Dendrogeomorphological anal y sis of the Sawicki Land slide in the Beskid Niski Moun tains (S Po land)
Marek KR¥PIEC1, Natalia NAWROCKA1, W³odzimierz MARGIELEWSKI2 and El¿bieta SZYCHOWSKA-KR¥PIEC1, *
1 AGH Uni ver sity of Sci ence and Tech nol ogy, Al. A. Mickiewicza 30, 30-059 Kraków, Po land
2 In sti tute of Na ture Con ser va tion, Pol ish Acad emy of Sci ences, Al. A. Mickiewicza 33, 31-120 Kraków, Po land
Kr¹piec, M., Nawrocka, N., Margielewski, W., Szychowska-Kr¹piec, E., 2019. Dendrogeomorphological anal y sis of the Sawicki Land slide in the Beskid Niski Moun tains (S Po land). Geo log i cal Quar terly, 63 (4): 801–810, doi: 10.7306/gq.1506 As so ci ate Ed i tor: Wojciech Granoszewski
We ap plied dendrochronological anal y sis to study an ex ten sive (~2.5 km long) com plex-type land slide in the Beskid Niski Moun tains (Pol ish Flysch Carpathians), which was re ju ve nated in his tor i cal time (the ear li est trans for ma tion oc curred in 1913). We used three com ple men tary meth ods (dat ing of ec cen tric ity of an nual growth rings, ec cen tric ity in di ces, and anal y - sis of re ac tion wood), based on seven spe cies of de cid u ous and co nif er ous trees. A to tal of 204 trees were sam pled and 408 cores col lected us ing a Pressler in cre ment borer. Based on the dendrochronological anal y sis, the ac tiv ity of var i ous parts of the land slide in the past cen tury is pre sented, which ap pears to have been di verse and in flu enced mostly by pre cip i ta tion.
The up per part of the land slide was ac tive from the 1920s un til the 1940s. In the mid dle sec tion of the land slide, in ten si fi ca - tions of grav i ta tional move ment were re peat edly re corded af ter in tense rain falls through out the en tirety of the last cen tury.
The high est land slide ac tiv ity was dendrochronologically de tected in the 1980s and 1990s. In creas ing land slide ac tiv ity dur - ing so-called “dry years” may have been re lated to quick dry ing of the bed rock, re sult ing in changes in rock strength pa ram e - ters. Our re sults in di cate sig nif i cant sus cep ti bil ity of this par tic u lar slope-val ley geosystem in re sponse to even slight hydrogeological changes (in clud ing dry ness), which can trig ger ground move ment.
Key words: dendrogeomorphological anal y sis, com ple men tary method, com plex-type land slide, Pol ish Flysch Carpathians, south ern Po land.
INTRODUCTION
An nual tree rings pro vide var i ous data re gard ing en vi ron - men tal con di tions and changes. Dendrochronological anal y sis is used to re con struct se quences of var i ous geo log i cal (Baillie, 2008; Bekker, 2010), geomorphological (McAuliffe et al., 2006;
Bollschweiler et al., 2009; Stoffel et al., 2012) and hy dro log i cal pro cesses (StGeorge and Niel sen, 2002; Stoffel and Wilford, 2012). Tree rings also re cord anthropogenic pol lu tion (Oleksyn, 1988; Malik et al., 2012). Dendrochronological anal y sis, as well as iso to pic anal y sis of subfossil trees, is used to re con struct palaeoenvironmental (mainly palaeoclimatic) changes (Pazdur et al., 2005; Edvardsson et al., 2012; Kr¹piec et al., 2016).
More over, liv ing trees re cord ev i dence of mass move ment ac - tiv i ties (i.e. ground destabilization) in their an nual rings (Ale - stalo, 1971; Shroder, 1980; Braam et al., 1987a, b; Kr¹piec and Margielewski, 1991, 2000; Stoffel et al., 2013). Ap pli ca tion of dendrochronological meth ods for the re con struc tion of past ac -
tiv ity of mass move ment en ables de ter min ing the dis tri bu tion of land slide move ments over time, and re con struct ing land slide his tory (Braam et al., 1987a, b; Kr¹piec and Margielewski, 1991, 2000; Corominas and Moya, 1999; Stefanini, 2004;
Stoffel et al., 2013; Silhan and Stoffel, 2015). The dendro - geomorphological stud ies aimed at iden ti fi ca tion of cli ma tic con di tions af fect ing the de vel op ment and ac tiv ity phases of land slides, con cern ing also nu mer ous land slides in the Beskidy Moun tains, and data for a large part of the Pol ish West ern Carpathians have been col lected (Kr¹piec and Margielewski, 1991, 2000; Kr¹piec and R¹czkowski, 2005; Kr¹piec et al., 2008; Kiszka, 2016).
Dendrogeomorphological anal y sis pre sented in the pa per was car ried out on an ac tive land slide (~2.5 km long) called the Sawicki Land slide, sit u ated in the Beskid Niski Moun tains within the Flysch Carpathians. This land slide has been al ready ana - lysed us ing one of the dendrogeomorphological meth ods, namely the deconcentricity method (Kr¹piec and R¹czkowski, 2005; Kr¹piec et al., 2008; Nawrocka, 2013; Kiszka, 2016). The aim of our study is to re con struct the his tory of Sawicki Land - slide ac tiv ity, us ing com plex three com ple men tary meth ods:
dat ing of ec cen tric ity of an nual growth rings, ec cen tric ity in di - ces, and anal y sis of re ac tion wood, based on seven spe cies of de cid u ous and co nif er ous trees.
* Corresponding author, e-mail: szycha@geol.agh.edu.pl Received: Frebruary 8, 2019; accepted: September 25, 2019; first published online: December 20, 2019
STUDY AREA
The study was car ried out on a vast ac tive com plex-type land slide called the “Sawicki Land slide” af ter its first in ves ti ga - tor, Ludomir Sawicki. The in ves ti ga tion lo cal ity is pre sented in Fig ure 1. Sev eral fac tors were con sid ered to chose of the sam - pling strat egy to meet the re search ob jects. They in clude the pres ence of dis tinct el e ments of the land slide re lief, traces of sec ond ary land slide mo tions, as well as the fact that the vast land slide area is for ested by a multi-spe cies stand of trees of
suf fi cient age to pro vide in for ma tion re gard ing grav i ta tional move ments over at least the last 100 years.
The land slide is lo cated in the Pol ish Flysch Carpathians, in an area of three tec tonic units (Magura, Dukla, Silesian) over - thrust to the north-east. All three units con sist of sand stone - -shale flysch that was de pos ited from the Late Cre ta ceous to the Early Mio cene (Œwidziñski, 1973; Kopciowski et al., 1997, 2014; Jankowski, 2007, 2015). A large part of study area is cov - ered by Qua ter nary sed i ments. Flysch mas sifs, oc cur ring in the study area, are strongly folded and faulted (Jankowski, 2015).
Tec tonic dis con ti nu ities, as well as cha otic com plexes (e.g.,
Fig. 1. Lo ca tion of the Sawicki land slide area: A – land slide lo ca tion; B – land slide on the dig i tal (LIDAR) ter rain model;
C – trees with de formed trunks over grow ing the land slide area
olistoliths, tec tonic mélanges) oc cur ring in the Beskid Niski Moun tains (Jankowski, 2007), are fa vour able for mass move - ment for ma tion in these parts of the Outer Carpathians. Re lief is gen er ally re lated to the geo log i cal struc tures, in clud ing dif fer - ences in bed rock strength (Starkel, 1960), po si tion of the flysch mas sifs in the struc ture of the Carpathian orogen (Jankowski and Margielewski, 2014), and the dis junc tive tec ton ics de ter - mined by the oc cur rence of nu mer ous faults, or even tec tonic mélange zones, of ten used by river val leys (Jankowski, 2007).
The study area is con spic u ous by the oc cur rence of re lief el e - ments typ i cal for both the Carpathian foot hills and Beskidy Moun tains (Kotarba, 1986). The typ i cal re lief type for the Beskids con sists of in versely carved ridges that formed within syncline or monocline struc tures, com posed mostly of thick - -bed ded weath er ing-re sis tant Magura sand stones. The ridges in the foot hill zones are more frag mented, and the hills are gen - er ally gen tle with broad and even pla teau tops and steep slopes (Starkel, 1960; Kotarba, 1986).
The Beskid Niski Moun tains is an area of ubiq ui tous land - slides that strongly trans form the re lief. In fact, the ef fects of mass move ment in this moun tain range are so well man i fested that it is the area where the term “land slide-type” re lief was first in tro duced (Starkel, 1960). In this area, slopes are heavily changed by mass move ment, and land slides lo cally cover up to 30% of their sur face (Kotarba, 1986; D³ugosz, 2011; Mro - zek, 2013), are formed also cur rently (£ajczak et al., 2014).
The most com mon are small land slides (usu ally <10 ha).
Large ar eas are oc cu pied by land slides that formed by the over lap ping and merg ing of smaller forms. In terms of the char ac ter of grav i ta tional dis place ments and type of ma te rial dis placed, these are mostly ro ta tional, translational and com - pound land slides, en com pass ing both the bed rock and slope cov ers (Zabuski et al., 2009). The vari a tion in bed rock li thol - ogy and strength (e.g., rigid thick-bed ded sand stones over ly - ing sus cep ti ble shales) and tec tonic con di tions (e.g., strongly jointed rock mas sifs cut by nu mer ous faults oc ca sion ally form ing tec tonic mélange – Jankowski, 2007, 2015) are con - du cive to the fre quent oc cur rence of mass move ment (Bober, 1984; Zabuski et al., 2009). In ten si fi ca tion of the mass move - ments in this part of the Beskid Niski Moun tains took place in the Early and Late Ho lo cene (Gil et al., 1974; Wójcik et al., 2006), when land slides formed, de vel oped and trans formed in cy cli cal pe ri ods of cli mate hu mid ity, for which ex treme or long-term pre cip i ta tion events were re corded (Gil et al., 1974;
Alexandrowicz, 1996; Starkel, 1997; Margielewski, 2006, 2018; Margielewski et al., 2010).
The land slide ana lysed in this study is lo cated north-west of Szymbark (Gorlice county) on the south ern slopes of Mt.
Maœla na Góra (753 m a.s.l.) (Fig. 1A). It is an old, large com - plex-type land slide, dated to the Mid dle Ho lo cene (Sawicki, 1917; Wójcik et al., 2006; R¹czkowski, 2007b; R¹czkowski and Zabuski, 2008). The land slide has a to tal sur face area of 151.1 ha and a length and width of 2846 and 500 m, re spec - tively (Fig. 1A, B). It ex tends at the el e va tion be tween 345 to 725 m a.s.l., stretch ing over al most the en tire Szklarka Creek catch ment.
The up per part of the Sawicki Land slide was formed in the thick-bed ded Magura sand stones of the Maœlana Góra Syn - cline, whereas the mid dle and lower parts were formed in the Ciê¿kowice and Mutne sand stones (in ter ca lated by var ie - gated shales) of the Szymbark Slice (Kopciowski et al., 1997, 2014). The low est part of the land slide is formed in the Ino - ceramian Beds.
The main scarp of the land slide is steep and al most 30 m high. Mul ti ple sec ond ary scarps, up to sev eral metres in height,
formed within the land slide body be low the main one dur ing sec ond ary move ments. The thick ness of the col lu vium is es ti - mated to be ~30 m (R¹czkowski, 2007b). The col lu vium con - sists of boul ders, clays with de bris, blocks, and de tri tal ma te rial (Fig. 1B; R¹czkowski, 2007b). In the mid dle of the land slide, damm ing of the creek by the col lu vial ma te rial has re sulted in the for ma tion of a small lake. The land slide is pe ri od i cally ac - tive, par tic u larly un der wet hydro meteoro logi cal con di tions. It was re ju ve nated in 1784 and com pletely re mod elled in 1913 (Sawicki, 1917; Kr¹piec et al., 2008).
MATERIALS AND METHODS
SAMPLING AND MEASUREMENTS
This study con cerns liv ing trees grow ing on the Sawicki Land slide, as well as sta ble ar eas in the close vi cin ity of the land slide. The sam pling, con ducted in two re search pe ri ods in 2003 and 2011, re sulted in the col lec tion of 408 sam ples (cores) from 204 trees us ing a Pressler in cre ment borer. The trees se lected for sam pling were vi su ally as sessed as healthy, dom i nat ing and de void of vis i ble can opy dam ages or loss of the assimilatory ap pa ra tus. The ex act lo ca tion of the in di vid u als was de ter mined us ing GPS.
A to tal of 174 trees of var i ous spe cies (both co nif er ous and de cid u ous, ring po rous and dif fuse po rous) were sam pled from the land slide area (Ta ble 1). The core sam ples were col lected from trees ex hib it ing vis i ble changes in the mor phol ogy of their trunks, such that they were de formed and ro tated by self-ad just - ment to a ver ti cal po si tion (Fig. 1C). For each tree in the dendrogeomorphological study, two sam ples were taken from both the slope-fac ing (upslope) and op po site (downslope) sides of the trunk at its max i mum bend.
Sam ples were also taken from trees grow ing on sta ble slopes with out any traces of grav i ta tional move ments. Two cores were ac quired from each of 10 trees of the fol low ing spe - cies: fir (Abies alba Mill.), Eu ro pean larch (Larix decidua Mill.), and Scots pine (Pinus sylvestris L.), yield ing a to tal of 60 sam - ples. These sam ples have been used in a com par a tive study, pro vid ing ma te rial to con struct lo cal chro nol o gies. Sam ple pre p a ra tion and anal y sis were car ried out in the Dendro - chrono logical Lab o ra tory of the AGH – Uni ver sity of Sci ence and Tech nol ogy in Kraków (Po land) us ing the DENDROLAB mea sure ment set. Mea sure ments of the an nual tree-ring width were made with 0.01 mm ac cu racy on a prop erly crafted cross-sec tion (i.e. transversal sur face) from the core to the
T a b l e 1 Tree spe cies sam pled on the Sawicki Land slide
Num ber of trees Sil ver fir (Abies alba Mill.) 99 Eu ro pean larch (Larix decidua Mill.) 45 Scots Pine (Pinus sylvestris L.) 33 Syc a more ma ple
(Acer pseudoplatanus L.) 15
Com mon Beech (Fagus sylvatica L.) 5 Nor way Spruce (Picea abies Karst) 6
Scots elm (Ulmus glabra Huds.) 1
TOTAL 204 trees (408 cores)
youn gest ring. The mea sured se quences were pro cessed us - ing TREE -RINGS (Krawczyk and Kr¹piec, 1995) and QUERCUS (Wala nus, 2005) pro gram pack ages. Trees grow - ing on land slides are rarely older than 100 years. The lon gest se quence con sisted of 137 tree rings and dated back to the year 1875.
DENDROGEOMORPHOLOGICAL ANALYSIS
In the case of pos si bil ity of de ter min ing the pe riod of grav i ta - tional bed rock dis place ments, the dendrogeomorphological ana l y sis was car ried out us ing three dif fer ent meth ods.
Based on the ec cen tric ity of an nual in cre ments, we used:
–anal y sis of pairs of tree-ring pat terns of trees grow ing on land slide ar eas (Braam et al., 1987a, b; Kr¹piec and Mar - gie le wski, 1991, 2000),
–re ac tion wood anal y sis (Alestalo, 1971; Shroder, 1980), –ec cen tric ity in dex eval u a tion (Malik and Wistuba, 2012).
Ref er ence chro nol o gies were also con structed for co ni fers.
Dendrochronological curves (dendrograms) were pro du - ced from the mea sured se quences of an nual in cre ments of trees grow ing on the land slide area. The dendrochronological curves for in di vid ual trees are pre sented in pairs, rep re sent ing
upslope and downslope sides of the trunks. The ec cen tric ity of the an nual growths on the dendrogeomorphological curves is ex pressed as a long-term di ver gence of dendrograms, when the width of the ring on one side of the trunk sud denly in - creases or de creases com pared with the op po site side (Braam et al., 1987b; Kr¹piec and Margielewski, 1991; Fig. 2).
A char ac ter is tic dis tri bu tion pat tern of the dendrochronological curves, con nected with their rapid di ver gence caused by a change in the ra tio of an nual growth widths in the upslope and downslope parts of the trunk, is con sid ered an in di ca tor of mass move ments, which al lows es ti ma tion of their time range (Shroder, 1980; Corominas and Moya, 1999; Kr¹piec and Margielewski, 2000). The most im por tant in for ma tion for dat - ing land slide events was the ini tial stage of this pro cess. Di ver - gence pe ri ods, as well as the years when re ac tion wood was formed, were iden ti fied for each pair of curves. Clear events (i.e. sharp di ver gence of dendrograms) were linked to the on - set of the mass move ment that caused de for ma tion to the tree trunks, whereas re ac tion wood was help ful to iden tify ep i sodes of land slide move ment (Kr¹piec et al., 2008). Spa tial anal y sis of the ac quired re sults al lowed track ing of the changes in the in ten sity of the land slide pro cesses across var i ous parts of the land slide. The re sults are the ba sis for the com pi la tion of land - slide ac tiv ity maps for in di vid ual years or pe ri ods of sev eral years.
Fig. 2. An ex am ple of dendrogeomorphological anal y sis based on sam ples taken from fir trees grow ing on the land slide area (Saw19)
and on the sta ble area (Saw80)
The pe ri ods in which the dendrograms from the downslope (A) and upslope (B) part of trunk di verged are marked in grey; the ex tent of re ac tion wood is marked on the axis
Within the frame work of re search car ried out on the Sawicki Land slide in 2011, the method of tree-ring ec cen tric ity in dex eval u a tion was used ac cord ing to the for mu las pro posed by Wistuba and Malik (2011; see also Malik and Wistuba, 2012):
Ex = Ux–Dx
Eix = (Ex/Dx) x 100% (if D > U) Eix = (Ex/Ux) x 100% (if D < U)
vEix = Eix–Eix–1
where: E – ec cen tric ity [mm], Ei – ec cen tric ity in dex [%], vEi – yearly vari a tion of the ec cen tric ity in dex [%], U – width of tree ring on the upslope side of tree trunk [mm], D – width of tree ring on the downslope side of tree trunk [mm], x – year/an nual tree ring.
The ec cen tric ity in milli metres and the ec cen tric ity in dex per cen tiles were cal cu lated based on mea sured se quences of an nual growth width from both sides of the trunks (i.e. down - slope and upslope). These in di ca tors were de ter mined both for each tree as a whole and sep a rately for each an nual growth ring. The year-to-year vari abil ity of ec cen tric ity in dex in di ca tors was used for dat ing the be gin ning of ground move ment. This was cal cu lated as the dif fer ence be tween in dex val ues in a given year com pared with the pre vi ous one. The thresh old val - ues for dat ing pe ri ods of grav i ta tional move ment were de ter - mined based on re sults for the trees grow ing in sta ble ter rains as an arith me tic av er age of the deconcentricity in dex.
RESULTS
LOCAL CHRONOLOGIES FOR CONIFEROUS TREES SPECIES
Based on 60 an nual growth se quences, ob tained from 30 trees of the three co nif er ous spe cies grow ing in ar eas un af - fected by deep mass move ments, sam ples with the high est sim i lar ity were cho sen for fur ther anal y sis. Dendrograms con - structed for the se lected trees are de void of dis or ders (e.g., growth de pres sions). Iden ti fi ca tion of the best fit ting in di vid ual se quences, the max i mum val ues of their cal cu lated cor re la tion co ef fi cients, and the vi sual re sem blance of curves were taken into ac count. In this way, at least 10 se quences were se lected for each spe cies. Dendrochronological pat terns were con - structed, af ter be ing av er aged, as the lo cal and stan dard chro - nol o gies (Szychowska-Kr¹piec, 2010), which pri mar ily re flect a cli ma tic im pact. Fur ther anal y ses ac counted for di ver gence of the dendrochronological curves and tree-ring ec cen tric i ties, which are di ag nos tic of the mass move ments, and were in - cluded in a def i nite trend as so ci ated with cli ma tic or anthro - pogenic fac tors (Kr¹piec and Margielewski, 2000).
COMPARISON OF PAIRS OF DENDROCHRONOLOGICAL CURVES WITH REACTION WOOD ANALYSES
A pair of dendrograms was drawn for each tree grow ing in the land slide area, and the ex tent of re ac tion wood oc cur rence was re corded. For ex am ple, three pe ri ods of dis per sion curves
Fig. 3. Dis tri bu tion of the land slide ac tiv ity sig nals gen er ated by: A – ini tial year of a long-term di ver gence of dendrograms; B – ec cen tric ity in dex eval u a tion method for each year
were iden ti fied for fir grow ing on col lu vium in the mid dle of the land slide (tree No. Saw19 – see Fig. 2). As the core sam pled from the upslope side of the trunk was in com plete, the be gin - ning of the first phase of grav i ta tional ground move ment could not be pre cisely iden ti fied. A dis tinct zone of re ac tion wood could be ob served in the pe riod 1955–1963. High ec cen tric ity of an nual growth and, in turn, sig nif i cant di ver gence of dendro - grams for the slope-fac ing and op po site sides of the trunk lasted un til 1980. An other re ac tion wood zone be gan to form in 1997 and con tin ued un til the youn gest ring of the sam pled core was col lected in 2011. Within this zone, the years in which di - ver gence of the dendrochronological curves be gan (i.e. 1997 and 2008) could be con sid ered di ag nos tic for ground move - ment. The in ter pre ta tion pre sented above was ap plied to all sam ples anal y ses from the in ves ti gated land slide. Anal y sis of dendrochronological curves also dem on strates that the graphs for the land slide slope and sta ble ar eas are sig nif i cantly dif fer - ent. Graphs for the ref er ence trees are de void of con sid er able pe ri ods of di ver gence from the dendrochronological curves.
More over, in most cases, there was no re ac tion wood. The curves have sim i lar and ap prox i mately par al lel courses, in which the pe ri ods of sin gle years with very low ec cen tric ity al ter - nate (Fig. 2).
A sum mary of the ini tial di ag nos tic pe ri ods (i.e. years for which the dendrochronological curves of the op po site ra dii drifted apart and re ac tion wood oc curred), that in di cate ini tial dis place ment dates, was used to il lus trate the move ment in ten - sity in in di vid ual years (Fig. 3A). The graph clearly marks the years for which many trees re sponded to ground move ment.
Less data are avail able for the pe riod prior to 1930, which is prob a bly the rea son for the small num ber of trees show ing a re - ac tion in the sum mary for those years. The old est sam pled tree with a re ac tion within an nual growth was dated to 1889. Fig ure 3A shows that the max i mum num ber of tree re ac tions, vis i ble as a char ac ter is tic dendrogram di ver gence from the op po site parts of the trunk, took place in 1981 (25 trees; 14% of those grow ing on the land slide) and 1997 (23 trees; 13%). A large num ber of trees re spon sive to ground move ment was also ob - served for 1971 (17 trees; 10%), 1977 (16 trees; 9%), 1961, 1973, 1986, 1988 (14 trees; 8%), 1951, 1965 (13 trees; 7%), 1953, 1993 (12 trees; 7%), 1989, 1998, and 2000 to 2002 (10 trees; 6%). Af ter 2000, the num ber of re spond ing trees is sig nif - i cantly smaller, but the land slide body is still not com pletely sta - ble (Fig. 3A, B). In 2010–2011, none of the sur veyed trees re - corded an ini ti a tion of the re ac tion (i.e. di ver gence of curves).
How ever, the an nual ring ec cen tric ity of nu mer ous trees could be ob served un til the fi nal anal y sis year (2011), which in di cates that these trees did not re turn to the equi lib rium state.
The vast ma jor ity of re ac tion wood zones in the an a lysed trees over lapped en tirely or partly with the de ter mined pe ri ods of dendrogram di ver gence. These dif fer ently de vel oped an nual growths oc curred most fre quently in the same years for which the dendrochronological curve di ver gence was iden ti fied or with one-year la tency, and, less of ten, two years later. These re sults dem on strate a def i nite de lay in the pro cess of de vel op ing re ac - tion wood within the an nual rings. This may be as so ci ated with the oc ca sion ally long pro cess of tree’s re turn to the up right po si - tion, from which it had been shifted due to ground move ment.
The re sults also in di cate that re ac tion wood is not al ways as so - ci ated with the ec cen tric ity of an nual growth.
The re ac tive trees (i.e. trees that ex hib ited deconcentric tree ring growth over a given time pe riod and/or the re ac tion wood) are sum ma rized on maps cre ated for ei ther de cades or in di vid ual years (Fig. 4). As a con se quence, it has been pos si -
ble to re con struct land slide ac tiv ity over the last 100-year pe - riod. How ever, this land slide, as shown by both the old est trees ex am ined in this study and data from the lit er a ture, was ac tive be fore this pe riod. From the 1920s to 1940s, the up per part (near the con cave scarp) and the mid dle part of the land slide were ac tive. Af ter 1951, move ment was re corded over al most the en tire area of the land slide body cov ered by the for est, from the scarp to its mid dle part. Move ment of the bed rock in sim i lar zones con tin ued un til the end of the cen tury. A clear de crease in the num ber of re ac tive trees in the up per and es pe cially in the head parts of the land slide is noted to have oc curred since 2000 when a de crease in land slide ac tiv ity took place. By com par ing the maps, it can be seen that the mid dle part of the land slide was the most ac tive seg ment from which mo tion was re corded over the en tire study pe riod. Gen er ally, the high est ac tiv ity was re corded in the 1980s and 1990s.
ECCENTRICITY INDEX EVALUATION The ec cen tric ity in milli metres, cal cu lated for co nif er ous trees grow ing on the land slide, ranged from –125.46 mm to 138.45 mm, and from –37.63 mm to 70.14 mm for the benchmarks. The ec cen tric ity in dex cal cu lated for the en tire tree trunk var ied from –114.29% to 113.06% for trees grow ing on the land slide, and from –16.88% to 33.34% for trees grow ing in a sta ble area.
Move ment in di ca tors were ob served in 33 co ni fers from the Sawicki Land slide based on the for merly cal cu lated thresh olds.
The old est sig nal of the mo tion oc curred in 1900. A to tal of 536 sig nals were dated; 265 (49.4%) were re lated to the upslope ec - cen tric ity and 271 (50.6%) to the downslope ec cen tric ity. In dex val ues for the bench mark trees grow ing on a sta ble sur face os - cil late around 0%, rarely ex ceed ing 100%. In dex val ues are high for trees grow ing on the land slide, which con firms that they grew on un sta ble ground. The max i mum ec cen tric ity in dex val - ues are 2588.9% (land slide) and 339% (benchmarks) for the upslope ec cen tric ity –2200%, and –463.6% for the downslope ec cen tric ity.
Vari abil ity of the num ber of dated sig nals re lated to grav i ta - tional mass move ments for in di vid ual years in the en tire pe riod of 1900–2011 was ana lysed (Fig. 3B and Ta ble 2). The great est num ber of ac tiv ity sig nals re corded in tree rings was seen in the years 1997, 1998 (14 spec i mens, about 42% of the ana lysed trees) and 1968 (13 spec i mens, about 39% of the ana lysed trees). A large num ber of sig nals was also ob served for 1975, 1977, 1978, 1979 and 1988 (12 spec i mens, 36% of the ana - lysed trees) and 1983, 1984, 1985 and 1989 (11 spec i mens,
~33% of trees).
DISCUSSION
Pe ri ods of grav i ta tional ground move ment, as de ter mined us ing dendrochronological meth ods, were cor re lated with cli - ma tic data, such as pre cip i ta tion re cords in the vi cin ity of the Sawicki land slide. The hu mid years were des ig nated based on pre cip i ta tion and tem per a ture data from 1968–2011 ob tained from the Pol ish Acad emy of Sci ence Me te o ro log i cal Field Sta - tion in Szymbark. The high est to tal pre cip i ta tion, ex ceed ing 1000 mm, was re corded in Szymbark five times dur ing that pe - riod, with the high est val ues in 1974 (1164 mm) and in 2010 (1137 mm). Only some of the years of strong tree re ac tion
could be di rectly re lated to high rain fall years or those fol low - ing high rain fall years in the vi cin ity of Szymbark. For in stance, 1981 was char ac ter ized by a strong tree re sponse that may have been con nected with ex treme to tal rain falls re corded in 1980, dur ing which the to tal veg e ta tion pe riod rain fall amoun - ted to 910.5 mm. Al though heavy rains caused flood ing in 1997 in south ern Po land, that year was only slightly more hu -
mid than av er age. How ever, a clear re sponse of trees from that year was noted, in di cat ing in ten si fi ca tion of grav i ta tional move ments of the ground. Our re sults dem on strate that the di - rect im pact of land slide ac tiv ity is re flected in trees al most im - me di ately in years of high to tal rain fall (e.g., 1973, 1997, 1998), or with a de lay of one year (e.g., 1971, 1981, 1986, 2011; Fig. 4 and Ta ble 2).
Fig. 4. Dis tri bu tion of trees on the land slide area, show ing the re sponse to the ground move ment
In other cases, slope ac tiv ity can be trig gered un der the in - flu ence of other fac tors, which are dif fi cult to in ter pret based on cli mate data. If fa vour able, these fac tors may lead to the re - newal of mass move ment pre cip i ta tion over a wider area, in and around the Beskid Niski and Beskid S¹decki Moun tains, as oc - curred in 1897, 1906, 1934, 1955, 1958, 1970, 1973, 1983, 1997, 1999 and 2002. Also in this case, ground move ment oc - curred only in some of the years, in 1973, 1997 (with a one-year de lay) and 1970 (move ment ep i sode in 1971), as reg is tered in an nual growths.
In the fol low ing “hu mid” years (dur ing which great pre cip i ta - tion was re corded) the ground grav i ta tional dis place ments were re flected in tree rings (Ta ble 2):
–1980 was re flected with a one-year de lay, very strongly in the anal y sis of pairs of dendrochronological curves, but less dis tinctly in the ec cen tric ity in dex method,
–1985 was re flected in the ec cen tric ity in dex eval u a tion, and with a one-year de lay in the anal y sis of pairs of dendro - chronological curves,
–1970 was re flected with a de lay of one year, very strongly in the anal y sis of the curves, less dis tinctly in the ec cen tric ity in dex method,
–1974 was the year of re cord to tal rain fall, al though it was re - flected with a one-year de lay only in the ec cen tric ity in dex method,
–2001 was not re flected in any of the meth ods used.
Nu mer i cal mod el ling of the slope of Mt. Maœlana Góra (R¹czkowski and Zabuski, 2008) dem on strated that even a slight in crease in rain fall could trig ger move ment in the up per part of the land slide, sub se quently in ten si fy ing and cov er ing wider parts of the land slide body. Our re sults seem to con firm that hy poth e sis – in hu mid and very hu mid years and in the im - me di ately fol low ing ones, as dis played by the an nual growths of trees grow ing on the en tire sur face of the land slide (e.g., 1981).
How ever, the strong re ac tion of the tree rings in “dry” years oc - cur ring af ter equally dry ones (e.g., 1977), or in years with low to tal an nual rain fall af ter the av er age ones, is puz zling.
The mass move ment ac tiv ity was usu ally as so ci ated with strong pre cip i ta tion (e.g., Dikau et al., 1996; Turner and Schuster, 1996; Starkel, 1997). Nev er the less, for ma tion of a land slide is as so ci ated with a dras tic, abrupt change of fac tors
that cause trans for ma tion of in ter nal me chan i cal prop er ties of rock mas sifs, such as, e.g., shear strength (Brunsden, 1979). Be sides the sud den in - crease in mois ture of rocks, these fac tors in clude dry ing of bed rock and low er ing of ground wa ter ta ble.
Abrupt dry ing can cause rapid shrink age of the rock me dium (mainly clays and silts, in which crack ing oc - curs due to dry ing; Brunsden, 1979). The most con - du cive to the for ma tion of mass move ments is the suc ces sive spread ing (ex pan sion) and con trac tion of the sub strate rocks as a re sult of changes in hu - mid ity such as in tense pre cip i ta tion, fol lowed by ex - tremely dry pe ri ods (Brunsden, 1979; Selby, 1993).
Ow ing to the com plex geo log i cal struc ture of the Sawicki Land slide bed rock (shales inter bedded by sand stones), de hy dra tion could have played an im - por tant role in the ini ti a tion of mass move ments. It is also pos si ble that land slide ac tiv ity in dry pe ri ods could be re lated to a low-mag ni tude earth quake (see Wistuba et al., 2018) in the Beskid Niski Mts. (Guterch and Lewandowska-Marciniak, 2002).
CONCLUSIONS
An in crease in the ac tiv ity of par tic u lar parts of the land - slide, reg is tered in the an nual in cre ments of trees in pe ri ods of high rain fall, among oth ers in 1934, 1970, 1972, 1980, 1997 and 2010 (of ten re corded in the an nual in cre ments one year later), seems un am big u ous. How ever, dendrochronological sig nals in di cate that the on set of ground move ments also oc - curs in dry years (e.g., in 1977) or low-rain fall years, which im - plies a sig nif i cant sus cep ti bil ity of this spe cific slope-val ley geosystem (Sawi cki Land slide trans forms the creek up-val ley to its sources) to slide move ments, even upon mi nor hy dro log - i cal changes. Among the ex ter nal fac tors cur rently dis cussed in the lit er a ture, ini ti a tion of the mass move ments could also be re lated to quick dry ing of the ground, re sult ing in changes in rock strength pa ram e ters. Due to the oc cur rence of land slides in hydrologically ac tive zones (spring ar eas, creek val ley with mi nor trib u tar ies), even a small amount of rain fall can re sult in disequilibration of the dy namic bal ance be tween land slide masses and the ground.
The dendrogeomorphological anal y sis, car ried out with three in de pend ent meth ods (ec cen tric ity of an nual growth rings, ec cen tric ity in di ces, and anal y sis of re ac tion wood), high lights their complementarity for the re con struc tion of pe ri ods of ground trans for ma tion re sult ing from mass move ments. Com - pa ra ble re sults ob tained us ing dendrogeomorphological anal y - sis car ried out for var i ous tree spe cies ex tend the pos si bil i ties of us ing dendrochronological meth ods for dat ing sec ond ary grav i - ta tional move ments.
Ac knowl edge ments. The study was car ried out as a part of stat u tory re search of the Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, AGH – Uni ver sity of Sci ence and Tech nol ogy, grant No. 16.16.140.315. Spe cial thanks are due to the Re view ers for sub stan tive com ments, which im proved the qual ity of this ar ti cle.
T a b l e 2 Com par i son of the years with the high est num bers of trees ex hib it ing
di ag nos tic changes iden ti fied by the anal y sis of pairs of dendrochronological curves of op po site ra dii
and by the ec cen tric ity in dex method
De cade Anal y sis of pairs of
dendrochronological curves Ec cen tric ity in dex
1950–1960 1951
1960–1970 1961, 1965 1968
1970–1980 1971, 1973, 1977 1975, 1977, 1978, 1979 1980–1990 1981, 1986, 1988, 1989 1983, 1984, 1985, 1988, 1989
1990–2000 1993, 1997,1998 1997
2000–2011 2000–2002
The years with the high est num ber of di ag nos tic ep i sodes for ground move ment are in bold
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