Restoring and maintaining deer habitat is a priority for forest management 

on the Tongass National Forest, and Sitka black-tailed deer are an important 

subsistence resource for communities in southeast Alaska (Brinkman et 

al. 2009, Hanley 1993 Schoen and Kirchoff 2007). Restoring deer habitat 

has focused on improving light conditions for understory forage plants in 

young-growth forest stands. Light is considered the primary limiting factor in 

understory plant growth in southeast Alaska (Alaback 1982, Hanley et al. 2014, 

Tappeiner and Alaback 1989). Therefore, manipulating overstory canopy cover 

to increase light penetration through forest canopies is a key objective of thinning and restoration treatments. 

The characteristics of high-quality deer habitat in old-growth forest serve as a 

guide to inform the habitat restoration in young-growth forests of southeast Alaska. 

High-quality habitat for deer results from the diversity of understory plant biomass 

and the ability of the forest canopy to intercept snowfall. Snow interception by the 

canopy leads to lower snow accumulation on the forest floor so that the highest 
2

RESEARCH PAPER PNW-RP-609

quality winter forage (low-lying evergreen forbs) can be available to deer. This forage resource can become unavailable to deer at snow depths of 10 cm and greater 

(Parker et al. 1999). At coarser scales, horizontal patchiness and forest edges 

increase these effects and provide structural habitat diversity (Schoen and Kirchoff 

2007). Fine-scale tree mortality from disease and wind events provide this patchiness by creating natural canopy gaps, and is one of the most common disturbance 

mechanisms in southeast Alaska (Harris 1999, Hennon and McClellan 2003). Ott 

and Juday (2002) found that these natural canopy gaps vary in size and shape and 

have an equivalent mean diameter of approximately 16 m. Structural complexity 

has also been shown to increase biodiversity and is a generally accepted principle 

of multiple-use forest management, or what has been called ecological forestry 

(Lindenmayer and Franklin 2002). 

The primary objective of thinning treatments is to improve tree growth and 

stand structure for future timber harvest. However, empirical studies of thinning 

treatments in young-growth forest stands in the stem exclusion stage have also 

shown promise for increasing understory plant biomass. By felling or girdling trees, 

thinning increases light penetration to the forest floor, which encourages understory 

plant growth. Without further management intervention, however, canopy closure 

can reoccur after 10 to 30 years, repeating the cycle of stem exclusion (Alaback 

2010, Deal and Farr 1994, Hanley et al. 2013). Thinning can also result in reduced 

structural complexity (Lindenmayer and Franklin 2002). These general patterns 

differ depending on site conditions, such as stand age, harvest practices, soils, etc., 

and the details of the individual thinning treatments.

Therefore, a significant challenge is to treat young-growth stands to increase 

understory plant growth, biodiversity, and structural complexity, and to extend 

the time before the return to the stem exclusion phase. Creating artificial canopy 

gaps has shown promise for meeting these multiple objectives. Nearly 600 canopy 

gaps were created in young-growth stands on Prince of Wales Island between 1983 

and 1993 (Alaback 2010). Additionally, an analysis of U.S. Forest Service stand 

data showed that 423 gaps were created in young-growth stands on Kruzof Island 

between 1991 and 1996. The Todd gap study described in this report is important 

as it represents the only study of gap treatments that were implemented in stands 

age 55 and older, and it is the longest term monitoring of gap treatments (23 years) 

in southeast Alaska. Therefore, it provides a unique opportunity to examine the 

long-term effects of this treatment type.

A significant challenge 

is to treat younggrowth stands to 

increase understory 

plant growth, 

biodiversity, and 

structural complexity, 

and to extend the time 

before the return to 

the stem exclusion 

phase. Creating 

artificial canopy gaps 

has shown promise for 

meeting these multiple 

objectives.
3

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Methods

Study Site

The study site, hereafter referred to as Todd, is located on southeast Chichagof 

Island on Peril Strait near the historic Todd Cannery (fig. 1). Historical aerial imagery, U.S. Forest Service geographic information system (GIS) data, and increment 

cores were used to determine forest stand history. Based on these multiple sources 

of information, we estimate harvest occurred between 1929 and 1936, resulting in a 

stand age of approximately 58 years when gaps were cut, and subsequently a stand 

age of 81 at the time of the last field data collection in 2013. Figure 2 shows the 

portion of the stand where gaps were created. 

A thorough review of available information on the Todd gaps reveals that 

specific gap locations within the stand were chosen subjectively based on professional judgment.1

 Criteria included: siting gaps where there was existing Vaccinium 

shrubs, having gaps evenly distributed throughout the stand (but avoiding the edges 

of the stand), and cutting gaps in a roughly oval shape with the north-south axis 

being longer. Slash was bucked and left on site. Although there was an intention to 

monitor the long-term plant response in the gaps, the gap locations were chosen to 

demonstrate the ability of canopy gaps to meet management objectives (improve 

understory forage resources for deer), and not under the guidance of an experimental design. There is no history of thinning in this stand, and the only management 

action following original harvest was the creation of the gaps.

The stand has a southern aspect, is at an elevation less than 100 m, and is 

within 400 m of the shoreline. The 13 gaps (of which 9 were monitored) described 

in this report were created in 1990. The gaps range in size from 150 to 430 m2. The 

mean width is 18.3 m (ranging from 10.7 to 23.6 m). The stand comprising the gaps 

is in the Tongass National Forest plant association of western hemlock/blueberrymenziesia (Martin et al. 1995). The plant association was determined by observing 

existing vegetation at the time of the 2013 survey. The Tongass National Forest 

GIS soils layer identifies the western portion of the stand as Kupreanof-Tuxekan 

soil series with a median slope of 20.5 percent, non-hydric, site index 105; and the 

eastern portion of the stand as Kupreanof gravelly silt loam series with a median 

slope of 65.5 percent, non-hydric, site index 105. Bedrock geology is Cretaceous 

quartz monzonite and quartz diorite.

1 Killinger, G.M. Unpublished memo dated September 19, 1990. Available from the authors. 

Also on file with: U.S. Department of Agriculture, Forest Service, Tongass National Forest, 

Sitka Ranger District, 204 Siginaka Way, Sitka, AK 99835.
4

RESEARCH PAPER PNW-RP-609

Juneau

Sitka Ketchikan

Todd study site

0 150

Kilometers

75

Figure 1—Location of the Todd study site on Chichagof Island, southeast Alaska.
5

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Field Data Collection

The percentage of understory plant-canopy coverage and gap characteristics were 

measured using the revised protocol developed by DeMeo2. East-west transects 

were established and permanently marked before gaps were cut. Thirteen gaps were 

cut in September 1990; 9 of these were subsequently monitored at 1 year, 5 years, 

10 years, and 23 years after treatment. Only 4 of the 9 gaps were monitored 15 years 

posttreatment. For each transect, understory plant cover estimates were recorded 

using 1 m × 2 m quadrats at 8 quadrats in the gap, 1 quadrat at the edge of the gap 

in the forest, and 8 quadrats in the adjacent untreated forest, as shown in figure 3. 

To minimize the effects of inconsistencies between the plant identification skills 

2 DeMeo, T. 1990. Gap Monitoring Program, Ketchikan Area, Revised April 1990. Unpublished protocols obtained from Greg Killinger, Sitka, AK: U.S. Department of Agriculture, 

Forest Service, Tongass National Forest. 13 p.

Figure 2—Composite aerial image (source image 2009, 30 cm resolution) showing the 13 artificial canopy gaps at the 

Todd study site, Chichagof Island, southeast Alaska. The nine gaps chosen for monitoring are identified. The clearcut 

in the center of the image occurred in 1975, and the gapped stands were harvested between 1929 and 1936. Gaps were 

cut in 1990, approximately 58 years after harvest.
6

RESEARCH PAPER PNW-RP-609

of observers, blueberry and huckleberry species (which included V. alaskaense, V. 

ovalifolium, and V. parvifolium) were all recorded as Vaccinium spp. The scientific 

names of plants for the Todd protocol are based on Hitchcock and Cronquist (1973).3

We recorded the percent canopy coverage of plants within the quadrat frame 

that would be available to deer, which we assumed to be at a height of 1.4 m or 

less. All “trace” observations were converted to 0.1 percent. In June 2013, we took 

hemispherical canopy photographs from gap center and from two locations along 

the transect in the adjacent forest, one at 8.1 m and one at 16.3 m from the gap edge 

in each of the gaps. All canopy photographs were taken on the same day, with the 

same exposure settings and a north orientation, from a tripod set to a camera height 

of 1.4 m, and levelled with a bubble level. 

3 For the Prince of Wales Island Commercial Thinning project regression equations, we 

used the online USDA Plants database for plant nomenclature (USDA NRCS 2017). We 

compared the two species lists and found no inconsistencies between the two sources for 

the species in this study.

Figure 3—Transect layout for monitoring the Todd study site, adapted from DeMeo (1990). Dimensions are approximate dimensions of 

the 9 monitored artificial canopy gaps.

Edge of gap

Gaps are approximately oval.

Diagram is not to scale.

Photo point with 

east view

spacing

Hemispherical 

canopy photo 

location at 8.1 m 

and 16.3 m from 

gap edge

Hemispherical 

canopy photo 

location at gap 

center

Downhill

S8

S7 S5 S3 S1 G1 G3 G5 G7

S6 S4 S2 E0 G2 G4 G6 G8

1 × 2 m 

plot frames

Rebar with flagging, 

photo point with east view

Plot Spacing

• E0 is on the forest edge, downhill side of transect

• G1 through G8 are equally spaced across the gap

• S1 and S2 are spaced at the same interval as 

 G1 through G8

• S3 through S8 are spaced at 3 m intervals

Rebar with 

flagging

mean width 18.3 m (range 10.7 to 23.6 m)

Approximate

North

mean length 

22.4 m (range 

18.0 to 26.7 m)
7

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Analysis

Key objectives of this publication are to assess the forage resources available to deer 

following gap treatments and to compare the effects of gap and thinning treatments. 

To do this, we used the understory canopy cover data to estimate plant biomass and 

Forage Resource Evaluation System for Habitat (FRESH) “deer days” per hectare 

(Hanley et al. 2012). From the field data, the mean percentage of canopy coverage 

for each species and vegetation group (e.g., shrubs, forbs) was calculated for each 

of the nine gaps and for the untreated forest adjacent to the gaps. Biomass (kg/ha) 

of the understory vegetation was estimated using plant cover-to-biomass regression 

equations developed from the Prince of Wales commercial thinning study data4. 

Data from that study were pooled from three thinning treatments and untreated 

controls to form the regression models. The Prince of Wales commercial thinning 

study regression equations were used because they are the closest available in stand 

age (50 to 70 years at treatment) to the Todd stand. Deer days per hectare were 

calculated by using the biomass results as input to the FRESH deer model (Hanley 

et al. 2012). The FRESH deer model was developed to provide insight into deer 

habitat quality based on the assumption that forage sets the potential upper limit 

on the abundance of deer. The FRESH model integrates available forage biomass, 

nutritional quality of the forage, and the nutritional requirements of deer with userspecified scenarios with constraints such as season, snow cover, and animal condition. From the model, deer days per hectare represent the number of animal-use 

days that can be supported if all of the current annual growth of plant material in 

the unit area that are above the specified nutritional quality threshold is consumed 

by the animal. Because the model does not account for variables such as animal 

behavior, population size, predation, or animal-plant interactions, it estimates only 

the theoretical maximum number of deer that can be supported by the available 

forage, and therefore serves as an index. 

We compared the deer days per hectare between the gap and adjacent untreated 

forest under two summer (one adult female and one adult female with one fawn) 

and six winter scenarios (snow levels of 0 cm, 20 cm, 40 cm, 60 cm, 80 cm, and 

100 cm). While forbs (forest herbs) are generally highly nutritious, evergreen forbs 

are especially important in the winter no-snow FRESH deer scenario. They are the 

most nutritious forage available in winter. At 20 cm of snow and above they are buried, so the only species available for forage are the much less nutritious shrub twigs 

4 Barnard, J.C.; McClellan, M.H. 2014. Unpublished data from the Prince of Wales 

commercial thinning study. On file with: U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station, 11175 Auke Lake Way Juneau, AK 99801.
8

RESEARCH PAPER PNW-RP-609

and conifer annual growth. In this study, we show the deer days for snow levels of 

0 to 100 cm. These levels are meant to illustrate the gradual burial of shrubs. In the 

FRESH model, shrub burial by snow is assumed to be complete at 100 cm. At that 

point, the only forage available would be taller conifers. In the results and discussion sections, we report and compare results for two summer scenarios, but we 

focus on the two winter scenarios of 0-cm and 20-cm snow depths as more critical 

for understanding deer habitat requirements. 

Statistical analyses were performed using SAS (2013) software. Distributions of 

the biomass and FRESH deer days output data were assessed with normal probability plots. Because the data were non-normally distributed, we used the Wilcoxon 

signed rank test to test for differences between paired data (between the gap and 

adjacent untreated forest quadrats), which is appropriate based on the study design. 

The α value for all tests was 0.05. We did not include the edge quadrats data in the 

analysis because there is only one edge quadrat per canopy gap transect. 

We processed the hemispherical images first by using SideLook (Nobis 2005) 

software to threshold the images using edge detection. We then processed images 

using HemiView software (2008) to estimate the proportion of visible sky. From 

this data we were able to calculate a canopy cover index (CCI) for each image. 

FRESH deer uses CCI to estimate forest canopy snow interception as part of the 

winter habitat scenarios. We used the means from the gap and untreated forest CCI 

in the FRESH deer scenarios. 

Results

Between 1991 and 2013, the mean total understory plant biomass in the gaps and 

adjacent untreated forest increased by a factor of 35.5 and 2.6, respectively (tables 

1 and 2). Vaccinium species mean biomass in the gaps and adjacent untreated forest 

increased by a factor of 17.0 and 1.7, respectively. Mean forb biomass was estimated 

at low levels, but also increased between 1991 and 2013 by a factor of 8.3 in the 

gaps and 4.3 in the adjacent untreated forest. In the gaps, Vaccinium species and 

other shrubs show an increasing trend without any apparent peak through the monitoring period (fig. 4). The category of “other shrubs” is predominantly composed 

of rusty menziesia (Menziesia ferruginea Sm.) and has small amounts (never more 

than 1 percent mean canopy cover for any year) of stink currant (Ribes bracteosum 

Douglas ex Hook.), salmonberry (Rubus spectabilis Pursh), red elderberry (Sambucus racemosa L.), and devils club (Oplopanax horridus (Sm.) Miq.). Conifer 

seedlings and saplings, predominantly western hemlock (Tsuga heterophylla (Raf.) 

Sarg.), were absent in the gaps and adjacent forests 1 year after treatment, then subsequently increased in mean biomass, but at a lower rate than shrubs. Sitka spruce 

Conifer seedlings 

and saplings, 

predominantly western 

hemlock, were absent 

in the gaps and 

adjacent forests 1 

year after treatment, 

then subsequently 

increased in mean 

biomass, but at a lower 

rate than shrubs.
9

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Table 1—Mean (and standard error) oven-dry biomass of major understory vegetation types following the 

creation of artificial canopy gaps at the Todd site, Chichagof Island, southeast Alaska. Gaps (n = 9; n = 4 for 

2005) were created approximately 58 years after harvest

Treatment Ferns Forbs Graminoids All shrubs

Conifer 

seedlings Total biomass 

Kilograms per hectare

1 year posttreatment (1991):

Untreated 2.21 (1.0) 0.35 (0.1) 0 (0) 9.92 (5.5) 0 (0) 12.48 (6.4)

Edge 0 (0) 0 (0) 0 (0) 6.67 (4.9) 0 (0) 6.67 (4.9)

Gap 0.48 (0.2) 0.57 (0.2) 0 (0) 4.77 (1.4) 0 (0) 5.83 (1.5)

5 years posttreatment (1995):

Untreated 1.18 (0.8) 0.28 (0.2) 0.85 (0.8) 5.27 (3.5) 1.28 (0.5) 8.86 (4.0)

Edge 0 (0) 0.04 (0) 0.11 (0.1) 5.13 (4.0) 6.0 (1.7) 11.29 (4.6)

Gap 3.04 (2.5) 0.21 (0.2) 4.56 (4.5) 9.73 (5.0) 15.36 (3.0) 32.89 (10.1)

10 years posttreatment (2000):

Untreated 0.98 (0.7) 0.62 (0.4) 0 (0) 8.76 (2.3) 8 .08 (2.09) 18.45 (5.0)

Edge 0.03 (0) 0.04 (0) 0 (0) 25.45 (14.3) 35.6 (9.9) 61.12 (21.3)

Gap 11.14 (5.7) 0.28 (0.2) 0.85 (0.6) 33.06 (4.4) 57.75 (9.6) 103.08 (10.1)

15 years posttreatment (2005):

Untreated 0.51 ((0.4) 0.05 (0) 3.81 (3.8) 2.93 (0.5) 0 (0) 7.3 (4.7)

Edge 0(0) 0 (0) 0 (0) 5.91 (3.2) 0 (0) 5.91 (3.2)

Gap 12.28 (7.8) 0.91 (0.7) 0 (0) 51.72 (5.8) 26.68 (15.4) 91.59 (24.1)

23 years posttreatment (2013):

Untreated 2.78 (1.6) 1.49 (1.0) 0.02 (0) 19.78 (7.2) 8.36 (1.7) 32.43 (8.6)

Edge 4.28 (3.0) 0.98 (0.6) 0 (0) 69.86 (24.9) 22.44 (5.9) 97.55 (26.8)

Gap 12.78 (5.3) 4.83 (1.9) 0 (0) 122.76 (16.2) 66.66 (23.5) 207.03 (23.9)
10

RESEARCH PAPER PNW-RP-609 Table 2—Mean (and standard error) oven-dry biomass of key forage species for Sitka black-tailed deer following the creation of artificial canopy gaps at the Todd site, Chichagof Island, southeast Alaska. Gaps (n = 9; n = 4 for 2005) were created approximately 58 years after harvest Coptis aspleniifolia Cornus canadensis Rubus pedatus Tiarella trifoliata Tsuga hetrophylla Vaccinium spp. twigs Vaccinium spp. leaves Mensiezia ferruginea twigs Mensiezia ferruginea leaves Kilograms per hectare Treatment: 1 year posttreatment (1991): Untreated NR NR NR NR 0 (0) 7.2 (4.1) 1.79 (1.0) 0.21 (0.2) 0.23 (0.2) Edge NR NR NR NR 0 (0) 5.34 (3.9) 1.33 (3.9) 0 (0) 0 (0)

Gap NR NR NR NR 0 (0) 2.48 (0.7) 2.01 (0.6) 0.14 (0.1) 0.15 (0.2)

5 years posttreatment (1995):

Untreated 0 (0) 0 (0) 0.01 (0) 0.01 (0) 1.28 (0.5) 3.62 (2.3) 0.9 (0.6) 0.34 (0.3) 0.38 (0.3)Edge 0 (0) 0 (0) 0 (0) 0 (0) 5.75 (1.6 1.98 (1.3) 3.07 (2.8) 0.02 (0) 0.02 (0)Gap 0 (0) 0 (0) 0 (0) 0 (0) 15.15 (2.9) 1.72 (0.8) 1.4 (0.6) 0.18 (0.1) 0.2 (0.1)

10 years posttreatment (2000):

Untreated 0.06 (0.1) 0.06 (0.1) 0 (0) 0.13 (0.1) 8.05 (2.9) 5.1 (1.2) 1.27 (0.3) 1.12 (0.5) 1.24 (0.6)Edge 0.03 (0) 0.03 (0) 0 (0) 0 (0) 35.48 (9.9) 18.37 (10.9) 4.59 (2.7) 1.18 (0.6) 1.31 (0.7)Gap 0 .05 (0) 0.05 (0) 0 (0) 0 (0) 57.03 (9.5) 12.16 (1.9) 9.88 (1.5) 4.6 (0.9) 5.11 (1.0)

15 years posttreatment (2005):

Untreated 0 (0) 0 (0) 0 (0) 0.02 (0) 0 (0) 1.99 (0.4) 0.5 (0.1) 0.21 (0.1) 0.23 (0.1)Edge 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 3.6 (1.9) 0.9 (0.5) 0.67 (0.5) 0.74 (0.5)Gap 0 (0) 0 (0) 0 (0) 0 (0) 26.68 (15.4) 17.97 (8.4) 14.6 (6.8) 7.13 (2.1) 7.52 (2.3)

23 years posttreatment (2013):

Untreated 0.02 (0) 0.02 (0) 0.05 (0) 0.13 (0.1) 8.24 (1.7) 12.49 (4.0) 3.12 (1.0) 1.97 (1.2) 2.19 (1.3)Edge 0 (0) 0 (0) 0 (0) 0 (0) 22.2 (5.8) 33.77 (12.7) 8.44 (3.2) 13.11 (5.8) 14.55 (6.5)Gap 0.02 (0) 0.02 (0) 0 (0) 0 (0) 63.92 (23.9) 42.17 (5.5) 34.27 (4.4) 21.13 (3.2) 23.46 (3.5)

NR = not recorded. 
11

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

(Picea sitchensis (Bong.) Carrière) was the only other conifer species observed 

and never represented more than 5 percent of the biomass of the combined conifer 

seedling category for any year. 

FRESH deer analysis showed that, for the 5 years immediately following treatment, there was no significant difference between deer days per hectare in the gaps 

versus the untreated forest (tables 3 and 4). At 10 years posttreatment (in 2000), 

deer days per hectare differed significantly between the gaps and adjacent forest for 

winter scenarios with 20 cm or more of snow. This was also the case for all summer and winter scenarios for data collected 23 years posttreatment (in 2013). At 23 

years posttreatment, estimates of deer days per hectare were higher in the gaps than 

the untreated forest by a factor of 8.1 times for the summer scenario with one adult 

female, 3.7 times for the winter 0-cm snow scenario, and 3.4 times for the winter 

20-cm snow scenario.

100

Understory biomass by plant group 

in the Todd Gaps 1991–2013

Understory biomass (kg/ha)

Years since treatment

80

60

40

20

0

0 5 10 15 20 25

Ferns

Forbs

Other shrubs

Vaccinium

Conifers

Figure 4—Temporal response, in biomass kilograms per hectare, of functional plant groups to the 

creation of artificial canopy gaps (n = 9) at the Todd study site, Chichagof Island, southeast Alaska. 

Bars represent the mean biomass in the 9 gaps (with standard error), for each monitoring event at 1, 

5, 10, 15, and 23 years following creation of the gaps. The gaps were created in an approximately 58- 

year-old young-growth forest stand.

At 23 years 

posttreatment, 

estimates of deer 

days per hectare were 

higher in the gaps 

than the untreated 

forest by a factor of 8.1 

times for the summer 

scenario with one 

adult female, 3.7 times 

for the winter 0-cm 

snow scenario, and 3.4 

times for the winter 

20-cm snow scenario.
12

RESEARCH PAPER PNW-RP-609 Table 3—Mean (and standard error) deer days per hectare estimated by the FRESH deer model for selected scenarios following the creation of artificial canopy gaps at the Todd site, Chichagof Island, southeast Alaska. Gaps (n = 9; n = 4 for 2005) were created approximately 58 years after harvest Treatment n Maintenance Maintenance + lactation 0 cm snow 20 cm snow 40 cm snow 60 cm snow 80 cm snow 100 cm snow Mean (SE) 1 year posttreatment (1991) Untreated 9 4.32 (2.1) 0.66 (0.2) 5.99 (3.4) 5.99 (3.4) 5.62 (3.2) 4.84 (2.8) 4.05 (2.3) 3.27 (1.9) Edge 9 1.33 (0.9) 0 (0) 4.44 (3.3) 4.44 (3.3) 4.16 (3.1) 3.58 (2.6) 3.0 (2.2) 2.43 (1.8)

Gap 9 3.24 (0.9) 0.80 (0.5) 2.06 (0.6) 2.06 (0.6) 2.0 (0.5) 1.72 (0.5) 1.4 (0.4) 1.17 (0.3)

5 years posttreatment (1995)

Untreated 9 3.67 (1.7) 1.19 (0.7) 4.19 (2.0) 3.06 (1.9) 2.83 (1.8) 2.43 (1.6) 2.04 (1.3) 1.64 (1.0)Edge 9 3.7 (3.3) 0.16 (0.1) 1.74 (1.0) 1.74 (1.0) 1.55 (1.0) 1.33 (0.9) 1.12 (0.7) 0.90 (0.6)Gap 9 10.05 (5.9) 4.48 (3.9) 2.71 (1.0) 1.48 (0.6) 1.39 (0.6) 1.20 (0.5) 1.0 (0.42) 0.81 (0.34)

10 years posttreatment (2000)

Untreated 9 5.64 (1.9) 0.84 (0.4) 7.01 (2.3) 4.24 (1.03)a 3.98 (1.0)a 3.42 90.8)a 2.85 (0.7) 2.32 (0.6 )aEdge 9 8.29 (4.3) 0.3 (0) 15.39 (9.0) 15.27 (9.0) 13.74 (8.6) 11.83 (7.4) 9.92 (6.2) 8.0 (5.0)Gap 9 34.46 (6.7) 2.61 (0.9) 11.24 (1.4) 10.11 (1.6)a 9.49 (1.5)a 8.17 (1.3)a 5.73 (1.0) 5.52 (0.9)a

15 years posttreatment (2005)

Untreated 4 4.99 (4.2) 2.29 (2.2) 1.95 (0.3) 1.66 (0.3) 1.56 (0.3) 1.34 (0.3) 1.12 (0.2) 0.91 (0.2)Edge 4 1.97 (1.1) 0 (0) 2.99 (1.6) 2.99 (1.6) 2.8 (1.5) 2.42 (1.3) 2.03 (1.1) 1.64 (0.9)Gap 4 45.88 (14.1) 8.88 (3.9) 35.1 (22.1) 14.94 (7.0) 13.79 (6.6) 12.07 (5.7) 10.12 (4.7) 7.85 (3.9)

23 years posttreatment (2013)

Untreated 9 12.81 (4.0)a 2.47 (1.1)a 18.45 (5.7)a 10.39 (3.3)a 9.75 (3.1)a 8.34 (2.7)a 7.03 (2.3)a 5.86 (1,8)aEdge 9 38.13 (14.7) 3.89 (2.1) 41.99 (18.1) 28.07 (10.6) 26.3 (9.9) 22.46 (8.5) 18.57 (7.2) 14.79 (5.8)Gap 9 104.29 (14.8)a 17.81 (5.3)a 68.51 (13.3)a 35.06 (4.5)a 32.91 (4.3)a 28.33 (93.7)a 23.74 (3.1)a 19.16 (2.5)a

a Results with mean differences that are significantly different from zero in pairwise tests. α = 0.05. Edge quadrats were not tested. SE = standard error.
13

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Table 4—Mean (and p value for tested comparisons), deer days per hectare estimated by the FRESH deer model for selected scenarios following the creation of artificial canopy gaps at the Todd site, Chichagof Island, southeast Alaska. Gaps (n = 9; n = 4 for 2005) were created approximately 58 years after harvestTreatment Maintenance

 Maintenance + 

lactation

 0 cm 

snow

20 cm 

snow

 40 cm 

snow

60 cm 

snow

 80 cm 

snow 

100 cm 

snow 

Mean (Pr > = S)

1 year posttreatment (1991):

Untreated 4.32 (0.94) 0.66 (0.57) 5.99 (0.29) 5.99 (0.29) 5.62 (0.46) 4.84 (0.46) 4.05 (0.46) 3.27 (0.46)Gap 3.24 (0.94) 0.80 (0.57) 2.06 (0.29) 2.06 (0.29) 2.0 (0.46) 1.72 (0.46) 1.4 (0.46) 1.17 (0.46)

5 years posttreatment (1995):

Untreated 3.67 (0.37) 1.19 (0.93) 4.19 (0.64) 3.06 (0.94) 2.83 (1.0) 2.43 (1.0) 2.04 (1.0) 1.64 (1.0)Gap 10.05 (0.37) 4.48 (0.93) 2.71 (0.64) 1.48 (0.94) 1.39 (1.0) 1.20 (1.0) 1.0 (1.0) 0.81 (1.0)

10 years posttreatment (2000):

Untreated 5.64 (0.1) 0.84 (0.1) 7.01 (0.16) 4.24 (0.01)a 3.98 (0.01)a 3.42 0.01)a 2.85 (0.05) 2.32 (0.01)aGap 34.46 (0.1) 2.61 (0.1) 11.24 (0.16) 10.11 (0.01)a 9.49 (0.01)a 8.17 (0.01)a 5.73 (0.05) 5.52 (0.01)a

15 years posttreatment (2005):

Untreated 4.99 (0.12) 2.29 (0.12) 1.95 (0.12) 1.66 (0.25) 1.56 (0.12) 1.34 (0.12) 1.12 (0.12) 0.91 (0.12)Gap 45.88 (0.12) 8.88 (0.12) 35.1 (0.12) 14.94 (0.25) 13.79 (0.12) 12.07 (0.12) 10.12 (0.12) 7.85 (0.12)

23 years posttreatment (2013):

Untreated 12.81 (>0.01)a 2.47 (>0.01)a 18.45 (>0.01)a 10.39 (0.01)a 9.75 (>0.01)a 8.34 (>0.01)a 7.03 (>0.01)a 5.86 (>0.01)aGap 104.29 (>0.01)a 17.81 (>0.01)a 68.51 (>0.01)a 35.06 (0.015)a 32.91 (>0.01)a 28.33 (>0.01)a 23.74 (>0.01)a 19.16 (>0.01)a

a = Results with mean differences that are significantly different from the test statistic in pairwise tests. α = 0.05. Edge quadrats were not tested. 
14

RESEARCH PAPER PNW-RP-609

The edge quadrat on the monitoring transect is included to monitor the effect of 

side lighting on understory vegetation response. Side lighting is oblique light from 

the canopy gap opening coming into the surrounding untreated densely shaded 

forest. Areas affected by side lighting increase the total area affected by the canopy 

gap treatments. There is only one edge quadrat per transect, so we did not conduct 

statistical tests comparing the edge with the gap or untreated forest data. The edge 

quadrats had higher total plant biomass than the untreated forest quadrats at 5, 10, 

and 23 years posttreatment.

We only had hemispherical photos taken in 2013 (23 years posttreatment) for 

use in this study, so we used the CCI from 2013 for the FRESH deer winter scenarios for all years analyzed. The average CCI in the both the gaps and untreated 

forest was estimated to be 89 percent (fig. 5). Qualitative observations of changes in 

canopy gaps following treatments throughout southeast Alaska allow us to suggest that CCI was likely lower; in other words, the canopy was more open in the 

Todd gaps prior to 2013. Using the higher CCI estimate (from 2013) for all years 

was unlikely to substantially influence estimates of available forage estimated by 

FRESH because it is at 90 percent and greater where CCI makes a large difference 

in the snow interception estimates by FRESH. 

Figure 5—Typical hemispherical canopy photos from the Todd study site of the untreated forest (left) and treated gap (right) from 2013, 

23 years posttreatment; stand age is approximately 81 years. Both images are 89-percent canopy cover index (CCI). 
15

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Discussion

In any discussion of habitat treatments, two of the most important considerations 

are magnitude and persistence of the treatment effect. Empirical studies of both 

thinning and gap treatments in southeast Alaska, including the Todd study, consistently show an increase in understory vegetation following treatment. For comparison, the only other study of canopy gaps in southeast Alaska is Alaback’s (2010) 

unpublished study of 76 gaps created in stands age 13 to 41 (median 23) years on 

Prince of Wales Island. He found a strong understory response in the gaps 20 years 

posttreatment, with forb and Vaccinium biomass significantly higher than controls. 

For the winter no-snow scenario, he used FRESH deer to estimate 44 deer days per 

hectare. He did not report winter results with snow. 

We can also draw some inferences about gap treatments in southeast Alaska 

by comparing with thinning studies in similar age stands. Figures 6 and 7 compare 

the FRESH deer days per hectare for the winter no-snow and winter 20-cm snow 

scenarios, from 3 thinning studies, 2 gap studies, and an old-growth forest reference condition from southeast Alaska. The old-growth reference is calculated from 

biomass data for the “blueberry-bunchberry” community type from Hanley and 

Brady (1997). 

The Tongass Wide Young Growth Study5 (TWYGS) experiment 4 treated 

stands at a mean age of 35 years. The data presented are the mean results from 

23-foot-spaced thinned stands with three different slash treatments (Hanley et al. 

2013). At 5 years posttreatment, TWYGS 4 biomass was 5 to 10 times greater than 

Todd, and estimated deer days per hectare were greater for the winter 0-cm snow 

scenario (77 for TWYGS versus 3 for the Todd gaps) and the winter 20-cm snow 

scenario (14 for TWYGS versus 1 for the Todd gaps). At 10 years posttreatment 

(see footnote 4), understory biomass was about 10 times greater than in the Todd 

gaps. Deer days per hectare for the winter 0-cm snow scenario were higher (214 for 

TWYGS versus 11 for the Todd gaps) but lower for the winter 20-cm snow scenario 

(5 for TWYGS versus 10 for Todd gaps). 

Results from the Prince of Wales commercial thinning study are available only 

for 5 years posttreatment. Biomass and deer days per hectare estimates are the 

mean of three thinning treatments in stands age 52 to 73 years at treatment: crown 

thinning (103 trees per acre, or tpa) thinning of dominants (134 tpa) and thinning 

from below (68 tpa). Biomass was greater than the Todd 5-year results, about six 

times overall. Deer days per hectare for the winter 0-cm snow scenario were higher 

5 Hanley, T.A. 2014. Unpublished data from the SGMP study. On file with: USDA Forest 

Service, Pacific Northwest Research Station, 11175 Auke Lake Way, Juneau, AK 99801.
16

RESEARCH PAPER PNW-RP-609

(63 for the commercial thinning study versus 3 for the Todd gaps) but lower for the 

winter 20-cm snow scenario (less than 1 for the commercial thinning study versus 

1 for the Todd gaps). 

The other thinning study of similar stand age is the second-growth management 

demonstration project (SGMP) (Zaborske et al. 2002). Results are from 14 years 

posttreatment. Treatments included thinning at 20- to 25-foot spacing in stands age 

49 to 64 at treatment. At 14 years posttreatment, thinned stands showed a strong 

treatment response compared to untreated controls. Hanley (footnote 5) recalculated 

the original published FRESH deer days results. The updated FRESH deer days 

results showed the same pattern as the other thinning studies. Compared to the 

15-year posttreatment Todd results, deer days per hectare for the winter 0-cm snow 

Deer days per hectare

0 cm snow

Deer days/hectare

Years since treatment

350

300

250

200

150

100

50

0

0 5 10 15 20 25 Old growth

Todd gaps

Todd untreated

POW CT thinned

POW CT untreated

SGMP thinned

SGMP untreated

TWYGS 4 thinned

TWYGS 4 untreated

POW gaps (Alaback)

POW thinned (Alaback)

Old growth

Figure 6—Comparison of deer days per hectare (with standard error) calculated by the FRESH deer model for the winter 0-cm snow 

scenario for the Todd gaps and adjacent untreated forest, with other thinning and gap studies, and old-growth conditions in southeast 

Alaska. The Todd gaps (n = 1) were created in a 58-year-old stand. The POW commercial thinning (with 3 types of thinning treatments), 

(n = 3) occurred in stands 52 to 73 years old. The SGMP thinning treatments (thinned n = 4, unthinned n = 5) occurred in stands >56 

years old. The TWYGS 4 treatments (thinning with 2 types of slash treatments, n = 17) occurred in stands with a mean age of 35 years 

old. The POW gaps are from Alaback’s (2010) study. The POW gaps (n = 76) were created in stands aged 13 to 41 years old. Alaback’s 

POW study also included measurements in thinned stands (n = 29) of the same age. Old growth (Hanley and Brady 1997) is a dataset for 

the “blueberry-bunchberry” plant community type for old-growth forest conditions.
17

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

scenario were higher (191 for SGMP versus 35 for the Todd gaps) but lower for the 

winter 20-cm snow scenario (6 for SGMP versus 15 for the Todd gaps).

These comparisons with thinned treatments show a general pattern. The three 

thinning studies discussed had more deer days per hectare than the Todd gaps for 

the two summer scenarios (not shown in figures) and winter no-snow scenario, but 

fewer deer days per hectare than the Todd gaps for the winter 20-cm snow scenario. 

The thinned and gap treatments all show a delayed forb response compared to the 

shrubs and conifers. At 20 cm of snow, the evergreen forbs are buried and only Vaccinium and a few other shrub twigs and conifer seedlings, saplings, and branches 

are available as forage.

Deer days per hectare

20 cm snow

Deer days/hectare

Years since treatment

Todd gaps

Todd untreated

POW CT thinned

POW CT untreated

SGMP thinned

SGMP untreated

TWYGS 4 thinned

TWYGS 4 untreated

Old growth

0 5 10 15 20 25 Old growth

60

50

40

30

20

10

0

Figure 7—Comparison of deer days per hectare (with standard error) calculated by the FRESH deer model for the winter 20-cm snow 

scenario for the Todd gaps and adjacent untreated forest, with other thinning studies, and old-growth conditions in southeast Alaska. 

The Todd gaps (n = 1) were created in a 58-year-old stand. The POW CT thinning (n = 3) (with 3 types of thinning treatments) occurred 

in stands 52 to 73 years old. The SGMP thinning treatments (thinned n = 4, unthinned n = 5) occurred in stands >56 years old. The 

TWYGS 4 treatments (thinning with 2 slash treatments, n = 17) occurred in stands 35 years old. Old growth (Hanley and Brady 1997) is 

a dataset for the “blueberry-bunchberry” plant community type for old-growth forest conditions.
18

RESEARCH PAPER PNW-RP-609

The thinning studies discussed for the most part have observations for only a 

few years posttreatment, so they do not yet offer much insight on the persistence 

of treatment effect. Alaback’s (2010) gap study on Prince of Wales Island included 

understory plant measurements in 29 thinned stands with similar site characteristics 

as the gaps in the same study. These 20-year posttreatment measurements showed 

that understory plants were nearly undetectable under the forest canopy that had 

closed in the years following the thinning treatments. 

The rapid growth of conifer seedlings in thinned and gapped stands has been 

suggested to reduce the habitat benefits of both treatments. For the Prince of 

Wales Island gap study, Alaback (2010) divided gaps into three size classes: small, 

medium, and large (<250 to 1100 m2

). His analysis found no significant differences 

in species richness, biomass, percentage of cover, or tree seedling density among 

the three groups of gaps. He had expected to see greater rates of tree colonization in the larger gaps but instead found them highly variable. This is significant 

because Deal and Farr (1994) observed that increased light into thinned or clearcut 

stands will promote the rapid growth of conifer seedlings (often called “hemlock 

flush”) that can shade out other understory vegetation. Cole et al. (2010) observed 

similar effects in a 7-year posttreatment study. The Todd gaps initially followed this 

pattern. The biomass of conifer seedlings exceeded shrub biomass for the first 10 

years following treatment, and subsequently trailed behind shrubs for the remaining 

measurements. We would expect understory plants in the small Todd gaps (150 to 

430 m2) to receive less light than in the Prince of Wales commercial thinning and 

TWYGS experiment 4 thinned stands with relatively wide spacing, so it is reasonable to expect the understory response to be greater at age 5 and 10 for the thinned 

stands. Additionally, with only 5 and 10 years of monitoring data available for the 

Prince of Wales and TWYGS experiment 4 studies, it is too early to reach conclusions about the eventual conifer growth in the treatments. 

We used canopy cover index (CCI) as input to the FRESH deer model to 

estimate the canopy interception of snow for this study. However, CCI does not 

characterize the light environment experienced by understory plants very well. 

Better light-environment information would come by collecting standard forestry 

overstory data and leaf area index measurements. Besides bringing more light into 

the treated area, we would assume that treatments bring more light into the adjacent 

untreated forest (side lighting). For our study, we did see greater shrub and conifer 

response compared to the untreated forest for the edge quadrats by year 23; but 

we did not do statistical analysis because of the small sample size. Alaback (2010) 

in his Prince of Wales Island gap study wasn’t able to detect any significant sidelighting effect on shrubs in his analysis. 
19

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Herbivory by deer can influence the measured plant response to thinning or 

gap treatments. Hanley and Barnard (2014) discussed the results of simulated 

herbivory experiments on understory plants and warn that the larger landscape 

context must be considered when designing silviculture treatments. Small, 

resource-rich areas such as canopy gaps surrounded by lower quality habitat can 

focus herbivory on the small treated areas, which could decrease the measured 

effects of the treatments. McClellan et al. (2014) visited various field sites in 

the Peril Strait area in 2006, including some of the Todd gaps. In the gaps they 

observed abundant light but lower than expected understory vegetation and 

evidence of heavy deer browse of the vegetation, including western hemlock 

Figure 8—Heavily browsed understory vegetation (western hemlock and Vaccinium) in one of the Todd study site artificial gaps on 

Chichagof Island, southeast Alaska. July 2006.
20

RESEARCH PAPER PNW-RP-609

seedlings (fig. 8). Similar to the results of the Hanley and Barnard (2014) study, 

they suggested that the large extent of forest in the stem exclusion phase that surrounded the gaps and the associated depauperate understory attracted deer to the 

more abundant understory vegetation available in the gaps. This browse activity 

suggests that our estimates of biomass and deer days per hectare in the Todd gaps 

are likely underestimates of the response. 

In addition to the differences in the light conditions experienced by understory 

plants, factors such as stand history, soils (and associated site productivity), topography, hydrology, and animal interactions, including herbivory, can play major roles 

in the observed variability. Further work is needed to help clarify these interactions. 

Additionally, the Todd gap treatment sites were chosen to maximize the understory 

response. Managers viewed the treatments as demonstrations of concept rather than 

experimental units. Even with these caveats, the most significant result of the Todd 

study is the persistence of the treatment effect. That is, understory plant biomass 

and deer days per hectare have steadily increased over the 23-year period following treatment and are consistently higher than in the adjacent untreated forest. The 

data show no apparent peak for both these measures.

Conclusions

Our results show that 23 years following the Todd gap treatments, understory plant 

biomass and forage resources for deer are higher in the Todd gaps than in the adjacent untreated forest. Estimates of both understory plant biomass and deer days per 

hectare in the gaps increased throughout the 23-year monitoring period without 

reaching an apparent peak. Comparing the Todd results with three thinning studies, the FRESH deer model estimated that deer days per hectare were lower in the 

Todd gaps for the summer and no-snow scenarios, but higher for the winter 20-cm 

snow scenario. However, we can only make limited conclusions about comparisons 

with thinning studies, because the thinning studies have data available only for 14 

years posttreatment. 

The results from the Todd gap study are from only one stand, so results should 

not be considered representative of southeast Alaska. This study adds important 

information to our understanding of canopy gap treatments and the dynamics of 

older young-growth stands. The persistence of the understory plant response in gap 

treatments is particularly encouraging. The empirical results from both the Alaback 

and Todd studies suggest that canopy gaps are a viable treatment option to improve 

forage resources for deer in young-growth forest stands, including older stands such 

as the Todd site.

Mike McClellan

The most significant 

result of the Todd study 

is the persistence of 

the treatment effect. 

That is, understory 

plant biomass and 

deer days per hectare 

have steadily increased 

over the 23-year period 

following treatment and 

are consistently higher 

than in the adjacent 

untreated forest.
21

Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Acknowledgments

This publication would not have been possible without the persistence and vision 

of Greg Killinger. He was instrumental in the implementation and subsequent 

monitoring of the Todd gaps. When monitoring resources became sparse, Greg 

reached out to non-U.S. Forest Service partners such as the Sitka Conservation 

Society to assist with the study and this report. We also thank all the participants of 

past monitoring trips, particularly Terri Suminski, who was a regular participant. 

Suresh Sethi provided statistical advice, and Paul Hennon, Paul Alaback, and Greg 

Hayward provided reviews of this manuscript. We greatly appreciate their critiques 

and suggestions, which were incorporated in the final version. We thank Tongass 

National Forest staff for their support of this publication. 

English Equivalents

When you know: Multiply by: To find:

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Hectares (ha) 2.47 Acres

Square meters (m2) 10.76 Square feet

Grams (g) 0.0352 Ounces

Kilograms (kg) 2.205 Pounds
22

RESEARCH PAPER PNW-RP-609

Literature Cited

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

hemlock forests of southeast Alaska. Ecology. 63: 1932–1948.

Alaback, P.B. 2010. An evaluation of canopy gaps in restoring wildlife habitat in 

second growth forests of southeastern Alaska. (A cooperative project with The 

Nature Conservancy—Alaska, Tongass National Forest, and POWTEC. 32 p. 

http://www.staneycreek.org/wp-content/uploads/2013/06/Gap-study-final-report.

pdf. (27 February 2017).

Brinkman, T.J.: Chapin, T.; Kofinas, G.; Person, D.K. 2009. Linking hunter 

knowledge with forest change to understand changing deer harvest opportunities 

in intensively logged landscapes. Ecology and Society. 14(1): 36.

Cole, E.C.; Hanley, T.A.; Newton, M. 2010. Influence of precommercial thinning 

on understory vegetation of young-growth Sitka spruce forests in southeastern 

Alaska. Canadian Journal of Forest Research. 40: 619–628.

Deal, R.L.; Farr, W.A. 1994. Composition and development of conifer regeneration 

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southeast Alaska. Canadian Journal of Forest Research. 24: 976–984.

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ecological indicator. Biological Conservation. 66: 61–67.

Hanley, T.A.; Barnard, J.C. 2014. Responses of southeast Alaska understory 

species to variation in light and simulated herbivory. Res. Pap. PNW-RP-599. 

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Research Station. 22 p. 

Hanley, T.A.; Borman, B.T.; Barnard, J.C.; Nay, S.M. 2014. Responses of 

Southeast Alaska understory species to variation in light and soil environments. 

Res. Pap. PNW-RP-598. Portland, OR: U.S. Department of Agriculture, Forest 

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Hanley, T.A.; Brady, W.W. 1997. Understory species composition and production 

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Canadian Journal of Botany. 75: 574–580
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Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

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Understory Plant Development in Artificial Canopy Gaps in an 81-Year-Old Forest Stand on Chichagof Island, Southeast Alaska

Appendix: Scientific and Common Names of Plant 

Species in This Publicationa

Scientific name Common name

Coptis asplenifolia Salisb. Fernleaf goldthread

Cornus canadensis L. Bunchberry dogwood

Menziesia ferruginea Sm. Rusty menziesia

Oplopanax horridus (Sm.) Miq. Devils club

Picea sitchensis (Bong.) Carrière Sitka spruce

Ribes bracteosum Douglas ex Hook. Stink currant

Rubus pedatus Sm. Strawberryleaf raspberry 

Rubus spectabilis Pursh Salmonberry

Sambucus racemosa L. Red elderberry

Tiarella trifoliata L. Threeleaf foamflower

Tsuga heterophylla (Raf.) Sarg. Western hemlock

Vaccinium ovalifolium Sm. Oval-leaf blueberry

Vaccinium parvifolium Sm. Red huckleberry

a

 Source: Hitchcock and Cronquist (1973) for the Todd protocol and online USDA Plants database for 

the Prince of Wales Island Commercial Thinning data.



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