DEPAR TMENT OF AGRICULT URE

Pacific Northwest 

Research Station

Research Paper

PNW-RP-599

April 2014

United States Department of Agriculture 

Forest Service

Responses of Southeast Alaska 

Understory Species to Variation 

in Light and Simulated Herbivory 

Thomas A. Hanley and Jeffrey C. Barnard
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Authors

Thomas A. Hanley is a research wildlife biologist, and Jeffrey C. Barnard is a 

fish biologist, Pacific Northwest Research Station, Forestry Sciences Laboratory, 

11175 Auke Lake Way, Juneau, AK 99801.

Cover: Upper photo shows a blueberry bush that has been heavily browsed by deer 

in a southeast Alaska forest; current use has greatly reduced its current annual 

growth of leaves and twigs, while past use has resulted in a “hedged” growth form 

of stunted plant size and shortened twigs. Lower photo shows an exclosure in an 

even-aged, young-growth forest on Admiralty Island in southeast Alaska. Constructed 21 years before the photo was taken, the exclosure has protected vegetation 

within it from herbivory by black-tailed deer.
Abstract

Hanley, Thomas A.; Barnard, Jeffrey 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.

Responses to variation in light intensity, simulated herbivory by clipping, and their 

interaction were studied over three seasons in a factorial experimental design. 

Six major species of southeast Alaska forest understories were studied, each as a 

separate experiment: bunchberry, Cornus canadensis L.; threeleaf foamflower, 

Tiarella trifoliata L.; oval-leaf blueberry, Vaccinium ovalifolium Sm.; salmonberry, 

Rubus spectabilis Pursh; devilsclub, Oplopanax horridus (Sm.) Miq.; and western 

hemlock, Tsuga heterophylla (Raf.) Sarg. Seedlings of each species were grown 

in plastic grow pots under three intensities of light in an outdoor garden experiment—full sun in the open (100 percent incident light) and under two natural forest 

canopies, red alder (Alnus rubra Bong.; about 17 percent incident light) and younggrowth conifers (about 1.7 percent incident light). Plants within each light treatment 

were subjected to simulated herbivory by clipping the current year’s growth at the 

end of the first and second growing season at three intensities of clipping—0-percent removal, 50-percent removal, and 100-percent removal (75-percent removal 

for western hemlock). At the end of the third growing season, mean percentage 

survival was determined and total biomass (both below- and aboveground ovendry 

weight) of each surviving plant was measured. Factorial analyses of variance, conducted at the α level of 0.10, indicated that for survival, light was a significant main 

effect for only three of the six species, and clipping was a significant main effect for 

only one of the species (salmonberry, which also was the only species having a significant light by clipping interaction effect). Treatment effects were much stronger 

for mean plant size, however, with light having a significant main effect on all six 

species, clipping having a significant main effect on all but one species (devilsclub), 

and three of the six species having significant light by clipping interaction effects. 

Overall, the results emphasized the predominant importance of light in determining 

plant responses to herbivory, although the light by clipping interaction effect was 

not as strong as hypothesized.

Keywords: Autecology, clipping, black-tailed deer, silviculture, survival, 

growth, biomass, browsing, grazing.
Summary

Responses to variation in light intensity, simulated herbivory by clipping, and 

their interaction were studied over three seasons in a factorial experimental design. 

Six major species of southeast Alaska forest understories were studied, each as 

a separate experiment: bunchberry, threeleaf foamflower, oval-leaf blueberry, 

salmonberry, devilsclub, and western hemlock. Principal forest management 

implications of the results are as follows:

• Light intensity within forest understories, especially in closed-canopy 

young-growth stands, is an overwhelmingly important environmental factor affecting both survival and, especially, growth of understory plants. 

Loss of plant tissue through herbivory is also an important environmental 

factor, but its interaction with light is frequently only an additive rather 

than interactive effect.

• Clipping has a stronger influence on reducing plant size than on reducing 

survival (at least within a 3-year timeframe) in most species. Reduction in 

plant size should interact with foraging efficiency to produce a negative 

feedback loop that reduces subsequent herbivory by animals the size of 

deer.

• However, foraging patch size (area of relatively uniform vegetation) and 

overall density of deer within the surrounding landscape will interact in 

determining the intensity of herbivory for individual plants and patches of 

vegetation.

• Therefore, all considerations of potential effects of herbivory within a silviculture context must be made at a landscape scale, considering the overall suitability of habitat and deer densities, because the level of herbivory 

experienced within any given stand will be strongly influenced by its larger 

landscape context. Herbivory is not a factor that can be evaluated and managed at the stand scale alone. 
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Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

Introduction

Young-growth, western hemlock (Tsuga heterophylla (Raf.) Sarg.)–Sitka spruce 

(Picea sitchensis (Bong.) Carrière) forests of southeast Alaska are commonly very 

depauperate in understory vegetation from the time the canopy closes in the regenerating stand (about 25 to 35 years) through anticipated age of harvest rotation at 

about 100 years or more (Alaback 1982, 1984). Availability of light in the understory environment appears to be the principal limiting factor affecting plant establishment (Tappeiner and Alaback 1989) and growth (Hanley et al. 2014, Rose 1990). 

Once plants are established, their growth rates are strongly dependent on 

the quantity of light, and species differ in their growth curves in relation to light 

(production of biomass as a function of intensity of light): greenhouse light trials 

with five common species showed substantial differences in growth curves in relation to light (Hanley et al. 2014). All four understory species (bunchberry, Cornus 

canadensis L.; oval-leaf blueberry, Vaccinium ovalifolium Sm.; salmonberry, Rubus 

spectabilis Pursh; and devilsclub, Oplopanax horridus (Sm.) Miq., grew more than 

western hemlock at low intensities (<4 mol m-2 day-1), while western hemlock did 

best at high intensities of light (>15 mol m-2 day-1). Growth rates among the nonconifer species also changed with light intensities: bunchberry was greatest relative 

to the others at lowest light, and blueberry and devilsclub were greatest at highest 

light. In manipulation field experiments with the same species, four of the five species showed greater production under a red alder (Alnus rubra Bong.) canopy than 

under a conifer canopy (bunchberry responded the same to both light treatments), 

but soil type was important only for two species (Hanley et al. 2014). Understory 

light intensity was low in both forest canopies but was more than a full order of 

magnitude greater under alder (about 19.5 percent of unobstructed incident light) 

than under conifers (about 1.7 percent). 

Interestingly, all five species in the above field experiments survived and grew 

(albeit slowly) when planted as seedlings under the conifer canopy, even though the 

conifer understory was nearly devoid of vascular vegetation. All plants were protected by fencing from mammalian herbivores. A similar understory response to 

fencing was observed by Hanley (1987) in an examination of understory response

to deer exclosures in southeast Alaska: the one exclosure that occurred in an evenaged stand (about 80 years old, at Pybus Bay on Admiralty Island) had a total vascular understory biomass of 782 kg/ha inside the exclosure and only 31 kg/ha outside. Although 60 percent of the biomass within the 21-year-old exclosure consisted 
2

research paper pnw-rp-599

of western hemlock seedlings, 309 kg/ha was in the form of shrubs and herbs. The 

low biomass of understory outside the exclosure was typical of that expected for 

stands of that age (Alaback 1982), yet the exclosure demonstrated that herbivory by 

Sitka black-tailed deer (Odocoileus hemionus sitkensis Merriam) had played a major 

role in the understory vegetation at the Pybus Bay site. The combination of the field 

experiments (Hanley et al. 2014) and the earlier Pybus Bay observations indicates 

that mammalian herbivory might be an important factor in understory dynamics at 

low levels of light such as commonly occur in young-growth, even-aged stands—

where plants can survive but cannot withstand browsing. However, until our experiments reported here, there have been no experimental tests of that idea.

We tested the hypotheses that both (1) survival and (2) growth of understory 

plants exposed to simulated herbivory by clipping are interactive with the intensity 

of light in the environment. Specifically, we expected that survival and growth 

would decrease with increasing intensity of clipping (amount of living tissue 

removed) and with decreasing intensity of light and, especially, that the interactive 

effects of high clipping and low light would be significantly greater than an additive 

effect.

Methods

The experiments were conducted in Juneau, Alaska, in an outdoor “garden” setting 

with all plants growing in 3.8-liter plastic grow pots and commercial potting soil 

through three growing seasons. We used six species of understory plants: the evergreen forbs bunchberry and threeleaf foamflower (Tiarella trifoliata L.); the shrubs 

oval-leaf blueberry, salmonberry, and devilsclub; and the conifer western hemlock, 

all of which are common understory species and significant in black-tailed deer 

diets in southeast Alaska. Western hemlock is a nutritionally marginal forage, but 

it is commonly eaten in winter (Hanley and McKendrick 1985, Parker et al. 1999), 

and it is a very important competitor with other understory species in young-growth 

forests (Deal and Farr 1994). We conducted the experiments separately for each 

species, subjecting them to three intensities of clipping and three intensities of 

light. The clipping intensities were 0, 50, and 100 percent removal of aboveground 

current year’s growth (leaves and current year’s twigs of shrubs and hemlock, all 

aboveground tissue of bunchberry and foamflower), except for western hemlock, 

where clipping intensities were 0, 50, and 75 percent removal because we thought 

100 percent removal would have caused death every time. Clipping treatments were 
3

Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

imposed twice (at the end of the first and second growing seasons) over the threegrowing-season timeframe.1

 The light intensities were full sunlight (100 percent 

unobstructed incident light) and much-reduced light intensities under the same 

forest canopies of red alder (about 20 percent) and young-growth western hemlock–

Sitka spruce (about 2 percent) used in the Hanley et al. (2013) light and soil experiments conducted a year earlier. The alder and conifer stands were natural stands, 

25 to 33 years old, one of each type, and were chosen simply to provide two widely 

different light intensities typical of young-growth forests of the region. Because the 

alder and conifer stands were not replicated (several stands of each), they cannot be 

considered representative of “red alder” or “conifer” overstories generally. 2

Each plant species was represented by 24 individual plants allocated to each of 

the three clipping by three light treatments (nine treatments total), for a total of 216 

plants per species. Each light treatment was spatially replicated twice within each 

stand, with 12 plants of each species (each plant being a subsample) in each of the 

two replicates. Although for statistical power it would have been preferable to have 

more replicates and fewer subsamples (for the same number of plants), we needed 

a relatively large number of subsamples per replicate for quantifying survival 

(percentage remaining alive). Clipping treatments were assigned randomly to each 

plant within each replication of each light treatment and remained constant for both 

clipping times. All of the 18 treatment areas (nine treatments × two replications) 

were fenced with hardware cloth, 1.5-m high, to protect them from herbivory by 

mammals. 

All plants were obtained from a commercial nursery as seedlings in excess of 

the number needed, and were sorted to select the 216 most similarly sized plants 

within each species for the experiments. The 216 plants of each species were transferred to 3.8-liter grow pots filled with horticultural-grade soil mixes (peat-based 

with vermiculite and lime added). Individually potted plants were then randomly 

assigned to each of the 18 replicated treatments (two replicates times three light by 

1

 Note that the clipping treatments were not intended to exactly replicate browsing by 

deer, in that the intensities of clipping (0, 50, 75 or 100 percent), the plant tissues removed 

(current annual growth), and the timing of clipping (end of growing season) were fixed for 

all species, not directed at species-specific plant parts or seasons. We wanted to be able to 

compare the six species in their responses to removal of current annual growth.

2

 Ideally, we would have replicated our experimental design with several alder and several 

conifer stands and considered the stands as replicates (instead of the spatial replication 

within each stand); then we could generalize our results to “alder” and “conifer” canopy 

types. However, only one stand of each was available to us for this work, so we must 

restrict our conclusions to only those two stands (i.e., a “case study” as far as canopy type 

goes). On the other hand, the two stands provided sharp contrasts in their light environments, which allowed us to test the species’ responses to light and clipping within light 

treatments, which was our principal objective.
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research paper pnw-rp-599

three clipping) with 12 plants in each replicate and were placed in the field within 

the second week of May of the first season. Fertilizing and watering were kept to a 

minimum throughout the experiments, once per season (general purpose fertilizer) 

and only as necessary (water), and then applied equally to all treatments. All plants 

were covered with straw during winter to protect them from hard frost.

In late August before leaf-fall in the first and second year, plants were subjected 

to the clipping treatment to which they had been assigned, visually estimating the 

percentage of biomass removed. At the end of the third growing season (same time 

in late August), survivorship (number of plants alive) was determined for all treatments and replicates, the experiments were terminated, and all plants were returned 

to the lab in their pots instead of being clipped. In the lab, each surviving plant 

was gently removed from its pot and all soil was washed from its roots. The total 

biomass (both above- and belowground) of each plant was then ovendried (100 °C 

for 24 hours) and weighed (to 0.01 gram) to provide a relative measure of its growth 

throughout the experiment. This was a relative (not absolute) measure of growth, 

because it was impossible to weigh each plant at the beginning of the experiment.

Ambient light was measured with a sunfleck PAR (photosynthetically active 

radiation—i.e., the wavelengths of light that plants are able to use in the process 

of photosynthesis) ceptometer (Decagon Devices, Inc.,3 Pullman, Wash.) at 30 

cm aboveground in the alder and conifer stands and adjacent opening at midday 

throughout each growing season. The mean light level within each of the two 

stands, expressed as a percentage of full sunlight in the open area, provided a 

measure of relative differences between the overstory types.

The survival and biomass data were tested for statistical significance in a 3 by 3 

factorial analysis of variance with the Mixed Procedure of SAS (SAS Institute, Inc. 

2004). Survival (mean percentage of original plants remaining alive) and biomass 

(mean ovendry weight per surviving plant, grams) were analyzed separately and 

independently for each species. Survival data (percentage) were transformed with 

the natural log transformation; biomass data (grams) were not transformed. We 

used the Tukey multiple comparisons test for determining significant differences 

among means when main effects or interactions were significant. Because statistical 

power was relatively weak with only two replicates per treatment, all tests of

3

 The use of trade or firm names in this publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of any product or service.
5

Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

statistical significance were conducted at an α level of 0.10, and the multiple 

comparisons tests of main effects were one-tailed: we expected both survival 

and biomass to decrease with decreasing light and increasing clipping levels.

Results 

Midday light intensities within the alder and conifer understories averaged 17.03 

(+ 1.07 SE) and 1.69 (+ 0.62 SE) percent, respectively, of that in the open (711.4 

μmol m-2 sec

-1). Those are virtually the same as the values reported for the same 

two stands in the Hanley et al. (2014) experiments conducted there 1 year earlier 

(19.5 and 1.7 percent, respectively, of 789 μmol m-2 sec

-1 in the open).

Although we had been surprised at the high survival rates of potted plants 

under the conifer canopy in the earlier one-season study of light and soils (Hanley 

et al. 2014), the low light intensity under the conifer canopy took a fairly heavy 

toll on survival of several species after three seasons in this study (table 1; fig. 1). 

Survival of bunchberry, foamflower, and devilsclub was not affected by light 

intensities (table 1; no statistically significant differences), but survival of blueberry, 

salmonberry, and western hemlock was lowest under the conifer canopy and ranged 

from only 1.3 to 28.2 percent. Surprisingly, clipping had a statistically significant 

effect on survival of only salmonberry, and that was also the only species that 

exhibited a significant interaction effect between light and clipping (table 1): 

salmonberry survival was virtually nil under the conifer canopy and was nil in the 

100 percent clipping treatment regardless of canopy type (fig. 1). Although survival 

of all species was unaffected by canopy differences during the first growing season 

(before clipping treatments were applied), differences in survival started becoming apparent after the second growing season, especially under the conifer canopy 

and regardless of clipping treatment (app. 1; data not analyzed statistically), and 

progressed further in year 3 (fig. 1). The strong effect of light, therefore, seems to 

compound with time in its effect on the survival of the more light-sensitive species, 

regardless of the potential effects of herbivory. And, at least in terms of survival, 

clipping was not a strong factor, either per se or in its interaction with light.

On the other hand, in contrast to survival, the effects of both light and clipping 

were much more pronounced on plant size (mean biomass per surviving plant, 

table 2 and fig. 2). All species exhibited statistically significant differences in the 

main effect of light; all but devilsclub exhibited significant differences in the main 

effect of clipping; and three species (bunchberry, blueberry, and salmonberry) had 

statistically significant interaction effects (table 2) with the combination of low light 
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research paper pnw-rp-599

Table 1—Mean survival (percentage of original plants surviving to end of experiment) 

within each of the main effects of the light by clipping factorial experiment and 

statistical significance of the interaction effect for each of the six species 

Light Clipping (percentage) Interaction 

Significance 

Species Open Alder Conifer 0 50 100 (Pr > F)

Bunchberry 100.00a 94.10a 49.77a 84.15a 83.40a 76.32a 0.895

Foamflower 94.02a 91.55a 71.47a 93.82a 90.34a 72.85a 0.506

Blueberry 79.10a 44.42a,b 28.20b 56.03a 56.75a 38.93a 0.877

Devilsclub 90.18a 53.07a 41.95a 72.60a 62.77a 51.83a 0.375

Salmonberry 16.95a 24.17a 1.27b 27.02a 15.37a 0

b 0.027*

Western hemlock 100.00a 98.83a 8.68b 72.17a 67.77a 67.58a 0.984

Note: The maximum clipping treatment for western hemlock was 75 percent, not 100 percent.

Values with the same superscript within treatments within rows do not differ significantly. Statistical 

significance of the interaction effect is indicated by the probability of the null hypothesis (no interaction) 

being greater than the calculated F value (“Pr > F”), with significance indicated with an asterisk. All tests 

of statistical significance were at the α = 0.10 level (i.e., Pr > F less than 0.10).

Figure 1—Mean (+ standard error) survival (percentage of original plants remaining alive) at the end of the experiment in year 3, after 

2 successive years of clipping treatments at the end of the first and second growing seasons. See table 1 for statistically significant main 

effects. Statistically significant interactions are indicated here with alphabetic labels over each bar (canopy-clipping treatment); bars 

with same letter do not differ at the α = 0.10 level (one-tailed; Tukey multiple comparisons test). Although survival data were natural 

log-transformed for analysis, they have been plotted without transformation, and standard errors are those calculated directly for each 

treatment combination rather than the pooled values from the analysis of variance to better show the actual means (untransformed) and 

the variation within each treatment.

100

80

60

40

20

Mean survival (percent)

0

Open Alder Conifer

Canopy

Foamflower

100

80

60

40

20

Mean survival (percent)

0

Open Alder Conifer

Canopy

Salmonberry

a

a

a

b

100

80

60

40

20

Mean survival (percent)

0

Open Alder Conifer

Canopy

Bunchberry

0% clipping

50% clipping

100% clipping

100

80

60

40

20

Mean survival (percent)

0

Open Alder Conifer

Canopy

Western hemlock

100

80

60

40

20

Mean survival (percent)

0

Open Alder Conifer

Canopy

Blueberry

100

80

60

40

20

Mean survival (percent)

0

Open Alder Conifer

Canopy

Devilsclub

b b b b

b

a

0% clipping

50% clipping

100% clipping

 (for Western 

 hemlock only, 

 75% clipping)
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Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

Table 2—Mean total biomass (ovendry grams) per surviving plant within each of the main 

effects of the light by clipping factorial experiment and statistical significance of the 

interaction effect for each of the six species 

Light Clipping (percentage) Interaction 

significance

Species Open Alder Conifer 0 50 100 (Pr > F)

Bunchberry 6.31a 1.90b 0.22c 4.24a 2.66b 1.54c 0.001*

Foamflower 8.28a 3.98b 0.71c 6.85a 4.06a,b 2.07b 0.326

Blueberry 2.79a 0.37b 0.09b 1.35a 1.33a 0.58b 0.090*

Devilsclub 48.02a 34.92a 6.49b 37.78a 29.82a 21.84a 0.193

Salmonberry 12.56a 10.84a 0.47b 12.22a 11.65a 0

b 0.021*

Western hemlock 27.97a 15.28b 1.65c 19.89a 13.46b 11.55b 0.164

Note: The maximum clipping treatment for western hemlock was 75 percent, not 100 percent.

Values with the same superscript within treatments within rows do not differ significantly. Statistical significance of the 

interaction effect is indicated by the probability of the null hypothesis (no interaction) being greater than the calculated 

F value (“Pr > F”), with significance indicated with an asterisk. All tests of statistical significance were at the α = 0.10 

level (i.e., Pr > F less than 0.10).

intensity and high clipping intensity resulting in strongly reduced plant sizes 

(fig. 2). Therefore, even the most shade-tolerant species in terms of survival (bunchberry, foamflower, and devilsclub) were strongly affected in size by light, and the 

size of virtually all species (except devilsclub) was affected by clipping, too—light 

providing the basis for accumulating biomass, and clipping removing biomass and 

reducing subsequent growth.

We are able to assess the combined effects of survival and growth for each 

species by analyzing the total biomass of all surviving plants (combined) of that 

species within each replicate of the experimental treatments. That measure of total 

biomass effectively quantifies what one would see visually or measure in fieldsampling biomass per unit area 3 years after the same initial starting conditions for 

each treatment, except that our data included belowground biomass. Those results 

are fairly similar to the effects on plant size for both light and clipping: significant 

main effect of light for all species; significant main effect of clipping for all species 

except devilsclub; and significant light-clipping interaction for bunchberry and 

blueberry (table 3). The overall pattern of response to treatments was similar to 

that for plant size in all species except salmonberry (compare figs. 2 and 3), where 

the relatively low survival rates greatly depressed total biomass per replication in 

both the open and alder light treatments, too (compare figs. 1 and 3). Thus, the total 

quantity of plant biomass growing within light and clipping treatments was most 

strongly affected by differences in plant size, which was strongly affected by both 

amount of light and intensity of clipping, but usually (four of the six species) in only 

an additive manner (i.e., light and clipping effects were mostly additive rather than 

strongly interactive).
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research paper pnw-rp-599

Figure 2—Mean (+ standard error) biomass (ovendry weight, grams) per surviving plant at the end of the experiment in year 3, after 2 

successive years of clipping treatments at the end of the first and second growing seasons. Biomass is entire plant weight, both aboveand belowground parts. See table 2 for statistically significant main effects. Statistically significant interactions are indicated here with 

alphabetic labels over each bar (canopy-clipping treatment); bars with same letter do not differ at the α = 0.10 level (one-tailed; Tukey 

multiple comparisons test).

Discussion

Our results were both consistent with and contradictory to our expectations regarding the roles of light and clipping on understory vegetation. They were consistent 

with our expectation that both light and clipping would be strong factors affecting 

plant growth (biomass), but were contradictory to our expectations that both light 

and clipping would also be strong factors affecting plant survival and, especially, 

that the interaction of light and clipping intensities would have a strong effect on 

virtually all measures of plant performance. Differences in these relations, of 
9

Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

course, existed among species, but overall we were surprised how light and plant 

size tended to predominate over clipping and survival and that light-clipping 

interaction effects did not predominate over all.

Western hemlock was the most shade-intolerant species, and bunchberry was 

the most shade-tolerant species at low levels of light in the earlier 1-year study in 

these same stands (Hanley et al. 2014). These were important findings, because 

western hemlock seedlings can be strong competitors with other understory vegetation (Alaback and Herman 1988, Deal and Farr 1994, Doerr and Sandburg 1986), 

and bunchberry is one of the most important yet difficult species to maintain in 

young-growth forests of the region (Hanley 1993, 2005). In this 3-year study, 

however, those differences in species responses were evident in survival (table 1) 

but not in plant size (table 2) or total biomass per replicate (table 3). Survival, size, 

and total biomass were strongly suppressed in western hemlock under the conifer 

canopy, but low light intensity also strongly suppressed size and total biomass of 

bunchberry. Clipping had relatively little effect on survival in both species (table 1), 

but both size and total biomass of both species were strongly affected by clipping 

(tables 2 and 3), with bunchberry showing significant interaction effects of light 

and clipping treatments (figs. 2 and 3). The other species were mostly intermediate 

in their responses to both light and clipping, although salmonberry was especially 

sensitive to both, devilsclub was least sensitive to clipping, and foamflower (a species not studied in the Hanley et al. 2014 study) was an especially strong survivor 

under both treatments (figs. 1 through 3). 

Table 3—Mean total biomass (ovendry grams) per replicate (sum of all surviving plants within a 

replicate) within each of the main effects of the light by clipping factorial experiment and statistical 

significance of the interaction effect for each of the six species 

Light Clipping (percentage) Interaction 

significance 

Species Open Alder Conifer 0 50 100 (Pr > F)

Bunchberry 75.75a 21.84b 2.25c 50.14a 31.75b 17.95c 0.001*

Foamflower 94.42a 45.69b 7.36c 78.73a 47.06b 21.69b 0.275

Blueberry 32.26a 2.61a 0.61b 14.79a 15.42a 5.27b 0.054*

Devilsclub 525.90a 377.30b 36.23c 343.10a,b 392.57b 203.75a 0.123

Salmonberry 38.53a 48.43a 0.47b 57.79a 29.60a,b 0.00b 0.409

Western hemlock 339.53a 185.43b 3.62c 233.73a 156.94b 137.91b 0.156

Note: The maximum clipping treatment for western hemlock was 75 percent, not 100 percent.

Values with the same superscript within treatments within rows do not differ significantly. Statistical significance of the interaction 

effect is indicated by the probability of the null hypothesis (no interaction) being greater than the calculated F value (“Pr > F”), 

with significance indicated with an asterisk. All tests of statistical significance were at the α = 0.10 level (i.e., Pr > F less than 0.10).
10

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Figure 3—Mean (+ standard error) total biomass (ovendry weight, grams) per replicate (all surviving plants combined within a replicate) 

at the end of the experiment in year 3, after 2 successive years of clipping treatments at the end of the first and second growing seasons. 

Biomass is entire plant weight, both above- and belowground parts. See table 3 for statistically significant main effects. Statistically 

significant interactions are indicated here with alphabetic labels over each bar (canopy-clipping treatment); bars with same letter do not 

differ at the α = 0.10 level (one-tailed; Tukey multiple comparisons test). This measure of biomass per replicate combines the effects of 

both survival (fig. 1) and biomass of surviving plants (fig. 2) into one measure of standing crop.

Clipping was intended to roughly simulate herbivory by a large herbivore like 

deer. However, the relatively extreme responses of salmonberry and devilsclub were 

probably related to an unrealistic way our clipping treatment (removal of current 

annual growth, both leaves and twigs) affected them. Both those species are eaten 

by deer only in summer, and leaves are virtually the only part eaten; deer eat very 

little, if at all, of the twigs of those two species (Parker et al. 1999). Thus, our clipping treatments of salmonberry were more severe than salmonberry would likely 
11

Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

experience under herbivory; and devilsclub twig growth was minor, so clipping 

mainly removed the leaf, which would have fallen relatively soon thereafter, anyway, as the deciduous plant went dormant at end of the growing season. Devilsclub 

biomass was strongly affected by light but not by clipping (tables 2 and 3).

These 3-year results (for all species) emphasize the importance of time in autecological field studies. Although studies of only one growing season can highlight 

important differences among species and major environmental factors, they cannot 

include effects that accumulate or compound with time. Our data for survival illustrate that most clearly, where survival was 100 percent for all species and habitats 

at the end of the first growing season (at which time the clipping treatments were 

then imposed), but began to decrease with the light effect (regardless of clipping) 

by the end of the second season (app. 1) and were pronounced by the end of the 

third season (fig. 1). We don’t have time-sequence data for biomass, but the relative 

difference in bunchberry response to one year (Hanley et al. 2014) versus 3 years of 

light treatment (figs. 2 and 3 for 0 percent clipping) indicates a strong compounding 

effect over time. Furthermore, there is no reason to assume that the compounding 

had reached a culmination after 3 years; it may be that none of our species could 

persist under our dense conifer canopy indefinitely. 

The overriding importance of light in both this study and the earlier one 

(Hanley et al. 2014) indicates that variation in overstory canopy must be especially 

important in forests of southeast Alaska, especially in the dark, closed-canopy 

forests of young-growth stands (with typically <2 percent of unobstructed incident 

light—Rose 1990, Tappeiner and Alaback 1989). Greenhouse light-gradient trials 

with these same species indicated approximate thresholds of about 4 mol m-2 day-1

below which western hemlock does more poorly than the other species and about 

15 mol m-2 day-1 above which it does much better than the other species in terms 

of growth (Hanley et al. 2014). Using data from table 1 of Hanley et al. (2014), the 

corresponding mid-day light intensities would be about 100 and 400 μmol m-2

 sec

-1, 

which would be approximately 13 and 53 percent, respectively, of an average 750 

μmol m-2 sec

-1 intensity of unobstructed incident light (average of the two studies 

for light in the open—see opening paragraph of “Results” section4). Therefore, the

4

 For perspective, measured average daily July irradiance (including effects of cloud cover) 

in southeast Alaska understory forests have ranged from <10 μmol m-2 sec-1 in closedcanopy young-growth forests, to 10 to 50 μmol m-2

 sec-1 in dense-canopied old growth, to 

50 to 300 μmol m-2 sec-1 in more open-canopy old growth and low-volume timber stands, 

and a maximum of 400 to 700 μmol m-2 sec-1 in open clearings (Rose 1990 and C.L. Rose 

unpublished data). Those values are about half of the maximum values measured on cloudfree days (Hanley et al. 2014, Rose 1990, Tappeiner and Alaback 1989).
12

research paper pnw-rp-599

low-light “window of opportunity” (Hanley et al. 2014) favoring understory species other than hemlock would be somewhat less than that under our alder canopy 

(17 percent), and the zone of relatively similar competitive abilities would range to 

much greater than that under our alder. These calculations are very simplistic, but 

they illustrate that a wide range of canopy variation within young-growth forests 

could be highly beneficial to understory species without necessarily tipping the 

competitive advantage to western hemlock seedlings. Even small canopy gaps 

and sunflecks could be a big improvement over the mean low light penetration in 

young-growth forests (Chazdon 1988).

High natural heterogeneity in site and disturbance is the norm for natural stands 

of forest in southeast Alaska, where major variation in soils commonly occurs at 

the scale of only a few (e.g., 20) square meters (Bowers 1987) and wind and disease 

are continuously at play (Harris and Farr 1974). Young-growth stands are much 

more homogeneous than old-growth stands in the region (Alaback 1984), but they 

nevertheless include substantial variation in tree spacing and size, overstory canopy 

coverage, and understory vegetation, especially at the entire stand scale (in contrast 

to small research plots) (Hanley 2005). That variation in stand structure provides 

variation in the understory light environment and results in strong effects on variation in understory species composition and biomass (Hanley and Barnard 1998). 

Our results from this study indicate that even small patterns of variation within 

a densely canopied young-growth forest would result in heterogeneous spatial 

patterns in survival, size, and biomass of understory species. Such heterogeneity 

is probably key to long-term persistence of understory species in young-growth 

forests of the region.

Spatial pattern, size, and biomass also are important factors affecting the 

foraging efficiency of herbivores the size of deer (Gross et al. 1993, Spalinger and 

Hobbs 1992), with clumping of forages (concentrated patches), large leaf size, and 

high biomass increasing foraging efficiency (net energy or nutrient intake per unit 

time spent feeding—i.e., time spent searching for, biting, chewing, and swallowing 

food), which has strong effects on net energy balance, body growth, and productivity (Parker et al. 1996, 1999). Of the three variables, however, biomass is the least 

important (Shipley and Spalinger 1992), and leaf size (equals bite size) is most 

important (Spalinger et al. 1988), although clumping certainly interacts with leaf 

size (Shipley and Spalinger 1992). Reasons are that time spent traveling between 

plants and time spent biting instead of chewing is lost, while the most profitable 

foraging time is spent chewing and swallowing, which is maximized by eating large 
13

Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

bites of plants in close proximity. Therefore, the effects of herbivory by deer, especially in combination with those of light intensity in relatively closed-canopy forest, 

create a self-dampening feedback loop where herbivory results in decreased plant 

size of most species and decreased survival (distribution) in many species, which 

in turn decreases foraging efficiency. The relative attractiveness of the browsed 

patch decreases as foraging efficiency decreases (Kohlmann and Risenhoover 1994, 

Langvatn and Hanley 1992, Roese et al. 1991), so herbivory decreases as foraging 

time decreases. This, of course, is all at the scale of habitat patches or stands of 

forest; patterns of habitat use can shift only when alternative patches exist.

Canopy gaps, especially in dense young-growth forest where understory vegetation is otherwise sparse, should be attractive to large foragers like deer, because 

understory plants there should be relatively abundant, large, and clumped, making 

the gaps especially favorable places for foraging. Deer should be expected to focus 

attention on such gaps until foraging efficiency is reduced by reduction in plant 

size and survival (the self-dampening feedback loop) to levels similar to alternative 

habitats. If light intensity is high enough to give a competitive advantage to conifers, and if the conifers are less palatable to deer than the other understory species, 

then the herbivory should hasten understory dominance by conifers (Hanley and 

Taber 1980), possibly even resulting in local exclusion of the palatable species. 

Management Implications

To date, there have been few studies of the autecology of understory species in 

southeast Alaska forests, but all those that have been conducted implicate light 

intensity as the primary limiting factor in virtually all stages (except germination) 

of plant life histories, especially under low light intensities of densely canopied 

forests: Once seedlings germinate, their success at becoming established is directly 

related to the light intensity of their microenvironment (Tappeiner and Alaback 

1989) and so is their subsequent rate of growth (Hanley et al. 2014, Rose 1990, 

Tappeiner and Alaback 1989, this study), vegetative reproduction (Tappeiner and 

Alaback 1989), and sexual maturation (in the case of blueberry, Alaback and 

Tappeiner 1991). Given that flowering, seed, and fruit production are carbon- and 

energy-intensive processes, it is highly likely that they, too, would be driven by light 

intensity, at least as threshold responses (Rose 1990, Waring and Schlesinger 1985). 

Thus, forest managers, especially silviculturists, who are concerned about understory plant communities and production need to think about the understory light 

environment not only at both stand and landscape scales but also even at microsite 

scales (i.e., within-stand heterogeneity), and not only at the present time but also in 
14

research paper pnw-rp-599

terms of dynamics within seasons and future years. Hanley et al. (2014) identified 

two thresholds of light intensity where non-conifer understory species are favored 

at low intensities and western hemlock seedlings are favored at high intensities. We 

estimated those thresholds to be about 13 and 53 percent, respectively, of canopyunobstructed incident light by very simplistic calculations (see “Discussion” section 

above). The exact values are not important; both are only very rough estimates. 

What is important is that they indicate a fairly wide range of latitude in managing 

light environments for understory vegetation before favoring western hemlock 

too strongly. However, when ground-layer plants such as evergreen forbs are of 

special concern (e.g., Hanley 1993), then shading effects of the shrub layer also 

come into play. The naturally high degree of site and canopy heterogeneity and the 

high-frequency, low-magnitude disturbance regimes of old-growth forests provide 

a naturally diverse, dynamic, and mid-intensity light environment for understory 

vegetation. Artificially creating and maintaining such rich light environments in 

young-growth forests might not be economically feasible, but the ecological principles are still the same and need to be considered throughout.

Our clipping treatments, to simulate removal of biomass by herbivory, were 

relatively moderate (50 percent removal) and severe (100 percent removal; 75 percent for hemlock). The 50-percent removal is comparable to the old adage of “take 

half, leave half” in range management (Stoddart et al. 1975) for species lacking 

more precise “proper-use factors,” whereas the 100-percent removal would be 

considered severe under any possible criterion. The clipping treatments imposed 

a significant effect on most species (except devilsclub) for growth but not survival 

(yet). Although the most severe treatment resulted in the smallest sized plants in 

all six species, it was significantly different from the moderate treatment in only 

half of the species (table 2). We suspect that the difference in treatment effects must 

compound with time, however, and that two clipping episodes (each at the end of 

a growing season) combined with the experimental error associated with variation 

between replicates was simply not enough to yield statistically significant differences yet. But the most important result from our clipping data was the finding that 

the clipping effect was only additive instead of interactive with light for four of the 

six species (table 3), meaning that clipping affected light-stressed plants in the same 

way that it affected open-grown plants. 

Now we can see how these understory plants interact with light and herbivory 

(both important environmental factors) in southeast Alaska forests: light predominates in determining virtually all life-history events, and understories of old-growth 

forests are so much more productive than those of dense-canopied young-growth 
15

Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

forests because they have much more light at the critical low-end of light intensities where growth rates are nonlinear and most sensitive to light (Hanley et al. 

2014, light-gradient trials). On the other hand, light intensities in closed-canopy 

young-growth forests are so low that even small variation in canopy structure (gaps, 

sunflecks) can make the difference of species’ persistence (survival) over time. Herbivory exerts a cost on the browsed plant, which even though only additive can be 

sustained much more easily in high-light environments than low-light environments 

simply because the growth margin is so much lower in low-light environments. 

Thus, for any given density of deer on a landscape, open environments (especially, 

productive, fast-growing recent clearcuts) should be able to sustain relatively high 

levels of herbivory (when measured as kilograms of forage removed per hectare), 

while dark, young-growth forests will be especially sensitive. The negative feedback loop between herbivory and foraging efficiency should provide some buffer 

for forage-poor understories, but if they occur as small patches within otherwise 

productive landscapes, they may experience a high level of herbivory disproportionate to their relative value as habitat simply because of the overall high density of 

deer in the general area (e.g., use by deer while traveling through the patch). Similarly, small patches of highly productive forage in an otherwise unproductive landscape (e.g., small gaps within a large area of closed-canopy young-growth forest) 

should be especially attractive to deer and therefore suffer high levels of herbivory 

and decreased growth rates disproportionate to what would be expected on the basis 

of their light environment and the overall low density of deer on the landscape (i.e., 

deer will actively seek them). Deer population densities, however, are in constant 

flux with annual variation in weather (e.g., especially winter snow), changing 

vegetation patterns with disturbance, and lag effects in population dynamics (body 

condition, fertility, natality, fawn survival, and demographic structure). Where 

deer population densities have remained high for extended periods of time (e.g., 

Coronation Island), herbivory can reach levels high enough to significantly reduce 

forage plant production in even the most productive and open habitats and affect 

plant community structure landscape-wide (Klein 1965, Lewis 1992). Over most 

of southeast Alaska, however, deer population densities have been low enough to 

not exert such a profound influence, and effects of herbivory have probably been 

most pronounced at the scale of small patches (forest stands or less) and deep-shade 

environments (Hanley 1987). 

The “interesting” examples of potential consequences of herbivory mentioned 

in the “Introduction” can now be understood more clearly: (1) the high survival of 

our experimental plants in the dense young-growth forest (Hanley et al. 2014) was 

observed because we were seeing only the first year of treatment, and one year 
16

research paper pnw-rp-599

alone is insufficient to judge the prospects for long-term survival; and (2) the high 

productivity of vegetation inside the deer exclosure at Pybus Bay indicates that 

the light intensity within that stand must surely have been greater than the light 

intensity under our conifer canopy in this experiment. The sharp contrast between 

inside and outside the Pybus Bay exclosure, however, illustrates the strong effect 

that herbivory can exert on a small stand of young-growth forest when deer population density is relatively high in the larger surrounding landscape (the Pybus Bay 

exclosure was in deer winter range on Admiralty Island, long known for its excellent deer habitat).

Finally, we offer the following points specifically for silviculturists who are 

developing plans and prescriptions that include consideration of deer habitat in 

southeast Alaska:

• Light is the most critical environmental factor in these forests, especially at 

the low intensities common in closed-canopy young-growth forests.

• Variation in canopy structure, and therefore light patterns within the understory, should be very important for maintaining and producing understory 

vegetation, including diversity in species composition.

• Beware of creating small, productive patches within otherwise marginal, 

forage-poor deer habitat, because herbivory focused there can negate the 

desired understory effects. This is a caution about “canopy gap” silviculture treatments for deer habitat on conifer-suitable soils (despite Alaback’s 

recent favorable report for gap treatments5

)—the overall density of deer is 

the especially critical factor. Also beware of investing limited resources in 

silviculture treatments to improve relatively small young-growth stands that 

occur within a larger landscape of relatively high deer densities, because 

the disproportionately high levels of herbivory (disproportionate to the foraging value of the stands) in those stands may negate the efforts to improve 

their light environments.

• All considerations of potential effects of herbivory within a silviculture 

context must be made at a landscape scale, considering the overall suitability of habitat and deer densities, because the level of herbivory experienced 

within any given stand will be strongly influenced by its larger, landscape 

5

 Alaback, Paul. 2010. An evaluation of canopy gaps in restoring wildlife habitat in second 

growth forests of southeastern Alaska. Unpublished final report to the Tongass National 

Forest, Craig and Thorne Bay Ranger Districts, Craig and Thorne Bay, Alaska. 32 p. 

http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5404637.pdf.
17

Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

context. In other words, deer herbivory is an important environmental factor affecting understory plants, but it is not a factor that can be evaluated 

and manipulated at the stand scale alone. When local deer densities have 

been monitored and are relatively well understood, that information will be 

especially useful. In the more usual case where deer data are few and relative density is judged on the basis of sign (abundance of fecal pellets, evidence of browsing), a suitable habitat evaluation model (e.g., FRESH-Deer, 

Hanley et al. 2012, especially its geographic information system application) could be a very useful complement. 

Acknowledgments

We thank the following people for their helpful reviews, comments, and improvements of an earlier version of the manuscript: Bernard Bormann, Elizabeth Cole, 

and Pat Cunningham. We thank Mark Miller for his help with conducting the 

fieldwork.

English Equivalents

When you know: Multiply by: To find:

Meters (m) 3.28 Feet

Hectares (ha) 2.47 Acres

Liters (L) .265 Gallons

Grams (g) .0352 Ounces

Kilograms (kg) 2.205 Pounds

Degrees Celsius (°C) 1.8 °C + 32 Degrees Fahrenheit

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Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

Appendix 1

Mean (+ standard error) survival (percentage of original plants remaining alive) at 

end of the second growing season, one year after clipping treatments were applied 

at end of first growing season. These data were not analyzed statistically. 
22

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Appendix 2: About the Experimental Design 

As explained in footnote 2, our experimental design lacked replication of the alder 

and conifer light treatments because we had only one stand of each available to us 

for this work. Therefore, although the two canopies provided very different light 

intensities for testing our hypotheses about light and its interaction with clipping, 

our analysis cannot say anything about differences between “alder” and “conifer” 

canopies per se. Also, we must acknowledge that, from a strictly technical point of 

view, our light-intensity treatment can be viewed as a case of “pseudoreplication” 

instead of true replication, because with only one stand of each overstory there was 

not spatial intermixing of the light treatments. It could be argued that some other, 

unspecified environmental factor may have varied spatially with canopy, too, and 

effects of that unknown factor would not be distinguishable from the effects of 

light. 

However, from a practical and much more realistic point of view, we point out 

that the difference in light intensities between the two overstories was a full order 

of magnitude, and the two stands were adjacent to one another and both on nearlevel ground (i.e., no difference in elevation, slope, exposure, etc.). Light intensity 

in the open area was nearly 6 times that of the alder understory and 60 times that 

of the conifer understory. Any other factor that we can think of (e.g., light wavelengths, relative humidity, air temperature) would have been independent of spatial 

replication and therefore not affected by “true replication” (keep in mind that all 

our plants were in pots, and truly replicated environments would have been similar 

among replicates). Therefore, unless one argues the potential existence of some 

unknown factor that depended on spatial location rather than overstory canopy, we 

argue that the case for bias from pseudoreplication is exceptionally weak here—far 

too weak to outweigh the insights from the factorial analysis of the data.

Problems from correlation among potential factors independent of true replication (e.g., light, relative humidity, and air temperature all varying together with 

stand type) are inherent in field experiments. They can’t be controlled by replication—they require a laboratory setting (e.g., growth chambers), but that makes the 

experiment even more removed from the natural environment. Our experiments, 

using replicated clipping treatments within pseudoreplicated light treatments were 

a compromise between the natural environment and a more rigorously controlled 

experiment. Our conclusions and implications must be evaluated accordingly.
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