Natural durability of timber species exposed to xylophagous fungi in southern Durango, Mexico

Collection site

Wood samples from Pinus cooperi, P. strobiformis, P. durangensis, Alnus acuminata, Juniperus deppeana, and Quercus sideroxyla were collected in the Ejido Brillante, municipality of Pueblo Nuevo, Durango, Mexico (23°39′39″ N and 105°26′20″ W; Fig. 1) in January 2018. Fagus sylvatica samples (control) were acquired in a lumber yard in the city of Durango, Mexico (Fig. 1).

Sawing and dimensioning of specimens

For each tree species, logs (one meter in length and 30 cm in diameter) were first collected and then immediately sawn to make boards, whose dimensions varied depending on the species. Boards were air dried for a month, under temperatures of 1 °C to 20 °C and relative humidity of 64%. A total of 420 samples (3 cm long, 1 cm wide and 0.5 cm thick) were obtained from all the boards, corresponding to 60 samples (30 for each of the two fungal species) for each of the seven timber species. The rings were considered to have an angle between 30° and 70° according to European Standard 113-1 (CEN EN 113, 1996).

Wooden sample drying and conditioning

Samples were dried in a stove at 103 °C until constant weight was reached; then dry mass was determined on an analytical balance to the nearest 0.01 g. Finally, the samples were conditioned to 20 °C and 65% relative humidity to achieve a moisture content of 12% as is normally found in wood in service.

Evaluated treatments

Petri dishes containing 0.50 mL of malt agar were prepared. A one cm inoculum of Coniophora puteana was placed in each of three Petri dishes. Five wooden samples of Pinus cooperi and one of Fagus sylvatica were placed around the inoculum at an equal distance of separation. Another three Petri dishes were prepared placing the six wooden samples but without inoculum, serving as controls (leaching). The same was done for each of the seven wood species (a total of 42 Petri dishes). Subsequently, this same experimental design was applied using Trametes versicolor as inoculum. For 4 months the Petri dishes were kept under controlled conditions in an incubator (23 °C and 65% relative humidity). Each month the Petri dishes were monitored to avoid contamination from other fungal species.

Quantification of mass loss

At the end of the growth period of the fungus, the anhydrous weight of the wooden samples of all tree species was determined. Mass loss was estimated using the following formula:

$$P_m=\frac{P_i-P_f}{P_i}\times 100$$where: Pm = Mass loss (%); Pi = Anhydrous mass of the specimen at the beginning of the test (g); Pf = Anhydrous mass of the specimen at the end of the test (g).

Classification of natural durability

The natural durability of each species was expressed according to the “X value”, which was obtained by dividing the average mass loss of each species by the mass loss of F. sylvatica, and then classified according to EN 350-1 mass loss criteria, where: X values f 0.15 is class 1 = Very durable; 0.15 < X values f 0.30 is class 2 = Durable; 0.30 < X values f 0.60 is class 3 = Moderately durable; 0.60 < X values f 0.90 is class 4 = Slightly durable and X values > 0.90 is class 5 = Not durable (CEN EN 350-1, 1994).

Statistical analysis

Descriptive statistics of mass loss for each tree species were obtained. Shapiro–Wilk normality and Levene homogeneity of variance tests were performed. Data were normalized by transforming to log10+1 (Zar, 1999). Since the data showed heterogeneity of variance, Welch ANOVA tests were used to observe differences of mass loss among tree species; Games–Howell tests were used to separate groups. Also, Welch t-tests were used to prove mass loss differences between fungi for each tree species. All tests were considered significant at P < 0.05 and were conducted in SPSS Ver.18.

Durability of wood exposed to Coniophora puteana

Significant differences in mass loss were observed among the tree species exposed to C. puteana (Welch = 240.78, df = 6, 43.10; P < 0.0001; Table 1, Fig. 2). Although this fungal species is considered to deteriorate softwood speces to a greater extent, in this study the highest mass losses were in Alnus acuminata, Q. sideroxyla and the control F. sylvatica, which are in the same statistical group. On the contrary, Pinus durangensis showed the lowest mass loss (x = 2.65%), followed by J. deppeana (x = 4.88%). Pinus cooperi and P. strobiformis, for their part, formed their own statistical group, both losing 8% of their mass.

Durability of wood exposed to Trametes versicolor

Significant differences in mass loss were observed among the tree species exposed to Trametes versicolor (Welch = 260.43, df = 6, 43.26; P < 0.0001; Table 2; Fig. 3). Juniperus deppeana, P. strobiformis and P. durangensis formed a statistical group with the lowest mass loss (around 6%), Table 1. The other three species (Q. sideroxyla, P. cooperi and A. acuminata) were similar in terms of losing a higher percentage of mass (around 18.5%), but not as much as the control F. sylvatica which lost 43.98% on average, thus showing that this fungal species causes greater damage in hardwood species.

Differences in mass loss among timber species for each fungus

The deterioration caused by fungi C. puteana and T. versicolor was high; some sections of the timber species A. acuminata, P. cooperi, and the control F. sylvatica were totally degraded (Fig. 4). According to the Welch t-test, Pinus strobiformis and Q. sideroxyla were significantly more susceptible to C. puteana damage; in contrast, P. durangensis, P. cooperi and the control (F. sylvatica) showed more damage due to T. versicolor (Table 3). Juniperus deppeana and A. acuminata showed a non-significant difference between fungal degradation effects (Table 3).

Differences in mass loss among timber species for each fungus

The timber species analyzed in this study showed statistically significant levels of durability, determined by the mass loss caused by the attack of the wood-degrading fungi Coniophora puteana and Trametes versicolor (Figs. 2 and 3). The results of this work are validated according to the mass loss of Fagus sylvatica used as the control, which was greater than 20% for both wood-degrading fungi; this value was established by European standard 350-1 (CEN EN 350-1, 1994). No P. sylvestris wood samples were used to validate the deterioration in softwood species; however, P. durangensis, P. strobiformis and P. cooperi species showed loss values from 2.65% to 18.92%. In general, these three species and J. deppeana showed from medium to higher resistance to deterioration due to the action of fungi compared to Q. sideroxyla and A. acuminata, which were the most susceptible species to these xylophagous fungi. Similar results to those obtained in this research for Q. syderoxila were presented by Ayata, Akcay & Esteves (2017) in an experiment where thermal treatments were used to increase the durability of Quercus petreae; they found mass losses of 21.54 and 11.20% by exposing this timber species to Pleurotus ostreatus and Coniophora puteana fungi, respectively. On the other hand, the mass loss caused by C. puteana in A. acuminata in this trial was higher than the 14.66% reported by Erazo et al. (2019); however, the values were lower than the 61.84% reported by the same authors when the wood was exposed to T. versicolor.

Classification of natural durability

When using the durability classification established by European standard EN 350-1 (CEN EN 350-1, 1994), it was found that Q. syderoxyla was not durable (Class 5) when in contact with C. puteana, while A. acuminata was slightly durable (Class 4), J. deppeana, P. cooperi and P. strobiformis were moderately durable (Class 3), and P. durangensis was very durable (Class 1). Using the same classification system, but for T. versicolor as a deterioration agent, A. accuminata, P. cooperi and Q. sideroxyla were classified as moderately durable (Class 3), while P. durangensis and P. strobiformis were durable (Class 2) and J. deppeana was very durable (Class 1). The low mass loss (Classes 3 and 1 when exposed to C. puteana and T. versicolor, respectively) of J. deppeana wood could be the result of a high content of extracts (Kirker, Bishell & Lebow, 2016). These non-structural wood components are often concentrated in different proportions in a radial direction and along the trunk of the tree, thereby avoiding biological attacks as established by Erazo et al. (2019). In Oregon, USA, Juniperus wood samples have shown many years of durability (>50), which is attributed to their extracts that provide a high inhibitory effect on the growth of Gloeophyllum trabeum and T. versicolor (Mun & Lynn, 2011). In contrast, several tree species produce non-toxic extracts that do not reduce damage by xylophagous fungi (de la Cruz Carrera et al., 2018). Regarding the timber species Alnus acuminata and Q. sideroxyla (Classes 4 and 5 when exposed to C. puteana and Class 5 when exposed to T. versicolor, respectively), their resistance ranged from slightly durable to not durable, with the composition of their extracts possibly being the cause. Similar studies conducted in Mexico found that T. versicolor caused a greater loss of mass (30.7%) in samples of Arbutus spp. and Q. sideroxyla; on the other hand, J. deppeana, P. strobiformis, P. durangensis, P. teocote and P. cooperi showed low mass losses (0.97%), placing these woods in Class 1, which corresponds to very durable (de la Cruz Carrera et al., 2018). The natural durability of the Mexican timber species tested in the present experiment was within or above the reported values for the yellow pine, Quercus petreae, Pinus caribaea and Eucalyptus saligna species (Fojutowski et al., 2014; Lana & Bernardes, 2019), which may give them an opportunity to be marketed in other countries where natural durability is an important selection factor.