How Different Are the Results Acquired From Mathematical and Subjective

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How different are the results acquired from mathematical and subjectivemethods in dendrogeomorphology? Insights from landslide movements

Karelilhn

Department of Physical Geography and Geoecology, Faculty of Science, University of Ostrava, Chittussiho 10, 710 00 Ostrava, Czech Republic

a b s t r a c ta r t i c l e i n f o

Article history:

Received 9 June 2015

Received in revised form 19 October 2015Accepted 20 October 2015

Available online 21 October 2015

Keywords:

Dendrogeomorphology

Landslide

Tree sensitivity

The Orlick hory foreeld

Knowledge of pastlandslide activityis crucial for understanding landslide behaviour and for modelling potential

future landslide occurrence. Dendrogeomorphic approaches represent the most precise methods of landslide

dating (where trees annually create tree-rings in the timescale of up to several hundred years). Despite the ad-

vantages of these methods, many open questions remain. One of the less researched uncertainties, and the

focus of this study, is theimpactof twocommonmethods of geomorphic signal extraction on thespatialand tem-

poral results of landslide reconstruction.In total, 93 Norway spruce (Picea abies (L.)Karst.)trees were sampled at

one landslide location dominated by block-type movements in the foreeld of the Orlick hory Mts., Bohemian

Massif. Landslide signals were examined by the classical subjective method based on reaction (compression)

wood analysis and by a numerical method based on eccentricgrowthanalysis.The chronologyof landslide move-

ments obtained by the mathematical method resulted in twice the number of events detected compared to the

subjective method. This nding indicates that eccentric growth is a more accurate indicator for landslide move-

ments than the classical analysis of reaction wood. The reconstructed spatial activity of landslide movements

shows a similar distribution of recurrence intervals (Ri) for both methods. The differences (maximally 30% of

the totalRiranges) in results obtained by both methods may be caused by differences in the ability of trees to

react totilting of theirstems by a specic growth response(reaction wood formation or eccentric growth).Final-

ly, the ability of trees to record tilting events (by bothgrowth responses) in their tree-ring series wasanalysed for

different decades of tree life. The highest sensitivity to external tilting events occurred at tree ages from 70 to

80 years for reaction wood formation and from 80 to 90 years for eccentric growth response. This means thatthe ability ofP. abies to record geomorphic signals varies with not only eccentric growth responses but also

with age.

2015 Elsevier B.V. All rights reserved.

1. Introduction

Landslides are very broadly extended geomorphic processes that

often occur in populated areas (Gutirrez et al., 2010; Van Den

Eeckhaut and Hervs, 2012), where they can present a signicant natu-

ral hazard. Therefore, knowledge of the chronology of past landslide ac-

tivity is important for modelling their future potential occurrence

(Borgatti and Soldati, 2010). Moreover, data regarding the spatial distri-

bution of landslideactivity are necessary for safe land-use planning. Un-

fortunately, direct information regarding past landslide activity is often

scarce, particularly in remote areas. Landslides inventory and mapping

is required to better understand where and when they may occur. Nev-

ertheless, as the dates of triggering of past events are often uncertain,

the application of absolute dating methods becomes necessary (Lang

et al., 1999).

Dendrogeomorphic methodsrepresent the most precise way of dat-

ing past landslide activity in areas with forest cover located in

temperate zones in the timescale up to several hundred years (Corona

et al., 2014), although some limitations exist. For example, a limitation

of dendrogeomorphic landslide dating is the age of the trees studied.

Since the introduction of dendrogeomorphic analysis, coniferous trees

have been favoured (Alestalo, 1971; Shroder, 1978; Bgin and Filion,

1988; Lopez-Saez et al., 2012a) probably due to their good visibility of

tree-rings, easy sampling, or good visible growth responses. Neverthe-

less, many landslide areas are populated by broad-leaved trees, and

studies investigating broad-leaved trees have become more frequent

in recent decades (Fantucci and Sorriso-Valvo, 1999; Guida et al.,

2008; Stefanini, 2004; ilhn et al., 2014). According to Shroder

(1978), the most frequent landslide-associated event affecting trees is

the tilting of their stems; nevertheless, damage to the roots or burying

of thestem base can occur as well (Stoffel and Corona, 2014). For conif-

erous trees, the dominant inuence on growth following tree tilting is

the formation of reaction (compression) wood on the lower side of

the stem. The cells of compression wood have thicker and more round-

ed walls of the tracheids, which is the reason why tree rings containing

compression wood are macroscopicallyidentiable(Timell, 1986). Nev-

ertheless, coniferous trees do not create reaction wood in all cases of

Geomorphology 253 (2016) 189198

E-mail address: [email protected].

http://dx.doi.org/10.1016/j.geomorph.2015.10.012

0169-555X/ 2015 Elsevier B.V. All rights reserved.

Contents lists available atScienceDirect

Geomorphology

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http://dx.doi.org/10.1016/j.geomorph.2015.10.012http://dx.doi.org/10.1016/j.geomorph.2015.10.012http://dx.doi.org/10.1016/j.geomorph.2015.10.012mailto:[email protected]://dx.doi.org/10.1016/j.geomorph.2015.10.012http://www.sciencedirect.com/science/journal/0169555Xhttp://www.elsevier.com/locate/geomorphhttp://www.elsevier.com/locate/geomorphhttp://www.sciencedirect.com/science/journal/0169555Xhttp://dx.doi.org/10.1016/j.geomorph.2015.10.012mailto:[email protected]://dx.doi.org/10.1016/j.geomorph.2015.10.012http://crossmark.crossref.org/dialog/?doi=10.1016/j.geomorph.2015.10.012&domain=pdf


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stem tilting, and a potential minimum tilting value for its creation had

not beendeneduntil recently (ilhn and Stoffel, 2015). Moreover, po-

tential delay in the compression wood formation after the tilting event

can occur (Shroder, 1978). However, compression wood identication

in tree ring series is a subjectiveapproach, and results can be inuenced

by a researcher error. Additionally, broad-leaved trees create reaction

(tension) wood on the upper side of their tilted stems. Unfortunately,

this type of reaction wood is practically unidentiable at a macroscopic

scale (Westing, 1965). In spite of potential limitations regarding com-pression wood analysis, many recent studies of landslide dating have

been based on this subjective approach (Lopez-Saez et al., 2012a,b;

ilhn et al., 2012, 2013). Another growth reaction to stem tilting is ec-

centric growth. Tilted coniferous trees start to produce wide tree-rings

on the lower side of the stem, whereas the upper side of the stem is

dominated by suppressed growth (Braam et al., 1987). The position of

wide tree-rings and suppressed growth is usually opposite in the case

of broad-leaved trees. Identication of a tilting event based on dating

of this reaction has a mathematical base, as the eccentricity is based

on the calculation from measured tree-ring widths, and subjective er-

rors can be excluded. Unfortunately, some limitations even for the ec-

centric growth analysis exist. The eccentric growth can occur even as a

response to non-geomorphic events (e.g. strong wind). Due to the

aforementioned difculties with identication of tension wood, the

analysis of tree-ring series of broad-leaved trees is dominantly focused

on eccentric growth as a reaction to stem tilting (Van Den Eeckhaut

et al., 2009;ilhn et al., 2014). To date, the resulting effect of these

two different approaches (reaction wood vs. eccentric growth) on re-

constructed chronologies or spatial reconstruction is not known. How-

ever, answering this question is crucial for increasing the accuracy of

past events identication. Therefore, as noted above,

dendrogeomorphology is characterized by some biases (while it is

very accurate), so that a comparison of different methods is highly

important.

Another open question is the changing sensitivity of trees to geo-morphic events with increasing age. For example,ilhn et al. (2013)

found the highest sensitivity of Crimean pine (Pinus nigra ssp.

pallasiana) to rockfall events at ages of 80 to 90 years. The highest sen-

sitivity ofP. nigrato landslides was found to be present in two phases,

between the ages of 40 to 60 years and 120 to 130 years (ilhn and

Stoffel, 2015) in the dependence on the dominant growth response (re-

action wood vs. eccentric growth). However, for Norway spruce (Picea

abies(L.) Karst.), a very common tree species occupying landslides in

the northern hemisphere, the most suitable ages for recording tilting

events have not been determined yet.

Based on the aforementioned open questions, the objective of this

study is to show the effects of using subjective and mathematical ap-

proaches (reaction wood and eccentric growth, respectively) on differ-

ent aspects of dendrogeomorphic reconstruction of landslide

movements. This objective will be achieved using the following strate-

gies: (i) evaluating landslide event chronologies obtained using both

Fig. 1. Location, geomorphology, andgeologyof thestudied area. A location of thestudyarea in theCzechRepublic,B geomorphicmap of thestudied landslide area (1position of the

topographic prolein Fig. 2; 2 sampled trees; 3 border of studied domains; 4 gully; 5 debris talus; 6 partial landslide block; 7 alluvialfan; 8 main scarp; 9 bedrock out-

crop; 10accumulationlobe), C geologyof thestudied landslide surroundings (1uvial sands andgravel,2 marlstoneand sandstone, 3 marlstone, 4 limestone claystones, 5

loess, 6

fault), and D

LiDAR image of hillslope affected by the studied landslide.

190 K.ilhn / Geomorphology 253 (2016) 189198


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approaches, (ii) comparing the effect of each approach on the recon-

structed spatial activity of landslides, and (iii) assessing the changes in

sensitivity to tilting events ofP. abies with increasing age for bothapproaches.

2. Study area

For the purpose of this study, a landslide area dominated by block-

type movements was selected. The studied landslide area is located in

the foreland of the Orlick hory Mountains (5006 N., 1612 E.;

~300 m asl;Fig. 1A). The probable pre-conditioning factors include

the climatic conditions and geological structure of the hillslope. The cli-

mate of the region can be characterized as moderately humid with a

mean annual temperature of 78 C and a mean annual precipitation

of 550700mm(Quitt, 1971; Tolasz et al., 2007). The landslide area de-

veloped in the sub-horizontal layers of limestone marls (ech, 2001) on

a hillslope located on theleft bank of the Divok OrliceRiver (Fig. 1C, D).

The existence of faults parallel with the slope direction and thepast un-

dercutting of the hillslope by the Divok Orlice river can serve as poten-

tial preparatory factors. The age of undercutting, which is evidenced bythe presence of an old river branch at the foot of the hillslope in some

places, is not known up to now.

The landslide area is characterized by a frontal shape with a widthof

~1.2 km anda NE orientation. Themaximumverticaldistance is approx-

imately 75 m. The landslide area can be dened as block-type move-

ments (according toCruden and Varnes, 1996) with several levels of

mildly inclined, subsided blocks separated by steep steps (Fig. 2E). The

northern part of the landslide area is represented by one subsided

block with several steps decreasing in height to the north (Figs. 1C

and2B). The middle part of the landslide area has up to three levels of

subsided blocks (Figs. 1C and2C) and, including its horizontal move-

ments, shows the greatest elongation in this section. The southern

part of the landslide area is dissected by individual blocks separated

by a network of gullies. At the mouth of the gullies, two long alluvial

Fig. 2.Topographic proles, geomorphic features, and disturbed trees on the landslide area. A main scarp with bedrock outcrop; B surface of subsided landslide block; C upslope

tilted trees on slightly rotated landslide block; D disrupted marl layers at the foot of the landslide area; E deformed trees on the steep parts of the landslide area.

191K.ilhn / Geomorphology 253 (2016) 189198


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fans(up to 100 m long)werecreated (Fig. 1B). Several man-made struc-

tures such as forest roads have been built on parts of the slope. The

fronts of some of the subsided blocks are compressively deformed

(Fig. 2D), resulting in the formation of accumulation lobes. The mainscarp is very steep (more than 35) and has bedrock outcrops in several

locations (Fig. 2A). Debris tali have formed below some of the outcrops.

There are several linear subsidence features that likely follow fault lines

on the plateau above the main scarp. The landslide area is covered by a

forest dominated by Littleleaf Linden (Tilia cordataMill.), European

Beech (Fagus sylvaticaL.), Norway Spruce (P. abies), European Larch

(Larix deciduaMill.), and Scots Pine (Pinus sylvestrisL.).

Because landslide reactivation often impacts only a limited part

of the whole landslide area (Floris and Bozzano, 2008; Lopez-Saez

et al., 2013;ilhn et al., 2013), the studied landslide area was divided

into ve quasi-homogenous domains (Fig. 1B). This approach is

particularly suitable for dendrogeomorphic research (ilhn, 2012).

Dendrogeomorphic research focused on the following parts of landslide

area (not described from upper to lower parts, but focusing on theanalysed areas). Domain I is the northern part with one subsided long

block; domain II is the middle part with three vertical levels of blocks;

domain III consists of a connective level of blocks bordering domain II

from the south; domain IV consists of two vertical levels of blocks

with a large debris talus in the middle of the domain; and domain V is

the southern part of the landslide area with blocks separated by a net-

work of gullies.

3. Methods

A total of 372 increment cores were extracted from 93 disturbed

P. abies trees in an area of block-type movements. Tree-ring series

were analysed using two different methods: mathematical-based anal-

ysis of eccentric growth and the subjective method of reaction wood

identication. Results from both methods were analysed separately to

assess their inuence on the dendrogeomorphic analysis of a landslide

area.

First, detailedeld geomorphic mapping (at 1:500 scale) was per-

formed. The mapping was produced usingeld investigation and a dig-

ital elevation model (DEM) created from LiDAR data. Along with other

map features such as landslide blocks, rocky scarps, gullies, and debris

tali, the positions of the disturbed (tilted) trees selected for samplingwere recorded.

For the purpose of this study, P. abieswere sampled. Trees were se-

lected by two criteria: their position on the geomorphic map with an ef-

fort to evenly distribute the sampled trees within the landslide area

(Corona et al., 2014) and visible external disturbance caused by land-

slide movements (tilted or bent stems). Trees were sampled preferably

on the surface of blockswith a variety of topographical surface slope an-

gles. Thesteepest parts of the main scarp were ignored. Trees were sam-

pled using a Pressler increment borer (max. length: 40 cm; diameter:

0.5 cm) at the height of maximum stem bending. Four increment

cores were extracted from each tree perpendicular to each other (one

from the lower side of the stem, one from the upper side of the stem,

and two parallel to the slope). Additionally, 30 individual specimens

ofP. abieswere sampled (two cores per tree) from a stable position

Fig. 3. Methodsused fordetermininglandslidemovement.A onset of reaction (compression) woodin coniferous trees;B extractionof thegeomorphicsignal based on abrupt changes

in the eccentricity of tree rings.

Table 1

The number and age ofP. abies in individual landslide domains.

Domain Trees (number) Maximal age Minimal age Mean age (stdev)

I 33 142 16 108.8 (31.9)

II 10 108 32 44.3 (28.1)

III 12 125 35 91.3 (33.5)

IV 27 133 68 108.4 (19.3)

V 11 136 66 115.4 (23.5)

Total 93 142 16 101.5 (32.9)



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outside the landslide area (stable plateau above the main scarp) to cre-

ate a reference chronology reecting only the effects of climatic condi-

tions or occasional insect outbreaks on tree growth (Cook and

Kairiukstis, 1990). The reference chronology was created using a

double-detrending procedure in the Arstan software (Cook, 1983).

The negative exponential curve (or linear regression) was used in the

rst step, and the cubic smoothing spline function was applied in the

second step (Holmes, 1994).

All samples were processed following a standard procedure

(e.g., Stoffel and Bollschweiler, 2008, 2009). The individual steps of

this procedure included sample drying, gluing samples into the woody

supports, sanding, tree ring counting, and measuring ring widths

using dendrochronological TimeTable and PAST4 software (VIAS,

2005). The increment curves were cross-dated withthe reference chro-

nology to identify and correct possible false or missing tree rings.Two dating approaches of tilting events were applied. First, a subjec-

tive analysis of reaction (compression) wood presence (Fig. 3A) was

carried out on all four increment cores from each tree using a binocular

microscope. Reaction wood canbe found in two forms (pronounced and

mild). Tracheids with compression wood anatomy (thicker and

rounded walls) can be visible in the latewood part of a ring in the case

of mild reaction wood, whereas pronounced reaction wood contains

such tracheids throughout the entire width of the ring (Lopez-Saez

et al., 2012a). The onset of both reaction wood types was considered

as a tilting signal. Second, a mathematical approach, based on tree ring

eccentricity calculation and analysis was applied. The determination of

a tree tilting event was based on eccentric growth using the approach

published byilhn et al. (2014)andilhn and Stoffel (2015). Eccen-

tricity was analysed by comparison of two tree ring series growing per-

pendicular to each other (one from the lower side of the stem and the

second perpendicular to therst). This strategy reduces the risk of miss-

ing a ring while still providing a chance at calculating tree-ring eccen-

tricity (Shroder, 1978). In the rst step, tree-ring eccentricity (e) was

calculated according toBraam et al. (1987):

e ac

a c 1

whereais the ring width from the upper side of the stem and cis the

ring width perpendicular toa. Eccentricity values were separated into

Fig. 4.Chronology of landslide movements in individual domains and for the total landslide area as reconstructed from reaction wood analysis.

Fig. 5.Chronology of landslide movements in individual domains and for the total landslide area as reconstructed from eccentric growth analysis.



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three intervals (marked as 0, 1, and 2), according to Fig. 3B. The occur-

rence of specic interval patterns (e.g., 0022; Fig. 3) allows determi-

nation of theintensity of thetilting signal. For the purpose of this study,

this pattern was used only to determine whether a tilting signal oc-

curred or not.

Landslide activity in individual domains was expressed using the

standard event-response index (It) according toShroder (1978):

ItX

Rt=X

Nt 100% 2

where Rt is the numberof treeswitha tilting signalin year tand Nt isthe

numberof all sampled trees living in yeart. The Itvalueswere calculated

individually for each domain and for each tree group. To dene a land-

slide event, at least two trees had to show a landslide signal and theItvalue had to be at least 7.5%.

The frequency of landslide events recorded by a single tree was

expressed by the recurrence interval (Ri), calculated as:

Ri At=X

LSt 3

Fig. 6.Proportion of dated landslide events based on data from reaction wood analysis and eccentric growth analysis in individual landslide domains.

Fig. 7.Spatial distribution of mean recurrence interval between two landslide movements, reconstructed based on data from reaction wood analysis and eccentric growth analysis.



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where LSt is the number of tilting signals in thetree rings of a tree andAtisthe totalage of the tree. The Ri valuesfrom each tree were spatially in-

terpolated using the ordinary kriging model due to irregular spatial pat-

tern of sampled trees locations, with the grid cell size of 1 m due to

minimal distance between trees, using the Surfer 8 software (Golden

Software, 2002). Resulting grids based on reaction wood data and ec-

centric growth data were subsequently subtracted from each other to

determine places on the landslide area with the highest differences be-tween results obtained by the two different approaches.

Finally, based on the assumption ofStoffel and Corona (2014),anat-

tempt was made to express changes in a tree ability to record tilting

events with increasing age. The number of tilting events recorded by

each tree was summarized for individual decades of tree live and

expressed as the number of recorded events per tree in each decade.

This evaluation was based on data from both reaction wood and eccen-

tric growth analysis.

4. Results

4.1. Number and age of sampled trees and growth disturbances

From the entire landslide area, 93 individualP. abies trees were sam-pled. In total, 372 increment cores were extracted. The highest number

of trees was sampled in domain I (33 trees) and the fewest number of

trees wassampled in domain II (10 trees). The mean ageof the sampled

trees was 101.5 years (stdev = 32.9 years); the oldest and youngest

trees were 142 and 16 years old, respectively; the oldest trees were

sampled in domain V (mean = 115.4 years), whereas the youngest

were sampled in domainII (mean = 44.3 years). Details about thenum-

ber and age of the sampled trees in the individual domains are listed in

Table 1.

Among the analysed tree-ring series ofP. abies, a total of 118 indica-

tions of reaction (compression wood) were identied. On the other

hand, based on the analysis of eccentric growth, a total of 149 tilting-

event signals were identied. Very strong event signals were identied

in 24 cases, strong signals in 28 cases, weak signals in 34 cases, and very

weak signals in 63 cases.

4.2. Chronology of landslide events

The chronology of landslide events based on reaction wood analysis

obtained from the tree-ring series ofP. abiescontained 20 total eventsacross all domains (Fig. 4). Twelve years are identied as years of

landsliding, as some events occurred in the same year. The oldest

event occurred in domain I in 1909 and the youngest event occurred

in thesame domain in 2002. The highest number of recorded events oc-

curred in domain V (six events), and only three events were recorded in

domains II and IV. The highest mean It value reached eventsin domainII

(26.7%), while the lowestItvalue came from events in domain I (9.5%).

Therst period of frequent landslide activity occurred between 1909

and 1932. This period was followed by 36 years without dated activity.

Between 1968 and 2002, nine event years were recorded, while during

the last 12 years no events were found.

Eccentric growth analysis discovered 39 total landslide events across

all domains (Fig. 5). These events occurred in 24 landslide years. Nine of

the 12 event years reconstructed from reaction wood analysis were alsorecorded by eccentric growth analysis (1932, 1968 and 1977 events

were not recorded). Moreover, eccentric growth analysis enabled the

dating offteen additional landslide years (1943, 1965, 1972, 1974,

1980, 1981, 1985, 1988, 1989, 2001, 2005, 2006, 2009, 2011, and

2012). The highest number of events recorded occurred in domain V

(10 events), whereas only ve events were dated in domain IV. As in

the reaction-wood-based chronology, the oldest event was found in do-

main I and dated to 1909, while the youngest events occurred in do-

mains I and II in 2012. The highest meanItvalue was found in domain

III (25.7%) and the lowest Itvalue occurred in domain IV (10.7%). The

chronology obtained from using reaction wood data demonstrates

two periods of higher landslide activity very similar to the chronology

based on eccentric growth data (19091943 and 19652012).

Fig. 8.Difference (absolute value) of the recurrence interval maps, reconstructed based on data from reaction wood analysis and eccentric growth analysis.

Fig. 9. Sensitivity ofP. abies to tiltingevents based on reaction wood analysisexpressedas: A number of dated tilting events in individual decadesof tree life; B numberof dated tilting

events per one tree in individual decades of tree life.



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Thirtyve per cent of all dated landslide years were discovered by

both methods simultaneously (not only by one method). However,

54% of event years were reconstructed using eccentric growth analysis

only. Using reaction wood analysis alone enabled reconstruction of

12% extra event years. The benets of using both methods for

reconstructing landslide events in individual domains are shown in

Fig. 6.

4.3. Spatial landslide activity

The spatial distribution of landslide movements was expressed by

the interpolated distribution of the treesRivalues. From data based on

reaction wood analysis (Fig. 7), it is clearly visible which parts of the

landslide area are most active. Generally, the most active part (with

the shortest periods between two events) occurs in the centre of the

landslide area in domains II and III with three vertical levels of subsided

blocks. This statement is supported by the results of the eccentric

growth analysis (Fig. 7). These domains contain the highest number of

vertical block levels and the greatest landslide area length. Moreover,

surface subsidence along the fault line above the main scarp is greatest

in this area (up to 40 m).The highest reconstructed landslideactivity re-

veals dynamic landslide area development in this location. Frequent

landslide movements broke the edge of the marl plateau into several

levels of blocks and moved these blocks the greatest distance fromtheir original locations (up to 100 m in comparison with surrounding

stable slope without landslidemovements).The area of the next highest

activity is located at the southern border of the landslide area in domain

V. Generally, low activity was recorded in domain IV, while domain I is

dominated by closely spaced changes in activity frequency.

Recurrence maps obtained from both approaches show very similar

results (coefcient of correlation = 0.89). Nevertheless, subtraction of

the two grids revealed locations with the highest differences between

the spatial distributions of both datasets. According toFig. 8, the maxi-

mum difference between the two grids is no higher than ~ 40 years,

which is ~ 30% of the maximumRiin either grid. This observation con-

rmsthe expected strongagreement between thegrids. Thehighest dif-

ferences are located at the southern border of the studied landslide area

in domain V, in the central part of domain IV on the debris talus, and inseveral locations within domain I.

4.4. The ability of P. abies to record tilting events in tree rings

Based on reaction wood analysis, the number of tilting events re-

corded during individual decades of tree life initially increases with

age. The highest number of identied tilting events was recorded at

tree ages of 60 to 80 years (Fig. 9A). After the age of 80 years, the num-

ber of reaction-wood-based tilting events abruptly decreases. The same

results are clearly visible even when the number of recorded tilting

events is weighed by the number of trees living in a particular decade

(Fig. 9B).

The number of dated tilting events based on eccentric growth anal-

ysis initially increases with age as well. However, the highest number of

recorded tilting events is shifted by about one decade in comparison

with data based on reaction wood analysis (i.e., to ages of 80 to 90

years). After this period, the number of events decreases. In general,

an above-average number of recorded tilting events based on eccentric

growth analysis was observed in trees aged 60 to 120 years (Fig. 10A).

Even the proportion of very strong and strong tilting signals is above av-

erage during this period with a peak at tree ages of 90 to 100 years

(Fig. 10B).

5. Discussion

5.1. Chronological consequences of mathematical and subjective method

use

As the landslides dominated by marly rocks are sensitive to

precipitation totals (Van Asch et al., 1999) the verication of

dendrogeomorphic dating can be done by comparison with extreme

rainfall events. The dating accuracy can be veried by the occurrence

of heavy precipitation events in most of the dated landslide years

(e.g., 1909, 1977, 1985, 1997, 1998, 2000; tekl et al., 2001) in the

study region. The precipitation total in these years exceeded at least

100 mm during one day.

Chronologies reconstructed from each method provide clear evi-

dence of signicant differences resulting from the use of the twomethods. Thechronology obtained from databasedon eccentric growth

analysis indicatedtwice thenumber of landslideevents as data based on

reaction wood analysis. Although some of the events were identied by

both methods, use of eccentric growth analysis alone identied 54% of

all events. ilhn and Stoffel (2015)had already demonstrated a higher

sensitivity of eccentric growth analysis compared to using macroscopic

observationsof reaction (compression) wood for P. nigra ssp.pallasiana.

Although a different tree species was used in this study, the results pre-

sented here fully support the conclusions of these authors. Differences

in the ability of trees to form compression wood vs. eccentric growth

are potentially caused by different tilting thresholds of tree stems re-

quired to initiate growth reaction. Based on the results of this study,

P. abiescan form eccentric growth at lower values of stem inclination

than necessary for the formation of reaction wood. In addition, the sen-sitivity of eccentric growth analysis is higher, even though the reaction

wood analysis was carried out on four radii of the stem, as a tree can be

tilted several times in different directions (although it is common to an-

alyse only two radii;Corominas and Moya, 1999; Carrara and O'Neill,

2003; Stefanini, 2004). Accordingly, further research should focus on

the relationship between the magnitude of stem deformation and the

intensity of eccentric growth/reaction wood formation.

5.2. Consequences of mathematical and subjective method use on spatial

reconstruction

Bothmethods of geomorphic signal extraction provided very similar

spatial distributions of recurrence intervals (Ri). Identical maximum

ranges of reconstructedRi depend on the maximum age of the trees.

Fig. 10.Sensitivity ofP. abiesto tilting events based on eccentric growth analysis expressed as: A number of dated tilting events in individual decades of tree life; B number of dated

tilting events per one tree in individual decades of tree life, and proportion of signal intensities.



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Tree age limits do not allow for the reconstruction of higherRivalues;

this means that the real period between two landslide events in the

less active parts of the landslide area could be higher than what was

found from the reconstructed chronology.

Based on the assumption that different thresholds of stem tilting

produce different growth reactions (reaction wood vs. eccentric

growth), it can be supposed that the greatest differences between re-

sults obtained by the different methods can be found in the trees with

low stem inclination. Data based on eccentric growth in these treesshould show higher frequency of tilting events in these cases. Accord-

ingly, areas with signicant differences between both grids probably

correspond to locations characterized by low stem inclination. This as-

sumption should be further investigated by research focusing on

dendrometric evidence.

5.3. Ability of P. abies to record tilting signals

One of the aims of this study was to analyse the changing ability of

P. abies trees to record tilting signals in their tree-ring series. This ques-

tion is quite new for landslide research, as the inuencing geomorphic

factors have not been intensively studied (ilhn and Stoffel, 2015).

However, solving this open question is crucial for establishing a gener-

ally recommended sampling strategy for a wide range of tree ages. Ini-tial results are known only for impacts of debris ows on P. nigra

(ilhn et al., 2015), rockfalls onP. nigra(ilhn et al., 2013), and for

landslides onP. nigraandF. sylvatica(ilhn and Stoffel, 2015). In this

study, the highest sensitivity ofP. abies to landslide movements was re-

corded at tree ages of 60 to 80 years for reaction wood formation and

ages 80 to 90 years for the formation of eccentric growth. These results

are quite different from the results ofilhn and Stoffel (2015).P. abies

trees seem to be better able to allocate their wood material along stem

circumferences during ages of limited ability to create reaction wood,

nevertheless the next research of this difference reason should be

realised in the future. The result is a shift of the highest sensitivity to

stem tilting from the formation of reaction wood toward eccentric

growth. An open question remains the effect of changes in tree size

(stem diameter) on the sensitivity of a tree to landslide movements.

6. Conclusion

This study highlights how the choice of a dendrogeomorphic meth-

od can imply some biases on the results. For this research, 93 trees of

P. abieswere sampled from one landslide area. The onsets of reaction

(compression) wood formation and abrupt changes in tree-ring eccen-

tricity were used as landslide signals.

The chronology of landslide movements based on eccentric growth

analysis indicated nearly twice the number of events compared to the

chronology based on reaction wood analysis. This means that the use

of eccentric growth is a much more sensitive approach to landslide dat-

ing than the classical analysis of reaction (compression) wood in the

tree-ring series ofP. abies. Nevertheless, each method can identifyonly a part of the landsliding events, and not necessarily the same

ones. Therefore, the combination of bothmethods forlandslideresearch

can be recommended. The assumption that trees react in different ways

to various values of stem inclination requires additional study, as it can

have consequences on the spatial reconstruction of landslide activity.

The sensitivity ofP. abiesto landslide movements via its ability to

create reaction wood or eccentric growth as response to stem tilting

changes with increasing tree age. The greatest ability to record tilting

events via reaction wood formation and eccentric growth occurs at

tree agesof 70 to 80 years and 80 to 90 years,respectively. This indicates

differences in a tree ability to allocate itswoody material in differentde-

cades of tree life. Future research should focus on changes in tree sensi-

tivity to landslide activity during variousage periods (not only decades)

or various intervals of stem diameter.

Acknowledgement

This study was funded by the Czech Science Foundation project no.

15-02067S. The English language was reviewed by Elsevier Language

Editing Services. I offer my sincere gratitude to Mauro Soldati and an

anonymous reviewer for their insightful comments and to Takashi

Oguchi for the thorough editing.

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