Abstract
In recent years, forest fires have become increasingly common, but also more damaging phenomena. These aspects are reflected in significant economic losses that affect the quality and quantity of wood volumes that can be used for industrial processing. For this reason, knowing the quality of the wood is important, especially in fire-affected trees. Because visual analyses cannot always reflect the quality of the wood inside the trunk, the present research aimed to evaluate the extent to which modern techniques based on the transfer of sounds can identify internal wood defects. In this sense, 42 tomograms made from beech trees affected by a litter fire were compared with the relative resistances of the wood to drilling and with the real condition of the wood inside the trunk, as made visible through the growth cores taken with a Pressler drill. From the cumulative interpretation of the results, it was found that the trees affected by the fire have serious defects, which lead to the downgrading of the wood and are not reproduced by the tomograph to their true extent. Conversely, sound transfer speeds through wood are influenced by the presence of beech red heartwood, which leads to an increase in sound transfer speeds through wood, and that can alter the accuracy of the tomogram.
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How Well Can Sound Tomograms Characterize Inner-Trunk Defects in Beech Trees from a Burned Plot?
Elena C. Musat *
In recent years, forest fires have become increasingly common, but also more damaging phenomena. These aspects are reflected in significant economic losses that affect the quality and quantity of wood volumes that can be used for industrial processing. For this reason, knowing the quality of the wood is important, especially in fire-affected trees. Because visual analyses cannot always reflect the quality of the wood inside the trunk, the present research aimed to evaluate the extent to which modern techniques based on the transfer of sounds can identify internal wood defects. In this sense, 42 tomograms made from beech trees affected by a litter fire were compared with the relative resistances of the wood to drilling and with the real condition of the wood inside the trunk, as made visible through the growth cores taken with a Pressler drill. From the cumulative interpretation of the results, it was found that the trees affected by the fire have serious defects, which lead to the downgrading of the wood and are not reproduced by the tomograph to their true extent. Conversely, sound transfer speeds through wood are influenced by the presence of beech red heartwood, which leads to an increase in sound transfer speeds through wood, and that can alter the accuracy of the tomogram.
DOI: 10.15376/biores.19.4.7530-7565
Keywords: Sound speeds in wood; Tomogram; Resistogram; Relative resistances to drilling; Beech; Forest fires; Romania
Contact information: Department of Forest Engineering, Forest Management Planning and Terrestrial Measurements, Faculty of Silviculture and Forest Engineering, Transilvania University of Brasov, Şirul Beethoven 1, 500123, Romania; *Corresponding author: elena.musat@unitbv.ro
GRAPHICAL ABSTRACT
INTRODUCTION
Globally, forest cover has seen fluctuations that have become increasingly visible and worrisome in recent decades (Palaghianu 2007; Meddour-Sahar et al. 2013), even if forests represent the most important renewable resource on the planet (Ene et al. 2013; Moskalik and Gendek 2019), having an essential role in the carbon cycle in nature (Palaghianu 2007; Moradi et al. 2022). One of the main factors that has led to the destruction of forests is man, who cut down forests to obtain areas suitable for agricultural crops (Palaghianu 2007; Meddour-Sahar et al. 2013). This has been practiced worldwide, but the largest changes in land use category recently have been recorded in the tropics (Palaghianu 2007; Armenteras et al. 2017).
Forest fires also have a devastating impact on forests (Meddour-Sahar et al. 2013; Foldi and Kuti 2016; Calviño-Cancela et al. 2017). Catastrophic fires affect the entire ecosystem (Palaghianu 2007; Guêné-Nanchen et al. 2021), starting from the soils (Brandstock 2008; Page-Dumroese et al. 2019), plants and animals, affecting water resources,as well as increasing greenhouse gases (Harrison et al. 2009). In addition, regardless of the type of fire and their aggressiveness, the role of the forest as a whole is endangered (Armenteras et al. 2017), and there is harm to the anti-erosion protection functions (Brandstock 2008) and the capabilities for storing rainwater and air purification (Földi and Kuti 2016; Hossain et al. 2020). It is known that forests represent the “lungs” of the planet through a net conversion of carbon dioxide to oxygen.
Forest fires represent calamities that affect the entire planet regardless of continent, such as in America (Gillet et al. 2004; Guêné-Nanchenet al. 2021), Asia (Harrison et al. 2009; Çoban and Eker 2010; Meddour-Sahar et al. 2013; Tian et al. 2013), Australia (Acuna et al. 2017), Europe (Dimitrakopoulos and Panov 2001; Pereira Domingues Martinio 2019)–or climate zone–boreal forests (Hély et al. 2000; Tian et al. 2013; Guêné-Nanchen et al. 2021),temperate forests (Adam 2007; Çoban and Eker 2010; Sivrikaya et al. 2015; Burlui and Burlui 2018), or tropical forests (Harrison et al. 2009; Meddour-Sahar et al. 2013; Armenteras et al. 2017). The devastating effects of fires are not limited to their local impact on the forest, with all its components (Sivrikaya et al. 2015), but also on people (Harrison et al. 2009; Calviño-Cancela et al. 2017), reaching losses of millions of Euros (Földi and Kuti 2016), and damage to millions of hectares of forest annually (Barreal et al. 2012). Even if forest fires have affected humanity and the environment since ancient times, they only occur in situations where three fire initiation conditions are met simultaneously, defining the “fire triangle” (Omi 2005; Lieberman 2008; Burlui and Burlui 2018), without which a fire could not occur, respectively, the ignition source, the combustible (the gas that ensures combustion), and the fuel (the combustible material).
The growing demand for wood at an international level includes industrial uses (Proto et al. 2020; Qu et al. 2020; Staže et al. 2021; Harvey and Visser 2022; Papandrea et al. 2022), as well as for obtaining energy benefits (Tenchea et al. 2019; Proto et al. 2020). The assessment of the impact of forest fires on the wood in the forests, especially on the remaining trees in the burned areas, plays an extremely important role, in particular for limiting or reducing the economic losses due to the depreciation of the wood (Rodríguez y Silva et al. 2012; Musat et al. 2020). In addition, knowing the quality of the wood inside the trunk can help establish measures to limit economic losses (Sandoz and Lorin 1996; Burlui and Burlui 2018), for example, using these trees as quickly as possible (Rodríguez y Silva et al. 2012). Such knowledge is needed even if this means that the trees are harvested before the age of exploitability (Rodríguez y Silva et al. 2012), at which it is considered that the best qualities and volumes of wood would ordinarily be obtained.This aspect is important especially because trees affected by fires, where the trunk or base show visible signs of burning (scorched or burned bark, fallen from the trunk, scorched or charred wood (Lawes et al. 2011; Odhiambo et al. 2014)), will vegetate in poor conditions (Musat 2017), but they will be weaker and, at the same time, more sensitive to the attack of external pathogens (Wuerther 2006; Musat et al. 2020). As a result of these attacks, the trunk and the base of the trunk can be affected by rot, which can develop much faster, especially along the trunk, leading to an increase in the volume of degraded wood (Feng et al. 2014; Sandak et al. 2020; Cristini et al. 2022; Harvey and Visser 2022).
The assessment of tree vitality and wood quality has been a topic of interest in numerous studies over time (Sandoz and Lorin 1996; Martinis et al. 2004; Punches 2004; Feng et al. 2014; Alves et al. 2015; Du et al. 2015; Espinosa et al. 2017; Wu et al. 2018; Cristini et al. 2022; Papandrea et al. 2022), and it continues to be important today.Thus, some studies evaluate the vitality of trees in various vegetation conditions, starting from visual assessments of the whole tree, with an emphasis on the crown, appearance, and quality of foliage (Ciubotaru and David 2011; David and Ciubotaru 2011; David and Enache 2011b). Studies have usually analyzed visible defects (David and Enache 2011a; Musat et al. 2014; Staže et al. 2021) or those hidden inside the trunk (Martinis et al. 2004; Wang et al. 2007; Deflorio et al. 2008; Du et al. 2015; Proto et al. 2020), and studies have used destructive (growth cores, cross-cutting, splitting), semi-destructive (resistograph), or non-destructive (sound waves, X-rays, etc.) methods to assess the quality of the wood inside the trunk (Sandoz and Lorin 1996; Martini et al. 2004; Wang et al. 2007; Deflorio et al. 2008; Feng et al. 2014; Alves et al. 2015; Du et al. 2015; Espinosa et al. 2017; Musat 2017; Proto et al. 2020; Cristini et al. 2022; Papandrea et al. 2022; Musat 2023).
Due to the strong impact of fire on wood and the need to know more precisely the implications on quality, the purpose of the present research was to evaluate the degree to which the analysis of the transfer speeds of sounds through the wood can be used to characterize the possible defects inside the trunk, as affected by a litter fire. To achieve the proposed goal, the sound tomograph will be used with the idea of obtaining information about the transfer speeds of sounds through wood. In addition, further investigations with the resistograph and the Pressler drill were carried out to provide more precise details regarding the quality of the wood.
MATERIALS AND METHODS
To conduct field investigations, a plot under the administration of the Runcu Forest District within the Gorj County Forestry Administration (Romania), a forest privately owned by the Plaiului Vălari Municipality, was chosen. This management unit was affected by a litter fire in 2017, which took 5 days to extinguish (Fig. 1). The fire that affected the plot came from an area bordering the forest, where the owner, wanting to clear the land of dry vegetation, set it on fire and the fire got out of control. From a geomorphologic point of view, the surface of management unit number 149 is characterized by an undulating configuration of the slopes, with a predominantly north-west exposure, with slopes varying between 24 and 30°. The 34.7 ha area of the unit has a large altitudinal variation, between 700 and 1100 m. In the study area, the trees affected by fire were 93-year-old beech trees.
Fig. 1. The extinguish fight against the litter fire that occurred in 2017 in management unit number 149
As part of the determinations, 21 beech trees affected by the fire were investigated, including some that showed clear signs of scorching or burning on the trunk (Figs. 2 through 4). For each individual tree, two levels were marked with forest spray to perform the determinations, respectively at 50 and 100 cm above the ground, considered upstream from the tree, resulting in 42 analyzed tomograms. The investigations in the field first assumed the analysis of sound transfer speeds through wood, in which the ArbotomRinntech v.2 sound tomograph was used (Fig. 5), using the same working methodology as in the case of previous Musat works (2017 and 2023), respectively, and Musat et al. (2020).
Fig. 5. The ArbotomRinntech tomograph and the sensors connection
An aspect that should be mentioned is that each emitting sensor had received 7 blows with the hammer provided with the device, the number being chosen in relation to the recommendations in the specialized literature (Divos and Divos 2005; Tarasiuk et al. 2007; Wang et al. 2007; Rinn 2014) and the ambient noise. In addition, before saving the measurements, sound transmission errors through the wood were checked to be less than 10% (Tarasiuk et al. 2007). Finally, the tomogram reconstructed by the tomograph based on the sound transmission speeds through the wood was briefly analyzed, which made it possible to obtain the points where the investigations could be carried out further with the IML RESI F500-S resistograph, equipped with a 50 cm long drill and 3 mm width of the cutting edge of the points where the growth cores will be extracted with the Pressler drill. The cores extracted were analyzed with the microscope to see where there was any modification of the wood, in the sense of any difference along the core and what defects could be identified. The information was used in the interpretation of the data. Changes were made apparent on the resistograms by marking delimitation lines between the different zones.
Thus, both the analyses with the resistograph (Fig. 6), as well as the sampling of growth cores (Figs. 7 and 8) were made on the direction of the sensors that indicated the greatest irregularities inside the trunk.
Of all the 21 investigated beech trees, only six are presented in the article.The detailed results and the rest of the tomograms are inserted as supplementary material (Appendix 1) at the end of the paper. The choosing of these six trees considers the diverse situations encountered in the field and in the data interpretation, allowing every case to be presented and commented on.
RESULTS
In the forest area, 21 beech trees (Fagus sylvatica L.) were investigated, whose diameters varied between 31 and 70 cm for the 50 cm level, between 28 and 61 cm for the 100 cm level; the diameter values were extracted from the program software, based on entering the position of the sensors on the circumference of the trunk. In the following, the most relevant results for the investigated trees are presented.
Upon visual inspection of beech tree number3 (Fig. 9), it was noted that the fire particularly affected almost half of the circumference of the trunk, between north and south, and was at a considerable height. There was also the presence of fruiting bodies of some fungi, also in the northern direction. In the southeast area, the trunk no longer had bark, and the presence of dead wood was noted. This could be justified by the fact that the fire not only affected the bark of the tree, but it led to the death of the cambial cells (Beldeanu 2001 and 2008).