Abstract
Variations in brown rot decay and proportions of heartwood and sapwood were investigated in eastern white cedar (Thuja occidentalis L.). This experiment tested the hypothesis that the incidence of brown rot decay depends on the site, tree age, tree height, and heartwood/ sapwood ratio. Forty-five trees were sampled and felled from three mature stands in the Abitibi-Témiscamingue region, Quebec, Canada. From each tree, disks were systematically sampled along the entire stem, and the heartwood, sapwood, and decay proportions and volumes were determined for each disk. Scanning electron microscopy showed that growth of fungi causing brown rot decay was limited and slower in latewood than in earlywood due to the narrow cell lumen, thick wall, and limited number of bordered pits in latewood tracheids. Site, tree height, and tree age had significant effects on the proportions of sapwood, heartwood, and decay. Heartwood and brown rot decay proportions decreased from the base of the tree upward, while the sapwood proportion increased. There was more decay in older trees and in those growing on moist versus dry sites; however, decay was not serious in trees younger than 80 years. In addition, brown rot decay proportion correlated strongly and positively with heartwood proportion and tree volume, but negatively with sapwood proportion.
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Variation of Brown Rot Decay in Eastern White Cedar (Thuja occidentalis L.)
Besma Bouslimi, Ahmed Koubaa,* and Yves Bergeron
Variations in brown rot decay and proportions of heartwood and sapwood were investigated in eastern white cedar (Thuja occidentalis L.). This experiment tested the hypothesis that the incidence of brown rot decay depends on the site, tree age, tree height, and heartwood/ sapwood ratio. Forty-five trees were sampled and felled from three mature stands in the Abitibi-Témiscamingue region, Quebec, Canada. From each tree, disks were systematically sampled along the entire stem, and the heartwood, sapwood, and decay proportions and volumes were determined for each disk. Scanning electron microscopy showed that growth of fungi causing brown rot decay was limited and slower in latewood than in earlywood due to the narrow cell lumen, thick wall, and limited number of bordered pits in latewood tracheids. Site, tree height, and tree age had significant effects on the proportions of sapwood, heartwood, and decay. Heartwood and brown rot decay proportions decreased from the base of the tree upward, while the sapwood proportion increased. There was more decay in older trees and in those growing on moist versus dry sites; however, decay was not serious in trees younger than 80 years. In addition, brown rot decay proportion correlated strongly and positively with heartwood proportion and tree volume, but negatively with sapwood proportion.
Keywords: Brown rot decay; Wood structure; Site effect; Within-tree variation; Heartwood; Sapwood
Contact information: Institut de recherche sur les forêts, Université du Québec en Abitibi-Témiscamingue, Québec, J9X 5E4, Canada; *Corresponding author: ahmed.koubaa@uqat.ca;Phone: 819 762-0971 Ext 2579; Fax: 819 797-4727.
INTRODUCTION
Eastern white cedar (EWC) (Thuja occidentalis L.), one of only two arborvitae species native to North America, is distributed over a vast territory extending from the Gulf of St. Lawrence in the east to southeastern Manitoba in the west and from southern James Bay in the north to the Lake States in the south (Burns and Honkala 1990; Little 1979). It grows in a wide variety of soils and on both uplands and lowlands, including swamps, stream banks, lake shores, and even dry rocky cliffs (De Blois and Bouchard 1995; Johnston 1990; Koubaa and Zhang 2008; Matthes-Sears et al. 1991).
EWC has outstanding commercial and ecological value (Archambault and Bergeron 1992; Denneler et al. 2008; Taylor et al. 2002). The timber of this species, especially the heartwood component, has a natural durability that enhances its utility in wooden structures exposed to constant moisture (Taylor et al. 2002). For example, the average service life of an untreated EWC heartwood post is 27 years, compared to just 5 years for an untreated, black spruce post (Koubaa and Zhang 2008). Hence, products such as shakes, shingles, fence posts, and mulch made from EWC have considerable potential market value (Behr 1976; Haataja and Laks 1995). The resistance of EWC to microbial attack and decay is attributed to toxic extractives present mainly in the tree’s heartwood tissues (DeBell et al.1997; Gripenberg 1949; MacLean and Gardner 1956; Rudloff and Nair 1964; Taylor et al. 2002). Even the most decay-resistant woods, however, are susceptible to rot under some combined moisture and temperature conditions.
Stems of maturing coniferous trees typically consist of cylinders of living wood, or sapwood, protected on the outside by bark, and surrounding a central core of physiologically dead wood, or heartwood. The living sapwood is generally resistant to decay as long as it remains alive, which helps protect the heartwood from invasion of decay fungi; however, this protective layer does not remain intact indefinitely. With age, various openings due to wounds, for example, are formed, through which heart-rot fungi may enter (Wagener and Davidson 1954). Additionally, minor discontinuities through which fungi may enter include sapsucker and woodpecker wounds, leaf and twig scars, lenticels and minor fissures, and dead or abscising twigs (Boddy and Rayner 1983).
Heartwood and sapwood proportions vary between and within species and have been related to growth rates, stand and individual tree characteristics, site conditions, and genetic control (Pinto et al. 2004). Both woods show several differences other than wood colour, such as wood moisture content, extractives content, and wood permeability (USDA 1999). These differences are critical factors in solid wood and fiber processing and in wood drying (Jozsa and Middleton 1994). For pulping, heartwood is at a disadvantage as its extractives can affect the process and product properties. For solid wood applications the different properties of heartwood and sapwood influence drying, durability, and aesthetic values for the consumer.
Sapwood and heartwood proportions within the stem have a significant effect on the rational utilization of timber (Nawrot et al. 2008). Although a few studies have examined sapwood and heartwood distribution in EWC and the effects on wood processing and in-service performance (USDA 1999; Jozsa and Middleton 1994), little information is available on within-tree distribution or site variation. In addition, the relationship between the relative proportion of sapwood and heartwood and tree growth is not known.
Fungal decay is a serious microbiological deterioration that can cause rapid structural failure (Downeset al. 2009). It is a very complex process, depending on the type of decay, the wood species, the environment, the wood structure, and the interactive competition between the fungi and the environment (Blanchette 2000; Clausen and Kartal 2003; Fabiyi et al. 2011; Green III and Highley 1997; Harju et al. 2003; Oliveira et al. 2010). In a natural environment, decay colonization is usually initiated by a limited number of spores or hyphal fragments. Thus, wood species containing high quantities of extractives can inhibit fungal colonization (Gripenberg 1949; Kim et al. 2009; Rudloff and Nair 1964; Taylor et al. 2002), thereby providing resistance to decay.
Any decay that becomes progressive in the central dead wood of a living tree may be termed a heart rot (Wagener and Davidson 1954). EWC is highly susceptible to heart rot, which can cause significant losses and predispose infected trees to windthrow (Hofmeyer et al. 2009; Johnston 1990). The most common type of heart rot in EWC is brown cubical rot caused by Postia balsamea and Phaeolus schweinitzii (Fowells 1965; Koubaa and Zhang 2008). It is widespread in old and damaged trees (Hofmeyer et al. 2009). In this respect, EWC presents a particularly interesting opportunity for study because this species is highly susceptible to brown cubical rot.
Heartwood contamination progresses from the pith outward (Amusant et al. 2004; Downes et al.2009). Although it varies with wood species, the decay generally occurs in a sequential process of incipient, intermediate, and advanced decay, inducing different physical, chemical, and morphological changes in the wood. Little attention has been paid to the effects of growing conditions on decay occurrence in EWC. Among the few studies, Hofmeyer et al. (2009) found decay across all drainage upland sites (well, moderate, and low drained), with the most severe decay (88 % to 97%) in well drained sites.
EWC is considered one of the most decay-resistant wood species of North America (Johnston 1990; Koubaa and Zhang 2008). The trees can attain considerable age, which helps researchers better understand the relationship between age and decay. No study to date, however, has investigated environmental or within-tree variation in decay in this species. The main objective of this study was therefore to investigate site and within-tree variation in heartwood subjected to brown rot decay in eastern white cedar wood.
MATERIAL AND METHODS
Three EWC stands in the Abitibi-Témiscamingue region in the province of Quebec (Témiscamingue, Abitibi, Lac Duparquet), Canada were selected to cover a wide range of soil moisture content conditions (dry, moderate, and moist sites). All stands were dominated by balsam fir and EWC; however, the Témiscamingue site also contained some spruce and yellow birch. The Abitibi site naturally regenerated from fires that occurred from 1760 to 1944 (Archambault and Bergeron 1992; Dansereau and Bergeron 1993). Climatic data for each site were obtained from weather data, from 1930 up to 2007 (Régnière and St-Amant 2007), and stand density was also calculated for each site (Paul 2011). Site location and tree characteristics are summarized in Table 1.
To assess site characteristics, five quadrats (4 m2) were systematically established at each site. Understory species and organic layer depth were measured at every 1 m2 quadrat. Five samples of organic horizon soil (organic layer) were collected from each site and analyzed for the following soil properties: pH, moisture and texture, and carbon (C), nitrogen (N), and phosphorus contents (Table 2). Samples were air-dried at 30°C for 48 h and ground to pass through 6-mm sieves. Substrate pH was analyzed in distilled water (Lafleur et al. 2011). The carbon and nitrogen contents were determined from dry combustion using a LECO CNS 2000 analyzer (LECO Corporation, St. Joseph, Michigan). The phosphorus content was determined by the Bray II method (Lafleur et al. 2011).
A total of 45 trees (15 per site) were randomly sampled from each site. The sampled trees were felled, and total height and diameter at breast height were measured using a steel tape (Table 1). From each felled tree, 10 cm-thick disks were sampled at 0.5, 1.3, and 3 m stem heights and at every 2 meters thereafter up to the top of the tree. In order to determine the age of the studied trees, a total of 90 cores (2 per tree) were taken from these trees at approximately 50 cm above the ground. This height represents a loss of about 3 to 5 years of growth in relation to total tree age (Savva et al. 2010). Ring number and density were determined on increment cores sampled at breast height using a QTRS-01X, a Tree-Ring X-Ray Scanner (QMC, Knoxville, Tennessee) according to the procedure described in Koubaa et al. (2002).
Heartwood diameter and sapwood thickness were determined visually for all sampled disks. The total diameter (TD) inside the bark and heartwood diameter (HD) were measured for each disk, along and perpendicular to the longest axis based on an established measurement rule. Sapwood thickness (ST) was computed from the half-difference between the total disk diameter and the heartwood diameter (ST=(TD-HD)/2) (DeBell and Lachenbruch 2009). When present, decay diameter (DD) was calculated in the same way at each sampling height to the nearest millimeter.
In stem cross section, the heartwood area decreased from the base to the top, following the stem shape. In the sampled discs, the shape of decay approximates a circle, as do tree sections; therefore, decay, sapwood, and heartwood areas were estimated assuming a circular shape, according to Alteyrac et al.(2006). Heartwood, sapwood, and decay volumes in the tree were estimated using a truncated cone formula (Alteyrac et al. 2006; Wernsdörfer et al. 2006).
Table 1. Stand and Tree Characteristics of Eastern White Cedar Grown in the Abitibi-Témiscamingue Region, Quebec, Canada
Table 2. Soil Properties of the Sampled Stands
Disks were air-dried for up to 9 months until sample preparation for scanning electron microscopy (SEM) analysis and other property measurement. SEM analysis was conducted on selected sound and decayed heartwood samples for comparison. Two small samples from each class of wood (sound wood, and initial and advanced stages of decay) were cut and prepared for scanning electron microscopy (SEM). Samples were first soaked in water overnight for softening. They were then oven-dried for 2 h at 100 °C. The area of interest was cut out with a razor and deposits of platinum followed by carbon were applied to the surface. The observations were performed using a scanning electron microscope (model JSM-840A).
Data were analyzed using Statistical Analysis System (Littell et al. 2006; SAS 2008). Sapwood, heartwood, and decay proportions were subjected to variance analysis using a mixed-model approach with cambial age as repeated measures (Littell et al. 2006), where the factors tree height, site, and cambial age (within each height) were considered as fixed effects and tree as a random effect. The hierarchical effects of individual tree and site were accounted for using two nested levels, with the tree effect nested within the site effect as follows (Eq. 1):
Yijk = μ + Si + Tj(i) + α k(j) + Al(kj)+ ε ijkl (1)
Where Yijk is the mean of the response variable for the kth height of the jth tree at the ith site, μ is the overall mean of the response variable of the kth height of the jth tree at the ith site, Si is the fixed effect associated with the ith site, T(j)i is the random effect associated with the jth tree at the ith site, αk(j) is the fixed effect associated with the kth height of the jth tree, Al(kj) is the fixed effect associated with the lth age at the kth height of the jth tree, and εijkl is the residual error. Because heights were not equally distributed, a parametric power transformation was applied to model autocorrelation effects using the Tukey–Kramer adjustment (Littell et al. 2006).
The MIXED procedure in SAS (Littell et al. 2006; SAS 2008) was used to fit models using restricted maximum likelihood (REML). Degrees of freedom were determined using the Kenward–Roger method. All data were log-transformed to achieve model assumptions such as homoscedasticity and residual normality. The statistical significance of fixed effects was determined using F-tests at P≤0.05. Z-tests were conducted to determine whether the random effect significantly differed from zero. Due to small site replication, results of the Z-tests must be considered indicative only (Littell et al. 2006). Tukey’s multiple range test was performed to estimate the statistical significance of tree height differences. Variance components were estimated as a percentage of the total variation (VAR) of all effects, using the VARCOMP procedure. Correlation analyses were also conducted using the CORR procedure to establish relationships between decay volumes, tree volume, tree age, heartwood volume, and sapwood volume.
RESULTS AND DISCUSSION
Macroscopic Structure of Sound and Brown Rot Decayed Wood
Figure 1 illustrates the macroscopic structure of sound wood (Fig. 1a), early stages of decayed wood (Fig. 1b), and advanced stages of decayed wood (Fig. 1c and 1d). Figure 1a shows the variations in shape and color between heartwood and sapwood. The heartwood of eastern cedar has a dark red color that distinguishes it from the sapwood due to the higher extractives content in the heartwood (Amusantet al. 2008; Freitag and Morrell 2001; Taylor et al. 2002).
The decay in sampled trees was limited to the heartwood (Fig. 1b), and was generally characterized by brown discoloration and a friable, cubically cracked texture when dry (Fig. 1b-d). Similar observations were reported by Eriksson et al. (1990). In advanced decay, cracks appeared in the decayed wood (Fig. 1c). In late stages of advanced decay, the wood was completely degraded, resulting in the appearance of a cavity in the center of the log (Fig. 1d). Merchantable losses due to brown rot decay can be extensive in older trees, because the most valuable part of the tree, the butt log, is the most seriously affected.