Karst Landscape Resources:‌

Summer monsoon occurs from May to August. Due to the high mountain terrain, the southwest wind is blocked and changes direction to northwest wind. This hot and dry wind causes many difficulties for production and forest protection.

In addition, there are East and Southeast winds blowing from the sea, which prevail from September to April of the following year. These winds often blow alternately with the Northeast monsoon and have low speed (except in the case of storms, the strongest wind speed can reach level 12).

5, Hydrological regime:

Located in the basins of the Chay River, Trooc River, Son River, ... are all the upstream of the Gianh River. Because the terrain here is a large limestone area, ancient Karst, strongly weathered and complexly divided, the phenomenon of underground water flow is common. Looking at the map, there are no large rivers and streams, on the ground there are some small streams flowing openly but interrupted when flowing underground through caves. In the rainy season, most rivers and streams have high water levels, creating large flows, causing local floods, but after the rain, the water quickly recedes through the "suction eyes". The flood season from September to November coincides with the months of heaviest rain. Big floods often appear between September and October of the year.

In addition to the main rainy season, the area is also affected by the minor monsoon rains in May and June. The minor monsoon rains sometimes cause major floods. During the dry season (from January to July), small streams in the area become "dead streams", and the Chay and Son rivers have very low water levels and minimal flows.

Observations of hydro-climatic data show that the dry season lasts from June to September, the rainy season is concentrated from August to November. Therefore, the most favorable tourist season is spring and summer (from March to September). Although the weather is hot during this period, it is very suitable for boating on the river to sightsee and visit caves.

However, during the rainy season, from September to October, sometimes lasting until December, there is a lot of rain concentrated in these months, high water, traffic jams and flooded caves. Therefore, it is completely unsuitable for tourism here, perhaps this is also true for tourism in Quang Binh and the North Central region.

With the weather and hydrological climate, we see that tourism in PN-KB National Park is completely seasonal and determined by natural conditions.

4.1.1.2 Geomorphological landscape resources (Karst):‌

(1) Landscape

According to the results of geological and geomorphological research over the past years by scientists, guidance documents, reports of Phong Nha - Ke Bang National Park and through a general survey of the whole area, the following results were obtained:

Karst terrain creates a natural landscape of limestone mountains of nearly 71,000 hectares, accounting for 82% of the area of the National Park. The complex endogenous and exogenous geological processes that have been taking place since the Triassic until now are the cause of the diversity of the terrain and geomorphology of the area with the following types:

 Non-Karst terrain: low hills, round tops, flat surfaces, abrasive terraces - accumulated along the Son and Chay river valleys and distributed along the edge of the central limestone massif.

 Transitional terrain, with complex interspersion between limestone blocks and terrigenous terrain. The Hung Lau - Table Rock Area (PKPHST), Hung Dang Area (PKBVNNI), PKDVHC.

 Karst terrain is typical of ancient tropical Karst, formed mainly in the Cenozoic, occupying about 2/3 of the area of the National Park, creating the largest limestone desert in the world (Pierre G. 1966).

Phong Nha - Ke Bang has a fairly developed karst system, with many types of geomorphology and unique karst landscapes, typical of humid tropical karst, with the following types of karst landscapes:

- Karren : These are very common karst terrains, including holes, hollows, crevices, grooves, etc., with strange shapes, sizes ranging from very small (1-2 mm) to quite large (5-10 m), jagged, sharp, and very difficult to travel. The deep hole at Km 18, Road 20 is very typical and is considered the deepest Kast sinkhole in Vietnam (with a depth of more than 255 m), possibly even the deepest in Southeast Asia. This sinkhole is estimated to be more than 400 million years old, which further affirms the global value of the area.

- Funnels, karst valleys : These are places with funnel-shaped terrain, measuring tens to hundreds of meters. Funnels caused by cave collapses often have vertical walls, caves at the bottom, surface water intakes, and are partly covered by clay, tree humus, and limestone boulders. (Km12, Km30, Road 20, BVNN subdivision)

- Karst valleys: are karst valleys that extend for hundreds of hectares, the bottom of which may have underground water sources and surface flows. This type of terrain is only found in Thung Tre with an area of approximately 198 hectares;

- Blind valley: is a blocked valley section, in the lower part there are a few drainage caves. When it rains heavily, water from all over pours in, these caves are blocked, cannot drain in time, flooding can occur like in the Gao Forest along Highway 20.

- Karst fields : are flat fields, can have surface runoff, typically Hung Tre area.

- Erosion plain : has relatively flat terrain, with remaining mountains such as in Phuc Trach, Xuan Trach, Son Trach.

- Elevated karst landforms: including peaks, ranges, blocks, towers, etc., varying in size and shape, rising high among low terrains such as the Hung Lau - Da Ban area of PKPHST.

(2) Cave

With the vast Karst terrain spanning millions of years of formation, it has formed the most unique cave and underground river system in the world. Experts have estimated that there are over 300 large and small caves here. Although the survey has not been complete, 128 caves have been identified in 3 cave systems: Phong Nha cave system, Vom cave system and Chay cave system.

Table 4.2 Statistics of cave systems in the study area

STT

Cave system name	Total number of caves	Total length length (km)
1	Phong Nha Cave System	68	70.4
2	Vault Cave System	31	40.5
3	Cave System	29	7.1
	Total length	128	118 km

Maybe you are interested!

Of the 128 caves investigated above, 37 large caves with a total length of over 87 km have been well known. Famous caves in the National Park include: Phong Nha Cave, Paradise Cave and most recently Son Doong Cave, the largest cave in the world, recently discovered by scientists.

Currently in 2009, it belongs to the Phong Nha cave system with a surveyed length of 6.5 km, 120 m high, 140 m wide. This is considered the largest cave in the world, far surpassing Deer Cave in Malaysia - the largest cave in the world at that time (2 km long, 90 m wide, 100 m high).

According to the assessment of local people through interviews and field surveys according to the following criteria: Attractive, Safe, Sustainable, Seasonal and Connected. 37 potential caves have been identified, however, with accessibility conditions, out of these 37 caves, only 10 caves have potential and can be exploited for tourism.

All geological and geomorphological values have been recognized by UNESCO as World Natural Heritage according to criterion (i) - 2002 (now Criterion (viii) - 2005).

4.1.1.3 Geology

Figure 4.1: Dark Cave - a type of karst cave Figure 4.2: Ancient Kasrt Tower is very popular Based on the results of geological research and World Heritage Records, it shows that

The history of the evolution of geological formations and the paleontological world, the evolution of geomorphology and the diversity of landforms are closely linked to the history of the development of the Earth's crust. Each stage of development is shaped by a characteristic structural type called an architectural plan. The region

Phong Nha - Ke Bang today is the result of the synthesis of 5 major development stages in the history of the Earth's crust development in the region:

1) Late Ordovician - Silurian period (463.9 - 430 million years ago): The earth's crust was broken, subsided, forming terrigenous rocks of the Long Dai formation, distributed in a linear manner extending in the Northwest - Southeast direction , containing Graptolithina fossils aged O 3 -S 1 .

2) Middle Devonian to Late Devonian period (386 - 362.5 million years ago): The second time the earth's crust sagged, the sea expanded. The sedimentary composition evolved from siltstone to argillite interbedded with limestone containing corresponding characteristic fossil assemblages.

3) Carboniferous - Permian period (362.5 - 245 million years): The stage of forming limestone blocks of Carboniferous - Permian age, the earth's crust in the PN-KB region was broken for the third time, creating a shallow, isometric basin (inland sea), containing fossils from the Lower Carboniferous to Middle Carboniferous ages, and finally Permian.

4) Mesozoic orogenic stage (Triassic, Jurassic, Cretaceous): PN-KB limestone massif rose above sea level, karst, weathering and erosion occurred.

5) Cenozoic period: The period of mountain building and formation of the ancient Karst cave system of Phong Nha - Ke Bang.

Figure 4.3 Karst topography is common throughout the region.

Here, geological events still exist that demonstrate the dynamic history of the Earth's crust development over 500 million years, from the Ordovician period to the present, "an outstanding model representing the main periods of development in the history of the Earth, containing evidence of life and ongoing geological processes of important significance."

important in the formation of land forms or topographic and geomorphological features” (UNESO-2003).

4.1.2 Biological resources - a typical form of TNDLST

The richness of geology and geomorphology has created Phong Nha - Ke Bang a diversity of ecosystems, including forest ecosystems, limestone mountain ecosystems, soil mountain ecosystems, cave ecosystems, surface river and stream ecosystems and underground river and cave ecosystems. Particularly, forest ecosystems with 15 large habitat types and 9 important vegetation types have been described and identified. The forest cover reaches 93.57% and the area of primary forest reaches over 83.74%, making the National Park with the largest coverage and proportion of primary forest among special-use forests in Vietnam.

Figure 4.4 Limestone mountain vegetation Figure 4.5 Vegetation on soil mountains There are many special habitats here that are of scientific and ecotourism significance.

The most notable are the relatively pristine natural forest habitats on limestone mountains with the largest area among the limestone mountain forests in Vietnam with nearly 71,000 hectares, accounting for 82% of the area. In particular, there exists a tropical evergreen closed forest on limestone mountains at an altitude of over 700m with an area of 22,500 hectares, which is the most unique forest type in Vietnam and in the world.

Figure 4.6 Primitive green cypress forest on limestone mountain >700 m Table 4.3: Area of vegetation types and habitats

Code

Carpet

Carpet type	Area area (ha)	Proportion %
1.1	Tropical humid evergreen closed forest Broadleaf trees on limestone mountains >700m	21,461.0	25.03
1.2	Tropical humid evergreen closed forest broadleaf trees on soil mountains >700m	2,316.4	2.70
1.3	Closed tropical coniferous forest on rocky mountains lime >700m	1,049.9	1.22
2.1	Tropical humid evergreen closed forest Broadleaf trees on limestone mountains below 700m	45,337.3	52.87
2.2	Tropical humid evergreen closed forest broadleaf tree on the hill	6,857.0	8.00
2.3	Anthropogenic secondary forest on limestone mountain	1,335.7	1.56
2.4	Man-made secondary forests on lowland mountains	1,731.0	2.02
2.5	Periodically flooded corridor forest	154.3	0.18
2.6	Shrubs and trees scattered on limestone mountains	1,289.3	1.50
2.7	Grasslands, shrubs and trees scattered on the dirt mountain	3,829.9	4.47
2.8	Habitat on other land	392.2	0.46
	Total	85,754	100 %

The National Park area located in the Northern Truong Son region has a very high biodiversity, and is considered one of the 200 biodiversity centers of the world (WWF, 2000). This is home to a large number of flora and fauna species, including many rare and endemic species of global conservation value.

About plants

Through statistics, the initial survey results have identified 193 families, 906 genera, 2,651 species of vascular plants. Of these, 116 species of threatened plants are listed in the Red List, of which 62 species are listed in the Vietnam Red Book, 79 species with a global threat level are listed in the IUCN World Red Book. The flora has up to 419 taxa endemic to Vietnam, of which the orchid group has up to 28 species with 3 species of Paphiopedilum that are still commonly found in nature. (See appendix for more information).

Table 4.4: Statistics of flora of Phong Nha - Ke Bang National Park

Taxon

Number of family	Number of expenses	Number of species
1. Psilotophyta	1	1	1
2. Lycopodiophyta	2	4	16
3. Equisetophyta	1	1	2
4. Polypodiophyta	23	73	176
5. Pinophyta	6	10	19
6. Magnoliophyta	160	817	2437
- Magnoliopsida	131	638	1909
- Liliopsida	29	179	528
Total	193	906	2,651

In particular, this region has a monospecific endemic genus Oligoceras belonging to the Euphorbiaceae family with the species Oligoceras eberhardtii. On the other hand, a narrow endemic species found only on limestone mountains in Vietnam is Calocedrus rupestris. In Phong Nha - Ke Bang National Park, Calocedrus rupestris grows almost exclusively on limestone forests at altitudes of over 700m, this forest type is of global importance.

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