Summary
The quantification of the intensities of tree growth responses to the impact of geomorphic processes is a modern research trend in dendrogeomorphology. It enables a more sensitive assessment of the activity of the studied geomorphic process compared to the traditional use of growth disturbances. The advanced definitions of individual intensity classes of growth disturbances are based exclusively on macroscopic observations. This study evaluates the possibility of anatomical quantification of compression wood (CW) intensity in the case of common spruce (Picea abies (L.) Karst.) and compares it with subjective macroscopic evidence of CW with respect to stem tilting intensity. In total, 25 disturbed (tilted) individuals of P. abies occupying a landslide active during July 1997 were sampled, and 21 of them were analysed. The intensity of external disturbance (stem tilting) was compared against the macroscopic (intensity and duration) and microscopic (quantitative change of the tracheid lumen area and the cell wall proportion) parameters of compression wood suitable for practical application in common dendrogeomorphic analysis. Generally, the macroscopic indices of CW were strongly correlated with stem tilting. The intensity of the anatomical growth response was stronger in the earlywood zone than in the latewood zone. Nevertheless, their dependence on stem tilting was not detected. Results suggest that CW classification based on the quantification of anatomical changes is not possible for dendrogeomorphic purposes. Nevertheless, based on the obtained results, the present study suggests preferring the most intensively tilted trees during future dendrogeomorphic research to obtain the most intensive macroscopic and subjective observable anatomical evidence for studying compression wood.
Introduction
Dendrogeomorphology (Alestalo 1971) is actually a very effective tool for the spatiotemporal analysis of past geomorphic processes (Stoffel et al. 2013). The principle of the method is based on the schema in which geomorphic processes cause external disturbances to trees (e.g., wounding of the stem), and trees subsequently react to specific growth disturbances (e.g., formation of scars) (Shroder 1978) that are identified and exactly dated using a dendrochronological approach (Butler et al. 1987). A wide spectrum of growth disturbances (GD) exists and is standardly used as evidence of the impact of geomorphic processes on trees (Stoffel & Corona 2014). Scars and associated callus tissues or traumatic resin ducts are responses to stem wounding (Stoffel 2005, 2008), stem burial is usually followed by abrupt growth suppression or release (under occasional conditions) (Strunk 1997; Chalupová et al. 2020;), and reaction wood and/or eccentric growth is the response to stem tilting (Pillow 1941; Westing 1965). Individual GD have been used in very simply ways in tree-ring-based chronologies (present or absent). The increasing requirements for precision and progress in methods (due to existing uncertainties) over the last decade has led to the introduction of the definition of GD intensities and weighting them. This trend is typical for the analysis of snow avalanches (Voiculescu & Onaca 2014; Chiroiu et al. 2015), debris flows (Kogelnig-Mayer et al. 2011), or floods (Ruiz-Villanueva et al. 2010).
The intensity of the studied geomorphic process using dendrogeomorphic methods is usually expressed by a specific tree-ring-based index (e.g., event-response index or rockfall rate (Shroder 1978; Stoffel et al. 2005)). The introduction of GD intensities even led to the creation of a new weighted event-response index (Kogelnig-Mayer et al. 2011). The definition of GD intensities in dendrogeomorphology is based on macroscopic observations. However, several GD types have exclusively anatomical characteristics. This is typical for the responses of tree roots to their exposure (Gärtner 2007) or tree stems to wounding (Bollschweiler et al. 2008). Ballesteros et al. (2010a) and Ballesteros et al. (2010b) studied cell size changes in pine, alder, oak, and ash associated with stem wounding during flooding. Similarly, Arbellay et al. (2010) studied changes in the vessel parameters of alders and birches associated with wounding caused by debris flow. Despite existing studies dealing with tree growth responses to geomorphic processes at the anatomical level, no classification of intensities of anatomical GD for dendrogeomorphic purposes exists. The question is if the quantitative anatomy of GD could bring more sensitive results compared to macroscopic subjective observations.
One of the most frequently used tree growth responses for dating geomorphic processes is reaction wood. It is a specific response to tilting of tree stems, enabling recovery to their original vertical position, and differs between broad-leaved trees (tension-type of reaction wood) and coniferous trees (compression type of reaction wood) (Westing 1965). Compression wood (CW) is macroscopically observable due to its reddish-brown colour caused by rounded cells with thick walls containing an increased content of lignin at the expense of cellulose, mannan, and xylan (Wardrop & Dadswell 1950). The use of CW in dendrogeomorphology is typical in landslide research (Šilhán 2020a) and can be used even in debris flow (Bollschweiler & Stoffel 2010), flood (Ruiz-Villanueva et al. 2010) or snow avalanche research (Chiroiu et al. 2015). In addition, its potential use lies even in other processes that can cause the stem of the tree to tilt (e.g., earthquakes, permafrost, and glacier-related processes). The use of CW in rockfall research is rather marginal (Stoffel & Perret 2006). Landslide movements mainly cause stem tilting due to undulation of its surface and destabilization of the stem base (Šilhán 2020a). Thus, CW caused by landslide movements is usually not accompanied by other GD types (e.g., scar, callus tissue of TRD) because the tilting is not caused by pressure or the impact of moving material. Nevertheless, the presence of other GD types than CW cannot be ruled out, and in the case of some landslide types, they can occur very frequently.
Donaldson et al. (2004) distinguished two types of CW as mild and severe. Some approaches to dividing CW into mild and severe forms are based on the proportion of CW to the tree-ring width (Pillow 1941; Low 1964; Seth & Jain 1978), while other more recent approaches use anatomical characteristics to make this division (Donaldson et al. 1999; Savić et al. 2016; Nedzved et al. 2018). Stoffel and Corona (2014) presented the proposal of CW intensity classification based on its macroscopically observable duration in tree-ring sequences. The only CW classification based on anatomical parameters was presented by (Yumoto et al. 1983), but it is not usable for dendrogeomorphic purposes due to its too detailed focus on the microscopic anatomy of tracheids (e.g., spiral grooves, form of bordered pits, or lignification in S2 (L) layer observable using UV absorbance microscopy). Moreover, despite its details, this classification is based on visual subjective evaluation. Overall, a quantitative assessment of the microscopic anatomy of CW and its correspondence with stem tilting intensity is missing for dendrogeomorphic purposes. A relationship outlining the anatomy of tension wood in the case of broad-leaved trees has been noted. Heinrich et al. (2007) and Heinrich and Gärtner (2008) detected the direct impact of stem tilting on vessel size in the tension wood of beeches and alders. Similarly, Šilhán (2021a) provided the same observation for birches. Nevertheless, the same research was not performed for compression wood of conifers. Providing the same evidence (if successful) for the quantitative anatomy of CW could supplement, increase the precision, or even replace the macroscopic evaluation of CW intensities for dendrogeomorphic purposes. Moreover, the definition of the anatomical parameters of CW that most strongly correspond to the intensity of external growth disturbance (stem tilting) could enhance the focused sampling strategy in future dendrogeomorphic research.
Based on the abovementioned state of the art, the aims of this study are (i) to detect possible changes in quantitative anatomy of CW, (ii) to assess the relationship between the quantitative anatomy of compression wood and stem tilting intensity, and (iii) to define parameters of compression wood (subjective or objective) that optimally correspond to the intensity of external growth disturbance. The last aim is solved via defining of mutual relationships among the intensities of external growth disturbances, subjective macroscopic parameters of compression wood, and subjective and objective microscopic parameters of compression wood. To eliminate the possible effect of other GDs on the results, individuals of common spruce (Picea abies (L.) H. Karst.) were sampled on the active landslide area.
Materials and methods
Study area
The selected locality (49°23′ N; 18°14′ E; 710 m a. s. l.; Fig. 1) is in the Vsetínské vrchy Mts. (the medium-high mountains in the Czech part of the Outer Western Carpathians). The Vsetínské vrchy Mts. hosts a thrust-fold structure that is Tertiary in age (Menčík et al. 1983). Individual nappes forming the mountains are formed by alternating layers of conglomerates, sandstones, mudstones, and claystones. The slopes of the mountains are locally deeply weathered (up several metres) with increased contents of swelling minerals (e.g., smectite) (Pánek et al. 2010). The average annual precipitation total in the studied region ranges between 1000 and 1200 mm. The maximal short-duration precipitation totals (up to 100 mm per 24 hours) occur during the summer months during short downpours. The mountain peaks are covered by snow 100–120 days per year on average. The mean annual temperatures reach 5–6°C in these locations (Tolasz et al. 2007). The combination of steep slopes and suitable geological and climatic conditions are the main causes of the high density of various types of slope processes in the studied region (Kirchner & Krejčí 1998; Krejčí et al. 2002). The vegetation cover is dominated by common spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica L.) that have colonized the mountain slopes since the end of the 19th century when extensive pasturing was terminated. The next dominant tree species occurring in the mountain forest are silver fir (Abies alba Mill.), sycamore maple (Acer pseudoplatanus L.), or European larch (Larix decidua Mill.).
Location of the studied locality in the Czech Republic (A) and within the landslide area (B) and cases of sampled tilted trees (C, D).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Location of the studied locality in the Czech Republic (A) and within the landslide area (B) and cases of sampled tilted trees (C, D).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Location of the studied locality in the Czech Republic (A) and within the landslide area (B) and cases of sampled tilted trees (C, D).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The study area is a part of a large complex landslide that was dendrogeomorphically analysed by (Šilhán et al. 2019b). The entire landslide area has experienced several extensive block subsidence/rotation events in its upper part and shallow movements of weathered material in its frontal zone. The sampling for the presented study was performed in the middle part of the locality, which was located just below the lower border of the lowest rotated block (Fig. 1). The character of slope movement shifts in this position from the rotation of large blocks to flow-like movements of shallow weathering mantle. The slope morphology is undulated with the presence of lobes, tension cracks, and transversal trenches. The selected zone of the large landslide had an approximately rectangular shape with an isoline length of ca. 100 m and a downslope direction distance of ca. 50 m. The landslide movements in the studied locality were evidenced exclusively in 1997, and no older or younger (regarding the age of trees) movement events were detected (Krejčí et al. 2002; Šilhán et al. 2019b). This year is well known for the extensive occurrence of landslides after extreme precipitation events from the beginning of July not only from the studied locality but even from the wider region (Kirchner & Krejčí 1998). Nearly all trees occupying this zone expressed stem tilting (Fig. 1) with various directions and intensities corresponding to the plastic character of slope movements (Šilhán 2015).
Methods
For the purposes of this study, 25 individuals of P. abies were sampled (autumn 2019). The sampling strategy suggested sampling trees with various intensities of stem tilting to enable the analysis of this factor effect on compression wood formation. Next, only trees with an absence of observable damage to the root system (i.e., trees without any visible exhumed, broken, or stretched roots), trees growing far from tension cracks (where root damage can be supposed to be minimal or none), and trees with an absence of visible external stem wounding were sampled. The position of each sampled tree was recorded by a GNSS device, and the stem tilting intensity was measured by a digital inclinometer with a precision of 1° at the lowest possible position on the bent stem, where the original angle of inclination can be assumed to be maintained, which decreases with increasing tree age and the formation of compression wood in the higher parts of the stem. Due to sometimes distinct bending of the stem, the measurement was performed on the tangent of the stem surface. The selection of trees for sampling was designed to cover the maximal range of stem tilting intensities. Sampling was performed using a Pressler increment borer (maximal length 40 cm; diameter 0.5 cm) at the height of the maximal stem bending (estimated visually). Two increment cores were extracted from each tree. The first core was extracted from the lower side of the tilted stem (location with the assumed presence of compression wood), and the other core was extracted from the opposite side of the stem. The reference chronology used for the precise cross-dating of the disturbed tree-ring series was built from 30 undisturbed trees located out of the studied landslide using Arstan (Holmes 1994). This reference chronology was already used in the study of Šilhán et al. (2019b), where it enabled the detection of 1997 as the landslide movement year in the studied locality.
Next, the subjective intensity and duration of compression wood starting in 1997 were determined. Following the suggestion of Šilhán (2020b), the intensity of compression wood was expressed as the percentage proportion of the CW structure in the entire width of the first complete tree ring after the tilting event (Fig. 2). In the P. abies species studied, the proportion of latewood is usually small, and therefore its wider range is considered as compression wood. Because tree tilting occurred as a response to landslide movements after heavy precipitation at the beginning of July 1997, the first complete tree ring that contained compression wood throughout its entire width was formed in 1998. The duration of compression wood was assessed visually as the number of following tree rings containing the macroscopically observable structure of CW (Fig. 2).
The macroscopic parameters of compression wood. (A) The duration of CW in the continuous row of years. (B) The intensity of CW as a percentage proportion of CW structure in the total width of 1998 tree rings.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The macroscopic parameters of compression wood. (A) The duration of CW in the continuous row of years. (B) The intensity of CW as a percentage proportion of CW structure in the total width of 1998 tree rings.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The macroscopic parameters of compression wood. (A) The duration of CW in the continuous row of years. (B) The intensity of CW as a percentage proportion of CW structure in the total width of 1998 tree rings.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
After the macroscopic analysis of compression wood was performed, the samples were prepared for microscopic analysis. The core segment containing the first tree ring before CW onset (1996), a tree ring with CW onset (1997), and a tree ring after CW onset (1998) was extracted (three tree rings per sample in total), and sectioning (thickness 10–15 μm) was performed using a G.S.L.1 microtome (Gärtner et al. 2014). Sections were chemically processed following Schweingruber (1978). The individual steps included the staining of the prepared samples in a mixture of safranin and astra blue, dehydration in an ascending alcohol row, rinsing with Diasolv, and fixing with Canada balsam. Finally, the processed sections were heated at 60°C for at least 12 hours. The prepared samples were photographed at 50 × magnification, and the images were processed using WinCELL software. The tracheid lumen area (TLA) and the cell wall proportion (percentage proportion of cell wall area) were measured following the procedure of Ballesteros et al. (2010b) separately in the early wood and late wood parts of each tree ring (1996, 1998: four measurement sites per sample; thus, only tree rings with entire development before and after the landslide event). Although the number of anatomical features associated with the occurrence of reaction wood is greater (e.g., spiral grooves or lignification of the S2 layer) than TLA and cell wall proportion, the use of these detailed anatomical features would be very demanding (in terms of time and material) and therefore unusable in dendrogeomorphic research. Therefore, this research focused only on anatomical parameters that can realistically be used in dendrogeomorphic analyses of natural hazards in the future (Stoffel & Corona 2014). Measurement was not performed in 1997 because the tilting event occurred in the middle of the growing season and thus the zone of early wood does not contain any compression wood cells.
Two published classifications of CW were applied in the following step as the fundament for their comparison with quantitative aspects (both are based clearly on the visual subjective observation). First, visual macroscopic observation (based on the amount of compression wood in tree-ring width; Pillow 1941; Low 1964) was used to define mild and severe CW according to Lopez Saez et al. (2012) (Fig. 3). Second, the compression wood was classified using the recorded microsections (based on anatomical features; Donaldson et al. 1999; Nedzved et al. 2018) following the suggestion of Yumoto et al. (1983) and modified by the process of Janecka et al. (2020) into three classes (weak, class I; moderate, class II; strong, class III). The criteria for the definition of each class were based on the subjective visual observation of (i) cell wall thickness, (ii) cell roundness, and (iii) the presence of intercellular spaces. For more details on the classification, see Janecka et al. (2020). The measured parameters of individual cells were compared against their fits with the subjectively defined classification.
Mild and severe forms of compression wood as defined macroscopically sensu Lopez Saez et al. (2012) (early wood zone of all tree rings contains normal wood) (A) and corresponding cases of moderate and strong compression wood as defined by visual observation of anatomy sensu (Janecka et al. 2020) (B).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Mild and severe forms of compression wood as defined macroscopically sensu Lopez Saez et al. (2012) (early wood zone of all tree rings contains normal wood) (A) and corresponding cases of moderate and strong compression wood as defined by visual observation of anatomy sensu (Janecka et al. 2020) (B).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Mild and severe forms of compression wood as defined macroscopically sensu Lopez Saez et al. (2012) (early wood zone of all tree rings contains normal wood) (A) and corresponding cases of moderate and strong compression wood as defined by visual observation of anatomy sensu (Janecka et al. 2020) (B).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Individual values of external growth disturbances (stem tilting intensity) were mutually compared with macroscopic parameters associated with compression wood (duration and intensity) and tree-ring eccentricity, and quantitative microscopic parameters (changes in tracheid lumen area and cell wall proportion) using a one-way ANOVA or nonparametric Kruskal–Wallis test, and Student’s t-test or nonparametric Mann–Whitney (M-W) test at significance level
Results
Tree ages and external growth disturbances
In total 25 trees were sampled, but the preparation of microsections was successful in only 21 cases. As anatomical analysis is crucial for the detailed assessment of mutual relationships between external, macro, and microscopic parameters, only the results of trees with the successful obtainment of sections are presented. The mean age of these trees was 32.2 years (SD 2.3 years). The oldest tree was 36 years old, and the youngest tree was 28 years old. This means that the mean age of trees in the landslide event year (1997) was 11.2 years. Only six sampled trees (28.6%) were younger than 10 years in the event year. All trees with only one exception expressed measurable (0.1° accuracy) stem tilting. The mean stem tilting was 4.5° (SD 3.2°). The maximal stem tilting was 12.9°.
Macroscopic parameters of compression wood and tree-ring eccentricity values
All analysed trees provided evidence regarding the presence of compression wood in 1997. Compression wood was not observed in 1996 in any of the samples analysed. Macroscopically, compression wood occurred in two basic forms (Fig. 3A). First, the structure of compression wood (detectable due to colour changes) was detected for the first time in the second part of the 1997 tree ring and to a similar extent in the following tree rings. This type of compression wood can be considered mild (Fig. 3). Second, the structure of compression wood was clearly visible since the middle part of the 1997 tree ring (a severe type of compression wood; Fig. 3), but the extent of compression wood in the following tree rings was very variable and reached 100% of the total tree-ring width after 1997. The abrupt onset of compression wood in these cases corresponds with the seasonal timing of landslide movements in 1997 (beginning of July). Generally, the duration of compression wood (assessed visually based on the macroscopic observation of colour changes) ranged between one and eight years. The mean CW duration was 4.3 years (SD 2.2 years). The mean intensity of CW, estimated based on the CW structure extent in the total width of the 1998 tree rings, was 66.2% (SD 35.8%) and ranged between 0 and 100%. The mean eccentricity of the 1996 rings was 0.51 (SD 0.07), whereas the mean eccentricity in the 1998 rings was 0.58 (SD 0.12). An increase in tree-ring eccentricity between 1996 and 1998 was detected in 14 trees (66.7%), whereas a decrease in eccentricity was detected in seven trees (33.3%). The mean change in tree-ring eccentricity between 1996 and 1998 increased by approximately 13.9% (SD 22.8%) (Fig. 4). The maximal detected eccentricity increase was 56%.
The chronological development of tree-ring eccentricity and its interannual changes in the analysed trees.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The chronological development of tree-ring eccentricity and its interannual changes in the analysed trees.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The chronological development of tree-ring eccentricity and its interannual changes in the analysed trees.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Anatomical changes in compression wood
The measurement of the anatomical parameters of tracheids (lumen area and cell wall proportion) in the zone of early wood and late wood in 1996 (normal wood) and 1998 (CW) rings was completed for 21 trees. Generally, the tracheid lumen area (TLA) decreased from 1996 to 1998 in the zone of the earlywood as well as latewood. In contrast, the cell wall proportion increased in both zones between 1996 and 1998. The mean values of both parameters from all trees from 1996 to 1998 differed significantly from those in the earlywood zone as compared to the latewood zone (Fig. 5;
Mean values microscopic CW parameters in the tree ring preceding the landslide event (1996) and the year following the landslide event (1998) as in the earlywood and latewood zones of the tree ring. (A) mean tracheid lumen area. (B) mean ratio of wall area.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Mean values microscopic CW parameters in the tree ring preceding the landslide event (1996) and the year following the landslide event (1998) as in the earlywood and latewood zones of the tree ring. (A) mean tracheid lumen area. (B) mean ratio of wall area.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Mean values microscopic CW parameters in the tree ring preceding the landslide event (1996) and the year following the landslide event (1998) as in the earlywood and latewood zones of the tree ring. (A) mean tracheid lumen area. (B) mean ratio of wall area.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Changes in the mean values of microscopic CW parameters between 1996 and 1998 in both the earlywood and latewood zones.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Changes in the mean values of microscopic CW parameters between 1996 and 1998 in both the earlywood and latewood zones.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Changes in the mean values of microscopic CW parameters between 1996 and 1998 in both the earlywood and latewood zones.
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Mutual relationships of parameters and their values in individual classes
The individual parameter values of the external disturbances (stem tilting intensity), macroscopic growth disturbances (duration and intensity of CW and tree-ring eccentricity), and microscopic growth disturbances (changes in TLA and cell wall proportion) were mutually compared (Fig. 7). Statistically significant relationships were found between stem tilting and the parameters of intensities of macroscopic growth disturbances (Fig. 7). In contrast, no significant relationship was found among stem tilting and the intensities of quantitative microscopic growth disturbances (Fig. 7).
The relationship of macroscopic and microscopic parameters of CW to the intensity of external disturbance (stem tilting).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The relationship of macroscopic and microscopic parameters of CW to the intensity of external disturbance (stem tilting).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The relationship of macroscopic and microscopic parameters of CW to the intensity of external disturbance (stem tilting).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The values of the studied parameters generally corresponded to the distribution of samples into three subjective anatomical classes of CW. The stem tilting, duration, and intensity of CW were significantly graded between all three classes (Fig. 8). Similarly, from the anatomical responses, the TLA changes in latewood were negatively graded, and the cell wall proportion changes were positively classified into classes. In contrast, the division of all samples into individual CW classes (based on the subjective assessment of microscopic anatomy) did not provide any significant grading or differences among classes for changes in tree-ring eccentricity or TLA changes and changes in cell wall proportion (both in early wood) (Fig. 8).
The distribution of macroscopic (A) and microscopic (B) CW parameter values in individual classes of CW (intensity defined by subjective observation of CW anatomy followed Janecka et al. 2020).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The distribution of macroscopic (A) and microscopic (B) CW parameter values in individual classes of CW (intensity defined by subjective observation of CW anatomy followed Janecka et al. 2020).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
The distribution of macroscopic (A) and microscopic (B) CW parameter values in individual classes of CW (intensity defined by subjective observation of CW anatomy followed Janecka et al. 2020).
Citation: IAWA Journal 44, 1 (2023) ; 10.1163/22941932-bja10098
Discussion
This study is focused on the detection of the most dependent parameters of compression wood (both macroscopic and microscopic) in P. abies on the intensity of external disturbance caused by landslide movements (stem tilting).
The analysed trees provided mean ages of 11.2 years at the moment of their tilting (1997). Young juvenile trees have more flexible stems than older trees (Stoffel & Bollschweiler 2008). During the first two decades of life, the tree is supposed to be highly susceptible to external growth disturbances causing stem instability (e.g., snow creep) (Stoffel & Bollschweiler 2008). More specifically, Šilhán and Stoffel (2015) and Šilhán (2021b) detected a highly increased ability of coniferous trees to form compression wood during the second decade of their life. Thus, the studied trees expressed the most suitable age for CW creation. In this regard, the presented study follows the standard research design applied, e.g., by Arbellay et al. (2012) or Ballesteros et al. (2010b). Nevertheless, it is not clear whether older trees (if analysed) would react in the same way. Although no relationship was found between the studied quantitative parameters and the ages of analysed trees, this should be verified in future research because the age variability of the studied tree dataset was not considerably wide.
Compression wood was very clearly macroscopically observable on the prepared (sanded) surface of each sample (increment core). Moreover, both of its macroscopic parameters (duration and intensity) expressed a very strong dependency on the intensity of stem tilting. This finding corresponds to the conclusions of Šilhán (2017), who detected a similar relationship in the case of surviving trees affected by catastrophic landslides. However, it is necessary to mention the possibility that in the following years after the main landslide event (1997) other smaller reactivations may have occurred. While this cannot be proven and is unlikely, a possible additional landslide event could affect the results, particularly in terms of CW duration and the intensity of stem tilting.
Trees generally responded with a significant increase in tree-ring eccentricity to landslide movement (Fig. 4). Next, the relationship between tree-ring eccentricity and the intensity of stem tilting was statistically significant but not as strong as that of the macroscopic parameters of CW (Fig. 7). This finding is surprising because tree-ring eccentricity is generally supposed to be a more sensitive growth response to stem tilting (Wistuba et al. 2013; Šilhán 2016; Fabiánová et al. 2021). On the other hand, Šilhán et al. (2019a) provided evidence regarding the ability of P. abies to form CW in response to slope movements on the order of several millimetres. Moreover, tree-ring eccentricity can be found as a response to stem tilting even without the presence of CW (Fisher & Marler 2006). However, an important note about the stem tilting measurement is necessary to mention here. Although the tilting measurement was performed as precisely as possible, some inaccuracies could occur due to the intensive bending of the stem or due to the deformation of the stem caused by the increased press due to the weight of the stem. Furthermore, although the measurements were taken as low to the ground as possible, i.e., where it can be assumed that the stem retains its original tilt and deforms upwards, there may have been a change in tilt even at the measurement position, which may contribute to the lower correlation coefficients found.
The results of the microscopic analysis of CW correspond with its generally known anatomy. The data evaluated in this study verify the decrease in tracheid lumen area (TLA) and increase in the cell wall proportion in CW. It should be noted here, however, that TLA generally varies between tree rings even without the presence of CW. However, since tree rings with CW occurrence and tree rings without CW occurrence were compared, changes in TLA can be largely attributed to CW. This anatomical effect is caused by structural changes in the function of woody cells, which have to mechanically support tilted stems (Westing 1965). This is performed by the increase in the thickness of the cell wall due to an increase in the lignin content at the expense of cellulose (Wardrop & Dadswell 1950; Bamber 2001). Generally, a stronger anatomical response (both in the TLA and cell wall proportion) was detected in the earlywood zone than in the latewood zone. This finding corresponds with the assumption of Ballesteros et al. (2010b), who found strong anatomical responses to wounding during flood events in the early wood zones of Pinus pinaster Ait rings. The latewood zone of a normal tree ring has the supportive function of trees (Björklund et al. 2017). Thus, the formation of CW does not change its basic function dramatically, resulting in rather less intensive quantitative changes in cell parameters. In contrast, earlywood tracheids primarily have conductive functions and generally larger lumen sizes (Björklund et al. 2017). The functional changes of these cells to supportive functions lead to more intensive quantitative anatomical changes. Moreover, a stronger anatomical response is supported by the larger amount of free space in the early wood structure that can be filled by lignin layers. For a more detailed insight into the issue of changes in compression wood cell wall parameters as a growth response to landslide movements, future research should focus on a separate analysis of changes in cell wall thicknesses in the radial and tangential directions as recommended, for example, by Savić et al. (2016). Furthermore, a controlled experiment in which the tree stem tilt is varied over a large range could help future research to detect the minimum value of stem tilt to which the tree will respond with anatomical changes.
The results of the present study suggest a fully independent microscopic anatomical growth response to the intensity of stem tilting. This finding is slightly in contradiction with Yumoto et al. (1983), who observed changes in the selected anatomical parameters of CW in trees with artificially tilted stems of various intensities. Nevertheless, their studies evaluated very detailed anatomical parameters, which are practically impossible to apply to dendrogeomorphology due to the time and equipment demand of the relevant analytical procedures. Nevertheless, the absenting dependence of microscopic CW parameters on the intensity of stem tilting is slightly surprising because this relationship was detected as strong in the case of vessel parameters in tension wood of broad-leaved trees. For example, Heinrich et al. (2007) and Heinrich and Gärtner (2008) found dependency in the case of Fagus sylvatica and Alnus glutinosa (L.) Gaertn. Similarly, Šilhán (2021a) concluded the same results for Betula pendula Roth. The obtained results can be affected by the characteristics of the studied site. Landslide movements do not only affect tree growth in the form of stem tilting, but their effect is probably more complex, i.e., affecting pressures in the soil, damaging tree roots, or changing soil hydrology (Šilhán 2021a). Although the sampled trees were carefully checked for the absence of externally visible disturbances, the mentioned factors could have remained hidden and can affect tree growth and anatomical response. As the present study was not performed under laboratory-like conditions, the obtained results more realistically correspond to the state that can be obtained from sampled trees during real dendrogeomorphic research on active landslides. Another explanation may be the relatively limited number of anatomical features studied, and the inclusion of more detailed parameters could confirm the results of Yumoto et al. (1983). However, practical analysis of these parameters (e.g., lignification of S2 layer) is not applicable in real dendrogeomorphological research due to time and material constraints, and therefore we studied only commonly used parameters.
The detected quantitative variations in anatomical growth responses suggest the possibility of their classification into several classes, as suggested by Heinrich and Gärtner (2008), for tension wood in broad-leaved trees. However, the absenting direct relationship with the intensity of external disturbance (stem tilting) excludes this possibility for practical use in dendrogeomorphology. This relationship could be analysed in more detail in follow-up research on a larger number of landslide events with different magnitudes. Nevertheless, the detected gradual relationships of selected quantitative parameters of CW anatomy with the visual classification of CW intensity based on subjective observation of its anatomy (as suggested by Janecka et al. (2020) following Yumoto et al. (1983)) permit the use of the anatomy-based classification of TW for dendrogeomorphic purposes. However, this classification cannot be based on the quantification of anatomical changes.
Finally, based on the results of this study, the selection of trees for future dendrogeomorphic sampling can be clearly focused on the most intensively tilted individuals. The macroscopically observable intensity of CW (intensity and duration) will likely correspond to the visual classification of CW anatomy in these individuals.
Conclusions
This study was focused on the assessment of the possible use of quantitative anatomy of compression wood in P. abies for dendrogeomorphic purposes and comparing its suitability with macroscopic parameters of compression wood. The intensity of external disturbance (stem tilting) was compared against the intensities of macroscopic parameters of CW (intensity and duration) and microscopic parameters of CW (percentage changes in tracheid lumen area and cell wall proportion — both in the earlywood and latewood zone of tree rings). Generally, macroscopically observed parameters of compression wood corresponded very well with the intensity of stem tilting. The anatomical growth response was tested between 1996 and 1998 tree rings. The response in the form of a decrease in TLA and an increase in the cell wall proportion was detected to be stronger in the zone of earlywood than in that of latewood. Nevertheless, no dependency of quantitative anatomical parameters on the intensity of stem tilting was detected.
The conclusions of this study suggest that the quantitative anatomy-based classification of CW is not usable for dendrogeomorphic purposes. Next, sampling of more severely tilted trees in future dendrogeomorphic research should provide the most intensive macroscopic evidence as well as the most intensive subjectively observable anatomical evidence of compression wood.
Corresponding author; email: karel.silhan@osu.cz
Acknowledgements
This research was funded by Czech Science Foundation grant number 22-12522S and University of Ostrava project no. sgs01/PřF/2022. Two anonymous reviewers are warmly acknowledged for providing constructive suggestions that improved the quality of the manuscript. The language was reviewed by American Journal Experts.
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Footnotes
Edited by Lloyd Donaldson