United States 

Department of 

Agriculture 

Forest Service 

Northeastern Forest 

Experiment Station 

Research Paper NE-679 

Protecting Red Oak Seedlings with Tree Shelters in 

Northwestern Pennsylvania 

Russell S. Walters 
Abstract 

Maintenance of oak species and a lack of oak regeneration are major silvicultural 

problems in many upland oak forests of the Eastern United States. Advance oak 

regeneration where present generally is small and lacks vigor. As a result, artificial 

regeneration is sometimes used, but planted oak seedlings usually do not grow well. 

Plastic tree shelters have been used to protect planted and natural oak seedlings 

from deer browsing and to promote seedling height growth and survival. This study 

examined the growth and survival of planted and natural red oak seedlings and 

seedlings from planted acorns within translucent tree shelters, fences, and 

unprotected controls with and without herbicide application under a shelterwood 

seed-cut stand. After 2 years, surviving control planted seedlings were significantly 

shorter than those protected by tree shelters and fences. The average height of 

seedlings planted within tree shelters and fences was 0.88 foot and was not 

significantly different. Planted control seedlings were shorter after the second year 

than the first, suggesting that these unprotected seedlings had been browsed by 

deer. The best survival of planted seedlings was inside the fences, with and without 

herbicide. Survival in shelters was 82 percent when herbicide was used and 40 

percent without herbicide, probably due to low light intensity. Survival of planted 

control seedlings was unsatisfactory whether or not herbicide was applied, likely the 

result of browsing. Only 16 percent of the acorns planted within shelters produced 

seedlings and none grew outside of shelters. Small mammals destroyed most 

planted acorns. Natural seedlings grew little and their height inside and outside of 

shelters was not different from that of planted seedlings. Recommendations based 

on these results should improve results when tree shelters are used. 

RUSSELL S. WALTERS is principal silviculturist and research forester at the 

Northeastern Forest Experiment Station's Forestry Sciences Laboratory at Warren, 

Pennsylvania. He received a B.S. degree from Michigan State University, an MS. 

degree from Oregon State University, and a Ph.D. degree from the State University 

of New York, College of Environmental Science and Forestry. He currently is 

engaged in research on silvicultural regeneration of oaks and Allegheny hardwoods. 

Manuscript received for publication 26 May 1993 

Northeastern Forest Experiment Station 

5 Radnor Corporate Center 

100 Matsonford Road, Suite 200 

Radnor, PA 19087-4585 

October 1993 
Introduction I 

Maintenance of oak species, particularly on the higher quality 

sites, is a major problem throughout the upland oak forests of 

Eastern North America. Successful natural regeneration of 

even-age oak stands requires that an adequate amount of 

sturdy, well-developed advance oak reproduction be present 

at the time of final harvest. Most oak regeneration failures are 

attributed to the absence of adequate amounts of advance 

regeneration or to the inability of oak to outgrow competition 

when released. 

The problem of inadequate advance tree regeneration under 

maturing hardwood forests is intensified in many areas by the 

presence of large populations of white-tailed deer, which 

have influenced forest vegetation development in 

northwestern Pennsylvania since the 1920's (Marquis and 

Brenneman 1981). As a result, understory vegetation is 

sparse in most uncut stands, and new seedlings appearing 

after cutting are browsed severely. New stand establishment 

after harvest cutting often is delayed or prevented entirely. 

Even where regeneration occurs, browsing results in shifts in 

woody species composition, reduced stocking, or extended 

rotations (Marquis and Brenneman 1981 ; Tilghman 1989). 

Inadequate advance tree regeneration has assumed even 

greater significance in the oak stands on the Allegheny 

Plateau of northwestern Pennsylvania due to insect 

infestation and drought. In 1988, the Allegheny National 

Forest reported significant oak mortality (in excess of 30 

percent) on more than 16,000 acres due to repeated gypsy 

moth defoliation between 1986 and 1988 followed by a 

severe drought in the summer of 1988. The weakened oaks 

were susceptible to attack by two secondary pathogens, the 

two-lined chestnut borer and the shoestring root rot fungus, 

which ultimately killed them.' A further consequence has 

been the accelerated shift in species composition from oak 

species. As overstory densities are reduced by mortality, the 

existing understory vegetation responds and captures the 

site. These understories are dominated by hayscented fern, 

striped maple, American beech root suckers, or, if tree 

seedlings are present, by black cherry, red maple, or sweet 

birch. There are few oak seedlings and these are not 

sufficiently vigorous to compete. 

This situation has heightened interest in planting to increase 

the population of oak seedlings in a stand understory where 

they are lacking or few in number. However, hardwood 

planting is futile in northwestern Pennsylvania unless the 

planted seedlings are protected against browsing. Also, 

planted oak seedlings are notoriously slow growing (Russell 

1971 ; Johnson et al. 1976; Hilt 1977; Wendel 1980). Thus, 

land managers must protect oak seedlings from browsing 

and help them compete with established vegetation. To 

' US. Department of Agriculture, Forest Service. 1992. FY 

'91 monitoring and evaluation report. Warren, PA: US. 

Department of Agriculture, Forest Service, Allegheny 

National Forest. 

accomplish this, it may be necessary to control the interfering 

vegetation to provide sufficient light for satisfactory 

development. Cutting, burning, and herbicide spraying all 

have been suggested, but none has proven consistently 

effective in oak regeneration programs (Nyland 1989). 

Using tree shelters to overcome browsing is a growing 

technology first developed in 1979 in Great Britain. Tree 

shelters are translucent, plastic tubes 4 to 6 inches in 

diameter and 4 to 6 feet tall that are placed around individual 

seedlings. They are expected to provide protection from 

browsing for 5 to 8 years. Tree shelters are reported to 

stimulate height growth during the first 1 to 3 years after 

planting and help support the seedling for up to 5 years. 

Seedling height growth inside tree shelters is reported to 

increase above normal unbrowsed growth rates by two or 

threefold due to a "greenhouse effect" that is created inside 

the tubes. Tree shelters also can screen planted seedlings 

from herbicide during a directed spray application (Tuley 

1983, 1985; Frearson and Weiss 1987; Potter 1988; 

Manchester et al. 1988). 

The reported success of tree shelters in promoting the 

survival and stimulating early growth rates of planted oak 

seedlings elsewhere suggested that this technology might be 

useful in planting oak on the Allegheny Plateau. The purpose 

of this study was to evaluate survival and height growth of 

planted and natural northern red oak seedlings and acorns 

within tree shelters. A knowledge of how red oak seedlings 

perform within shelters will aid in assessing the possibility of 

adapting tree-shelter technology to forest conditions in 

northwestern Pennsylvania and adjacent areas. 

Methods 

Site Description 

The study site located on the Allegheny Plateau in Warren 

County, Pennsylvania, is highly representative of a large 

proportion of upland forests in northwestern Pennsylvania. 

The elevation is about 1,900 feet and the site's 

medium-textured soils (Aquic Fragiudults) developed on 

broad plateaus not subjected to glacial action. Annual 

precipitation measures 43 inches and is distributed evenly 

throughout the year. Average rainfall is about 4 inches each 

month in April through September, so moisture deficits are 

rare during the growing season. 

This study was installed in 1990 in a 75-year-old upland 

mixed oak stand that had received a shelterwood seed cut in 

1984. When sampled in 1990, the residual overstory 

contained 98 square feet of basal area per acre with a 

relative stand density of 53 percent of the average maximum 

stocking expected in an undisturbed stand of similar size and 

species composition. Oak species made up 53 percent of the 

basal area; red maple, white ash, and black cherry accounted 

for most of the balance. There was a dense ground cover of 

hayscented fern over 60 percent of the area. Although the 

site contained about 14,000 seedlings of commercial species 

per acre, only about 200 were northern red oak. 

Three-quarters of the commercial seedlings and all of the 
oaks were less than 1 foot tall. Once they emerge from fern 

cover, seedlings are unable to grow taller because they are 

browsed continually by white-tailed deer (Marquis and 

Brennerman 1981). 

Plantings 

Seedlings and acorns were planted at 114-chain intervals in a 

check-row grid; that is, distances were measured carefully 

between rows and within rows to facilitate relocation. The 

planted seedlings were bareroot, 1-0 northern red oak grown 

in the Pennsylvania Bureau of Forestry's Penn Nursery from 

Pennsylvania acorns. At the time of planting in early May 

1990, the seedlings were top-pruned at 6 inches and rootpruned at 8 inches. Their average root collar measured 0.25 

(20.05) inch. The study acorns were collected in the 

Moshannon State Forest in Clearfield County. To ensure 

soundness, they were separated by float testing and then by 

visual examination. All acorns were pregerminated and 

planted approximately 1 inch deep. 

Three protection treatments for planted seedlings and 

acorns, with and without herbicide application, provided 12 

treatment combinations. These treatments were applied in a 

complete random arrangement where the 12 treatments were 

assigned in a row at random. The same random arrangement 

was then repeated 50 times throughout the grid row by row to 

ensure that the treatments were scattered across the study 

site. 

One protection treatment provided was by 6-foot-tall, tan tree 

shelters as described. Although white tree shelters are now 

available, only tan shelters were available for this study. A 

second protection treatment was small fences 5 feet tall by 1 

foot in diameter made from 1 -inch mesh chicken wire 

supported by two stakes. The third treatment was an 

unprotected control. The herbicide treatment was glyphosate 

applied as a directed spray to an area within a 3-foot radius 

of the seedling at the equivalent rate of 1 Ib a.i. per acre. 

Spraying control in mid-August 1990; this was within the 

recommended period for adequate control of herbaceous 

weeds (Horsley 1981). 

The three protection levels and two herbicide levels also 

were applied to natural northern red oak seedlings within the 

study-grid area. Half of these seedlings were top-clipped at 8 

inches (to simulate top-clipping of the planted seedlings) 

while the other half were left unclipped. Again, these 

treatments were assigned at random with one random start 

and the same random arrangement then repeated. A total of 

180 seedlings (1 5 for each treatment) were treated. The 

location of each treated natural seedling was then referenced 

to the check-row grid as described in order to relocate them. 

Measurements of first-and second-year growth response for 

planted seedlings, planted acorns, and natural seedlings 

were taken in early October in 1990 and 1991. Each group 

was examined separately. Data were collected on seedling 

mortality, number of growth flushes, and height growth. Data 

on height growth were examined by analysis of variance with 

the Bonferroni procedure to test differences among treatment 

means. The herbicide treatment was not significant (P = 

0.371). These data were then combined for the comparisons 

among the three protection treatment means, The 0.05 level 

of probability was accepted as significant. Survival data were 

compared against an assumed threshold level of satisfactory 

survival of 80 percent. 

Results and Discussion 

At the time of planting, the initial oak seedling caliper 

averaged 0.25 inch and ranged from -0.14 to 0.40 inch. 

Because the planted seedlings were top-clipped, initial 

heights were not measured. The average caliper was less 

than the minimum-size guideline of 0.354 inch (318 inch) 

recommended by Johnson and others (1976). In fact, only 5 

of the 300 seedlings planted in this study met or exceeded 

that minimum diameter. However, because we did not have 

larger stock available, we decided to plant this stock. 

There were significant differences among protection 

treatments each year (Table 1). The herbicide treatment did 

not have a significant influence during either year; therefore, 

the herbicide and no herbicide data were combined. After the 

first year, the average height of seedlings planted inside tree 

shelters was 0.85 foot. They were significantly taller than 

either the fenced or control seedlings at 0.78 and 0.76 foot, 

respectively. The following year, the average height of 

sheltered and fenced trees was 0.88 foot while the control 

seedlings were significantly shorter at 0.68 foot. We found no 

trees with more than one growth flush in either year. 

Table 1. - First- and second-year mean heights (&.E.) 

for northern red oak seedlings planted in 1990, by 

protection treatment 

Year Tree shelter Fence Control 

---------------------------Feet ........................... 

1990 .88 k.02 b -78 p.02 a .76 1t.02 a 

1991 .88 2.03 a .88 e.02 a .68 2.02 b 

Values in each row followed by the same letter are not 

significantly different (P < 0.05). 

The first-year results seemed to indicate an expected trend. 

The seedlings planted in tree shelters were significantly 

taller, though by only a small amount, than either the fenced 

or control seedlings (Table 1). However, by the second year, 

the protected seedlings (tree shelter and fence) were the 

same height while the control seedlings were 0.2 foot shorter. 

The earlier trend of faster seedling growth inside the tree 

shelters had disappeared. The fence protection was not 

expected to influence seedling growth other than protection 

from browsing. Therefore, since the control seedlings are 

now shorter than those inside the fences, the controls likely 

were browed by the white-tailed deer. The overall 

second-year height across all treatments in this study is the 
Table 2. - First- and second-year survival over 2 years for northern red oak seedlings 

planted in 1990, by herbicide and protection treatment 

same (0.82 foot) as that reported by Zastrow and Marty 

(1 991) for two-year-old, 1-0 northern red oak seedlings 

planted in an old field. However, they also found that their 

seedlings inside tree shelters averaged 1.45 feet. Our 

sheltered seedlings did not grow better than the controls or 

fenced seedlings. 

First-year survival across all treatments averaged 96 percent; 

the range among treatments was 90 to I00 percent (Table 2). 

Accepting 80 percent survival as the threshold of success, 

survival for all treatments was successful. Survival was 

poorest (90 percent) in the tree shelters without herbicide 

treatment. By the second year, survival changed drastically 

and chi-square tests revealed significant differences among 

treatments (range: 94 to 40 percent). Survival in half of the 

protectionlherbicide treatment combinations still exceeded 

the success threshold: fence with and without herbicide at 90 

and 94 percent, respectively, and tree shelter with herbicide 

at 82 percent. Survival was poorest (40 percent) for seedlings 

in tree shelters without herbicide. 

The high survival rates observed at 1 year were not 

surprising. Early survival of planted red oak seedlings 

generally is good and these observations were made in the 

fall of 1990, before the dormant season when winter weather 

conditions can be harsh and deer browsing intense. The low 

survival of the control trees compared to those in fences 

(Table 2) is best explained by exposure to deer browsing. 

The fences were not expected to modify the seedlings' 

environment other than to exclude deer. However, the low 

survival rate for trees planted in tree shelters with no 

herbicide is best explained as a response to low light 

intensity. Being translucent, tree shelters reduced the amount 

(Table 3). The average height of these seedlings was 0.48 

foot. After the first year (1990), 98 percent of the acorns 

without shelters and 62 percent of those inside shelters were 

missing. Every tree shelter from which the acorn was missing 

had a hole approximately 1 by 1.5 inch chewed in the side. 

The chewed hole always was near the bottom of the shelter 

where the lower support tie passes through shelter side and 

around the stake. The size of the hole and the size of the 

incisor marks suggest that they were made by a species of 

deer mouse attracted to the acorn inside. 

Table 3. - Fate of planted acorns after two growing 

seasons, by protection treatment 

Seedlings Tree shelter Fence Control 

..................... Percent .................... 

Live 16.0 0.0 0.0 

Dead 220 1 .O 4.0 

Missing (1 990) 62.0 99.0 96.0 

Table 4. - Second-year mean heights (2S.E.) and 

survival of top-clipped and unclipped natural red oak 

seedlings, by protection treatment 

Item Tree shelter Fence Control 

Height 

of light reaching the seedling by two-thirds of that entering clipped (feet) .50 k.04 .50 e.06 -43 t.04 

through the shelterwood canopy, hence, the reduced survival Unclipped (feet) .59 k.06 53 e.03 .56 k.05 

(82 percent) for seedlings in tree shelters with herbicide 

treatment. But where tree shelters also were shaded by fern Survival 

and other understory vegetation (no herbicide), the light level Clipped (%) 56.7 75.9 61.3 

inside was further reduced by half, resulting in unsatisfactory Unclipped (%) 50.0 86.7 96.6 

mortality. In an understory of similar composition, 

interference from hayscented fern with survival and growth of 

black cherry seedlings by altering the availability of light close 

to the forest floor has been reported (Horsley, in press). Second-year height of the natural seedlings observed in this 

study averaged 0.52 (t.02) foot across all treatments and 

The direct seeding of acorns was a failure regardless of ranged from 0.43 to 0.59 foot (Table 4). None of the 

treatment. After 2 years, there were no seedlings from acorns . differences among the protection, top-clipping, or herbicide 

planted as controls or inside fences and only 16 percent of treatment means were significant (P < 0.05). The overall 

acorns planted inside tree shelters had produced seedlings treatment mean is not different from the pretreatment 
average height of 0.51 (k.01) foot for these seedlings. The 

mean height of the unclipped seedlings for all protection 

treatments was greater than for top-clipped seedlings. This 

difference, though small and not significant, is consistent 

across all treatments. However, the probability of this 

difference being due only to chance is small (P = 0.055), 

suggesting that top-clipping had a real effect on the 

top-clipped seedlings. 

Of the naturalseedlings observed in this study, only 71 

percent are alive, and there are just as many surviving in the 

nonherbicide plots as in those herbicided. The survival range 

among the protectionltop-clipping treatment combinations 

was 59 to 97 percent (Table 4). Only the unclipped control 

(97 percent) and unclipped fenced (87 percent) seedlings 

exceeded the 80-percent survival threshold. However, the 

top-clipped seedlings in all three of the protection treatments 

and those unclipped inside of tree shelters had survival rates 

below 80 percent. It is especially notable that only slightly 

more than half of the natural seedlings with tree shelters are 

still alive after 2 years. Wendel (1 980) also reported that topclipping resulted in lower survival rates and reduced height 

growth. 

Oak advance seedlings are not uncommon in many eastern 

upland hardwoods and in some stand they are relatively 

abundant. However, these seedlings generally are small 

(less than 1 foot) and slow growing, and live only for a couple 

of years (Arend and Gysel 1951; Weitzman and Trimble 

1957; McGee 1967; Sander 1972; Nyland et al. 1982). The 

height growth of oak seedlings is directly related to the 

number of growth flushes experienced in a season, and 

multiple flushing usually is attributed to the high availability of 

light (Phares 1971). The natural seedlings in this study fit this 

pattern. They were initially short (0.5 foot average) and did 

not increase in height in 2 years. They experienced no 

multiple growth flushing and many died. In fact, imposing the 

additional light restriction of a tree shelter is the likely cause 

of this high mortality and slow growth. If natural advance oak 

seedlings are to be a part of the next forest, they need help 

by release and browsing protection. But not all seedlings can 

respond; only those that are at least of average height or 

taller and whose previous-year's growth was at least average 

for their age class should be treated. Small, slow-growing 

seedlings are unable to respond (Waiters 1963). This study 

also suggests that the light levels inside tan tree shelters 

under. even a partial canopy of a shelterwood seed cut may 

be too limiting for oak seedlings to respond. The seedlings 

should not be top-clipped. 

Management Implications 

The failure to replace oak forests with oak forests is cause for 

concern over the net loss of oak-dominated forests and the 

loss of landscape biodiversity. The conditions and processes 

that led to the establishment of oak forests on our better sites 

are no longer operating. Thinning and shelterwood methods 

are not sufficient to maintain oak in the nefl stand and are 

stimulating northern hardwood advance regeneration. On the 

Allegheny Plateau of northwestern Pennsylvania, the result is 

virtually monospecific stands of black cherry because of its 

prolific seeding and the preference for other species by the 

high deer population (Marquis and Brenneman 1981; 

Tilghman 1989). 

Interest by land managers in using tree shelters as a way of 

increasing the oak component in newly emerging forest 

stands has increased sharply. Not only do tree shelters 

protect natural and planted seedlings from browsing but they 

are reported to promote both survival and height growth. 

Although results are not conclusive and costs are high, 

averaging about $760 to plant 200 seedlings per acre 

(Crothers 1991), enrichment planting of oak seedlings in tree 

shelters has been used on the Allegheny National Forest for 

the past 3 years. With establishment costs of this magnitude, 

it is necessary to take every conceivable precaution to 

ensure success. 

The results of this study are not encouraging but should not 

be considered as critical of the tree-shelter concept. Rather, 

this study serves as a reminder that tree shelters are not a 

cure all for the difficulties in planting or regenerating oaks. 

The following precautions based on observations from this 

study should improve the performance of oak in tree shelters: 

Plant only large (z 318 inch diameter), 2-0, undercut 

seedlings as suggested by Johnson and others (1986). 

Our planting stock was small and did not perform well. 

Do not plant seedings in tan tree shelters under a dense 

overstory. Oak seedlings do not grow well where light is 

limited. Johnson and others (1986) suggested a 

shelterwood overstory density no greater than 55 to 65 

percent. The overstory density should be even less for 

planting with the three shelters. 

Manufacturers recommend using white tree shelters 

when planting under a shelterwood because they 

transmit more light than tan shelters. White tree shelters 

have been used locally but their success has not been 

evaluated fully. 

At least for the first year, controlling competing 

vegetation with herbicide within at least 2-foot radius of 

the tree shetter may be helpful. 

If tree shelters are to be placed around natural 

seedlings, select only seedlings that are at least 1 foot 

tall. The natural seedlings in this study averaged only 0.6 

foot and did not respond. 

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1 84-1 87. 

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AU. S. GOVERNMENT PRINTING OFFICE: 193 3- 7 50- 12 3/80000 
Walters, Russell S. 1993. Protecting red oak seedlings with tree shelters 

in northwestern Pennsylvania. Res. Pap. NE-679. Radnor, PA: U.S. 

Department of Agriculture, Forest Service, Northeastern Forest Experiment 

Station. 5 p. 

Examines the growth and survival of planted and natural red oak seedlings 

and seedlings from planted acorns within translucent tan tree shelters, fences, 

and unprotected controls under a shelterwood seed-cut stand. Seedlings 

planted within tree shelters and fences were inside tree shelters. Natural 

seedlings grew very little and their height inside and outside of tree shelters 

did not differ. Recommendations based on these results should improve 

results from the use of tree shelters. 

Keywords: artificial regeneration; seedling growth; Quercus rubra 
Headquarters of the Northeastern Forest Experiment Station is in Radnor, Pennsylvania. Field laboratories are maintained at: 

Amherst, Massachusetts, in cooperation with the University of Massachusetts 

Burlington, Vermont, in cooperation with the University of Vermont 

Delaware, Ohio 

Durham, New Hampshire, in cooperation with the University of New Hampshire 

Hamden, Connecticut, in cooperation with Yale University 

Morgantown, West Virginia, in cooperation with West Virginia University 

Orono, Maine, in cooperation with the University of Maine 

Parsons, West Virginia 

Princeton, West Virginia 

Syracuse, New York, in cooperation with the State University of New York, . 

College of Environmental Sciences and Forestry at Syracuse University 

University Park, Pennsylvania, in cooperation with The Pennsylvania State University 

Warren, Pennsylvania 

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discrimination on the basis of race, color, national origin, age, religion, sex, or 

disability, familial status, or political affiliation. Persons believing they have been 

discriminated against in any Forest Service related activity should write to: Chief, 

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"Caring for the Land and Serving People Through Research"