United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

Research Paper

PNW-RP-576

June 2008

Influence of Four Tree Shelter

Types on Microclimate and

Seedling Performance of

Oregon White Oak and

Western Redcedar

Warren D. Devine, Constance A. Harrington
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Authors

Warren D. Devine and Constance A. Harrington are research foresters, Forestry

Sciences Laboratory, 3625 93rd Avenue SW, Olympia, WA 98512-9193.

Cover

Four tree shelter types tested in this study. From left to right: blue unvented, finemesh fabric, white unvented, and whited vented.
Abstract

Devine, Warren D.; Harrington, Constance A. 2008. Influence of four tree

shelter types on microclimate and seedling performance of Oregon white oak

and western redcedar. Res. Pap. PNW-RP-576. Portland, OR: U.S. Department

of Agriculture, Forest Service, Pacific Northwest Research Station. 35 p.

Four types of tree shelters were evaluated in southwestern Washington for their

effects on seedling microenvironment and performance of two tree species. Shelter

types were fine-mesh fabric shelters, solid-walled white shelters with and without

vent holes, and solid-walled blue unvented shelters. Summer mean and daily

maximum air temperatures were increased by 0.8 °C and 3.6 °C, respectively, in

solid-walled tree shelters. Shelter color and shelter venting did not influence air

temperatures. Tree shelters only affected vapor pressure deficit late in the growing

season. Midday photosynthetically active radiation within shelters ranged from 54

percent of full sun in fine-mesh fabric shelters to 15 percent of full sun in blue

solid-walled shelters. In the first year after planting, height and diameter growth of

western redcedar (Thuja plicata Donn ex D. Don) were significantly increased by

all shelter types, with blue solid-walled shelters resulting in the greatest height

growth. However, in blue solid-walled shelters, photosynthesis and stem diameter

growth of Oregon white oak (Quercus garryana Dougl. ex Hook.) seedlings were

significantly less than for unsheltered seedlings.

Keywords: Tree shelter, microclimate, photosynthesis, Thuja plicata, Quercus

garryana.

Summary

Tree shelters are used to protect planted seedlings from herbivory and to create a

microenvironment conducive to rapid seedling growth. Numerous types of tree

shelters are commercially available; the most common varieties are plastic solidwalled and mesh-walled shelters. This study, conducted in southwestern Washington, was designed to examine the effects of solid-walled shelters, fine-mesh shelters,

vented shelters, shelters of different colors, and unsheltered controls in the context

of the regional climate. Shelters tested were fine-mesh fabric (ME), solid-walled

white unvented (WU), solid-walled white vented (WV), and solid-walled blue

unvented (BU). Tree shelters were applied to seedlings of two frequently browsed

tree species native to the Pacific Northwest: Oregon white oak (Quercus garryana

Dougl. ex Hook.) and western redcedar (Thuja plicata Donn ex D. Don).
Relative to ambient conditions, summer mean and maximum daily air temperatures were increased by 0.8 °C and 3.6 °C, respectively, in solid-walled tree shelters. On the five hottest days of the summer, temperatures in solid-walled shelters

reached an average maximum of 42.9 °C, significantly hotter than ambient conditions (40.0 °C). Air temperatures in the ME shelter treatment did not differ from

ambient conditions except on hot, clear days when air was cooler within the ME

shelters. Venting of solid-walled shelters had no effect on interior air temperature,

and shelter color affected air temperature only in September and October when

blue shelters were cooler than white shelters. Leaf temperature, measured in early

September, was somewhat increased by solid-walled shelters and decreased by mesh

shelters. Although none of the tree shelters had a significant effect on vapor pressure

deficit between June and August, in September, vapor pressure deficit in solidwalled shelters was greater than that of ambient conditions. Intensity of midday

photosynthetically active radiation within shelters, relative to full sunlight, was 54

percent in the ME treatment, 38 percent in the WU and WV treatments, and 15

percent in the BU treatment. The ratio of red to far-red light was significantly lower

for the BU shelter than for the other shelters, which did not differ from one another.

In the first year after planting, both height and diameter growth of western

redcedar were significantly increased by all shelter types. The BU shelter treatment

resulted in significantly greater height growth and height:diameter ratio compared

to the other shelter treatments. For Oregon white oak seedlings, however, the BU

shelter treatment also was associated with a reduced rate of photosynthesis and

reduced stem diameter compared to the unsheltered treatment. The various tree

shelter designs altered microclimate in a variety of ways, particularly in regard to

the light environment. These effects on microclimate, and the differing growth

responses of species to the same shelter types, suggest that tree shelter selection

should be based on the light requirements of the planted species, among other

factors.
Contents

1 Introduction

3 Microenvironment Hypotheses

3 Seedling Growth Hypothesis

3 Photosynthesis Hypothesis

3 Methods

3 Study Area

4 2005 Study

7 2007 Study

8 Data Analysis

9 Results

9 Shelter Effects on Microclimate

21 Shelter Effects on Seedlings

25 Discussion

25 Shelter Effects on Microclimate

27 Shelter Effects on Seedlings

29 Conclusions

30 Acknowledgments

31 Metric Equivalents

31 Literature Cited

35 Appendix: Tree Shelter Types
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Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Introduction

Tree shelters were first developed in 1979 to protect planted seedlings from

animal browse and to create a greenhouse-like environment for the seedlings

(Tuley 1985). It soon became apparent that tree shelters significantly increased

early survival and height growth of some species of tree seedlings (Potter 1988).

Although tree shelters are expensive relative to most other establishment costs, they

may be justified where browse pressure is severe or where a high value is placed on

the success of individual tree seedlings. Although the most common type of tree

shelter is a 0.6- to 1.5-m-tall plastic tube, numerous varieties are commercially

available including shelters with single- and double-layer walls, shelters with

vented walls, plastic mesh shelters, and fabric shelters.

The effect of a tree shelter on a seedling’s microenvironment is a function

of shelter design and local climatic conditions. Although the ventilation of meshwalled tree shelters attenuates their effects on microclimate, the microclimate

within a solid-walled shelter is often substantially different than ambient conditions. For solid-walled tree shelters, interior air temperature is greater than ambient

conditions during the daytime (Bellot et al. 2002, Evans and Potter 1985, Kjelgren

and Rupp 1997, Ponder 1995). In some regions, air temperature in tree shelters is

high enough to negatively affect seedling performance (Kjelgren and Rupp 1997);

however, the addition of vents to solid-walled shelters has been shown to moderate

shelter air temperature (Bellot et al. 2002, Swistock et al. 1999). The environment

within a solid-walled tree shelter is typically more humid than ambient conditions,

particularly when a shelter contains a large, transpiring seedling (Kjelgren et al.

1997, Minter et al. 1992, Potter 1988). Carbon dioxide (CO2

) concentration also is

affected by solid-walled tree shelters. The CO2

 concentration rises above the

ambient level at night as a result of soil respiration but then decreases rapidly in the

morning and remains below the ambient level during the day owing to the combination of assimilation by the seedling and a lack of air circulation (Dupraz and

Bergez 1999, Peterson et al. 1995). Tree shelter color and material also significantly affect the amount and quality of solar radiation reaching the seedling (Jacobs

and Steinbeck 2001, Kjelgren 1994, Sharew and Hairston-Strang 2005). In a

Mediterranean climate with a dry growing season, solid-walled tree shelters have

been shown to improve soil water availability owing to the condensation that flows

down the inside of the shelter and enters the soil in the vicinity of the seedling (del

Campo et al. 2006).

The microclimate

within a solidwalled shelter is

often substantially

different than ambient conditions.
2

RESEARCH PAPER PNW-RP-576

The effect of solid-walled tree shelters on seedling height growth is positive

and often dramatic (e.g., Burger et al. 1992, McCreary and Tecklin 2001, Potter

1988, Sharrow 2001), but the effects of shelters on other growth variables differ

widely by study. For example, diameter growth of seedlings has been increased

(Dubois et al. 2000, Sharrow 2001) or decreased (Quilhó et al. 2003, Sharew and

Hairston-Strang 2005) by solid-walled shelters. Root growth also has been shown

to increase (Bellot et al. 2002, Ponder 1995) or decrease (Burger et al. 1997,

Mayhead and Boothman 1997) for seedlings grown in tree shelters. This variation

in seedling response to tree shelter is apparently the result of multiple interacting

variables including tree species, environment, and shelter type.

To predict how a tree shelter will influence seedling performance, it is necessary to understand how the shelter affects the seedling microclimate under local

environmental conditions and how this altered microclimate will affect seedling

development. Potential shelter effects on tree seedlings are numerous and include

form, phenology, and water use. For example, the reduced photosynthetic photon

flux density (PPFD; i.e., sunlight) and lack of stem movement within many solidwalled shelters were suggested to be the cause of the tall, slender form of sheltered

seedlings (McCreary and Tecklin 2001). Where cool air temperatures limit seedling

development, tree shelters may provide a warmer microclimate leading to earlier

budbreak (Sharrow 2001, Svihra et al. 1993). Additionally, the increased humidity

that typically occurs within tree shelters reduces the vapor pressure deficit (VPD),

which may substantially reduce seedling water use (Bergez and Dupraz 1997,

Kjelgren 1994).

In this study in southwestern Washington, our objective was to determine how

four tree shelter types influence microclimate and performance of western redcedar (Thuja plicata Donn ex D. Don; redcedar) and Oregon white oak (Quercus

garryana Dougl. ex Hook.; oak) seedlings in the context of the mild maritime

climate typical of the coastal Pacific Northwest. These species were selected

because they are the preferred browse species of black-tailed deer (Odocoileus

hemionus columbianus) and therefore would likely benefit from tree shelter application. The two species differ in shade tolerance, with oak rated moderately

intolerant and redcedar rated tolerant (Minore 1990, Stein 1990). We examined

vented and unvented, white- and blue-colored solid-walled shelters, a mesh-walled

shelter, and an unsheltered control treatment. We tested the following hypotheses

regarding shelter effects on microclimate and seedlings:
3

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Microenvironment Hypotheses

Hypothesis M1: Summer daytime air temperatures within solid-walled tree

shelters are greater than ambient air temperatures owing to the combination of

insolation and a reduction in convective heat loss.

Hypothesis M2: Daytime VPD will be decreased in solid-walled shelters

relative to ambient conditions owing to increased humidity from the transpiration

of seedlings and reduced air movement.

Hypothesis M3: Fine-mesh fabric shelters will reduce insolation relative to

ambient conditions while allowing mixing of internal and external air; thus,

summer daytime temperatures in fine-mesh shelters will be lower than ambient.

Hypothesis M4: Venting of solid-walled tree shelters will moderate their effects

on air temperature and VPD.

Seedling Growth Hypothesis

Hypothesis G1: Seedling height growth will be increased by tree shelters as a

result of the shelters’ effect on light, specifically the reduction of the ratio of red

to far-red light. Thus, the positive height growth effect will be greatest for shelter

types that have the greatest reduction in the ratio of red to far-red light.

Photosynthesis Hypothesis

Hypothesis P1: Previous research showed substantially greater height growth

for outplanted Oregon white oak seedlings in solid-walled tree shelters, regardless

of shelter color (Devine et al. 2007); thus, we expect rates of net photosynthesis to

be greater for seedlings in solid-walled tree shelters than for unsheltered seedlings

because of shelter microclimate conditions including warmer temperatures.

Methods

Study Area

This research was conducted in two phases, hereafter referred to as the 2005 study

and the 2007 study. Both studies took place at the Forestry Sciences Laboratory in

Olympia, Washington (46° 57 ́ N; 122° 58 ́ W) at an elevation of 50 m above mean

sea level. The study area was level and unshaded; soil was a Cagey loamy sand

(mixed, mesic Aquic Xeropsamment) (Soil Survey Staff 2006). Annual precipitation in Olympia averages 1291 mm, with 191 mm occurring between 1 May and 30

September (Western Regional Climate Center 2007). Mean annual air temperature

This research was

conducted in two

phases at the

Forestry Sciences

Laboratory in

Olympia, Washington
4

RESEARCH PAPER PNW-RP-576

is 10 °C, and mean temperatures in January and July are 3 °C and 17 °C, respectively (Western Regional Climate Center 2007). Competing vegetation in the area

of the 2005 study was controlled by a layer of water-permeable landscape fabric

covered with bark mulch. In the 2007 study, weeds were pulled by hand throughout

the growing season. The study was conducted in a fenced area that prevented

damage from deer browse on the unsheltered seedlings.

2005 Study

Study design—

The 2005 study followed a randomized, complete-block design with 12 blocks.

Each block consisted of 21 seedling locations (the experimental unit) on a 1.0- by

1.0-m grid. Treatments, arranged in a factorial design, were tree shelter (five levels)

and species (two levels). The tree shelter treatment consisted of four shelter types1

(table 1) and a nonsheltered control (NO). The two species were Oregon white oak

and western redcedar. Within each block, the 10 treatment combinations were

randomly assigned to two locations each. The extra location in each block was

assigned the unsheltered treatment, with oak in six blocks and redcedar in six

blocks.

Oak seedlings were 2 + 0 bare-root nursery stock purchased from Mineral

Springs Ornamentals nursery in Carlton, Oregon. The seed source was trees adjacent to the nursery. Western redcedar seedlings were Plug + 1 bare-root stock

(Puget Sound seed zone) purchased from the Washington Department of Natural

Resources Webster Nursery in Olympia. Throughout the study, seedlings were

grown in containers set in the ground to prevent rodent damage and to provide

the option of root biomass sampling. Ultimately, sampling of root biomass was

excluded from the study when it was determined that the majority of oak seedlings

grew poorly. The bare-root seedlings were potted in plastic containers, 10- by 10-

cm square and 55-cm deep, between 24 February and 3 March 2005. Potting media

was Sunshine Mix #2 (SunGro, Vancouver, Canada). To achieve uniform length,

oak seedling taproots were pruned at 30 cm below the root collar prior to potting.

A piece of copper-impregnated fabric at the base of the container allowed drainage

but prevented roots from growing beyond the container. After potting, seedlings

remained outside until 5 to 7 April 2005, when containers were buried at their

1

 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

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

specified planting locations so that the soil level within the pot was equal to that of

the surrounding ground, and only the lip of the pot (≤1.0 cm) protruded. After pots

were placed in the ground, tree shelter treatments were installed. Shelters were

supported with wooden stakes located on their north side. The WU, WV, and BU

shelters were attached to stakes with plastic cable ties; the ME shelter design

included friction clips, which were used to secure the shelter to the wooden stake.

The base of each shelter was flush with the ground, and soil was mounded to a

height of 2 to 3 cm around the shelter base to prevent air exchange at the bottom of

the shelter.

Seedlings were irrigated as needed throughout the growing season (approximately once per week) to maintain adequate soil moisture, and were fertilized at an

8-week interval with water-soluble 20-20-20 fertilizer (The Scotts Company,

Marysville, Ohio).

Seedling growth—

After potting, seedling height and stem diameter (at a marked location 15 cm above

ground level) were measured. Midway through the growing season (13 July 2005),

and at the end of the growing season, seedling height and stem diameter at 15 cm

were remeasured.

Table 1—Four types of tree shelters tested

Manufacturer/type

Freegro® Tree Pro® Tree Pro® ProTex®

Pro/Gro

Parameter fine mesh unvented vented solid tube

Abbreviation ME WU WV BU

Wall typea 2-mm mesh Double layer Double layer Single layer

with holesb

Colorc Beige White White Blue

Height (cm) 91.4 121.9 121.9 121.9

Diameter (cm) 15.2 11.4 11.4 10.2

Vented Yes No Yes No

Priced $1.88 $2.79 $2.79 $1.91

a

 All shelters are made of polyethylene.

b

 Vent holes are circular, 0.8-cm diameter, and spaced approximately 9 cm apart. Holes are present on

the upper 61 cm of the shelter.

c

 All shelters are translucent.

d

 Price per unit for an order of 500 units in 2007; price varies with quantity purchased. Price does

not include stake or other installation materials.
6

RESEARCH PAPER PNW-RP-576

Air temperature and relative humidity—

Air temperature and relative humidity were measured at 30-min intervals in all

treatments using Hobo®

 Pro Series 08 temperature/relative humidity dataloggers

(Onset Computer Corp., Bourne, Massachusetts). Two dataloggers per tree shelter

treatment (one per seedling) were installed on 8 April 2005, and two additional

dataloggers per tree shelter treatment were installed on 3 June 2005, for a total of

four dataloggers per shelter treatment, with two per species-shelter combination.

Dataloggers were installed inside randomly selected shelters at a height of 30 cm

above ground level. A small plastic shield was used to protect the humidity sensor

from direct precipitation. In the NO treatment, dataloggers were mounted approximately 15 cm west of the seedling at a height of 30 cm. Dataloggers in the NO

treatment were protected from direct sunlight and precipitation with an 18-cmdiameter, round white plastic shield. This shield was placed above the datalogger

and did not impede air movement in its vicinity. Vapor pressure deficit was calculated from simultaneous temperature and relative humidity readings (Lee 1978).

Leaf temperature—

Leaf temperature and air temperature inside shelters were measured in each of the

five shelter treatments using paired type-T thermocouples and a Campbell CR10X

datalogger and AM25T multiplexer system (Campbell Scientific, Logan, Utah).

Two oak seedlings from each shelter treatment were selected for this measurement. On each seedling, one leaf near the top of the terminal shoot was selected for

measurement. A type-T thermocouple (38-gauge wire) was woven through the leaf

and mounted flush with the abaxial side of the leaf so that it gently pressed against

the surface. An identical thermocouple was mounted 30 cm above the seedling to

measure air temperature. This thermocouple was shaded by a 4-cm-wide square of

white plastic mounted 1 cm above it. Temperatures were recorded every 30 minutes

during early September.

Photosynthetically active radiation—

Photosynthetically active solar radiation, measured as PPFD, was recorded simultaneously in the five tree shelter treatments using five LI-190 Quantum sensors (LICOR Biosciences, Lincoln, Nebraska) and two LI-1400 dataloggers (LI-COR

Biosciences). For the purpose of these measurements, a group of shelters was

installed without seedlings. Quantum sensors were installed and leveled 30 cm

above the ground in the center of each shelter. For the NO treatment, an unsheltered

sensor was installed at the same height to receive full sunlight. Data were recorded

every 15 minutes from 5:15 until 20:00 PST during August and early September

2005.
7

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Photosynthesis rate—

Photosynthesis rate (net carbon assimilation; An

) was measured on four oak seedlings per treatment between 10:00 and 14:00 PST on 26 September 2005 using an

open-system gas analyzer (CIRAS-1 Portable Photosynthesis System with automatic

Parkinson leaf cuvette, PP Systems, Amesbury, Massachusetts). Conditions were

clear, and air temperature averaged 22.0 °C during measurements. For sheltered

seedlings, an 8.0- by 8.0-cm door was cut in the side of the shelter at the location

of the terminal shoot. This door was taped shut other than when the cuvette was

inserted to make measurements. On each seedling, measurements of An were made

at 2-s intervals for at least 120 s on one leaf from the terminal shoot.

2007 Study

Study design—

Owing to generally poor growth of the oak seedlings in the 2005 study (see results),

a followup study was implemented in 2007 to again examine the influence of tree

shelters on the rate of photosynthesis of Oregon white oak seedlings and to determine how shelters affected seedling morphology. This study was conducted on the

same site as the 2005 study, but seedlings were grown in 57-cm-tall raised beds

constructed in February 2007. The 24 oak seedlings in this study were 2-year-old

bare-root stock from an Olympia, Washington, seed source that were grown in

Webster State Nursery in Olympia and planted in the raised beds in March 2007. In

early May, prior to budbreak, eight each of the ME and BU shelters were installed

around seedlings. An additional eight seedlings were designated as the NO treatment. The 24 seedlings were irrigated daily from July through September using a

drip irrigation system (0.63 liters per seedling per day over a period of 20 minutes).

Seedling growth and morphology—

Seedling height and diameter at groundline were measured after planting. The

seedlings were harvested on 17 October 2007 and destructively sampled. For each

seedling, we recorded height, diameter, average leaf area of 10 typical first- and

second-flush leaves (LI-3100 area meter, Li-Cor, Lincoln, Nebraska), number of

leaves in first flush, number of leaves in subsequent flushes, and dry weight of stem,

foliar, and root components. Dry weights were determined by drying samples to

constant weight at 65 °C. Predictions of pre-growing-season dry weight were based

on data from 56 seedlings that were destructively sampled at the time the 24 study

seedlings were planted. The destructively sampled seedlings were measured for

height and diameter, divided into root and shoot components, dried, and weighed.
8

RESEARCH PAPER PNW-RP-576

Data from the destructively sampled seedlings were used to create equations to

predict shoot and root dry weight of the 24 study seedlings at the time of planting,

based on measurements of height and diameter. Equations were:

WS

 = 0.8175 + 0.0017D2H

R2

 = 0.93

WR = - 5.5448 + 2.5664D - 0.062D2

R2 = 0.66

where WS

 and WR are dry weights of above- and belowground woody components,

respectively, D is seedling diameter, and H is seedling height. Stem and root dry

weight growth increments for each seedling in the 2007 study were estimated based

on predicted dry weight at the beginning of the growing season and the measured

weight at the end of the growing season.

Ratio of red-to-far-red light—

An S2000 spectrometer (Ocean Optics, Inc., Dunedin, Florida) was used to measure

the light spectra within ME, WU, and BU tree shelters containing no seedlings that

were installed near the 2007 plantings. The sensor was installed within the shelter at

45 degrees from vertical, facing due south. Four measurements were made in each

treatment within 2 hours of solar noon on 26 June 2007. From these measurements

of irradiance across spectra (310 to 780 nm), we calculated the ratio of red (655 to

665 nm) to far-red light (725 to 735 nm) for each reading in each shelter treatment.

Photosynthesis rate—

On 3 July 2007, An was measured on the study’s 24 oak seedlings between 10:00

and 14:00 PST using the CIRAS-1 Portable Photosynthesis System. The sky was

clear, and air temperature averaged 21.0 °C during measurements. For these measurements, the same procedure as that used in 2005 was followed. On 9 July 2007,

four sets of photosynthesis measurements were made at the following times: 6:30 to

7:30 PST, 9:00 to 10:00 PST, 11:15 to 12:15 PST, and 15:30 to 16:30 PST. On this

date, conditions were clear and air temperature averaged 24.9 °C. On 24 August

2007, photosynthesis measurements were made on fully developed second-flush

leaves between 12:00 PST and 14:00 PST. Conditions were clear and air temperature averaged 23.8 °C.

Data Analysis

Shelter air temperatures and VPD were analyzed by month and for selected dates

using repeated-measures analysis of variance (ANOVA), with tree shelter treatment

as a fixed effect and block as a random effect (PROC MIXED, SAS Institute

An S2000 spectrometer was used to

measure the light

spectra within ME,

WU, and BU tree

shelters.
9

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

2005). For monthly analyses, day was the repeating unit of time. For single-day

analyses, days representative of typical weather conditions were chosen, and all

measurements within 4 hours of solar noon were analyzed. Degree-day accumulation (base temperature = 5.0 °C) was calculated but not analyzed statistically

because data from April and May, and therefore all cumulative data, had a sample

size of only two sensors per treatment. Leaf temperature and PPFD were assessed

graphically but not statistically, as there were only two replicates. Seedling growth

and photosynthesis were analyzed with ANOVA. In all ANOVA models, assumptions of normality and equal variance were met. Post-ANOVA mean separations

were performed using contrasts (Zar 1999). Contrasts included mesh shelter (ME)

versus ambient conditions (NO), solid-walled shelters (WU, WV, BU) versus

ambient conditions (NO), vented (WV) versus unvented (WU) solid-walled shelters, and white (WU) versus blue (BU) solid-walled shelters. Significance was

judged at the P = 0.05 level.

Results

Shelter Effects on Microclimate

Air temperature—

Mean ambient air temperature (i.e., NO treatment) peaked during July and

August, averaging 18.6 oC (table 2). The coolest months of the study were April

and October, when the ambient temperature averaged 10.6 oC and 11.0 oC, respectively. For all months analyzed (June through October), mean air temperatures

within the three solid-walled tree shelter treatments were significantly higher than

the ambient temperature. The greatest temperature difference occurred in July when

the solid-walled shelters averaged 0.9 oC warmer. There were no significant differences between the mean ambient air temperature and air temperature in the mesh

shelter or between the vented and unvented treatments, but the mean air temperature

in the white shelter (WU) was significantly warmer (a difference of 0.3 oC) than the

blue shelter (BU) in October.

Maximum daily air temperature was greatest in August (33.6 oC in ambient

conditions) (table 3). From June through October, maximum air temperature was

significantly higher in solid-walled tree shelters than in the NO treatment, with a

difference of 3.6 oC and 3.3 oC in July and August, respectively. There were no

significant differences in maximum air temperature between the unsheltered and

mesh shelter treatments or between the vented and unvented treatments. Maximum

air temperature in the white shelter was significantly greater than that in the blue

shelter in September and October.

From June through

October, maximum

air temperature was

significantly higher

in solid-walled tree

shelters than in the

ambient conditions.
10

RESEARCH PAPER PNW-RP-576

Table 2—Mean monthly air temperature (2005) for five tree shelter treatments, with contrasts testing differences

among treatment combinations

Treatmenta

/ Month

contrast Apr May Jun Jul Aug Sep Oct

– – – – – – – – – – – – – – – – – – – – – – °C – – – – – – – – – – – – – – – – – – – – – –

NO 10.6 14.5 15.8 ±0.2 18.5 ±0.2 18.7 ±0.2 13.4 ±0.2 11.0 ±0.1

ME 10.5 14.1 15.7 ±0.2 18.3 ±0.2 18.5 ±0.2 13.4 ±0.2 10.9 ±0.1

WU 11.6 15.3 16.7 ±0.2 19.6 ±0.2 19.6 ±0.2 14.2 ±0.2 11.4 ±0.1

WV 11.9 15.6 16.8 ±0.2 19.3 ±0.2 19.3 ±0.2 14.1 ±0.2 11.3 ±0.1

BU 11.4 15.0 16.4 ±0.2 19.2 ±0.2 19.3 ±0.2 13.9 ±0.2 11.1 ±0.1

– – – – – – – – – – – – – – – – – – – – – P > F – – – – – – – – – – – – – – – – – – – – – –

NO vs. ME — — 0.844 0.679 0.654 0.921 0.308

NO vs. WU, WV, BU — — <0.001 0.003 0.015 0.002 0.003

WU vs. WV — — 0.644 0.481 0.520 0.615 0.411

BU vs. WU — — 0.231 0.254 0.518 0.270 0.020

Note: n = 2 for April and May; n = 4 for June through October. Standard error is shown where n = 4. Significant contrasts are bolded. a

 Treatments are no shelter (NO), a fine-mesh fabric shelter (ME), a solid-walled white unvented shelter (WU), a solid-walled white

vented shelter (WV), and a solid-walled blue unvented shelter (BU).

Table 3—Maximum daily air temperature, by month, for five tree shelter treatments in 2005, with contrasts

testing differences among treatment combinations

Treatmenta/ Month

contrast Apr May Jun Jul Aug Sep Oct

– – – – – – – – – – – – – – – – – – – – – – °C – – – – – – – – – – – – – – – – – – – – – –

NO 19.5 24.0 25.9 ±0.4 30.9 ±0.5 33.6 ±0.5 27.1 ±0.4 17.7 ±0.2

ME 20.1 23.7 26.2 ±0.4 30.9 ±0.5 33.4 ±0.5 27.2 ±0.4 18.0 ±0.2

WU 24.0 27.4 29.6 ±0.4 34.6 ±0.5 37.1 ±0.5 31.7 ±0.4 21.4 ±0.2

WV 25.9 28.7 30.6 ±0.4 35.2 ±0.5 37.3 ±0.5 32.3 ±0.4 21.3 ±0.2

BU 22.9 26.3 28.8 ±0.4 33.7 ±0.5 36.3 ±0.5 30.2 ±0.4 19.7 ±0.2

– – – – – – – – – – – – – – – – – – – – – P > F – – – – – – – – – – – – – – – – – – – – –

NO vs. ME — — 0.597 0.998 0.859 0.878 0.153

NO vs. WU, WV, BU — — <0.001 <0.001 <0.001 <0.001 <0.001

WU vs. WV — — 0.108 0.441 0.733 0.294 0.862

BU vs. WU — — 0.190 0.205 0.300 0.016 <0.001

Note: n = 2 for April and May; n = 4 for June through October. Standard error is shown where n = 4. Significant contrasts are bolded. a

 Treatments are no shelter (NO), a fine-mesh fabric shelter (ME), a solid-walled white unvented shelter (WU), a solid-walled white

vented shelter (WV), and a solid-walled blue unvented shelter (BU).
11

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

On the five hottest days of the summer (as measured in the NO treatment),

the solid-walled shelters reached a significantly (P < 0.001) higher maximum air

temperature (42.9 °C) than the unsheltered treatment (40.0 °C). On the five hottest

days, there were no significant differences in maximum temperature between

unsheltered and mesh-shelter (39.6 °C) treatments, between unvented and vented

treatments (43.0 and 43.3 °C, respectively), or between white (43.0 °C) and blue

(42.2 °C) shelter treatments. The total number of hours for the 2005 growing

season during which the air temperature was above 35.0 °C were 73, 72, 199,

200, and 170 for the NO, ME, WU, WV, and BU treatments, respectively. The

total number of hours during which the air temperature was above 40.0 °C were

5, 3, 47, 51, and 34 for the NO, ME, WU, WV, and BU treatments, respectively.

The highest recorded temperature was 45.2 °C, occurring on 19 July in the WV

treatment.

On a typical, partly cloudy June day, air temperature in solid-walled tree

shelters was significantly warmer than ambient conditions (P = 0.001), particularly

during periods when the sky was clear (afternoon temperature peaks shown in the

larger graph in fig. 1). Air temperature in the mesh shelter treatment did not differ

from ambient conditions (P = 0.156), and air temperatures among the three types

of solid-walled tree shelters did not differ. On a very hot, clear July day, air

temperatures in solid-walled shelters were below ambient from 6:00 until 10:00

PST; after 10:00 PST, air temperatures in solid-walled shelters remained above

ambient for the rest of the day (fig. 2). Air temperature in the mesh-shelter treatment was cooler than the ambient temperature until 16:00 PST, and remained

above ambient until 24:00 PST.

On a typical clear day with morning fog (1 September), air temperatures in

solid-walled shelters began near ambient temperature but by afternoon had warmed

to approximately 5 °C above ambient (fig. 3). Air temperature in the mesh-shelter

treatment was generally cooler than the ambient temperature (P = 0.035) and

surpassed ambient only in the late afternoon. On an overcast day (4 September), air

temperature in the solid-walled tree shelters was 1 to 5 °C above ambient temperature (P < 0.001), whereas air temperature in the mesh shelter did not differ significantly from ambient temperature (P = 0.733) (fig. 4).

From 9 April through 31 May, 852 degree-days accumulated in the NO treatment. During the same interval, degree-day accumulation in the WV, WU, BU, and

ME shelter treatments was 115, 111, 108, and 97 percent of that in the NO treatment, respectively (fig. 5). For the period 9 April through 31 August, degree-day

On the five hottest

days, there were no

significant differences in maximum

temperature.
12

RESEARCH PAPER PNW-RP-576

Figure 1—Air temperature in four tree

shelter treatments, relative to no shelter,

on 23 June 2005, a typical, partly cloudy

day. Inset graph shows air temperature

with no shelter (ambient condition).

Treatments are no tree shelter (NO; the 0

line in the large graph), a fine-mesh fabric

shelter (ME), a solid-walled white

unvented shelter (WU), a solid-walled

white vented shelter (WV), and a solidwalled blue unvented shelter (BU).

Figure 2—Air temperature in four tree

shelter treatments, relative to no shelter,

on 19 July 2005, a very hot, clear day.

Inset graph shows air temperature with

no shelter (ambient condition). Treatments

are no tree shelter (NO; the 0 line in the

large graph), a fine-mesh fabric shelter

(ME), a solid-walled white unvented

shelter (WU), a solid-walled white vented

shelter (WV), and a solid-walled blue

unvented shelter (BU).
13

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Figure 3—Air temperature in four

tree shelter treatments, relative to no

shelter, on 1 September 2005, a clear

day with morning fog. Inset graph

shows air temperature with no shelter

(ambient condition). Treatments are

no tree shelter (NO; the 0 line in the

large graph), a fine-mesh fabric

shelter (ME), a solid-walled white

unvented shelter (WU), a solidwalled white vented shelter (WV),

and a solid-walled blue unvented

shelter (BU).

Figure 4—Air temperature in four

tree shelter treatments, relative to no

shelter, on 4 September 2005, an

overcast day. Inset graph shows air

temperature with no shelter (ambient

condition). Treatments are no tree

shelter (NO; the 0 line in the large

graph), a fine-mesh fabric shelter

(ME), a solid-walled white unvented

shelter (WU), a solid-walled white

vented shelter (WV), and a solidwalled blue unvented shelter (BU).
14

RESEARCH PAPER PNW-RP-576

accumulation for the WU and WV treatments was 108 percent that of the NO

treatment, and degree-day accumulation in the BU and ME treatments was 106 and

99 percent of the NO treatment, respectively.

Vapor pressure deficit—

Maximum daily VPD was greatest during July and August but was generally similar

among treatments (table 4). The only significant treatment effect occurred in

September when the solid-walled shelter treatments had a higher maximum VPD

than the NO treatment (3.0 vs. 2.5 kPa). Diurnal patterns in VPD also were similar

among treatments (fig. 6). Under very hot, clear conditions (19 July 2005), VPD in

the NO treatment increased earlier in the day than in the other treatments, and the

WV treatment reached the greatest maximum VPD in the afternoon. However, these

treatment differences were not statistically significant.

Figure 5—Accumulated degree days (base temperature = 5.0 °C) relative to ambient conditions

(NO treatment; the 0 line) during the 2005 growing season for four tree shelter treatments.

Treatments are a fine-mesh fabric shelter (ME), a solid-walled white unvented shelter (WU), a

solid-walled white vented shelter (WV), and a solid-walled blue unvented shelter (BU).
15

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Table 4—Maximum daily vapor pressure deficit, by month, for five tree shelter treatments in 2005, with contrasts

testing differences among treatment combinations

Treatmenta

/ Month

Contrast Apr May Jun Jul Aug Sep Oct

– – – – – – – – – – – – – – – – – – – – Kilopascal – – – – – – – – – – – – – – – – – – – –

NO 1.2 1.7 2.1 ±0.2 3.3 ±0.3 4.0 ±0.3 2.5 ±0.2 0.7 ±0.1

ME 1.3 1.6 2.1 ±0.2 3.1 ±0.3 3.8 ±0.3 2.5 ±0.2 0.7 ±0.1

WU 1.6 1.7 2.2 ±0.2 3.3 ±0.3 4.0 ±0.3 2.9 ±0.2 0.8 ±0.1

WV 1.7 2.0 2.5 ±0.2 3.7 ±0.3 4.4 ±0.3 3.2 ±0.2 1.0 ±0.1

BU 1.6 1.7 2.3 ±0.2 3.4 ±0.3 4.2 ±0.3 2.9 ±0.2 0.8 ±0.1

 – – – – – – – – – – – – – – – – – – – – – P > F – – – – – – – – – – – – – – – – – – – – –

NO vs. ME — — 0.791 0.602 0.524 0.743 0.703

NO vs. WU, WV, BU — — 0.252 0.702 0.577 0.026 0.081

WU vs. WV — — 0.272 0.307 0.373 0.332 0.128

BU vs. WU — — 0.630 0.788 0.564 0.809 0.685

Note: n = 2 for April and May; n = 4 for June through October. Standard error is shown where n = 4. Significant contrasts are bolded. a

 Treatments are: no shelter (NO), a fine-mesh fabric shelter (ME), a solid-walled white unvented shelter (WU), a solid-walled white

vented shelter (WV), and a solid-walled blue unvented shelter (BU).

Figure 6—Vapor pressure deficit (VPD)

in five tree shelter treatments on four days

in 2005. Treatments are no tree shelter

(NO), a fine-mesh fabric shelter (ME), a

solid-walled white unvented shelter (WU),

a solid-walled white vented shelter (WV),

and a solid-walled blue unvented shelter

(BU).
16

RESEARCH PAPER PNW-RP-576

Figure 7—Difference between leaf temperature (abaxial side of Oregon white oak seedling leaf) and

air temperature in the same treatment for seedlings (n = 2) in various tree shelter treatments on a clear

day with morning fog (1 September 2005) and on an overcast day (4 September 2005). Treatments

are no tree shelter (NO), a fine-mesh fabric shelter (ME), a solid-walled white unvented shelter

(WU), a solid-walled white vented shelter (WV), and a solid-walled blue unvented shelter (BU).

Data for NO on 1 September are missing.
17

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Leaf temperature—

During the three hours prior to solar noon on a clear day, leaf temperatures in each

of the solid-walled shelter treatments declined slightly relative to air temperatures in

the same treatments (fig. 7). During the afternoon, leaf temperatures in solid-walled

shelter treatments remained 1 to 3 °C below air temperatures in the same treatments. On an overcast day, leaf temperature remained similar to air temperature in

all treatments.

Relative to leaf temperatures in the unsheltered treatment, leaf temperatures in

the solid-walled shelters were approximately 3 to 5 °C higher in late afternoon on

a clear day, with the WU treatment reaching the highest temperature (fig. 8). Leaf

temperature in the mesh-shelter treatment was as much as 4 °C cooler than the

unsheltered treatment during late morning. On an overcast day, leaf temperature of

seedlings in the solid-walled shelters remained approximately 2 °C above that of

the unsheltered seedlings.

Photosynthetically active radiation—

On a clear day when maximum PPFD reached 1473 mmol·m-2

·s-1 at solar noon,

PPFD within the ME, WU, and BU shelter treatments remained at approximately

600 to 800, 600, and 250 mmol·m-2·s-1, respectively, during daylight hours (fig. 9).

Photosynthetic photon flux density in the ME, WU, and BU treatments averaged

54, 38, and 15 percent of full sunlight (i.e., the NO treatment) during the 2-hour

interval centered on solar noon, but these percentages increased earlier and later in

the day. On an overcast day, where maximum PPFD exceeded 700 mmol·m-2·s-1

only for short periods of time, PPFD in the shelter treatments remained relatively

constant when expressed as a percentage of full sunlight (fig. 10). Photosynthetic

photon flux density in the ME, WU, and BU treatments averaged 53, 40, and 20

percent of full sunlight, respectively. The relationship between PPFD under full

sunlight and PPFD within shelters is shown in figure 11. As ambient PPFD levels

increased, all shelters showed a tendency toward a lower fraction of PPFD admitted.

Ratio of red-to-far-red light—

The ratio of red (655 to 665 nm) to far-red light (725 to 735 nm) on a clear day

(26 June 2007) was significantly lower inside the BU shelter (0.46 ± 0.01) than

within the ME shelter (0.67 ± 0.01) or the WU shelter (0.69 ± 0.01). The latter

two shelter treatments did not differ significantly.

As ambient PPFD

levels increased,

all shelters showed

a tendency toward

a lower fraction of

PPFD admitted.
18

RESEARCH PAPER PNW-RP-576

Figure 8—Leaf temperature (abaxial side of Oregon white oak seedling leaf) for seedlings (n = 2)

in four tree shelter treatments, relative to leaf temperature of seedlings in ambient conditions (NO

treatment; the 0 line) on a clear day with morning fog (1 September 2005) and on an overcast day

(4 September 2005). Tree shelter treatments are a fine-mesh fabric shelter (ME), a solid-walled

white unvented shelter (WU), a solid-walled white vented shelter (WV), and a solid-walled blue

unvented shelter (BU).
19

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Figure 9—Photosynthetic photon flux density (PPFD) in five tree shelter treatments and PPFD

relative to ambient conditions (NO treatment) on 7 September 2005, a clear day. Treatments are

no tree shelter (NO), a fine-mesh fabric shelter (ME), a solid-walled white unvented shelter (WU),

a solid-walled white vented shelter (WV), and a solid-walled blue unvented shelter (BU).
20

RESEARCH PAPER PNW-RP-576

Figure 10—Photosynthetic photon flux density (PPFD) in five tree shelter treatments and PPFD

relative to ambient conditions (NO treatment) on 4 September 2005, an overcast day. Treatments

are no tree shelter (NO), a fine-mesh fabric shelter (ME), a solid-walled white unvented shelter

(WU), a solid-walled white vented shelter (WV), and a solid-walled blue unvented shelter (BU).
21

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Shelter Effects on Seedlings

Seedling survival, growth, and morphology—

The survival rate for oak seedlings in the 2005 study was 79 percent after 1 year,

with similar rates among tree shelter treatments. Growth of oak seedlings in the

2005 study was generally very poor: only 22 percent of seedlings had height growth

greater than 5.0 cm. This poor performance was attributed to planting shock probably owing to a scarcity of fine roots on most of the planting stock. Because seedling performance was atypically poor, growth results for oak seedlings are not

presented. However, a small number of oak seedlings grew well and appeared

vigorous throughout the growing season; these seedlings were used for An assessments. No animal damage occurred during the 2005 or 2007 studies.

Figure 11—Photosynthetic photon flux density (PPFD), as a percentage of ambient conditions (NO treatment), in three tree shelters under a range of ambient PPFD values. Data

were collected on three clear days (6 to 8 September 2005). Treatments are a fine-mesh

fabric shelter (ME), a solid-walled white unvented shelter (WU), and a solid-walled blue

unvented shelter (BU).
22

RESEARCH PAPER PNW-RP-576

All oak seedlings in the 2007 study survived the growing season. Diameter

growth in the BU treatment was significantly less than that in the NO and ME

treatments in the 2007 study, although height growth did not differ significantly

among treatments (table 5). Estimated belowground biomass growth was significantly greater in the NO treatment than in the BU treatment; aboveground biomass

growth followed a similar trend but did not differ significantly among treatments.

Estimated leaf area of first and second flushes did not differ among treatments, nor

did specific leaf area.

Height growth of redcedar was significantly affected by shelter treatments

(P < 0.001). The ranking of height growth among treatments did not differ between midseason (13 July) and the end of the growing season (fig. 12). Height

growth in all shelter treatments was greater than that of unsheltered seedlings,

with the BU treatment yielding the greatest growth (34.9 cm). Redcedar diameter

growth was significantly greater in all shelter treatments (mean = 2.3 mm) compared to the unsheltered treatment (1.6 mm) (fig. 13). At the end of the growing

season, redcedar seedlings in the BU treatment had a significantly greater heightto-diameter ratio (126.7) than the WU (115.4), WV (111.3), ME (110.6), or NO

(101.9) treatments. During the growing season, the height-to-diameter ratio increased by 11.2 percent in the BU treatment, remained virtually unchanged in the

WU and WV treatments, and decreased in the ME and NO treatments by 6.7 and

11.3 percent, respectively. Only one redcedar seedling died during the growing

season.

Physiological variables—

On 9 July 2007, An

 was significantly higher for unsheltered oak seedlings than for

seedlings in the ME treatment at 7:00, 11:45, and 16:00 PST. There was no significant difference between these two treatments at 9:30 PST (fig. 14). At all four

measurement times, An

 in both the unsheltered and the ME treatments was significantly greater than that in the BU treatment. Measurements of An on second-flush

leaves (12:00 to 14:00 PST on 24 August 2007) showed that the NO and ME

treatments did not differ (7.3 ± 1.2 and 7.9 ± 1.3 μmol CO2·m-2·s-1, respectively),

but that both were significantly greater than the BU treatment (0.8 ± 1.7 μmol

CO2·m-2·s-1). When An for the three shelter types (2005 and 2007 data) was expressed relative to An

 in the NO treatment, there was a strong relationship between

net assimilation of CO2

 and PPFD (fig. 15).

Height growth in all

shelter treatments

was greater than

that of unsheltered

seedlings.
23

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Figure 12—Cumulative height growth (with

standard error) at midseason and at the end

of the 2005 growing season for western redcedar seedlings in five tree shelter treatments.

Contrasts compare treatment combinations for

both measurement periods. Treatments are no

tree shelter (NO), a fine-mesh fabric shelter

(ME), a solid-walled white unvented shelter

(WU), a solid-walled white vented shelter (WV),

and a solid-walled blue unvented shelter (BU).

Table 5—Growth and morphological response of Oregon white oak seedlings in 2007 to three tree

shelter treatments

Tree shelter treatmenta

Variable NO ME BU

Diameter growth (mm) 3.9 ± 0.5 a 3.7 ± 0.5 a 1.8 ± 0.5 b

Height growth (cm) 38.5 ± 9.0 32.2 ± 9.0 29.4 ± 8.9

Estimated belowground growth (g) 15.8 ± 1.8 a 10.5 ± 1.8 ab 7.3 ± 1.8 b

Estimated aboveground growth (g)b 9.3 ± 1.7 7.5 ± 1.7 5.3 ± 1.7

Total foliar dry weight (g) 17.6 ± 3.0 16.5 ± 3.0 11.1 ± 3.0

Estimated leaf area, 1st flush (cm2)

c 589 ± 91 366 ± 91 520 ± 89

Estimated leaf area, 2nd flush (cm2)

c 999 ± 304 609 ± 276 708 ± 320

Specific leaf area, 1st flush (cm2

/g) 184 ± 22 147 ± 22 226 ± 21

Specific leaf area, 2nd flush (cm2

/g) 131 ± 12 127 ± 11 169 ± 13

a

 Treatments are no shelter (NO), a fine-mesh fabric shelter (ME), and a blue unvented shelter (BU). Means with

standard error are shown. Within each row, values not followed by the same letter are significantly different (P <

0.05).

b

 Woody components only.

c

 Average leaf area of 10 leaves multiplied by the total number of leaves.
24

RESEARCH PAPER PNW-RP-576

Figure 13—Diameter growth (with standard error) for

western redcedar seedlings in five tree shelter treatments

after 1 year in the 2005 study. Contrasts are shown

indicating the significance of various treatment comparisons. Treatments are no tree shelter (NO), a fine-mesh

fabric shelter (ME), a solid-walled white unvented shelter

(WU), a solid-walled white vented shelter (WV), and a

solid-walled blue unvented shelter (BU).

Figure 14—Net assimilation of CO2 (An) via photosynthesis for Oregon white oak seedlings on

9 July 2007 in three treatments: no tree shelter (NO), a fine-mesh fabric shelter (ME), and a solidwalled blue unvented shelter (BU). Different letters within each time of measurement indicate that

means differ significantly (P < 0.05) at that time.
25

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Discussion

Shelter Effects on Microclimate

Solid-walled tree shelters increased air temperature and leaf temperature relative to

ambient conditions. Maximum daily air temperatures in solid-walled shelters were

often at least 4 to 5 °C higher than ambient conditions. These results support

Hypothesis M1 regarding greater air temperatures in solid-walled shelters. Although these temperature increases may have influenced seedling physiology, they

were not great enough to have caused mortality. Compared to previous studies, the

difference in VPD between the unsheltered condition and unvented solid-walled

shelters was small (Kjelgren 1994, Kjelgren et al. 1997). The finding of high VPD

in solid-walled shelters in our study is contrary to Hypothesis M2 in which we

anticipated decreased VPD in solid-walled shelters owing to seedling transpiration

and reduced air movement. This result suggests that the seedlings produced little

water vapor through transpiration or that this water vapor was produced but lost

Figure 15—Photosynthesis rate, measured as net CO2 assimilation (An), for Oregon white oak

seedlings in three tree shelter types, relative to that of unsheltered seedlings (NO treatment), as a

function of photosynthetic photon flux density (PPFD). Tree shelters are a fine-mesh fabric

shelter (ME), a solid-walled white unvented shelter (WU; 2005 data only), and a solid-walled blue

unvented shelter (BU). Data were collected on 2 clear days: 26 September 2005 and 3 July 2007. Maximum daily air

temperatures in

solid-walled shelters

were often at least 4

to 5 °C higher than

ambient conditions.
26

RESEARCH PAPER PNW-RP-576

from the shelter through air circulation. For shelters with oak seedlings, the seedlings were small and transpiration rates were low owing to limited leaf area and

root development. Solid-walled tree shelters were constructed of a flat sheet that

was formed into a tube using tabs but not sealed along the vertical joint; thus, there

was an unknown amount of air exchange through this seam through which humid

air may have been lost.

Mesh-walled tree shelters had no significant effect on monthly air temperatures,

although there was a trend in which midday temperatures were slightly cooler in

mesh shelters than in unsheltered conditions on clear summer days. Although the

lack of significant air temperature reduction in the mesh shelter failed to support

our hypothesis (Hypothesis M3), the mesh-walled shelter was the only shelter to

cause a reduction in seedling leaf temperature relative to unsheltered seedlings. Previous studies examining various types of mesh-walled shelters have reported that

summer air temperatures within mesh shelters are closer to ambient conditions than

temperatures within solid-walled shelters, where mean and maximum temperatures

are consistently higher than ambient (Bellot et al. 2002, LePage and Banner 2005,

Sharew and Hairston-Strang 2005). Although not significant, VPD values in meshwalled shelters were generally slightly lower than in the other tree shelter treatments, apparently owing to slightly (but not significantly) cooler air temperatures.

Mesh-walled shelters had higher PPFD levels than solid-walled shelters and a redto-far-red ratio that was closer to unobstructed sky than that of the BU shelter.

Thus, mesh-walled shelters may have advantages relative to solid-walled shelters

for growing shade-intolerant species.

The presence of vent holes in solid-walled tree shelters did not significantly

reduce monthly air temperature or VPD as we had hypothesized (Hypothesis M4),

although leaf temperatures were generally slightly lower in the vented shelter. The

trend toward slightly higher afternoon VPD in vented shelters (fig. 6) may be due

to the venting of water vapor produced by seedling transpiration (Bergez and

Dupraz 1997). Other shelter designs with more vent holes or larger vent holes have

produced greater effects on the interior microclimate (vents in the WV shelters

were circular holes, 0.8-cm in diameter, spaced approximately 9 cm apart). Twelve

1.3-cm diameter holes reduced maximum daily air temperature in 1.5-m-tall

shelters by an average of 2.8 °C in Pennsylvania (Swistock et al. 1999). As few as

four 2-cm-diameter holes reduced air temperature by approximately 5 °C in 0.6-mtall shelters in southeastern Spain (Bellot et al. 2002). Small (0.6-cm diameter),

closely spaced (5-cm) vent holes significantly reduced humidity but not air temperature or PPFD for tree shelters in Virginia (Peterson et al. 1995).

Mesh-walled shelters may have advantages relative to

solid-walled shelters

for growing shadeintolerant species.
27

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

The primary effect of shelter color on the microenvironment was the reduced

PPFD within the blue (BU) shelter compared to the white (WU) shelter. Although

the blue shelter showed a trend toward slightly cooler leaf and air temperatures

than the white shelter, differences were not significant, even on hot summer days.

Studies in Indiana and Maryland showed summer air temperatures among translucent white and translucent nonwhite shelters to be similar (Minter et al. 1992,

Sharew and Hairston-Strang 2005). In Utah, air temperatures were slightly higher

in white shelters than in brown shelters (Kjelgren et al. 1997).

Shelter Effects on Seedlings

Both height and diameter growth of redcedar were increased by the fine-mesh

fabric shelters relative to unsheltered seedlings. A positive effect on redcedar height

growth for this type of mesh shelter also was reported in coastal British Columbia

(LePage and Banner 2005), but effects of this shelter type on diameter growth have

not previously been reported. Some species perform poorly within mesh shelters

owing to lateral shoots becoming entangled in the mesh, but the fine-mesh fabric of

the shelters in this study did not allow foliage to penetrate the wall of the shelter.

The larger diameter of the fine-mesh shelters (i.e., 15.2 cm) also may have permitted greater development of branches compared to smaller diameter (10 cm) shelters

commonly used such as Vexar®

 mesh shelters, which may sometimes result in

growth losses because of physical restriction of seedlings (Brandeis et al. 2002).

Height growth of redcedar increases rapidly from 0 to approximately 20 percent full sunlight but then does not respond to further increases after reaching 30

percent full sunlight (Drever and Lertzman 2001). Thus, we can infer that even the

BU shelter, with the lowest PPFD, did not limit height growth of redcedar owing to

its light environment. Rather, the ranking of shelter PPFD relative to full sunlight

was inversely related to the ranking of redcedar height growth (figs. 9, 10, and 12).

Height growth differed significantly between two solid-walled shelter treatments

(BU and WU) of similar size and with similar temperature and VPD patterns; the

growth difference may have been caused by different light environments within

these two shelter types. In addition to reducing PPFD to a greater extent than the

white shelter, the blue shelter had a red:far-red ratio of 0.46, which was substantially lower than that of the white shelter (0.69) or the fine-mesh fabric shelter

(0.67). The red:far-red ratio of the blue shelter is somewhat closer to that of the

forest understory (0.29) (Sharew and Hairston-Strang 2005). Western redcedar has

a very high tolerance of shade (Minore 1990); its shade tolerance may be an

The blue shelter

had a red:far-red

ratio of 0.46, which

was substantially

lower than that of

the white shelter

(0.69) or the finemesh fabric shelter

(0.67).
28

RESEARCH PAPER PNW-RP-576

adaptation to not only the low PPFD in a forest understory but also to the red:farred light conditions typical of those environments. Overall, the growth response

of redcedar to tree shelters supported our hypothesis (Hypothesis G1) that height

growth would be greatest in the shelter in which the ratio of red to far-red light

was lowest.

In the 2007 study, Oregon white oak seedling growth did not follow patterns

reported previously in which substantial height growth increases were observed

for seedlings in BU shelters compared to those in mesh shelters or unsheltered

seedlings (Devine et al. 2007). Contrary to our hypothesis (Hypothesis G1), height

growth in the 2007 study was similar across treatments; additionally, diameter

growth was significantly less in the BU treatment (table 5). Aside from potentially

warmer soils of the raised beds that may have improved root growth, the primary

difference between the 2007 study and the research reported by Devine et al.

(2007) is that, in the 2007 study, seedlings were irrigated daily throughout the

summer, whereas in the previous studies, seedlings were generally not irrigated and

water was a growth-limiting factor. Tree shelters have been shown to reduce

seedling transpiration rates compared to seedlings in unsheltered conditions (Bergez

and Dupraz 1997, Kjelgren 1994, Kjelgren and Rupp 1997). Bergez and Dupraz

(1997) concluded that lower solar radiation within tree shelters was one of the

primary factors reducing evaporative water loss and increasing water use efficiency.

We observed dramatic reductions in solar radiation within tree shelters, and it is

possible that the positive growth effects of tree shelters reported previously for

nonirrigated Oregon white oak were due to increased water use efficiency of

sheltered seedlings. But because soil water was abundant across treatments in our

2007 study, we would not expect to observe growth effects owing to differences in

seedling water use efficiency.

The significantly lower An

 rate for oak seedlings in solid-walled tree shelters

was contrary to our hypothesis (Hypothesis P1) and unexpected because of the consistently greater height growth rates previously reported in these shelters, specifically in the BU shelter type (Devine et al. 2007). The lower An observed in the

solid-walled shelter types in this study was clearly related to the reduced light

levels within those shelters (fig. 15). This relationship between light and An is in

agreement with the findings of Hinckley et al. (1978) who reported, for eastern

white oak (Q. alba L.), relatively high An at PPFD values above approximately 350

mmol·m-2

·s-1 and rapidly declining An at lower light intensities. In our study, PPFD

in the BU treatment averaged only 124 mmol·m-2

·s-1 during An measurements,

The significantly

lower An rate for oak

seedlings in solidwalled tree shelters

was unexpected

because of the consistently greater

height growth rates

previously reported

in these shelters.
29

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

whereas the WU, ME, and NO treatments averaged 373, 619, and 1502

mmol·m-2

·s-1, respectively. The BU shelters, which had the lowest An rates, also

had the lowest diameter growth and estimated belowground growth (table 5).

Additionally, there was no significant difference in estimated leaf area among

shelter treatments for either first or second flushes, indicating that the lower An

 was

not compensated for by greater leaf area of seedlings in the BU shelters. Thus,

under conditions where soil water was nonlimiting, light availability appears to be

the factor that limited photosynthesis and growth of Oregon white oak seedlings.

Net assimilation of carbon by white oak has been shown to decline when air

temperatures reach 35 to 40 °C (Sharkey et al. 1996) and to cease when leaf

temperatures reach 44.5 °C (Hinckley et al. 1978). Given the daily maximum

temperatures in this study (table 3), and the similarity between eastern white oak

and Oregon white oak, it is likely that temperature-induced reductions in An

occurred on the hottest days, probably to a greater extent for oak seedlings in

solid-walled shelters than for those in other treatments. Based on air temperatures

throughout the growing season, full cessation of An owing to temperature probably

occurred little or not at all. Maximum recorded air temperatures in the NO, ME,

WU, WV, and BU treatments were 42.0, 41.5, 46.4, 48.0, and 44.9 °C, respectively, and leaf temperatures during the hottest part of the day (late afternoon) were

generally slightly lower than air temperatures (fig. 7).

Conclusions

There has been little research on fine-mesh fabric tree shelters, but there are several

characteristics that may make this shelter type particularly well-suited for certain

sites or species. The ventilation permitted by this shelter type apparently prevented

the increased air and leaf temperatures produced by solid-walled shelters. Also,

light availability within the mesh shelter was greater than within solid-walled

shelters, which may be advantageous for shade-intolerant species. Although tree

shelters with large mesh openings (e.g., Vexar® shelters) are problematic for some

species because of entanglement or herbivory of branches that grow through the

mesh, the small opening size of fine-mesh fabric shelters in this study prevented

growth of branches through shelter walls.

Solid-walled tree shelters increased air temperatures as anticipated, and significantly increased height and diameter growth of redcedar. Although we observed no

effect of shelter venting, vent holes larger than the ones in this study may provide

greater air circulation, thus moderating temperature increases caused by solidwalled shelters. The most dramatic effect of solid-walled tree shelters was on the

Fine-mesh fabric

tree shelters may be

well-suited for some

sites or species.
30

RESEARCH PAPER PNW-RP-576

light environment, with a greater than twofold difference in PPFD between white

and blue shelters. However, because the shelter with lowest PPFD produced

redcedar seedlings with the greatest height growth and no reduction in diameter

growth, it is clear that this shade-tolerant species is well-suited to the microenvironment of blue shelters. Alternatively, Oregon white oak, which is less tolerant of

shade, had less stem diameter growth within the low-light environment of bluecolored shelters than in an unsheltered environment, when grown under conditions

of no water limitation. Under field conditions, where water is often limiting, solidwalled shelters, regardless of color, increased the rate of Oregon white oak height

growth (Devine et al. 2007). We would suggest, however, that the reduction in

PPFD in solid-walled blue shelters has the potential to limit growth of some shadeintolerant species. Although certain combinations of seedling, site, and environmental conditions may cause seedling growth to be more limited by factors other

than light, it seems prudent to consider shade tolerance when selecting shelter

color.

Our findings illustrate several examples of the numerous influences that tree

shelter design can have on microclimate and seedling performance. Because shelter

design may alter microclimate in a variety of ways, and because optimal microclimate conditions differ among species, selection of an appropriate tree shelter design

must involve these and other considerations, such as cost, durability, and ease of

installation. There are currently many varieties of tree shelters commercially

available, encompassing a wide range of dimensions, materials, and designs (appendix). These shelters, and the option of constructing shelters from other materials,

provide numerous options for altering seedling microenvironment while protecting

from animal damage.

Acknowledgments

We thank the former and current members of the Silviculture and Forest Models

Team who assisted with this research, particularly Kimberly Littke who performed

much of the fieldwork and data collection in the 2005 study. We also thank Douglas

Runde for providing expertise on tree shelters during the early stages of this research and for supplying some of the experimental material and Jeffrey Foster for

providing instrumentation for measurement of photosynthesis. We thank Webster

Forest Nursery (Washington Department of National Resources) for growing the

oak seedlings for the 2007 study. We thank Douglas McCreary, Felix Ponder, Leslie

Brodie, and Kimberly Littke for reviewing a draft manuscript.

Oregon white

oak, had less stem

diameter growth

within the low-light

environment of

blue-colored shelters than in an

unsheltered

environment.
31

Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

English Equivalents

When you know: Multiply by: To find:

Millimeters (mm) 0.0394 Inches

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Square centimeters (cm2) 0.155 Square inches

Degrees Celsius (C) 1.8 C + 32 Degrees Fahrenheit

Kilopascals (kPa) 0.145 Pounds per square inch

Millimole per square meter 10.76 Millimole per square foot per

per second (mmol·m-2·s-1) second

Micromole per square meter 10.76 Micromole per square foot per

per second (μmol·m-2·s-1) second

Liters 0.265 Gallons

Nanometer (nm) 39.370×10-9 Inches

Square centimeters per gram 0.228 Square inches per ounce

(cm2/g)

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Influence of Four Tree Shelter Types on Microclimate and Seedling Performance of Oregon White Oak and Western Redcedar

Appendix: Tree Shelter TypesTable 6—Examples of some commercially available tree sheltersShelter Type Manufacturer Web siteFreegro® Fabric mesh Freegro, Prince Rupert, BC, http://www.freegro.com/

Canada

Tree Protector & Miracle Tube® Solid-walled Tree Pro, West Lafayette, IN http://www.treepro.com/

ProTex® Pro/Gro Solid-walled Norplex, Inc., Auburn, WA none

Vexar® Plastic mesh Norplex, Inc., Auburn, WA none

Tubex® Treeshelters and NetGuard® Solid-walled and mesh Tubex, Ltd., Aberaman Park, http://www.tubex.com/

Aberaman, South Wales, U.K.

Blue-X® Treeshelters Solid-walled, two-layer Blue-X Enterprises, Inc., http://www.growtube.com/

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