Forest

Products

Laboratory

General 

Technical 

Report

FPL–GTR–224

United States Department of Agriculture

Forest Service

In cooperation 

with the

United States

Department of

Transportation

Federal 

Highway 

Administration

Evaluating Naturally 

Durable Wood Species for 

Repair and Rehabilitation 

of Above-Ground 

Components of 

Covered Bridges

Grant T. Kirker

Carol A. Clausen

A.B. Blodgett

Stan T. Lebow
July 2013

Kirker, Grant T.; Clausen, Carol A.; Blodgett, A.B.; Lebow, Stan T. 2013. 

Evaluating naturally durable wood species for repair and rehabilitation

of above-ground components of covered bridges. General Technical Report 

FPL-GTR-224. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 40 p.

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Contents

Introduction..........................................................................1

Objective and Scope ............................................................2

Materials ..............................................................................2

 Wood Selection ................................................................2

 Controls............................................................................5

Research Methods................................................................6

 Field Testing.....................................................................6

 Chemical Extraction and Characterization ..................... 7

 Gas Chromatography–Mass Spectrometry (GS–MS) 

 Analysis........................................................................... 7

Data Analysis...................................................................... 7

Laboratory Bioassays.......................................................... 7

 Disk Assay Inhibition Test (Fatty Acids)........................ 7

 Disk Assay Inhibition Test (EtOH:Tol Extractives)........ 9

 Preparation of Extractive-Free Wood.............................. 9

 Soil Bottle Experiments (Extracted vs. Unextracted 

 Naturally Durable Samples)............................................ 9

Results and Discussion ....................................................... 9

 Field Tests....................................................................... 9

 Chemical Extractives Characterization........................... 9

Laboratory Decay Resistance ............................................ 12

Disk Assays........................................................................ 12

Laboratory Termite Resistance .......................................... 14

Laboratory Soil Block Assays............................................ 15

Summary and Discussion................................................... 17

 Field Tests...................................................................... 17

 Chemical Characterization............................................. 19

 Laboratory Bioassays..................................................... 19

Conclusions and Recommendations.................................. 19

References.......................................................................... 20

Appendix I—Above-Ground Field Evaluation and 

GC-MS Analysis of Naturally Durable Wood 

Species ............................................................................... 22

Appendix II—The Role of Extractives in Naturally 

Durable Wood Species....................................................... 35

Abstract 

More than 1,500 covered bridges remain in the United 

States. They are a unique part of our history; thus, replacement of bridge components is an equally important part 

of preserving this uncommon style of craftsmanship. The 

goal of this project was to evaluate seven wood species for 

their durability in above-ground field exposure. Chemical 

analysis was also conducted using gas chromatography–

mass spectrometry (GS–MS) for fatty acids and terpenoids 

in an attempt to correlate extractive content with durability. 

Extracts removed from the durable wood species were also 

tested in laboratory bioassays to determine their biological 

activity against wood decay fungi and termites. This report 

serves as a guide for the use of these naturally durable wood 

species for rehabilitation of above-ground components of 

covered bridges and incorporates the results of field and 

laboratory tests into the final recommendations.

Keywords: Natural durability, above-ground testing, decking, wood decay fungi, weathering, subterranean termites.

This study is part of the Research, Technology and Education portion of the National Historic Covered Bridge 

Preservation (NHCBP) Program administered by the 

Federal Highway Administration. The NHCBP program 

includes preservation, rehabilitation and restoration of 

covered bridges that are listed or are eligible for listing 

on the National Register of Historic Places; research for 

better means of restoring, and protecting these bridges; 

development of educational aids; and technology transfer to disseminate information on covered bridges in 

order to preserve the Nation’s cultural heritage.

This study is conducted under a joint agreement between 

the Federal Highway Administration–Turner 

Fairbank Highway Research Center, 

and the Forest Service – Forest Products Laboratory.

Federal Highway Administration Program Manager– 

Sheila Rimal Duwadi, P.E.

Forest Products Laboratory Program Manager– 

Michael A. Ritter, P.E.
Evaluating Naturally Durable Wood Species 

for Repair and Rehabilitation of AboveGround Components of Covered Bridges

Grant T. Kirker, Research Forest Products Technologist

Carol A. Clausen, Supervisory Research Microbiologist

A.B. Blodgett, Biological Science Technician

Stan T. Lebow, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin

Introduction

More than 1,500 covered bridges remain in the United 

States. They are a unique part of our Nation’s history; thus, 

replacement of bridge components is an equally important 

part of preserving this uncommon style of craftsmanship. 

Covered bridges are designed for protection from the elements, but it is difficult to protect all wood components 

from moisture exposure. Some areas of the structures, such 

as weatherboarding and wood members near the ends of 

the bridges, are very particularly susceptible to wind-driven 

precipitation. Chronic wetting or wet–dry cycling can create 

conditions that are conducive to biodeterioration. Consequently, biodeterioration from decay and insects is frequently responsible for replacement of above-ground covered 

bridge components. Treatment of susceptible components 

with chemical wood preservatives is one way to protect 

them from biodeteriorating organisms. However, naturally 

durable domestic wood species, particularly underutilized 

or invasive species, could provide a more suitable alternative to chemically treated wood products for above-ground 

replacement components. 

The original structural elements of covered bridges were 

not treated with preservatives. Historically, covered bridges 

were constructed from locally sourced old-growth stands 

of local wood species and wood species known to be more 

resistant to rot, such as white oak (Quercus alba L.) (Pierce 

and others 2005). Subsequently, covered bridges in the 

Eastern United States were commonly made from eastern 

hemlock (Tsuga canadensis), white pine (Pinus strobus), 

and spruce (Picea spp.), whereas Douglas-fir (Pseudotsuga 

menziessii) was used most often in the west, and southern covered bridges were mostly built with southern pine 

(Pierce and others 2005). Whereas old-growth (first-growth) 

southern pine (Pinus spp.), spruce, and Douglas-fir were 

commonly used and adequately durable in the 18th and 19th 

centuries to build covered bridges, second-growth stands of 

the same wood species are typically nonresistant to decay 

(Clausen 2010) and should not be considered for replacement components unless they 

are pressure treated with a wood preservative. In lieu of 

preservative treatment, locally sourced, naturally durable 

wood species should be considered for replacement components, such as floor elements and trusses, when rehabilitating a covered bridge. By design, the risk of fungal decay on 

interior components of a covered bridge that are not exposed 

to wetting is low. That risk is highly dependent on wood 

species, exposure to rainfall, local temperature, and ability of the component or structural design to shed moisture 

quickly. Decay risk can be elevated by trapping water or 

condensation in joints between components, around bolts, or 

by connectors that channel water to the interior of the bridge 

enclosure. 

Chemical extractives have long been recognized as key 

features that impart natural durability in the heartwood of 

certain wood species (Scheffer and Cowling 1966; Taylor 

and others 2002). The field of naturally durable wood is a 

huge body of literature, but excellent reviews can be found 

in Taylor and others (2002), Singh and Singh (2012), and 

Yang (2009). The review by Taylor and others (2002) is 

especially useful for understanding heartwood extractives 

and their natural roles. Extractives from some wood species 

have been evaluated for use as environmentally friendly 

preservatives or additives to coatings (Laks and McKaig 

1988; Van Acker and others 1988; Kennedy and others 

2000; Smith and others 1989; Chedgy and others 2009), but 

little is known about the chemical makeup of many wood 

extractives as it relates to natural durability. Unlike extractives from a well-known durable wood such as western 

redcedar (Thuja placata), extractives from lesser-known 

wood species believed to be responsible for high resistance 

to decay and insects have not been isolated, identified, and 

characterized. Correlating chemical properties of extractives in laboratory tests with field performance may increase 

opportunities for utilizing lesser known wood species as locally sourced materials for repair and rehabilitation of covered bridge components in protected outdoor exposure. The 

chemical component(s) responsible for imparting durability 

could also provide the basis for future research on environmentally benign wood protection systems for above-ground 

components of covered bridges.
General Technical Report FPL–GTR–224

2

Objective and Scope

Above-ground performance of treated and untreated wood 

has been the subject of numerous studies worldwide (Carey 

and Bravery 1986; Carey and others 1981; Carey 2002; 

Fougerousse 1976; Highley 1984, 1993; Morris and McFarling 2007; Scheffer 1971; Savory and Carey 1979; Williams 

and others 1995). Above-ground tests are useful for evaluating the natural durability of building components intended 

for applications protected from the environment (Clausen 

and Lindner 2011; Clausen and others 2006; Eslyn and others 1985; Highley 1995; Räberg and others 2005). A number 

of standardized and alternative test designs have been evaluated; e.g., post-rail, cross braces, Y-joints, simulated deck 

test, L-joints, and lap joints (DeGroot 1992; Hedley and others 1995; Morrell and others 1998; Morris and McFarling 

2007), with L- and lap-joints most commonly used. One 

alternative test assembly, the simulated deck test, addresses 

the need for accelerated test methods to speed up the otherwise lengthy period of time necessary to acquire sufficient 

performance data for new preservatives (Williams and others 1995; Morris and McFarling 2007). In this study, field 

performance for natural durability was assessed through 

comparative above-ground simulated deck tests that approximate in-service conditions in Mississippi (Severe decay 

hazard zone 5) and Wisconsin (Moderate decay hazard zone 

2). Under conditions of high decay hazard, Southern Pine 

typically fails in above-ground exposure in just 12 months. 

Tannins, phenols, and other resin acid extracts from certain 

species of wood have been found to be fungitoxic and are 

believed to be responsible for imparting natural durability to certain wood species. The chemical composition of 

pine cone extracts (Micales and others 1994), pine needle 

extracts (Zinkel and Magee 1991), and the efficacy of pine 

and spruce extractives against brown-rot and white-rot fungi 

have been evaluated (Celimene and others 1999). Western 

redcedar extractives are perhaps the most studied and best 

understood of the highly durable wood species in the United 

States (Stirling and others 2007; Morris and others 2012). 

However, little is known about the phytochemical composition of durable underutilized or invasive wood species. 

The three main objectives of this project were the following:

•	 To assess field performance of naturally durable wood 

species in accelerated above-ground field tests in high 

and moderate decay hazard zones.

•	 To assess the effectiveness of chemical extractives from 

those wood species in laboratory tests against decay fungi 

and termites. 

•	 To characterize the chemical components of wood 

extractives that successfully inhibited biodeteriorating 

organisms for future development as “green” wood 

preservatives.

Materials

Wood Selection

Table 1 shows basic information about the species chosen 

for this study. Considerations for the selection of naturally 

durable timber species for covered bridge replacement 

components must also include workability and regional 

availability. The following excerpts on the characteristics 

of the wood species selected for this study are taken from 

Wiemann and others (2010) and Alden (1995, 1997). Distribution maps are adapted from U.S. Geological Survey 

tree species range maps (USGS 2012) and the United States 

Department of Agriculture Natural Resources Conservation 

Service plants database (www.plants.usda.gov/).

Alaska Yellow Cedar

Alaska yellow cedar (Chamaecyparis nootkatensis) is distributed in the coastal forests from southwestern Alaska 

through British Columbia to northern California (Fig. 1). 

Alaska yellow cedar trees can attain heights of 120 ft 

(36.6 m) and diameters of 6 ft (1.8 m). These trees are frequently over 300- y- old to more than 700 y with a record of 

more than 1,040 y. The sapwood is narrow and sometimes 

slightly lighter than the bright yellow heartwood. The wood 

is moderately heavy, soft, fine-textured, straight-grained, 

and durable. It has moderate strength, stiffness, and hardness, shrinks little during drying, and is dimensionally stable 

after seasoning. Alaska yellow cedar is easily worked by 

Table 1. Botanical and common names for the wood species evaluated in this study 

and their classification as underutilized or invasive species

Abbreviation Common name Botanical name Category 

AYC Alaska yellow cedar Chamaecyparis nootkatensis UUa

BL Black locust Robinia pseudoacacia UU 

CAT Catalpa Catalpa spp. UU 

ERC Eastern redcedar Juniperus virginiana UU 

HM Honey mesquite Prosopis gradulosa ISb

PAW Paulownia, princess-tree Paulownia tomentosa IS 

SYP Southern pine Pinus palustris Negative control 

WCJ West Coast juniper Juniperus occidentalis UU 

WRC Western redcedar Thuja plicata Positive control a

Underutilized. 

b

Invasive species. 
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

3

both hand and machine tools. The heartwood is rated as 

resistant to very resistant to decay. It has many commercial 

uses such as bedding for heavy machinery, boat building, 

bridge and dock decking, cabinetry, cooling towers, framing, furniture, flooring, marine piling, paneling, stadium 

seats, and utility poles, to name a few.

Black Locust

Black locust (Robinia pseudoacacia) has been extensively 

naturalized in the United States and is native to 

the Appalachian Mountains and Ozark Mountain regions 

(Fig. 2). It also grows in southern Illinois and Indiana. Black 

locust trees can reach a height of 100 ft (30.5 m) with a 

diameter of 3 ft (0.9 m). The sapwood of black locust is 

creamy white and the heartwood varies from greenish yellow to dark brown. The wood turns reddish brown when 

exposed to air. It has a high density and decay resistance. It 

shows slight shrinkage and has high bending strength. Because black locust is one of the hardest woods in America, 

it is difficult to work with hand tools, but it nails well. It is 

rated as exceptionally resistant to heartwood decay. Common uses include fencing, furniture, and mine timbers. 

There are reports of dermatitis from direct skin contact with 

the wood.

Catalpa spp.

Catalpa is native to the central eastern United States, but 

is naturalized throughout the United States and Canada 

(Fig. 3). There are two commercial North American 

Catalpa species and the wood characteristics are similar for 

both species. The sapwood is narrow and gray, while the 

heartwood is grayish brown, tinged with lavender. It is ring 

porous, straight-grained, light, and soft. It may be confused 

with ash. Catalpa dries quickly and is easy to season. It 

works very well with hand and machine tools, although it 

requires care to sand well. The heartwood of Catalpa is very 

resistant to decay, even when it is in contact with soil. It is 

commonly used for fence posts and rails, general construction, interior finish, picture frames, handles, cabinetry, and 

fuel wood.

Eastern Redcedar

Eastern redcedar (Juniperus virginiana) is native to the eastern half of the United States (Fig. 4). It has the widest distribution of any other conifer in the eastern Unites States and 

can reach heights of 120 ft (36.6 m) and a diameter of 

4 ft (1.2 m). It is known as a pioneer species as it is one of 

the first trees to invade disturbed areas. It grows very slowly. 

Eastern redcedar has thin, white sapwood, and the heartwood is red to deep reddish-brown. The sapwood may be in 

stripes, alternating with the heartwood. It has a fine, uniform 

texture and a straight grain, except where deflected by knots, 

making it easy to work with hand or machine tools. Eastern 

redcedar splits easily but holds nails well.

The wood is moderately low in strength and stiffness, but 

high in shock resistance. It has good dimensional stability 

and shrinks little during drying. The heartwood is highly 

Figure 1. Alaska yellow cedar (Chamaecyparis nootkatensis) distribution in the 

continental United States and Alaska (Little 1971).
General Technical Report FPL–GTR–224

4

resistant to decay and insects. It is commonly used for fence 

posts, chests, closet linings, pencils, carvings, pet bedding, 

furniture, flooring, small boats, and household items. It may 

cause dermatitis and respiratory problems.

Honey Mesquite

Honey mesquite (Prosopis glandulosa) is one of three North 

American species of mesquite. Its native range is shown in 

Figure 5. It reaches heights of 40 ft (12.2 m), with diameters 

varying from 10 in. (25.4 cm) to 4 ft (1.2 m). The sapwood 

is a lemon yellow, while the heartwood is deep reddish 

brown. The wood is dense, close-grained, very hard and 

heavy, but somewhat brittle. It is exceedingly resistant to 

heartwood decay, with thin sapwood. It contains high concentrations of tannins and has been reported to cause dermatitis. Honey mesquite is used for buildings, cabinetry, posts, 

charcoal, fuel, railway crossties, and paving blocks.

Princess-tree

Princess-tree (Paulownia tomentosa) is a popular ornamental and widely recognized as an invasive species and specifically as a noxious weed in some states (Fig. 6). The growth 

rate is rapid and trees reach their maximum height of 60 ft 

(18.3 m) and 2 ft (0.6 m) in diameter. They are used for fuel, 

lumber, and naval stores. 

Western Juniper

Juniperus occidentalis is native to the mountains of the 

Pacific Coast region (Fig. 7). It reaches heights of 35 ft 

Figure 2. Black locust (Robinia pseudoacacia) distribution in the United 

States (Little 1971).

Figure 3. Distribution of Catalpa spp. in the United States (Little 1971, 1977). 
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

5

(10.7 m) with exceptional trees reaching heights of 87 ft 

(26.5 m), with a diameter of more than 13 ft (4 m). Older 

trees may live for 1,000 years. The heartwood of western 

juniper is light red to reddish-brown. It is durable, fragrant, 

close-grained, moderately dense, lightweight, relatively brittle, and splits easily. The heartwood is reported to have good 

natural durability. It is easy to work and finish. It is used for 

fence posts, fuel wood, novelties and pencil wood. Western 

juniper may cause dermatitis and respiratory problems, so 

appropriate protection should be used when handling this 

wood.

Controls

Western Redcedar

Western redcedar (Thuja plicata) was selected as the positive control in this study. It is lightweight, moderately soft, 

subject to compression, splinters easily, and is low in 

strength. These characteristics make it undesirable for replacement components in covered bridges compared with 

other naturally durable wood species. However, western 

redcedar is renowned for being very resistant to decay, and 

volumes of published literature have characterized the 

Figure 4. Eastern redcedar (Juniperus virginiana) distribution in the United States 

(Little 1971).

Figure 5. Honey mesquite (Prosopis glandulosa) distribution in the United 

States.
General Technical Report FPL–GTR–224

6

extractives responsible for its natural durability making it a 

model comparative wood species. 

Southern Pine

Longleaf pine (Pinus palustris), designated as SYP, was 

selected as the negative control in this study. The sapwood 

is usually wide in second-growth stands and is not resistant 

to decay. Southern Pine is a term used for a group of pines 

that includes longleaf pine, shortleaf pine, loblolly pine, 

and slash pine and has numerous other common names. The 

southern pines are native to southeastern United States. Because the otherwise nondurable sapwood is quite permeable 

to preservative treatments, Southern Pine makes up 75% of 

the treated wood market in the United States. 

Research Methods

Field Testing

Above-ground durability was assessed at the Forest Products Laboratory exposure site near Madison, Wisconsin, 

(decay hazard zone 2) and in the Harrison National Forest in 

Gulfport, Mississippi, (decay hazard zone 5) using a modification of the E25-08 field test decking method (AWPA 

2011a). Five untreated decking specimens (5.1 by 15.2 by 

45.7 cm long) or (2-in. by 6-in. nominal by 18-in. long) cut 

Figure 6. Princess-tree (Paulownia tomentosa) distribution in the United States 

(NRCS 2012).

Figure 7. Western juniper (Juniperus occidentalis) distribution in the United States 

(Little 1971).
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

7

from each wood species were fastened to a simulated deck 

structure 91.4 cm (36 in.) above the ground at each test site. 

The exposure platform was designed so that the front edge 

of each specimen hung over the platform and the back edge 

of each specimen was flush with the platform and abutted a 

pine feeder (5 cm by 15 cm (2-in. by 6-in.) nominal) positioned at a 90° angle to the edge of the test specimen. Each 

year for 3 y, the specimens were visually evaluated for decay, insect attack, dimensional stability (cupping and checking), and overall appearance. Specimens were rated according to E25-08 (AWPA 2011a) with additional measurements 

for cupping and checking recorded during each evaluation 

(Table 2).

Chemical Extraction and Characterization

Wood samples were Wiley-milled to 40 mesh size, extracted 

using various solvents (details follow) using a soxhlet apparatus, concentrated on a rotary evaporator and re-suspended 

in hexane or methanol (depending on target analytes) for 

analysis. The chemical analysis focused on two major 

groups of chemical compounds: (1) fatty acids and (2) terpenoids. Fatty acids were chosen because previous research 

indicated the presence of antifungal and antibacterial properties at high concentration, and we hypothesized that the 

more durable species might contain and retain higher concentrations of certain fatty acids. Terpenoids were an obvious second choice; they have been implicitly linked to the 

durability of western redcedar (Morris and Sterlling 2012) 

and have demonstrated antifungal properties (Abad and others 2007). The goal of the fatty acid analysis was to observe 

the changes in concentration as the samples weathered.

Gas Chromatography–Mass Spectrometry 

(GS–MS) Analysis

Fatty Acids

Fatty acids were extracted in 2:1 chloroform: methanol, 

derivitized using methanolic hydrochloric acid, and quantified using custom c7-c26 fatty acid methyl ester (FAME) 

standards (Restek) (Fig. 8). Standards and samples were 

fractionated using solid phase extraction (SPE) columns to 

separate saturated, monounsaturated, and polyunsaturated 

fatty acids. Fatty acid concentrations were quantified for 

years 1–2 and compared with unexposed samples. GS–MS 

conditions are presented in Table 3.

Terpenoids

Terpenoids were the second target in the analysis of extractives of these naturally durable wood species. Terpenoids 

are a large class of chemicals that are widely distributed 

in many different plant species. Terpenoids are chemically 

modified terpenes and are also sometimes referred to as isoprenoids. The name terpene refers to turpentine, which is the 

most famous of the compounds derived from conifers and 

frequently used as a solvent and to make other chemicals. 

Pure terpenoids were selected for analysis and used to create 

a standard curve for quantitation and an analytical method 

was produced that fully separated the peaks of interest 

(Fig. 9). Ethanol: toluene extractives from the preparation 

of extractive-free wood (See Chemical Analyses section) 

were analyzed using the terpenoid method and quantified. 

Additional terpenoids not included in the standard set were 

identified using the GS–MS spectral library. 

Data Analysis

GS–MS data were compiled into tables to compare fatty 

acid and terpenoid composition across species and time intervals. Fatty acid profiles were compared from unexposed 

and exposed samples to determine depletion of fatty acid 

methyl esters (FAME)s over time. FAME composition was 

also compared between durable species in an attempt to 

correlate initial FAME composition with field durability. 

Terpenoid composition of quantified targets was compared 

between species for unexposed samples and additional terpenoids not in the standard curve were identified using the 

GS–MS library and tabulated.

Laboratory Bioassays

Disk Assay Inhibition Test (Fatty Acids)

Mycelial inoculum was prepared by aseptically inoculating 

100 ml of modified Bailey’s minimal medium in a 250-ml 

Erlenmeyer flask with an agar plug from the margin of active fungal growth in a Petri dish culture of four wood decay 

fungi (two white-rot fungi: Irpex lacteus (HHB 7823) and 

Trametes versicolor (Mad 697), and two brown-rot fungi: 

Tyromyces palustris (TYR 6137) and Gloeophyllum trabeum (Mad 617). Stationary cultures were incubated at 

27 °C for 3 wk until a mat of mycelial growth had covered 

the surface of the flask. The mycelial mat was aseptically 

placed in a sterile blender cup with 100-ml sterile deionized 

(DI) water and blended briefly to create a mycelial suspension of the individual test fungi. Petri dishes of 2% malt 

extract agar were inoculated with 1 mL of mycelia suspension from individual test fungi. Extractives obtained from 

naturally durable wood species using ethanol: toluene (See 

preparation of extractive-free wood (Chemical analyses section)) was used to treat sterile Whatman disks. 100 μL 

of each ethanol: toluene fraction was added to Whatman 

cellulose discs and allowed to dry. Plates were incubated at 

27 °C and inhibition of fungal growth was recorded at 1, 3, 

and 7 days. 

Table 2. Rating system for 

above-ground deck test 

Scale Condition of specimen 

10 Sound 

9.5 Trace/Suspicion of decay 

9 Slight attack 

8 Moderate attack 

7 Moderate /Severe attack 

6 Severe attack 

4 Very severe attack 

0 Failure 
General Technical Report FPL–GTR–224

8

Figure 8. Fatty acid methyl esters (FAMES) used for quantification of durable wood species. Standards 

were fractionated to separate saturated FAMES (no double bonds), monosaturated (1 double bond), and 

polyunsaturated (>1 double bond) FAMES. 

Figure 9. New targets for terpenoid analyses. 1. Alpha-Pinene, 2. Limonene, 3. Gamma.-Terpinene, 4. Terpinolene, 

5. Citronellal, 6. 1-Nonanol, 7. Geraniol, 8. Cedrene 9. Thujopsene. 
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

9

Disk Assay Inhibition Test 

(EtOH:Tol Extractives)

Extractives were removed from test blocks of the nine naturally durable wood (NDW) species using an ethanol: toluene 

mix according to ASTM standard D1105 (ASTM 2012). 

The removed extractives were added to sterile assay disks 

and aseptically placed on agar plates covered with mycelia 

of four wood decay fungi (two white-rot fungi: Tyromyces 

palustris and Trametes versicolor, and two brown-rot fungi: 

Irpex lacteus and Gloeophyllum trabeum). Inhibition of fungal growth was recorded at 1, 3, and 7 days. 

Preparation of Extractive-Free Wood

Ten millimeter cubes were cut from each species of wood. 

They were numbered and conditioned 1 wk at 27 °C and 

30% relative humidity (RH) before obtaining initial weights. 

Half of the numbered blocks were extracted following 

ASTM D1105-96 with minor adaptations as follows: blocks 

were extracted in 150 mL of a 1:0.43 mixture of 95% ethanol to toluene in soxhlet extraction apparatus, 24 blocks per 

soxhlet by species, at 100 °C for 6 h. Blocks were rinsed 

in 95% ethanol and allowed to air-dry overnight. After 

air-drying, blocks were extracted in soxhlet extraction apparatus for 6 h in 150 mL 95% ethanol. Blocks were again 

rinsed in 95% ethanol and allowed to dry overnight. The 

following day, blocks were boiled at 100 °C three times 

consecutively in 1-L portions of distilled water. Blocks were 

rinsed with hot distilled water and allowed to dry overnight. 

Blocks were allowed to condition for 1 wk then weighed to 

determine loss caused by extractive removal. Because each 

soxhlet apparatus could only hold 24 blocks, the extraction 

was repeated with a second set of 24 blocks.

Soil Bottle Experiments (Extracted vs. 

Unextracted Naturally Durable Samples)

Termite Soil Bottle Choice Test

Termite soil bottle choice tests were conducted with one 

extracted and one unextracted block (10 mm) exposed to 1-g 

termites in triplicate jars following a modification of AWPA 

E1-07 choice test bioassay (AWPA 2007). Weighed conditioned blocks were added to plastic cups containing 50 g of 

sterile washed and sifted sand plus 9 ml of deionized water. 

After 4 wk, the percentage of termite mortality was visually approximated. Blocks were removed, then conditioned 

for 1 wk at 26 °C and 30% humidity and final weights were 

obtained.

Laboratory Soil Block Cultures

Laboratory soil block cultures were set up according to a 

modification of AWPA standard E10 (AWPA 2010). Three 

extracted and un-extracted blocks were challenged in duplicate soil bottles with one of six wood-decay Basidiomycetes, which included three brown-rot fungi: G. trabeum

(Pers.: Fr.) Murr. (Mad 617), Postia placenta (Fr.) Lars. & 

Lomb. (Mad 698), T. palustris (Berk. & Curt.) Murr. (TYP 

6137), and three white-rot fungi: 2 strains of I. lacteus (Fr.: 

Fr.) Fr. (HHB7328 and Mad 517), and T. versicolor (L.: Fr.) 

Pil. (Mad 697). Southern pine and sugar maple were used 

as feeder strips for brown-rot and white-rot fungi, respectively. Test blocks were pre-sterilized with propylene oxide 

(AWPA E10.13.3.3) in glass tubes separated by species and 

extraction process to prevent volatiles from crossing over 

between extracted and nonextracted wood specimens. Soil 

block cultures were placed in an incubation room at 27 °C 

and 70% humidity for 8 wk to decay. After 8 wk, blocks 

were removed and mycelium was brushed off. Blocks were 

conditioned 1 wk before final weights were obtained.

Results and Discussion

Field Tests

Over the 3-y study, above-ground decking samples became 

bleached in appearance because of normal UV exposure, 

more heavily in Mississippi than Wisconsin (Fig. 10). Some 

fungal growth characteristic of incipient decay was noted 

as early as 12 months in Wisconsin (Fig. 11). Similar patterns of weathering were seen in Wisconsin and Mississippi. 

SYP was moderately attacked by decay fungi (Fig. 12) and 

showed the highest degree of cupping and checking (See 

Figs. 13, 14). Paulownia (PAW) was slightly attacked and 

showed moderate cupping and checking. The entire cedar 

group (western redcedar (WRC), eastern redcedar (ERC), 

West Coast juniper (WJN), and AYC) showed no signs of 

deterioration and low levels of cupping and checking. The 

durable hardwoods, honey mesquite (HM) and black locust 

(BL), showed very little deterioration or cupping, but BL 

did have moderate checking on the wood surface. CAT was 

slightly attacked in Mississippi, but remained sound in Wisconsin. CAT also exhibited moderate cupping, being more 

pronounced in Mississippi than in Wisconsin. 

Chemical Extractives Characterization

The phytochemical composition of the wood species selected for field tests in this study were chemically extracted and 

both extractives and the extractive-free wood were individually evaluated for resistance to decay fungi and termites. 

Extractives were analyzed by gas chromatography–mass 

spectroscopy (GS–MS). Components were characterized 

on the basis of their GS–MS spectra compared with corresponding known spectra in the GS–MS library. Two classes 

of compounds were singled out for this study: (1) fatty 

acids, analyzed as FAMES and terpenoids. Both classes of 

chemical have documented fungicidal properties and are 

thought to play a protective role in natural durability.

Fatty Acid Methyl Ester (FAME) Analysis

For quantification of common fatty acids, the Restek Food 

Industry FAME mix was used and targets were quantified 

and compared with unexposed samples (Fig. 15). Initial 
General Technical Report FPL–GTR–224

10

Figure 10. Visual comparison of decking specimens at installation (top row) and after 3 y above-ground 

exposure at the Valley View test site in Madison, Wisconsin (bottom row). Note the checking of SYP and 

BL.

Figure 11. After 12 months outdoor exposure in Wisconsin 

(Decay hazard zone 2) the underside of the Southern Pine 

controls with fruiting bodies present and stringy bleached 

undersides, indicating colonization by white-rot fungi 

along with black mold.

0

2

4

6

8

10

12 Wisconsin Mississippi

Mean decay rating

AYC

BL

CAT

ERC

HM

PAW

SYP

WCJ

WRC

Wood species

Figure 12. Mean decay ratings of each nondurable 

wood species after 3-y field exposure in Wisconsin and 

Mississippi.
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

11

0

0.5

1

1.5

2

2.5

AYC

BL

CAT

ERC

HM

PAW

SYP

WCJ

WRC

Cupping (mm)

Wisconsin Mississippi

Wood species

Figure 13. Mean cupping in millimeters from level of each 

wood species after 3-y field exposure in Wisconsin and 

Mississippi.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

AYC BL CAT ERC HM PAW SYP WCJ WRC

Wisconsin

Mississippi

Wood species

Mean visual rating

Figure 14. Mean visual ratings of degree of checking on 

a scale of 1–5 by wood species after 3-y field exposure in 

Wisconsin and Mississippi.

0

2

4

6

8

10

12

14

16

18

20

AYC BL CAT ERC HM PAW SYP WCJ WRC

Lignoceric

Tricosanoic

Behenic

Heneicosanoic

Arachidic

Stearic

Margaric

Palmitic

Pentadecanoic

Myristic

Tridecanoic

Lauric

Undecanoic

Capric

Caprylic

Wood sample

Saturated FAME concentration (ng/sample)

Figure 15. Concentration of saturated FAMEs in unexposed wood samples.
General Technical Report FPL–GTR–224

12

fatty acid analysis found highest amounts of target fatty acids in AYC and SYP (both 19 ng/mg wood). Lowest amount 

was in WCJ (5.5 ng/mg wood). Unexposed SYP had the 

highest diversity of FAMEs (9). The largest amount of any 

one fatty acid was behenic acid found in AYC (5.7 ng/mg 

wood). 

After 2-y exposure at both test sites, the amounts of fatty 

acids diminished significantly, particularly SYP and ERC 

(Figs. 16, 17). Saturated fatty acids were markedly low in 

the 2-y Wisconsin samples (Fig.16). Unsaturated fatty acids 

(cis-10-heptanoic, oleic, cis 11eicosanoic, 13-docosenoic, 

and nervonic acids) were still detectable in several of the 

species samples after 12 months of outdoor exposure. The 

unsaturated fatty acids varied considerably among the different wood species. For a more complete explanation of 

this portion of the study, refer to Appendix I for the complete results of the FAME analysis.

Analysis of Terpenoids

Common terpenoids were selected as standards and used 

to create a standard curve for analysis. Concentrations of 

the targeted compounds in unexposed samples are shown 

in Figure 18. Western redcedar had the highest diversity of 

terpenoids (5) and the hardwood species, not surprisingly, 

contained no terpenoids, as they are typically asociated with 

conifers. Additional terpenes not included in the standard 

curve are shown in Table 4. Beta sitosterol was the most 

widely isolated compound, present in all but SYP, WRC, 

and ERC. It is likely also present in those species and may 

not have been detectable in those samples because of coelution or masking by other compounds. Several unique 

compounds were found in AYC (trans-shisool, cubenol, 

tau-muurolol, alpha-cadinol, coniferyl aldehyde) and WRC 

(isopugelol acetate, flavone, and nortrachelogenin). Terpenoid analysis was carried out on unexposed samples only, 

but future analyses will look at compounds over time in 

weathered samples.

Laboratory Decay Resistance

Disk Assays

Derivatized FAME fractions were used in a disk assay 

inhibition test to evaluate possible antifungal activities of 

saturated and unsaturated FAMEs against four wood decay 

fungi. No inhibition of any of the test fungi was found using 

the FAME fractions. Based on these results, fatty acids do 

not seem to have biological activity at the concentrations we 

have found in the NDW species. 

In a second experiment, extractives that were removed from 

test blocks of the nine NDW species using an ethanol: toluene mix according to ASTM standard D1105-96 were used 

in a similar disk assay. The removed extractives were added 

to sterile assay disks and aseptically placed on agar plates 

covered with mycelia of four wood decay fungi (two 

-2

0

2

4

6

8

10

12

14

16

18

AYC BL CAT ERC HM PAW SYP WCJ WRC

Lignoceric

Tricosanoic

Behenic

Heneicosanoic

Arachidic

Stearic

Margaric

Palmitic

Pentadecanoic

Myristic

Tridecanoic

Lauric

Undecanoic

Capric

Caprylic

Saturated FAME concentration (ng/sample)

Wood sample

Figure 16. Concentration of saturated FAMEs (fatty acid mothyl esters) after 2-y field 

exposure in Wisconsin.
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

13

0

2

4

6

8

10

12

14

16

18

Lignoceric

Tricosanoic

Behenic

Heneicosanoic

Arachidic

Stearic

Margaric

Palmitic

Pentadecanoic

Myristic

Tridecanoic

Lauric

Undecanoic

Capric

Caprylic

AYC BL CAT ERC HM PAW SYP WCJ WRC

Wood sample

Saturated FAME concentration (ng/sample)

Figure 17. Concentration of saturated FAMEs after 2-y field exposure in Mississippi.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

SYP PAW CAT HM BL WCJ AYC WRC ERC

alpha.-Pinene

alpha.-Terpinen

D-Limonene

Terpinolene

beta.-Citronellol

alpha.-Cedrene

Thujopsene

Saturated FAME concentration (ng/sample)

Figure 18. Quantities of standard terpenoids in EtOH. Tol extracts of unexposed naturally 

durable wood species. Note high number of compounds present in WRC, which exhibited 

inhibitory activity against 4 test fungi in disk assays. 
General Technical Report FPL–GTR–224

14

white-rot fungi, Irpex lacteus and Trametes versicolor, and 

two brown-rot fungi, Tyromyces palustris and Gloeophyllum 

trabeum). Extracts of WRC and AYC showed direct antifungal activity against all four fungi tested (Fig. 19).

Laboratory Termite Resistance

The results of the choice termite bioassays showed higher 

wood consumption in the extracted blocks compared with 

unextracted blocks (Fig. 20) in all wood species except for 

G. trabeum I. lacteus T. versicolor T. palustris

WRC

Blank

AYC

Figure 19. Disk assay experiment evaluating ethanol.toluene extractives from WRC and 

AYC against Gloeophyllum trabeum (Gt), Irpex lacteus (Il), Trametes versicolor (Tv), and 

Tyromyces palustris (TP). Inhibition (zones of clearing) of each fungus was apparent after 

7-d exposure compared with solvent-only controls (blank).

Table 4. Additional identified compounds found in NDW WtOH: Tol extractions. WRC and 

AYC had several unique compounds listed beneath the table columns 

Identified chemical compounds in Etoh: Tolune extraction 

CMPD SYP PAW CAT HM BL WCJ AYC WRC ERC 

(-)-Myrtenol Y Y Y Y 

.beta.-Sitosterol Y Y Y Y Y Y Y 

.gamma.-Sitosterol Y Y Y 

alpha-cedren Y Y Y Y 

Andrographolide Y 

Aromadendrene Y Y Y 

Campesterol Y 

Ferruginol Y Y Y Y Y 

Longipinocarveol, trans- Y Y Y 

Phenanthrene Y Y Y Y 

Stigmasterol Y Y 

Thujopsene Y Y 

Thymene Y Y 

Vanillin Y Y Y Y 

Unique compounds Trans-Shisool Isopulegol acetate 

 Cubenol Flavone, 5,7-dihydroxy-8- methoxy- 

 .tau.-Muurolol (-)Nortrachelogenin 

 .alpha.-Cadinol 

 Coniferyl aldehyde, methy ether 
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

15

honey mesquite, which had high mortality in the soil jars 

presumably because of insecticidal toxins present in the unextracted blocks. WRC-, PAW-, AYC-, and SYP-extracted 

blocks had the highest mass loss because of consumption 

(over 30% weight loss), whereas BL extracted wood had the 

lowest weight loss (under 10%) (Fig. 21). 

Laboratory Soil Block Assays

The results of this study stress the importance of extractive 

content when evaluating natural durability of wood. Blocks 

from the naturally durable wood species were extracted 

following ASTM D1105-96, to the point where virtually 

all extractives were removed, then exposed to six common 

wood rot fungi along with unextracted blocks. The extracted 

wood blocks were significantly more susceptible to decay 

fungi than the unextracted blocks, and many became as 

susceptible as the SYP nondurable controls. Differences in 

fungal colonization of the blocks could be seen by 4 wk in 

the soil bottles (Figs. 22, 23). Overall, brown-rot fungi were 

more efficient at degrading blocks than white-rot fungi, 

causing 30% to 60% weight loss compared with 10% to 

15% weight loss, respectively (Figs. 24–26). Many durable 

wood species, AYC, ERC, WRC, CAT, and BL, became 

significantly less durable when extractives were removed. 

In both the termite and fungal block assays, BL and HM remained quite durable even after extractives were removed.

Durability indices were calculated for both extracted and 

unextracted species in this study according to EN 350-1 

(1994) using SYP as the reference specimen. Wood species 

were averaged separately across brown- and white-rot fungi 

to calculate the indices. For brown-rot fungi, unextracted 

BL, HM, ERC, and WRC were classified as very durable 

AYC

BL CAT

ERC HM

PAW

SYP

WCJ

WRC

Figure 20. Visual comparisons of extracted and unextracted naturally durable woods exposed to eastern 

subterranean termites (Reticulitermes flavipes Kollar) in a choice bioassay. Blocks on the right side of each 

image are unextracted controls, and blocks on the left have extractives removed by solvent extraction. 

Virtually no feeding was seen in HM (center) because of high mortality in the soil jars. 
General Technical Report FPL–GTR–224

16

(class 1) while AYC, CAT, and WCJ were classified as durable (class 2). Even with extractives removed, BL, HM, 

and WCJ remained moderately durable (class 3) to durable. 

For white-rot fungi, unextracted AYC, BL, CAT, HM, WCJ, 

and WRC were all classified as very durable while ERC was 

only durable. AYC was still very durable after extractive 

removal, and HM, WRC, BL, ERC, and WCJ were durable 

to moderately durable. The Wood Handbook (Clausen 2010) 

(table 14-1) classifies all of these durable wood species as 

resistant, except for BL, which is classified as very resistant. 

PAW is not listed in the table; however, we found it to be 

moderately durable against white-rot fungi and not durable 

(class 5) to brown-rot fungi.

Of particular interest in the brown-rotted blocks were the 

patterns noted on unextracted Alaska yellow cedar. There 

was an area of “cubical rot” on the underside of the block 

that was in direct contact with the feeder strip, suggesting 

that the fungus was actively attempting to colonize the block 

but was unsuccessful. The presence of fruiting bodies is 

indicative of the fungal response to a lack of nutrients from 

encountering an unsuitable substrate (Rayner and Boddy 

1988).

-15

-5

5

15

25

35

45

55

65

AYC BL CAT ERC HM PAW SYP WCJ WRC

Weight loss (%)

Extracted

Unextracted

Wood sample

ef

ab

bc

a a a a a

f

ab ab

cd

a

f

ef

ab

cd

de

d

Figure 21. Percentage of weight loss (PWL) of wood 

blocks because of termite attack of nine wood species 

in a modified E7 termite choice bioassay. Letters above 

columns indicate statistical groupings (a is least PWL, f is 

highest PWL). Termites readily consumed all species when 

extractives were removed with the exception of honey 

mesquite (HM), which caused high mortality (96%).

SYP

CAT

AYC

WRC

Figure 22. Soil blocks exposed to G. trabeum at 8 wk. Blocks on the left side of 

each image have extractives removed and right side are unextracted. 
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

17

Summary and Discussion

Field Tests

The results of the field test indicate that with the exception 

of PAW and SYP all of the “durable” wood species are sufficiently decay resistant for use in above-ground conditions. 

All of the softwood NDW species (ERC, WRC, AYC, and 

WCJ) exhibited excellent performance in our tests. Of the 

hardwood NDW species, HM and BL performed exceptionally; CAT was slightly less durable and was also prone to 

cupping. Black locust had moderate checking at both field 

test sites, which does diminish the overall appearance of 

the wood and would be more of a problem underfoot for 

deck replacement than when used in vertical orientation, 

such as cladding. The visual observations of these decking specimens do indicate that bleaching is inevitable for 

uncoated lumber in this particular situation and a surface 

protection (i.e., stains or clear polyurethane) would minimize the bleaching that is no doubt caused by UV exposure. 

Clear coatings would also slow leaching of extractives from 

the wood and prolong the useful service life of the aboveground components. As demonstrated by our laboratory 

SYP HM

AYC WCJ

Figure 23. Soil blocks exposed to I. lacteus at 8 wk. Blocks on the left side of each 

image have extractives removed and right side are unextracted.

Figure 24. Unextracted blocks of AYC after 8 wk of soil 

bottle exposure to G. trabeum. Note localized patterns of 

brown-rot only on the underside of block that abutted the 

feeder strip. 
General Technical Report FPL–GTR–224

18

-5

5

15

25

35

45

55

65

AYC BL CAT ERC HM PAW SYP WCJ WRC

Weight loss (%)

Tyromyces Palustris ATCC6137

Extracted

Unextracted

*

*

*

*

*

-5

5

15

25

35

45

55

65

AYC BL CAT ERC HM PAW SYP WCJ WRC

Gloeophyllum trabeum Mad617

Extracted

Unextracted

*

*

*

*

*

*

-5

5

15

25

35

45

55

65

AYC BL CAT ERC HM PAW SYP WCJ WRC

Postia placenta Mad698

Extracted

Unextracted

*

*

*

*

*

Weight loss (%)

Weight loss (%)

Figure 25. Mean percentage of weight loss for extracted 

and unextracted blocks of nine wood species exposed to 

(i) Tyromyces palustris (TYP 6137), (ii) Gloeophyllum trabeum

(Mad 617), and (iii) Postia placenta (Mad 698) for 8 wk in an 

E-10 soil bottle test. Asterisks indicate where extracted mean 

weight loss for a wood species was significantly different 

from the corresponding unextracted mean weight loss. 

-5

0

5

10

15

20

25

30

35

40

45

AYC BL CAT ERC HM PAW SYP WCJ WRC

Irpex lacteus HHB7328

Extracted

Unextracted

*

-5

0

5

10

15

20

25

30

35

40

45

AYC BL CAT ERC HM PAW SYP WCJ WRC

Irpex lacteus Mad517

Extracted

Unextracted

*

-5

0

5

10

15

20

25

30

35

40

45

AYC BL CAT ERC HM PAW SYP WCJ WRC

Trametes versicolor Mad697

Extracted

Unextracted

*

*

Weight loss (%)

Weight loss (%)

Weight loss (%)

Figure 26. Mean percentage weight loss for extracted 

and unextracted blocks of nine wood species exposed to 

(i) Irpex lacteus (HHB 7328), (ii) Irpex lacteus (Mad 517), and 

(iii) Trametes versicolor (Mad 697) for 8 wk in an E-10 soil 

bottle test. Asterisks indicate where extracted mean weight 

loss for a wood species was significantly different from the 

corresponding unextracted mean weight loss for the wood 

species.
Evaluating Naturally Durable Wood Species for Repair and Rehabilitation of Above-Ground Components of Covered Bridges

19

tests, extractive content is a critical component of natural 

durability, and diminished extractive content diminishes the 

durability of the wood.

Chemical Characterization

Overall, the chemical analyses conducted on these wood 

species netted a significant amount of information. Our 

initial GS–MS analysis found upwards of 150 compounds 

in each sample, thus the reasoning behind narrowing our 

focus to fatty acids and later terpenoids. Because fatty acids 

were severely diminished in the wood samples after 2-y 

exposure and exhibited no biological activity in our laboratory disk bioassays, we conclude that fatty acids do not play 

a direct protective role in naturally durable wood species. 

Terpenoids look more promising as having a protective role 

in natural durability; two of the species having the highest amount and diversity of our targeted terpenoids were 

classified as durable (AYC and WRC). For these same two 

species, antifungal activity was determined for EtOH: Tol 

extracts against several wood decay fungi (See Fig. 19). For 

the softwood species, terpenoids seem to be a major contributing factor to durability, but the durable hardwoods likely 

contain additional compounds, such as flavonoids, polyphenols, and alkaloids. Terpenoid-rich resins (Van Acker and 

others 1999; Kennedy and others 2000) have been used in 

the literature to treat nondurable sapwoods, so it is possible 

to use terpenoids as wood protectants.

Laboratory Bioassays

The termite-resistance assays indicate the importance of extractive content in naturally durable wood species especially 

emphasizing the maintenance of high extractive content. 

Once the extractives were removed, two of the most durable 

species (AYC and WRC) were consumed by termites to a 

higher degree than the extracted nondurable control (SYP). 

Similarly, once extractives were removed, NDWs showed 

significantly higher weight loss when challenged by brownrot fungi. Many of the wood species chosen for these studies were softwoods, which are typically not preferred by 

white-rot fungi. However, the general pattern was observed 

and was more prominent in hardwoods when exposed to 

white-rot fungi. Overall, most wood species were rendered 

considerably less durable when extractives were removed, 

but some of the naturally durable wood species remained 

durable to moderately durable after extraction. It is possible 

that with honey mesquite and black locust this is due to 

the inability to fully extract the durable components, either 

because of insolubility or the presence of physical barriers 

such as densely packed cells, resins, or waxes. (For a more 

complete discussion of the soil bottle assays, refer to Appendix II, page 35.)

Conclusions and Recommendations

The overall findings of this study indicate that several of the 

species evaluated have benefits for use in above-ground replacement components for covered bridges. 

•	 Alaska yellow cedar exhibited good performance in the 

above-ground test. It was slightly more prone to discoloration in the field, but showed excellent resistance to decay 

and was not prone to cupping or checking. Extractives 

from Alaska yellow cedar showed some direct antifungal 

activity against several of our test fungi in laboratory 

bioassays. Laboratory soil bottle tests indicated Alaska 

yellow cedar is durable to decay fungi and termites, but 

durability is severely diminished once extractives are removed. It is always important to source lumber with sufficient heartwood content and protect wood from weathering and leaching with a sealant. 

•	 Black locust exhibited similar durability in the field with 

quite a bit more checking. Black locust also performed 

similarly in laboratory assays and it maintained more termite and fungal resistance after extraction. Black locust 

appears to be difficult to extract possibly because of the 

wood structure, which could also be contributing to inhibition of degradation by termites and fungi and makes it a 

very good choice for replacement repairs.

•	 Catalpa has shown good durability in the field, more so in 

Wisconsin than Mississippi, but with quite a bit of cupping and some checking. It showed very good resistance 

to termites and fungi, but upon leaching, durability was 

severely diminished. Catalpa could perform well in the 

field with the benefit of weather protectants. 

•	 Eastern redcedar has exhibited good performance in the 

above-ground decking study. The wood is durable but 

shows some cupping and cracking. Extractive content is, 

however, an important factor to consider when selecting 

cedar as a naturally durable wood. Our tests concluded 

that eastern redcedar also becomes quite susceptible to 

fungal and termite attack once extractives are removed. 

As with the other cedars evaluated in this study, it is important to source lumber with sufficient heartwood content and protect wood from weathering and leaching with 

a sealant.

•	 Honey mesquite exhibited excellent durability in the field, 

with low cupping and checking. Both with and without 

extractives present, Honey mesquite showed excellent 

fungal resistance in laboratory bioassays and caused high 

mortality in termites. Microscopic examinations showed 

plugged vessels in honey mesquite test blocks, combined 

with the tight cellular arrangement; its structure could inhibit extraction or even inhibit accessibility by fungi. The 

scarcity of large dimensions of honey mesquite may limit 

its usefulness for covered bridge components. A better 

understanding of the gums and waxes in honey mesquite 

as well as other chemical components will help us better 

understand its excellent decay resistance. 

•	 Paulownia tomentosa cannot be recommended based on 

the results of this study. Above ground, the wood was susceptible to decay, cupping, and checking and performed 
General Technical Report FPL–GTR–224

20

only slightly better than Southern Pine controls. Laboratory tests indicated very little resistance to test decay fungi 

and once extracted, it performed worse than Southern 

Pine against several decay fungi and termites. Additional 

in-ground tests using P. tomentosa all failed within a year 

and faster than Southern Pine controls. 

•	 Western juniper exhibited good performance in the field 

tests with some checking by year 3. In laboratory bioassays, Western juniper showed excellent durability to termites and fungi and even maintained some durability after 

stringent extraction making it another excellent choice.

•	 Western redcedar also exhibited good performance in the 

field tests with some cupping by year 3. Extractives from 

Western redcedar showed some direct antifungal activity 

against several of our test fungi in laboratory bioassays. 

Western redcedar does diminish in durability once extractives are removed, so the same recommendations would 

be made for western redcedar as the other cedars. Always 

make sure that the material is mostly heartwood to ensure 

highest possible extractive content. Sealants may also improve the durability of western redcedar as it would slow 

leaching of extractives caused by weathering.

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Appendix I—Above-Ground Field Evaluation and GC-MS 

Analysis of Naturally Durable Wood Species
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Appendix II—The Role of Extractives in Naturally Durable Wood Species
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