**5. Discussion**

The karst features are subsoil karren (these are grike and rootkarren on sandstone pseudokarren), subsidence dolines or sinkholes (mainly suffosion dolines), drawdown dolines, collapse dolines, caprock dolines, ponors with blind valleys, cave-ins, spring caves reflecting a thermal effect of various degrees, shafts, caves with wreathing, pseudokarst caves, freshwater limestones (in brook channels) and spring cones.

Among karst features, subsidence dolines are the most widespread. They occur in patches (altogether, in 22 patches, out of which, 19 patches are in the Northern Bakony) and in small numbers (there are about 700 subsidence dolines). Their size and density is small and a lot of them are inactive filled dolines. They occur at sites where the horst is covered with permeable sediment and where the caprock thinned out or was thin in the first place. These sites are the floors of creeks, terrains between exposed mounds, the covered mounds of the bedrock and limestone sites veneered with loess at which a larger quantity of water arrives from the surrounding impermeable rock or from the impermeable and wedging caprock [26] (Figure 7). On its soil-covered karst, some drawdown dolines and collapse dolines also occur (Tapolca Karst and the environs of Devecser). On the mixed allogenic–autogenic karst, ponors with blind valleys occur at the termination of the basalt cap (Figure 8). Caprock dolines also developed on the basalt by the collapse of the rock [32].

**Figure 7.** Development of subsidence dolines [33]. Legend: Development of subsidence dolines at various development environments: (**a**) above the mound of the bedrock at low accumulation cover thickness, (**b**,**c**) at cover that thinned out in an erosional way, (**d**) at the termination of covered impermeable cover, (**e**) at the margin of covered, intercalated non-karstic rock, 1. limestone, 2. nonkarstic rock, 3. permeable cover, 4. impermeable cover, 5. reworked, partly impermeable cover, 6. fault, 7. water flow, 8. infiltration, 9. subsidence doline, 10. shaft.

**Figure 8.** Ponor of Macskalik cave (Kab Mountain).

Shaft caves are common and of relatively great dimension in the mountains (102 shaft caves) (Figure 9). Among them, there occur caves with significant length, for example, the Alba-Regia Cave which is on the Tés Plateau is 3.6 km long. Shafts open out of subsidence dolines and are of dissolution origin.

**Figure 9.** Öregköves ponor shaft cave [34].

Among their caves, cave-ins are widespread (about 225 caves), which mostly occur in epigenetic-antecedent valley sides (about 212 caves); there are few spring caves (about 10 caves), and they were affected by thermal effects of various degrees. They are situated at the hypogene branch of the regional flow. The pseudokarren of the sandstone are also related to a former hypogene branch (Szentbékálla, Figure 10). Here, the silica that precipitated from warm water solution cemented Pannonian sand [35]. Amorphous silica that developed by precipitation is dissolved much more intensively than crystallised

silica [9,36,37]. Karren (mainly kamenitzas) resulted from the dissolution of amorphous silica [32].

**Figure 10.** Karren near Szentbékkálla. Legend: 1. sandstone polygon, 2. kamenitza, 3. rinnenkarren.

Pseudokarst caves are most widespread on basaltic terrains. From the ascending waters of the early hypogene branch, spring cones were built on the Tihany Peninsula (Figure 10) during the precipitation of dissolved limestone and from the diatom skeletons of lakes [38]. Materials of spring cones grew round and round space and thus, cavities developed (Figure 10).

The karst and karstification of the individual horst types are different, which can be traced back to the variations in geology, elevation, expansion, coveredness and hydrology of the horsts.

At the surface of horsts in the summit position (for example K˝oris Mountain, Som Mountain), there are no local flows due to the distribution of Triassic Main Dolomite without impermeable intercalations, but descending branches of regional flows are present. Surface karstification is of low intensity; disregarding karren formation (on Dachstein limestones), it is only present at sites where sediment patches occur on the surface of such horsts.

The complete lack of surface karst features (drawdown dolines) can be traced back to the fact that in their area, the presence of low-inclined terrains is insignificant or they are completely absent. On steep terrains, no drawdown dolines develop. Based on literary data, according to Zámbó [39], at a surface with an inclination larger than 20◦, no solution dolines occur. According to other data, doline development is the most intensive on surfaces with an inclination of 2–7◦ and there are no dolines on surfaces steeper than 13◦ in the Mecsek Mountains [40]. Telbisz et al. [41] state that in the Serbian Miroˇc Mountains, only 23% of surfaces with an inclination of 12◦ have dolines. All this can be traced back to the fact that on steep surfaces, the ratio of surface runoff increases and infiltration decreases. To our knowledge, there is only one subsidence doline group on this horst type (the Eleven-Förtés doline group on K˝oris Mountain) where dolines developed on a terrain of low inclination, on a superficial deposit patch [42]. The infiltration and the survival of the superficial

deposit was promoted by the fact that it is situated in a depression of the bedrock. Here, the Keszthely Mountains can also be mentioned, which are a lower elevated variety of these horst types (some mounds are at an altitude of 400–500 m), where 20 subsidence dolines occur [26]. There are also some inactive shafts on this horst type (for example, the Ördög-lik of K˝oris Mountain). These were formed below subsidence dolines, but they became truncated after the denudation of the superficial deposit.

The epigenetic valleys of horsts deepened into the rock below the former regional groundwater level and exposed inactive, presently dry cavities (cave-ins, the medium section of Cuha Valley and the northern section of Gerence Valley).

In the mountains, the number and expansion of their horsts, which are horsts elevated to the summit position, is large. On these horsts, perched water tables overlie impermeable intercalations. Local flows developed at them, their descending branches are fed by the water of brooks percolating on valley floors, the water flowing into subsidence dolines that occur on the horst surface and by the infiltrating meteoric water. Waters of the outlet branch emerge in karst springs situated high above the regional groundwater level or they get into the regional groundwater along the fractures or faults that dissect the aquifuge (Figure 5). The subsidence dolines and cave-ins of the horst belong to the karst systems of local flows. Permeable superficial deposits are widespread on them and locally thin at several places for the already mentioned reasons.

The majority of subsidence dolines occur on horsts belonging to this type. As regards the horsts of Northern Bakony, subsidence dolines occur on 30% of horsts elevated to the summit position, while they can be found only on 9.1% of the horsts in the summit position and on 12.5% of cryptopeneplains [26]. These features developed at places where the cover is thin or thinned out (Figure 7). Since the shafts of the mountains occur below subsidence dolines, 90–95% of shafts can also be found on this horst type. Tés Plateau is a good example of the distribution of subsidence dolines and shafts, with 137 dolines and 46 shafts. However, the majority of cave-ins (estimated 80–90%) can also be found on the horsts of this type.

Cave-ins and cavities also occur subordinately in the sides of some epigenetic-regression valleys, but their majority and those longer than 1–2 m are exclusively in the walls of epigenetic-antecedent valley sections without exception. These valleys are formed on uplifting horsts which are surrounded by cryptopeneplain. The streams of the cryptopeneplain carve an antecedent valley section in the uplifting area of the horst. With their percolating water, streams generate a perched water table and trigger its cavity formation, then they deepen and open up the cavities at the top (inactive part) of the perched water table (cavein). However, the perched water table can continue to accumulate if there is an aquifuge below the antecedent valley section. The Dudar Basin (cryptopeneplain) and the S ˝ur ˝u hill group (horst elevated to summit position) constitute such a system where, for example, in the Ördög-árok there are more than 40 cave-ins of various sizes.

With their streams, cryptopeneplains have an important role in the hydrological and karstic development of the horsts in their environs. However, in the area of some cryptopeneplains (Hárskút Basin, Lókút Basin, Porva Basin), the material of the impermeable beds (Csatka Gravel Formation) has been partially eroded and the limestone outcrops became partially covered with loess. At these sites, for example, on the floor of epigenetic valleys, subsidence dolines, shafts and inactive shafts are common.

On the basaltic horst such as the Kab Hill, mixed allogenic–autogenic karst developed with ponors with blind valleys (Figure 8) and with inflow cave-like shafts (Figure 9). With the lack of erosion, they are of dissolution origin [32,43]. These features may have formed at the margin of the basalt cap or at the limestone outcrops within the basalt cap. On the basalt cover caprock, dolines are also common, which may develop into ponors with blind valleys [26], but on terrains with loess, subsidence dolines also occur.

On threshold surfaces, drawdown doline groups (Tapolca Karst, the environs of Devecser) are typical. The dolomite terrains of this horst type are dissected into mounds, but some dolines of small depth also occur on these surfaces (between Márkó and Hajmáskér). Some spring caves can also be mentioned from this horst type. These are connected to a former (Lóczy Cave) or present (the spring cave of Lake Hévíz) hypogene branch, but cavities opened up due to anthropogenic activity can also be found (Cserszegtomaj Caves). In case of the latter case, as well as the spring cave of Hévíz and the Lóczy Cave in Balatonfüred, a hot water effect is dominant, but in the development of the Tapolca cave system, lukewarm waters and cold waters also played a role. The landscape of threshold surfaces also includes the spring cones (Tihany Peninsula, Figure 10) and pseudokarren (Szentbékkálla, Figure 11).

**Figure 11.** (**A**) spring cone, (**B**) exposed cavity (Tihany Peninsula).

In Hungarian karst literature, both the Transdanubian Mountains with the Bakony Region (which it is a part of) together are regarded as an independent karst type [44,45]. The Transdanubian Mountains are called Transdanubian type, while the other karst areas are called Aggtelek type. A more appropriate term is character instead of type. (Characteristics of karsts of Aggtelek character coincide with those of temperate soil-covered karst.) The difference may be traced back to the fact that the Transdanubian Mountains are poor in karst features, but hypogene caves are characteristic [46] and erosion caves and solution dolines are subordinate (they are absent). In contrast to the karsts of Transdanubian character, erosion caves, rootkarren and solution dolines are typical of the karsts of Aggtelek character [44]. However, morphological characteristics have become more accurate as compared to an earlier classification (for example rootkarren can also be found in the Transdanubian Mountains); basically, this differentiation is substantial.

The karst features of the Bakony Region are not widespread uniformly, but in patches of variable size. Although it is the most intensively karstified part of the Transdanubian Mountains, the frequency and size of their karst features is limited. The above characteristic feature can be traced back to its separation into horsts and to recent uplift. As a result of the former, karstification sites are localised (and thus, for example no ponors with significant catchment area developed), while, as a result of the latter karstification being young, there was little time for the development of karst features.

Karstic patches are constituted by subsidence dolines because this doline type is related to permeable superficial deposit patches, and the water infiltrated into the caprock ensures abundant (the water does not flow down low-inclined surfaces), permanent and uniform water supply to the epikarst (the already mentioned low resistances are evidence for this). This favours cavity formation and thus, shaft development in the epikarst. Abundant infiltration ensures favourable conditions for the transportation of superficial deposits into the karst. At sites where impermeable beds occur, neither subsidence dolines, nor other karst features occur. However, the small size of the horsts also results in a patchy appearance. On the other hand, the occurrence of solution dolines, a dominant variety of temperate soil-covered karst, is subordinate. All this results from the fact that, as has already been mentioned, the extension of low-inclined terrains is restricted in its uncovered terrain. In spite of the position of threshold surfaces at low elevation, this doline type occurs on them since their surface is low-inclined and hardly dissected.

Impermeable caprock is widespread in the mountain region. If it is the material of the Csatka Gravel Formation, a stream network develops on it because it is of clayey composition (cryptopeneplain). Their stream and valleys stretch across the surrounding horsts as epigenetic-regression valleys, on the floor of which, significant percolation may take place. However, percolation sites are not ponors. It may occur that the gravel cover already terminates in the area of the horst and its flowing water arrives at the margin of the cover. However, since the cover is of small extension, not even valleys develop on its surface. Waters arriving at the termination of the gravel cover form subsidence dolines on the loess surface that interacts with the cover. Ponors with blind valleys only developed at the termination of the basalt cover of the Kab Hill. Therefore, ponors with blind valleys are subordinate and inflow caves that constitute the continuation of such features are absent in the mountains. Concretions are subordinate either in the caves or on the surface, while they are missing from the poljes.

As mentioned above, the most widespread cave type in the mountain region is the cave-in. These are widespread in the sides of epigenetic-antecedent gorges. Their evolution contributed to the erosion of the impermeable beds (along which phreatic cavity formation took place) and of the stream since the area of the surrounding cryptopeneplain ensured abundant water supply to this.

Shafts are also characteristic; their frequency is comparable to that on other temperate karsts. The reason for this is the relatively high number of subsidence dolines. Subsidence doline development and shaft development interact. Shafts enable the local transportation of superficial deposits into the karst and thus, the development of indentations, while the developing subsidence dolines collect surface waters and transmit them into the epikarst. This favours the transformation of epikarst cavities into shafts.

There are no erosional inflow caves in the mountain region. The development of such caves can only be expected on Kab Hill, but the low inclination of the basaltic terrain and the lack of gravel do not favour erosion. Spring caves are also absent except for some which were transformed by a warm water effect. The reason for this is that there are only some karst springs in the mountains since their karst water, as mentioned above, flows in the surrounding basins [20], and the water emerged gallery-like and not point-like before karstwater extractions [18]

The stability of the surface karst of the mountains is also promoted by the fact that there has been a land use change and return to forestry and grazing in the past decades. These cultivation methods decrease the transport of superficial deposits into karst features. Thus, these features become filled to a lesser degree, or, if they do become filled, it takes place by natural processes.

Intensive mining activity took place in the mountains which could only be practiced safely by artificially sinking the karstwater level. In the past decades, the karstwater level has been rising and gradually reaching its former elevation. Thus, the operation of springs and spas (including medicinal waters), which are fed by the karstwater of the mountains, is ensured in the long term.

#### **6. Conclusions**

The karst of the mountains can primarily be related to various local flows, at which epikarst systems developed.

The karstification of horst types and thus their karst features are different. The karstification of horsts in the summit position is not significant since the water leaves the area of these horsts, and there is no permeable cover, or it hardly ever occurs in their area. The karstification of horsts elevated to the summit position is diverse where subsidence dolines are predominant because the permeable cover is widespread. The development of cave-ins was enabled by the impermeable intercalations of the horst and intensive valley deepening.

Cryptopeneplains only karstify at sites where their impermeable cover was denuded. Streams starting from these terrains cause gorge development on horsts elevated to the summit position and their cavity formation. Low-inclined surfaces of mountain marginal

threshold surfaces favoured the development of some solution dolines and spring caves. Features reflecting a hot water effect (spring caves, sandstone karren, spring cones) also occur on this horst type. On the basaltic Kab Hill, ponor with blind valleys also developed.

Predominant karst features of the mountains are subsidence dolines, cave-ins and shafts. One of the important directions of future research may be the study of the epikarst, which helps obtain a better understanding of the development or lack of some karst features on different horst types.

**Funding:** This research received no external funding.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Conflicts of Interest:** The author declare no conflict of interest.

#### **References**


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**Xiaona Guo 1, Ruishan Chen 1,\*, Michael E. Meadows 2,3, Qiang Li 1,4, Zilong Xia <sup>3</sup> and Zhenzhen Pan <sup>5</sup>**

<sup>1</sup> School of Design, Shanghai Jiao Tong University, Shanghai 200240, China


**Abstract:** Globally, the loss of forest vegetation is a significant concern due to the crucial roles that forests play in the Earth's system, including the provision of ecosystem services, participation in biogeochemical cycles, and support for human well-being. Forests are especially critical in mountains environments, where deforestation can lead to accelerated biodiversity loss, soil erosion, flooding, and reduced agricultural productivity, as well as increased poverty rates. In response to these problems, China has implemented a series of ecological restoration programs aimed at restoring forests. However, there is a lack of knowledge as to whether the forest cover is increasing or decreasing, as well as the relative roles played by natural and human factors in forest change. Here, we aim to address these issues by analyzing the pattern and process of the forest changes in Guizhou province, a typical mountainous karst area with a fragile environment in southwestern China, between 1980 and 2018, and evaluating the extent to which these forest changes were influenced by natural and anthropogenic driving forces. Using a temporal sequence of satellite images and a Markov model, we found that the forest cover increased by 468 km2, and that over 33% of the cropland in Guizhou province was converted into forest between 1980 and 2018, with the most significant increases in the forest cover occurring in Qiandongnan. Through correlation analyses and generalized linear model (GLM) regression, we demonstrate that management factors exerted a more significant positive impact on the forest cover than climate change. While the mean annual precipitation and temperature were mostly stable during the period studied, the effects of population and gross domestic product (GDP) on the forest changes weakened, and the influence of land-use change markedly increased. These findings provide valuable information for resource managers engaging in forest protection, deforestation prevention, and ecological restoration in similar regions.

**Keywords:** factors; forest change; Guizhou
