Large-scale permafrost degradation as a primary factor in Larix sibirica forest dieback in the Khentii massif,northern Mongolia

David Jurˇicˇka·Jitka Novotna´·Jakub Housˇka·Jana Parˇı´lkova´·Jan Hladky´·Va´clav Pecina·Hana Cihla´rˇova´·Marcela Burnog·Jakub Elbl·Zdena Rosicka´·Martin Brtnicky´·Jindrˇich Kynicky´

Abstract The objective of this study is to investigate the potential causes of widespread Larix sibirica Ledeb.mortality observed in the Khentii massif of northern Mongolia.The ratio of deadwood to living trees in affected stands in the Goricho region,the southernmost study site situated close to the Gobi Desert,was as high as 3.6:1.Moisture fluctuations monitored over 2 years using electrical impedance spectrometry revealed that the Goricho study site had higher soil moisture levels than the two less affected sites Barun Bayan and Dzun Bayan.High soil moisture was recorded in an area characterized by highly skeletal soils,ones with more than 35%by volume of rock fragments,and comparatively shallow soil horizons,from valley to mountains.The layer of permafrost influencing hydrogeological processes is much deeper in the Goricho region compared to the undisturbed study sites. Redundancy analysis confirmed a significant number of dead L.sibirica on sites with developed soils.Live forest stands,however damaged,grow in this region on well-drained scree slopes or on rocky bastions.The mass mortality observed for L.sibirica may be directly linked to accelerated permafrost thaw in the area bordered by the Tuul and the Terelj Rivers.Our assumption is that L.sibirica root system necrosis occurred as a result of long-term waterlogging of developed soils with high spatial heterogeneity,normally able to absorb high quantities of groundwater.The areas unaffected were scree fields and rocky bastions characterized by adequate drainage.All of our findings support the primary stages of large-scale permafrost thaw,i.e.,correlating increases in soil moisture with increasing permafrost active layer thickness.

Keywords Larix sibirica·Mortality·Permafrost thawing·Waterlogging·Mongolia

Introduction

Mongolia is located in Central Asia between 41°35′N and 52°09′N latitude and 87°44′E and 119°56′E longitude and covers an area of 1.56×106km2.Permafrost is found in 67%of the territory(Tumurbaatar and Mijiddorj 2006)and primarily in the north and northwest regions of Khentii,Arkhangai,Kho¨vsgo¨l aimags and in the Altai Mountains(Gravis 1974). Relict permafrost structures (ice-wedge casts and cryoturbation structures)have been found in the Gobi Desert of southern Mongolia(Owen et al.1998).The border of continuous permafrost is in the Khentii and Kho¨vsgo¨l aimags with a lower limit between 1400 and 2000 m a.s.l.The lowest limit of sporadic permafrost is between 600 and 700 m a.s.l.,and the average thickness in the continuous and discontinuous zones is 100-250 m in the mountains where mean annual temperatures fluctuate between-1 and-3°C.In valleys and depressions,the average thickness is 50-100 m with average annual temperatures from-1 to-2°C.The active layer of permafrost consists of 4-6 m of coarse material and 1-3 m of fine-grained soils.Evidence of ongoing degradation of warmer permafrost in Mongolia includes thermokast development created by thawing and an increase of the active thickness layer(Sharkhuu et al.2007;Bohannon 2008;Kynicky´et al.2009).

Forest cover in Mongolia is roughly 11.9%of the total forest cover in the country or 18,592 thousand ha(FAO 2015),and is concentrated in the north between 47-52°N and 89-116°E. These stands are dominated by Larix sibirica(60%),Pinus sibirica Du Tour,Picea obovata Ledeb.,Abies sibirica Ledeb.,Betula spp.and Populus spp.Most of these northern forests grow on permafrost.In the southern parts(42-45°N and 91-109°E),saxaul forests(salt tolerant,drought resistant species)are formed mainly by Haloxylon ammodendron C.A.Mey.Bunge or black saxaul(Batkhuu et al.2011).

The structure and presence of forest cover have a significant impact on the permafrost and its active layer characteristics and vice versa.Late successional stages of forest stands are invariably linked to the steady cyclic thawing and freezing mode of active permafrost layer(Tutubalina and Rees 2001;Bohannon 2008;Genxu et al.2009,2012;Kokelj et al.2010).Damage to ground vegetation can provoke serious changes in soil hydrologic processes at regional scales(Walker et al.2003).Intensive permafrost thawing leads to ground surface subsidence and ultimately to waterlogging of forests growing on permafrost soils(Drury 1956;Woo 1992;Vitt et al.1994;Osterkamp et al.2000;Jorgenson et al.2001;Crawford 2008;Genxu et al.2012;Runyan and D'Odorico 2012).

Long-term waterlogging from surplus water from thawing permafrost reduces root zone aeration necessary for growth(Osterkamp et al.2000;Jorgenson et al.2001).Water logged soils create anaerobic conditions which limit oxygen supply to root systems and cause trees to die by suffocation,particularly if this happens during the growing season(Wilde et al.1953;Coutts and Phylipson 1977;Davison and Tay 1985;Iowa State University 2008).For species intolerant to waterlogging (Sinclair and Lyon 1987),1 month of oversaturation of the soil can be enough to cause root system death(Coutts and Phylipson 1977).The vast continuous forests in Mongolia are largely populated by species unable to tolerate intensive waterlogging for extended periods of time(Lieffers and Rothwell 1986;Sinclair and Lyon 1987).

Vegetation in Mongolian forests has undergone dramatic changes as a result of climate changes over the past 25 years and, at regional and local levels, by human activities.Climatic change in Mongolia is most clearly reflected in the increased frequency of dust storms(Natsagdorj et al.2003)and the gradual increase in average annual temperatures by 2.1°C over the last 70 years(Oyuntuya et al.2015).It has been projected that,by 2030-2050,average global temperatures will increase by 1-2°C(Lioubimtseva et al.2005)which will induce precipitation pattern changes(Marin 2010),notably a decrease in annual rainfall(Khishigjargal et al.2014).Unfortunately,an additional scenario has been predicted for the Khentii region which is a projected decrease in the total volume of precipitation(Sato et al.2007).As noted by Smith and Riseborough(2002),climate change has a direct impact on permafrost temperatures and active-layer thickness,and may eventually lead to its complete elimination.There is evidence of permafrost degradation in other areas of the world,(Arctic regions,Eastern Siberia,Alaska),which has resulted in higher atmospheric concentrations of greenhouse gases and reduced carbon sequestration(Wei et al.2016).The permafrost degradation process,however,can be very different at different altitudes.High-altitude permafrost degradation is more pronounced than in lowland areas because these regions experience more rapid changes in temperature(Oliva and Fritz 2018).

Recent human activities have led to unprecedented levels of deforestation,forest fires and intensive overgrazing of both forests(Tsogtbaatar 2004)and grasslands(Maasri and Gelhaus 2011).According to Batkhuu et al.(2011),as many as four million hectares of Mongolian forests have been harvested in recent decades.The most dramatic forest degradation resulted from harvesting between 1990 and 2000 when the rate of deforestation increased to 60,000 ha a-1(Batkhuu et al.2011),while deforestation by illegal logging accounted for about 80%(Ykhanbai 2010).Weakened forests are more prone to invasion by non-native forest pests such as the gypsy moth(Lymantria dispar L.),which can potentially cause further significant damage to live stands (Hauck et al. 2008;Dulamsuren et al.2010).

In this study,new observations are reported of severe degradation of L.sibirica stands in the taiga forests of the Khentii Mountains in northern Mongolia.Based on our results,we propose an outline of what is needed to sustainably develop this degraded habitat and suggest possible solutions to this critical situation.

The hypothesis underlying this study,previously unexplored in Mongolia,is that large-scale permafrost degradation in the Goricho area,situated between the Tuul and Terelj rivers where capillary water action is most prevalent,is actively contributing to the necrosis of Siberian larch root systems as a result of long-term waterlogging.We evaluate this hypothesis by comparing the affected forested area in Goricho with two relatively undamaged reference sites,Barun Bayan and Dzun Bayan,characterized by low anthropogenic influences and stable forest conditions.More than 300 soil moisture measurements were collected from 30 soil pits and a large number of shallow testing soil pits.

Materials and methods

Initial calibration,dendrological mapping and soil moisture measurements were conducted in July and August,2015.The more targeted research of forest mortality was carried out July 2016.

Study area

Research focused on locations Goricho,Barun Bayan and Dzun Bayan,all situated in the Khentii massif(Fig.1).This mountain range belongs to the biogeographic Holarctic realm,the Circumboreal Region and the Transbaikal province,forming the southernmost part;Khentii is one of 21 aimags(provinces)of Mongolia.The mountains of Khentii are unique in that they are predominately populated by an unchanged tree species composition,and are ecologically distinct from the rest of Mongolia.Located in northern Mongolia,the Khentii massif represents a transition zone between the Siberian taiga forests and grass steppes(Mu¨hlenberg et al.2012).

The Goricho research site is the southernmost of the sites and closest to the Gobi desert.It lies approximately 30 km east of the capital city of Ulaanbaatar.Forests in Goricho are characterized by closed stands covering entire slopes and frequently populating lower-order small valleys(i.e.,where precipitation collects to valleys of higher order at lower altitudes).The Goricho forests represent a typical example of habitats that have been severely damaged by intensive steppe and forest grazing(heavy to very heavy grazing pressure was reported by Reimoser et al.(1999),intensive timber harvesting to large-scale clearcutting,and forest fires.Pasturing in Goricho takes place throughout the year.

Fig.1 Study site(ArcGIS 10.2.)

The Barun Bayan area is situated in the valley of the Barun Bayan river(Right Bayan)and includes the surrounding mountains.Forests cover large areas mainly on the hillsides of a wide valley.This site is situated 43 km north-northeast of Goricho and approximately 58 km northeast of Ulaanbaatar.Barun Bayan ecosystems are generally healthier than those in Goricho.This is mainly due to the remoteness of the site which is primarily used by herdsman as a wintering place for cattle.Therefore,the forests and steppes have only been minimally affected by light to zero grazing pressure(Reimoser et al.1999).Timber harvesting is almost exclusively to meet local needs for firewood and materials for wintering buildings sparsely scattered across the landscape.

The northernmost site of Dzun Bayan is located in the Dzun Bayan river valleyto the west of Bayan and includes the surrounding mountains approximately 10 km east of Barun Bayan and 68 km northeast of Ulaanbaatar.Forest stands exhibit similar distribution as those of Barun Bayan.The location has the same grazing regime as Barun Bayan,and shows no signs of anthropogenic disturbance.It can be characterized as a healthy intact habitat.

Forest characteristics

According to our findings,stands in Goricho are composed mainly of L.sibirica(stand basal area SBA 12.1 m3ha-1)and Betula platyphylla Sukaczev(SBA 3.5 m3ha-1).Birch commonly occurs on lower sites below 1540 m a.s.l.that have been waterlogged,while larch is dominant in areas up to 1915 m a.s.l.The Goricho site characteristics correspond to the description provided by Tsedendash(1995)and forms a transition between a steppe and mountain foreststeppe.James(2011)reported that these are very sensitive ecosystems that offer protection for north occurrence of vegetation,effectively mitigating steppe expansion and land aridization.

Forest stands in Barun Bayan at 1626-1923 m a.s.l.consist of L.sibirica(SBA 32.8 m3ha-1),B.platyphylla(SBA 1.98 m3ha-1),and Pinus sibirica Du Tour(SBA 1.3 m3ha-1).L.sibirica was the dominant species within the entire range. P. sibirica occurs at altitudes of 1827-1893 m a.s.l.Birch grows in the narrow range of 1756-1923 m a.s.l.

The Dzun Bayan forest at 1616-2159 m a.s.l.is composed of L.sibirica(SBA 37.6 m3ha-1),B.platyphylla(SBA 1.3 m3ha-1),P.sibirica(SBA 1.2 m3ha-1)and Populus tremula L.(SBA 0.7 m3ha-1).P.sibirica and L.sibirica extended outward up to 2160 m a.s.l.B.platyphylla occurred at 1703-1961 m a.s.l.Continuous growth of P.tremula was found at altitudes of 1840 m a.s.l.,but only on one site,likely profitting from competition-free ecological niches which allowed the species to replace previously uprooted L.sibirica.

Considering the landscape in a wider context,both the Barun Bayan and Dzun Bayan sites can be characterized as previously reported by Hilbig and Knapp(1983),Ermakov et al.(2002)and Mu¨hlenberg et al.(2012)as transitional zones between light taiga(B.platyphylla and related species,L.sibirica and Pinus sylvestris L.)and dark taiga(Picea obovata Ledeb.,Abies sibirica Ledeb.，P.sibirica and L.sibirica).

Geological-pedological characteristics of surveyed areas

All locations have coarse-grained alkali Mesozoic granite belonging to the Khentii synclinorium predominated by weakly metamorphosed and strongly silicified fine-grained terrigenous sediments.Large bodies of miarolitic pegmatite have been observed in all three sites(Antipin et al.1976).

Thirty soil pits were dug in the survey areas of Goricho,Barun Bayan and Dzun Bayan.Cambisols,dystric cambisols and rankers or soils developed over non-calcareous material,evolved on granitic rocks with a horizon depth of up to 40 cm.These horizons were moderate(20-50%)to strongly skeletal(＞50%of rock fragments).Subsequently,they transferred into a heavily skeletal B horizon or‘‘subsoil''.The C horizon extended to a depth of 3 m to bedrock.

Permafrost is present in all three locations and forms the impermeable bedrock as a hydrogeological insultor in the fissure environment of coarse-grained granites and soil profiles.In Barun Bayan and Dzun Bayan,permafrost was identified at a depth of 0.2-1 m below ground.Exploration of the previous pegmatite mining tunnels in Goricho revealed that permafrost occurred at depths ≥4 m below the surface(Fig.2).

Climate data

The study area is characterized by considerable temperature fluctuations between day and night.The minimum average annual temperatures in the mountainous region of northern Mongolia(Altai,Khangai and especially Khentii)are known to drop below-50°C(Batkhuu et al.2011).This northern region is characterized by particularly low average annual rainfalls less than 400 mm.Most rainfall typically occurs in the summer months between June and August (Marin 2010) and later between August and September(Gradel et al.2017).

Design of measurements

Twenty plots 10×10 m were established on eac h locations(Kooijman et al.2000;Bellingham et al.2016)using a subjective approach to ensure that each site represented a cross-section of forest ecosystems and natural topographical conditions with particular focus on exposure and position on the slopes.We conducted dendrometric measurements, forest regeneration evaluation, deadwood mapping,soil sampling,and soil moisture measurements using a Z-meter III.No measurements were made during or after the rainy season.Measurements before the rainy season ensure that the origin of soil moisture is only from underground water.This minimizes possible influence of precipitation.

Dendrological-dendrometric mapping

Dendrometric measurements are important for determining the structure of the forest stands and the health status of individual trees on a given site.Effectively,this allowed the division of the stands into two distinct categories:prosperous,and declining.All measured parameters,in the context of other factors,indicated deterioration of specific natural conditions.The number of trees on each site were examined:tree attributes(TA),combining measurements of height(m),crown projection area(m)and diameter at breast height(cm);qualitative parameters(QP),including physiological vitality or degree of viability ranging from 1 to 5, total biomechanical vitality which expresses the potential degree of reduced or threatened viability caused by mechanical failure of an individual,ranging from 1 to 5,dendrological potential of research areas-assessing the degree of stability and further perspectives of individual specimens,ranging from 1 to 3,and tree damage parameters(TDP):degree of trunk injury(1-3),crown damage(1-3),presence of decaying fungi,rot or cavities(1-3),irregular branching(1-3),misaligned center of gravity and geometric structure(1-3),overly dry parts of the crown(1-3),symptoms of root disturbance(1-3),and additional damage(1-3).The lower the value,the better the tree condition(Pejchal and Sˇimek 2012,2015).In total,438 individual trees were measured in Goricho,Barun Bayan and Dzun Bayan.The qualitative parameters were determined using a confirmed estimation method based on Pejchal and Sˇimek(2012,2015).Total tree heights were measured using an altimeter Sylva CM-1015-2025 and a Nikon Laser Forestry Pro Rangefinder.

Dead individual trees were ranked into two categories separating standing deadwood with trunk diameters from10 to 50 cm and ＞50 cm, and fallen deadwood with diameters between 10-50 cm and ＞50 cm measured at breast height.

Fig.2 Typical soil profiles observed in Goricho(left)and Dzun Bayan(right)

Basal area was calculated from diameters using 0.0007854×diameter at breast height2(He´dl et al.2009).The SBA was calculated by summing up BA from all sites within a specific area and converting this to the area of one hectare.

EIS measurement

In the context of studying soil-fluid systems,electrical impedance spectrometry(EIS)is a reliable method for collecting error-free kinetic and mechanistic data using a variety of techniques and output formats.It is recognized worldwide for providing highly accurate soil moisture measurements(Kaya and Fang 1997;Yuan et al.2010).

For this reason,EIS has increasingly become the tool of choice for studying soil environments with different degrees of water saturation.It is the principle in measuring electrical impedance,and the basic property characterizing electrical alternating-current circuits, which are always higher than or equal to the real electrical resistance(R)in the circuit. Apparent resistances, i.e., inductance-the reactance XL of an inductor and capacitance,the reactance XC of a capacitor,form the variable and consequently the frequency-dependent element of electrical impedance.Electrical impedance,therefore,is composed of both real and theoretical parts.The real part is formed by the resistance R,which is frequency-independent.The water content in the soil significantly affects its conductivity.Lower resistance indicates higher humidity levels at a given site.The theoretical part is formed by the reactance X,which is frequency-dependent. Electrical impedance Z can be expressed using the Ohm relation for alternating-current circuits,i.e.,the relation between the phasor of electrical voltage U and the phasor of electrical current I(Parˇı´lkova´and Radkovsky´2011;Callegaro 2012).

Measurements on plots

On each of the 60 experimental sites,following dendrological mapping,five soil moisture measurements using a Z-meter III were carried out,after three repetitions with one measurement at a frequency of 4000 Hz.Fifty verification measurements were done in 2015 and 300 in 2016.All measurements were before the rainy season(June/July)when the climatic conditions were similar in all locations,i.e.,unaffected by rain.Due to the high skeletality of local soils and thus a high risk of error,vertical measurements were taken at depths up to 10-20 cm in the A horizon using a paired probe. Similar physical soil properties were observed in the A horizon, particularly rock fragment content, in all locations. In specific hydro-geological pedological locations of interest,(highly skeletal B and C horizons with factors of fissure permeability of granites render permafrost an impermeable bedrock),at the time of measuring,the water from thawing permafrost is the only source of groundwater and the EIS method for measuring the A horizon is a reliable way to determine the degree to which a site has been waterlogged,and to measure the resulting increase in groundwater supply.

Data analyses

Basic raw data processing was performed using Excel®program.The Kruskal-Wallis test,a statistical method for data with abnormal distribution,was used to determine any significant differences between the data from EIS measurments taken in the three study areas.

An ordinal redundancy analysis(RDA)was performed for testing environmental effects-soil moisture, soil character and other natural conditions of forest stands,for dead L.sibirica trees,qualitative parameters of live trees and of damaged trees.The data was centered and standardized at 500 permutations before performing a Monte Carlo permutation test.

Results

The results further confirm the poor condition of forest stands at Goricho with the highest occurrence of mortality and the lowest number of live trees(Fig.3).The number of standing deadwood far exceeded the number of fallen trees(ratio 4.8:1),particularly with trees 10-50 cm diameter with corresponding heights between 18 and 21 m.The ratio between standing and fallen dead trees can be important for determining the intensity of destructive external factors,e.g.,sudden versus slow.

Forest health and future survival prospects,the potential prosperity of individuals on a given site,can be derived from the qualitative dendrological parameters.L.sibirica and B.platyphylla had the least favorable parameters of all parameters measured in the Goricho plots(Fig.4).These individuals are characterized by reduced vitality, with medium to rarely long-term survival projection(Pejchal and Sˇimek 2012,2015).

Our results confirm the stable conditions of forests in both the Barun Bayan and Dzun Bayan locations.Live trees largely predominated over dead ones(4.2:1;Fig.3).Dead P.tremula individuals were found in limited numbers.Trees on both sites were undamaged or only slightly damaged and showed signs of maintaining their current health(Fig.5).

The lowest values of electrical impedance measures expressing the highest soil moisture levels were observed at Goricho(Fig.5),i.e.,the location with the highest mortality of L.sibirica and the most unfavorable overall health of L.sibirica and B.platyphylla.A statistically significant difference was confirmed when data between Goricho and Barun Bayan,and between Dzun Bayan and Barun Bayan were compared (Kruskal-Wallis test,p=0.000).Comparative analysis between the Goricho and Dzun Bayan sites showed no statistically significant difference(p=0.46).

Fig.4 Qualitative parameters of species(mean from all sites in the locations)at the Goricho(GOR),Barun Bayan(BB)and Dzun Bayan(DZ)locations

Fig.5 Values of electrical resistance ρ[Ω m]at the Goricho(GOR),Barun Bayan(BB)and Dzun Bayan(DZ)locations

Fig.3 Total number of live trees and deadwood at the Goricho(GOR),Barun Bayan(BB)and Dzun Bayan(DZ)locations

Fig.6 RDA ordination diagram for L.sibirica at Goricho

An additional statistical assessment,focused specifically on the Goricho location,revealed the highest moisture levels and mortality rate of L.sibirica.The RDA ordination diagram(Fig.6)for larch shows that data comprise two ecological gradients (blue arrows in the graph),differentiating between(1)the parameters of health condition/wood vitality;and,(2)the number of dead trees on the site.Acute mortality of L.sibirica，particularly numbers of dead standing trees,is related to the subgrade(rock,soil;Fig.6,-green point).‘‘Rock''and‘‘soil''represent locations with different hydric conditions.‘‘Rock''indicates excellent drainage in areas of scree fields;‘‘soil''represents substantially poorly drained soil types (Fig.2). The occurrence of dead trees corresponds almost exclusively to areas with soils,while scree field areas and rocky bastions show only a limited tree mortality(Figs.7,8).Further gradients are the position on the slope,exposure,and moisture,which are closely related to overall health and vitality,of live trees.Below the median value of 7933.81[Ω m](EIS measurements),i.e.,on half of the wetter areas from the total number of sites,there is only 30%from the total number of living trees,and 78%of dead individuals from the total number of dead trees.Moisture proved to be a determining factor with universal negative impacts on L.sibirica in Goricho;sites with soils characterized by poor drainage experienced total forest destruction.Trees growing on scree fields,which typically are well-drained,continue to survive despite their worse health condition.

Discussion

Degradation of Khentii massif forests

This study in Goricho revealed high mortality of L.sibirica,which is directly linked to high soil moisture levels(Fig.9).Our assumption is that this phenomenon originates from long-term degradation of permafrost in the Tuul and Terelj river basins(Fig.8).The findings specific to Goricho suggest that a regional groundwater drainage mechanism exists which was previously bound in the permafrost(Fig.9).

Several authors have reported that thawing permafrost,an increased thickness of the active layer over permafrost,can cause drastic changes in hydrological conditions and transform the original forest ecosystem to a swamp(Drury 1956;Woo 1992;Vitt et al.1994;Osterkamp et al.2000;Jorgenson et al.2001;Crawford 2008;Genxu et al.2012;Runyan and D'Odorico 2012).In the example of L.sibirica，Crawford(2008)reported that a decline in larch forests growing on permafrost may lead to the transformation of habitats into swamp or wetland communities.

Water from thawing permafrost as a cause of high moisture levels in upper soil layers has been shown by high soil moisture contents in Goricho despite its unfavorable hydrological properties, i.e., shallowness with a weak humus horizon,and considerably high skeletalization.This does not suggest the presence of capillary water but rather gravitational groundwater(Brahy et al.2000).The natural drainage of groundwater from infiltrating local rainfall may be discounted,as waterlogging was detected both in the lower parts of valleys and at higher altitudes.You et al.(2017)have also reported groundwater from the increasing active layer of thawing permafrost as the main driver of increased moisture in soil structures.

Fig.7 Dead trees L.sibirica(left)and a live forest growing on a scree field(right)in Goricho

Fig.8 Area affected by acute permafrost thaw(left)and details of distribution of dieback and live forests in the surrounding landscape(right)

Fig.9 Primary and secondary phases of permafrost thaw occurring at the Goricho location

It is assumed that constantly high soil moisture over consecutive years in Goricho were a primary factor in the mass mortality of larch forests,which are better adapted to less rainwater and lower ground water levels.Given that the permafrost provided a degree of hydrogeological insulation in Goricho stands at a depth of ≥4 m,it may therefore be presumed that the saturated zone above the active layer of non-degraded permafrost,before the influx of higher volume of groundwater from the thawing permafrost,was originally much deeper than it is currently.Interestingly,there were heavily water-saturated upper soil layers which would otherwise be comparatively dry.The undamaged Barun Bayan and Dzun Bayan sites showed permafrost at depths from 0.2 to 1.0 m.

Siberian larch is a species that can tolerate short-term waterlogging(Sinclair and Lyon 1987).However,prolonged waterlogging can cause extensive necrosis in these root systems(Wilde et al.1953;Davison and Tay 1985;Sinclair and Lyon 1987;Osterkamp et al.2000;Jorgenson et al.2001;Iowa State University 2008).Abaimov(2010)stated that L.sibirica thrives in well-drained sandy to rocky soils,and experiences stress when subjected to long periods of high water saturation,which was observed in Goricho.Decline in health and subsequent mortality due to high soil moisture contents originate not only from increased annual rainfall;the roots adapt to seasonal fluctuations in the volume of rainwater supply(Chenlemuge et al.2015).The occurrence of mass mortality and the pronounced degree of stand damage is instead indicative of a long-term process wherein hydrogeological conditions have changed,and were followed by consistent flows of large volumes of groundwater.The character of dead larch forests points to the time and common factor of disturbance,as the ratio of standing and fallen deadwood is 4.8:1,favoring standing deadwood. It can be deduced that forest degradation occurred uniformly across the affected area at approximately the same time. The presence of a high water table relates to the significant representation of birch as late successional species(Otoda et al.2013).Birch colonizes a wide variety of habitats but is sensitive to dry environments(Puhua 2014).This is not the case in Goricho where the primary stage of thawing permafrost has resulted in an acute increase in groundwater.Surviving stands,despite declining health of L.sibirica on the Goricho site,were most successful growing n scree fields,and to a lesser extent in ills and steep,rocky outcrops(Figs.7,8).Scree fields facilitate excellent drainage and are rarely affected by increasing groundwater.Lloyd et al.(2003)found that the negative effect on forests of permafrost thaw and subsequent waterlogging was not related to well-drained upland soils. Dead L. sibirica stands occurred almost exclusively in areas characterized by poor drainage but where water retention is relatively strong.These soils are also composed of deep layers of hyperogeneous granite which,as reported by Jones and Graham(1993),Sternberg et al.(1995)and Witty et al.(2003),absorbs and retains a significant amount of groundwater.

Regarding regional hydrogeological characteristics,Goricho is located among hydrogeological structures formed by the Khentii massif which are naturally conducive to drainage.The extent of permafrost degradation on a regional scale,i.e.,drainage from large hydrogeological structures,is evidenced by the spatial distribution of damaged trees(Fig.8).

Sulphur air pollution,originating from Ulaanbaatar and Nalaikh(heating plants,power plants,transportation)on forest degradation can be excluded,considering the mortality distribution of L.sibirica in the landscape.Mortality is strictly linked to forests bordered by hydrogeological barriers(Tuul and Terelj rivers),and in this specific area,by scree fields.

A scenario that may be compared to Goricho can be found in North America.It corresponds to the primary stage of permafrost thawing,i.e.,waterlogging.Jorgensosn et al.(2001)reported that in the Tanana Flats within the Tanana River valley lowlands in central Alaska,a partial modification of the original birch forest ecosystem occurred over a span of more than 45 years,notably in a change of direction toward groundwater-driven wetlands.The total forest area decreased by 35%and the fen area increased by 29%; the new environment was rapidly colonized by aquatic herbaceous plants.Further,Oliva and Fritz(2018)stated that these changes took place at different speeds,depending on altitude.Due to climate change,higher altitude environments such as the Artic,East Siberia,Alaskan mountains,experience accelerated increases in average annual temperatures.Osterkamp et al.(2000)reported that in the Mentasta Pass area of central Alaska,permafrost thawing resulted in the gradual change of the forest ecosystem into wet sedge meadows,thermokarst marshy ponds and lakes.A study by Payette et al.(2004)showed that subarctic peatland areas on the eastern coast of Hudson Bay,northern Que´bec,Canada have experienced significant loss of permafrost,by 5.3%a-1from 1993 to 2003.This has resulted in the gradual change of peatland,colonized Picea mariana，to pond conditions.Baltzer et al.(2014)describe their findings in the Northwest Territories of Canada of vast permafrost thaw which has waterlogged surrounding soil environments,resulting in forest fragmentation and has led to tree mortality.

Permafrost degradation in conditions that closely resemble the areas of Mongolia in this study can be found in southern Transbaikalia,Russia.This is shown in Fig.8 on the western macroslope of the Ikatskii Ridge of the Stanovoe Range,described by Anenkhonov and Krivobokov(2006).The area of Sokhondo Mountain,southeast of Lake Baikal, Russia, is similarly described by Kozyr(2014).Both studies note that,due to permafrost thaw,soil moisture levels have increased in lower sections of the slope.However,increased soil moisture was not recorded in the upper sections.

Perspectives

We can assume that further major hydrological changes will arise in Goricho,given that large-scale permafrost thaw is ongoing.An increased thickness of the active layer is indicative of the continuing initial phase of permafrost thawing. Following the subtracting static groundwater levels from thawing permafrost and subsequent deepening of underlying hydrogeological insulation provided by the permafrost,we can expect a significant drop to occur in groundwater levels. Declining groundwater levels will strongly affect the upper part of the Goricho valley which will render groundwater far less available to trees in this area (Fig.9). Consequently, ecosystems will become increasingly dependent on natural rainfall. Given that precipitation levels are already relatively low and appear to be declining(Sato et al.2007;Oyuntuya et al.2015),this scenario will inevitably result in heavy drying of the surrounding environment.

Water table fluctuation at all stages of permafrost thaw causes strong stress to trees and significantly weakens surviving individuals.These weakened trees are then susceptible to pathogen infections and to attacks by non-native insects(e.g.,Lymantria dispar),which often results in severe defoliation,reduced growth,dieback and sometimes death.

Transformation of such severely degraded forest-steppe habitats into steppes is irreversible if effective measures are not applied soon.One way to encourage or facilitate the growth of larch outside the affected habitats may be to use of the example of successful growth of L.sibirica on dumps after small scale pegmatite mining located away from the closed forest growing on the study sites.These self-watering,cumulus shape structures,working on the principle of condensing of air humidity,are guarantee an optimal water supply where larch in particular would thrive.This would make it possible to create viable closed stands,in contrast to the environment of arid steppes,meadows,and waterlogged forests(Jurˇicˇka et al.2016).Such measures,however,would be ineffective without implementing strict forest grazing regulations.

Conclusions

In addition to observations of mass mortality of L.sibirica,the lowest and most revealing qualitative parameters,namely physiological vitality,total biomechanical vitality,dendrological potential of research areas,used to determine the survival prospects of trees were found in stands in the southernmost and most disturbed Goricho study site located in the Khentii massif between steppes and mountain steppes.The other two locations,Barun Bayan and Bayan Dzun,situated further north between mountain steppes and light taiga,were only minimally disturbed and showed good values of all measured parameters.

The highest soil moisture levels,significantly higher than values recorded in the undamaged areas,were found in Goricho.High soil moisture was found at both lower and higher altitudes,and was statistically confirmed to negatively affect quality parameters and parameters of tree damage.It was detected in Goricho despite the site's unfavorable parameters,shallowness and high degree of soil skeletality.Likewise,the hydrogeological insulator,the source of groundwater formed by permafrost,is in Goricho at ≥4 m below ground and deeper;for Dzun Bayan and Barun Bayan,this occurs from 0.2 m and deeper.Live closed L.sibirica stands in poor health growing in Goricho exhibited higher survival capacity only when growing in well-drained scree fields;mortality rates correlated directly to areas characterized by poorly drained soils.

In the study locations,a major source of water present in the soil is thawing permafrost which leads us to conclude that intensive thawing dramatically increases soil moisture at regional levels.We believe that root system necrosis in L.sibirica occurs as a direct result of prolonged waterlogging over consecutive years.

AcknowledgementsWe wish to thank the reviewers and editors for their insightful comments that led to improvements of the manuscript.We are grateful for the support provided by the project Development of forests and the gene pool of local forest tree ecotypes in Mongolia 2015-2017(a part of the international cooperative development effort of the Czech Republic).We also thank students Michal Vojtek,Michael Bobula,Krysˇtof Klimpar,Miroslav Trneˇny´,Petr Hledı´k,Luka´sˇVa´gner,and Diana Sychova´for help in gathering data in the Mongolia territory..