Pearl River Delta

Pearl River Delta

formerly known as the Yuejiang Plain. Located on the eastern coast of Guangdong Province, China, it is a general name for the large delta formed by the Xijiang and Beijiang rivers and the small delta formed by the Dongjiang river. It is a delta complex with radial branches. It is in the shape of an inverted triangle, with the base being a line from Sanshui City in the west, Guangzhou City in the east to Shilong, and the apex at Yamen Bay. It covers an area of ??approximately 11,000 square kilometers. The alluvial layer is thin, generally 20 to 30 meters. The ground is undulating and surrounded by hills, mountains and islands, accounting for 30% of the area. The central part is a plain, distributed in the south of Guangzhou City, the north of Zhongshan City, the east of Jiangmen, and the west of Humen. The average annual sediment load of the Pearl River system reaches more than 80 million tons, and the delta near the river mouth is still extending to the South China Sea. In the estuary area, it can extend by an average of 10 to 120 meters per year, making it one of the key reclamation areas in China. The delta has a subtropical climate, which is warm and humid all year round. The annual average temperature is 21 to 23°C, with the coldest month of January having an average temperature of 13 to 15°C, and the hottest month of July with an average temperature of over 28°C. From June to October, typhoons are often affected, rainfall is concentrated, and the weather is the hottest. The average annual precipitation is more than 1500 mm. The rainy season coincides with the high temperature season, the soil is fertile, and the rivers are crisscrossed, which is beneficial to agriculture. Rice yield per unit area ranks among the best in China. There are more than 50 kinds of tropical and subtropical fruits, including lychees, citrus, bananas, pineapples, longans, star fruits, mangoes, grapefruits, lemons, etc. Three-dimensional agricultural structural forms such as mulberry-based fish ponds, fruit-based fish ponds, and sugarcane-based fish ponds have been developed and become a model of ecological agriculture in China. It has sugar, silk weaving, food, paper, machinery, chemical, building materials, shipbuilding and other industries, and is known as the Pearl of the South China Sea.

Ecological Characteristics

Phytoplankton biomass and composition in the Pearl River Delta (mg/L): total phytoplankton 1.63, diatoms 87.4, green algae 4.9, cyanobacteria 1.9, dinoflagellates 1.5, Chrysophycoxanthellae 0.4, Euglena 7.8.

Zooplankton species composition: 34 protozoa (species), 38 rotifers (species), 26 cladocerans (species), 49 flexipoda (species), total (species) 147.

Number and biomass of benthic animals: Number of individuals in 1981 (individuals/㎡) 249.601, Biomass in 1981 (g/㎡) 29.654, Number of individuals in 1982 (individuals/㎡) 163.548, 1982 Biomass (g/㎡) 28.492, average number of individuals (individuals/㎡) 206.575, average biomass (g/㎡) 29.073.

Main economic fish: anchovy, anchovy, anchovy, seven-threaded anchovy, whitebait, eel, eel, blue carp, grass carp, anchovy, anchovy, red-eye trout, Hainan red bream, giant bream Red-eyed bream, () striped fish, Cantonese bream, bream, yellow-tailed bream, spiny bream, southern white turtle, dace, lip fish.

The physical geographical characteristics of the Pearl River Delta

The Pearl River Delta is a tropical delta because its geographical location is south of the Tropic of Cancer (calculated as a small delta). Between 23°40′ and 21°30′ north latitude, most of it belongs to the tropical range. From a climate perspective, the large delta also belongs to the tropical region (see Zhu Kezhen et al., "Phenology", 1962). The landform development also has this characteristic, and the vegetation landscape is even more affected by it, developing into tropical monsoon rainforest vegetation. And because the north of the delta is a mountainous area in northern Guangdong, it acts as a barrier to the cold current from the north, allowing tropical vegetation to invade the mountainous areas north of the Tropic of Cancer along the valley.

The Pearl River Delta region includes Guangzhou, Shenzhen, Zhuhai, Foshan, Jiangmen, Zhongshan, Dongguan and Huicheng District, Huiyang, Huidong and Boluo of Huizhou City, as well as Ruizhou District, Dinghu District and Zhaoqing City. Gaoyao, Sihui and other places. The area of ??the district accounts for 23.4% of the province's total area, and its population accounts for 31.4% of the province's total population (in 1994). In recent years, its GDP has accounted for about 70% of the province's GDP. The Pearl River Delta is one of the regions with the fastest economic development in the country.

With the rapid economic development, the region's social development is characterized by a high degree of rural industrialization and rapid urban-rural integration.

From the topographic boundary of the Pearl River Delta, the Luoping Mountains are its western and northern boundaries. That is, to the west of the Luoping Mountains is the Xijiang Valley area, which is customarily called the Western Guangdong Mountains; to the north of the mountains is the Beijiang River system. Or called the mountains of northern Guangdong. The Luofu Mountain District on the east side is the eastern boundary of the delta.

(1) Geomorphological characteristics

The tropical characteristics of the Pearl River Delta are reflected in the river network, which is large water volume, low sediment content, many branching and radiating river channels, and the development of wide and deep water channels. However, because its development history began after the Middle Pleistocene and the amount of subsidence was not large, the biggest difference between it and the Yangtze River and Yellow River deltas is that it has a short formation history, small sediment thickness, and rapid advancement toward the bay. The Pearl River Delta is formed by sedimentation on multiple river channels in Drownu Bay, so it is called a composite delta. For example, Guangzhou is known as the "Three Rivers Collection". However, its area is not large, so its development potential is not as good as that of the Yangtze River Delta.

The formation of Drowning Valley Bay in Guangzhou was caused by the formation of a depression between the Guangning Yunfu uplift and the river source Huiyang uplift due to the interaction of the Pacific and Indian plates. During the Mesozoic Era, red basins such as Sanshui, Longgui, Dongguan and Xinhui were deposited in the depression. It is surrounded by hills formed by Paleozoic Era strata. It was also invaded by Mesozoic granite and volcanic rocks (such as Xiqiao Mountain, which is a Tertiary ancient volcano). After leveling at the end of the Tertiary Period, it rose in the Quaternary Period, which is now the 1,000-meter quasi-plane of the mountains surrounding the delta. For example, the top of Beidaluo Mountain in Qingyuan has a gentle hilly landform, called 12 hills, which is a good base for off-season crops. The main peaks of the Luoping Mountains are mostly around 1,000 meters, such as Gaoyao Jilong Mountain (1,007 meters), Qingyuan Daluo Mountain (1,024 meters), Fogang Apo Mountain (1,224 meters), Xinfeng Qingyun Mountain (1,246 meters), Lianpingjiu Lianshan (1279 meters) and so on. Xijiang, Beijiang and Dongjiang are all inherited quasi-plain rivers. The collapse of the Pearl River Delta has turned it into a low, hilly and platform area. During the Quaternary transgression, the continental shelf descended and the continent rose, so the Quaternary terraces tended to sink toward the sea, resulting in the existence of buried terraces at the base of Drowning Valley Bay. Delta deposition began after the Middle Pleistocene (approximately 40,000 years ago). The hilly platform protruding from the sea became hillocks on the delta plain, and genetic valley landforms such as "Danxia topography" and "Shimen" were formed where the bedrock was exposed. Platforms or terraces can reach level 5 and are distributed over a large area. The plain sediments also thicken toward the sea, from 25 meters to more than 60 meters. The thickest is 63.6 meters of Quaternary sediments at Dengrensha. There are marine layer 2 and continental layer 1, indicating that there are old and new layers in the delta. Two issues.

The development of the delta plain towards the sea can be divided into the following three stages:

1. During the delta formation period in the lower reaches of each river, the Xijiang River is outside the mouth of Sanrong Gorge, and the river branch branches, flowing into the Beijiang River in the northeast and the Beijiang River in the south. When it enters the Gaoming River, it was silted up into the Antelope Gorge in the Song Dynasty. After the Beijiang River leaves Damiao Gorge, it radiates and divides, forming the lower reaches of the Beijiang River delta. Today, there is still a main line of the Beijiang River left. The Dongjiang River divides after it leaves the Tianluo Gorge (Boluodong). It was branched into the lower reaches of the Dongjiang River delta. At that time, the deltas were not yet connected to each other. There are also the development of small deltas in the lower reaches of Zengjiang, Suijiang, Tanjiang, etc., which were also formed in prehistoric times.

2. Composite delta formation period In the early historical era, deltas in the lower reaches of various rivers began to unite. For example, the newly formed deltas in the lower reaches of the Xijiang and Beijiang rivers were already the product of the cooperative siltation of the Xijiang and Beijiang rivers; the lower reaches of the Dongjiang River The delta developed, and a new delta was formed downstream, which was also formed in cooperation with the Zengjiang River. Today's composite delta form was formed during this period. In addition to the delta plains, there are also "point plains" deposited according to the sea surface. They are different from river flood plains. First, the river is a two-way tidal river. Second, there is no false terrace landform with high banks and low water. Third, the plains are not It shows an inclined slope and a curved river course. It is the main tourist scenic resource of the delta and has high development value. For example, the famous "Lizhi Bay" has become one of the eight scenic spots in Yangcheng in the Ming Dynasty. Today it is a major agricultural area, called "enclosed field area".

3. The development period of the flushed delta. After the Tang and Song Dynasties, river trunks mostly cut through the mountainous areas at the entrance of the delta front.

Outside the entrance, the river branches branch out radially, forming new deltas. For example, the Jiangmen River cuts through the Jiangmen hilly area, forming a delta in Xinhui, which is called the Xinhui flush delta; the Xijiang main road cuts through the Ganzhutan hilly area, That is, the Zhongshan outflow delta is formed in Zhongshan City; the Beijiang trunk road forms the Panyu outwash delta in Panyu County after cutting through the Shiqiao platform and Shunde hilly land; the Xijiang trunk road cuts through the Muzhou hilly land and Dieshi hilly land. Later, the Doumen thrust delta was formed in Doumen County. The area known today as "Shatin District" is one of my country's rice and grain production bases. It is also a major large-scale reclamation area in my country. There are 1.2 million acres of tidal flats outside the gate of 8 Dakou with a depth of -3 meters; 400,000 acres of shoals within 1 meter; and the coastline advances by an average of 60-150 meters per year. It varies, with annual siltation height ranging from 10 to 20 centimeters.

The hilly platform landforms are older; therefore, the weathered crust has mostly developed into red soil, which is a brick red soil. Only higher hills can develop red soil, which is conducive to the growth of tropical monsoon rain forests.

(2) Coastline changes

The Pearl River Delta has shallow bedrock and a large amount of sand, so it spreads rapidly toward the sea, causing Panyu, Zhuhai, and Doumen to become counties and cities with increased fields. The coastline continues to stretch toward the sea. The delta was roughly formed during the Atlantic period about 6,000 years ago. At that time, the coastline could be determined by the coastal topography at the foot of the hills and slopes of the delta, such as sea cliffs, sea caves, sea erosion platforms, and sand banks. Today, these coastal landforms can often be seen at the foot of the delta plain hills, such as the Qixinggang sea cliffs and sea erosion platforms in Guangzhou. Therefore, the coastline of Drowning Valley Bay in Guangzhou can basically be determined by the contact line between hills and plains. Deltas began to develop at the front edges of these coastal landforms. For example, the plains in the lower reaches of the Xijiang River have extended to the vicinity of Guangli. The lower age of the shell mounds and the Qulim Burial Neolithic site there is 7170±140 years ago, while the pottery sherds are 5680 years old. In ±284 years (thermoluminescence), the Boluo Hulu Mountain shell mounds are also on the plains, which means that deltas have developed in the lower reaches of each river. According to the distribution of marine diatoms (saltwater species), foraminifera and silt layers, the delta areas in the lower reaches of each river basically reflect the range of transgression in the Atlantic period, that is, the coastline 6,000 years ago reaches the Qingyuan Basin in the north and the Zhaoqing Basin in the west (Zhao Huanting, 1990), east to the Boluo Basin and Tonghu Lake.

From the perspective of Neolithic sites, the scope of the delta in the Dongjiang Delta area can basically be regarded as the coastline. Since there are no rocky islands in the middle of the Dongjiang Delta, most of it is still sea surface, so the surrounding areas of the Dongjiang Delta are 4000 years old. Upper and lower Neolithic shoreline. The top of the Xibei River Delta has also begun to form, because the Jinli Maogang Water Ganlan Site is 4140±90 years old (C14 dating), the rotten wood layer is 3970±110 years old (South China Normal University Geomorphology Office, 1988), and the Xiqiao Mountain Shell Mounds The age ranges from 6120±130 years (ibid.) to 4905±100 years, and more than 53,000 stone tools have been unearthed (Zeng Qi, 1991), which shows that there is a large area of ??land nearby. The age of the blue clams growing under the sea erosion cliff of Shijie in the South China Sea is also 4640±280 years. That is, the Northwest River Delta has reached south of the Xiqiao Mountain and Foshan line in the south (the Shuiteng silt is 3997±190 years old).

2000 years ago (Qin and Han Dynasties), the Dongjiang Delta merged from the Dongjiang and Zengjiang deltas, extending down to Zhongtang (where there is the Yingtang Temple of the Han Dynasty), and the south is still the sea. The Northwest River Delta extends northeastward to the north of the Nanhua Waterway (i.e. the East China Sea Channel). Han Dynasty pottery pieces and other cultural relics have been found buried 2 meters deep in Xingtan (Fengjian Village). Freshwater Malayan crocodiles have grown in Leliu, and the age is 2540±120 years. Shiyong is the hometown of Lu Jia, Prime Minister of South Vietnam, so the surrounding area should be a plain. Chencun also became a land in the Han Dynasty.

1000 years ago (Tang Dynasty), the Dongjiang River reached Dongguan City, that is, the top of the Dongjiang Delta had developed. But most of it is still a pearl pond. The top of Panyu's flushed delta has developed, because "Yuanhe County Chronicles" says "it's seventy miles from the south of Guangzhou to the sea", so it can be seen that the land is right on the shore between present-day Shawan and Shunde. The top of the Zhongshan outburst delta has also developed. Because Huangchao has stationed troops in places such as Rongqi, Guizhou, and Maqi today, it can be seen that there is already a large paddy field nearby. The Xinhui outburst delta also has a top plain. For example, Xinhui was the seat of the state government in the Sui Dynasty, indicating that a large plain was formed at that time.

Most of the formations of the Tang Dynasty buried the formations of the Han Dynasty. The delta coastline did not advance much during the millennia of the Han and Tang Dynasties, which may be related to the period of sea level rise during this period.

700 years ago (the end of the Song Dynasty), the coastline moved southward to the center of the flooded deltas. The Dongjiang River reached Mayong, Dafen, and Daokiao (according to genealogy); Panyu reached Lanhe, Yu Along the vortex line, Xiqiao Yong has been recorded (1233); the Zhongshan coastline is along the Henglan, Fuwei (now Fusa), Huangpu, and Tanzhou lines (according to records); the vicinity of Xinhui has become a tidal field in the Song Dynasty, and Li The Le and Waihai lines became sand; the Twin and Huangchong lines of the Tanjiang River developed rapidly during this period. This was the result of the influx of immigrants from Zhuji Port from south to the delta during the Song Dynasty to build embankments and develop the delta. The tidal fields were changed to flat fields, and the sea level dropped. It also has an impact. Jiao is the waterway connecting the two rivers.

400 years ago (at the end of the Ming Dynasty), due to the construction of dikes in the Song Dynasty to divert water back into the trough, the coastline was silted up into sand at a faster pace. For example, Zhongshan belonged to Dongguan in the early Song Dynasty, and Guangzhou began to enter the Southern Song Dynasty. Xiangshan and Panyu are three hundred miles apart from each other by the sea, so it is not as convenient as going to Dongguan. This shows that during the Southern Song Dynasty, the Zhongshan outburst delta extended to Shiqi and the port area, that is, the sixteen sands in the East China Sea and the eighteen sands in the West China Sea were formed. The Panyu delta has reached Xiahengli (Yisha), and the entrance gate of Hongqili has just been completed. The Xinhui outburst delta has reached the southern edge of Jiuzisha, and the coastline has moved southward from Liyue to Xiongzishan (i.e. Xiongzhou). Most of the Dongjiang Delta became land in the late Song Dynasty. In the late Ming Dynasty, it extended to the east of Zhangpeng, and the southern branch also extended to the north of Houjie. The accelerated advance of the delta coastline in the Ming Dynasty was also due to artificial influence. Instead of building embankments to protect the fields like the Song Dynasty, the Ming Dynasty built embankments into fields and planted reeds and grasses on the beaches to promote siltation. The top of the Doumen Delta has begun to develop, and now Da'aosha has been formed, and Muzhou and Sanjiangkou have also risen. Huangbu and Dasha have been formed, and they only enter the island south of Zhuzhou and Fenzhou.

In the 100th year (late Qing Dynasty), the front coastline was advanced again, and the outer edge of the Doumen delta was cut out at the Modao Gate, namely Bamboo Pavilion Sand, Lantern Sand East Head and West Head Wai; Xinhui entered the west of Sanjiangwei The shores of Yinzhou Lake and the Panyu coastline have filled up the Wuzhu Ocean, and tens of thousands of hectares of sand have reached ten peaks. The progress of the Dongjiang River is slow due to the strong tidal force of the Lion Ocean.

In short, the Pearl River Delta coastline continues to advance seaward, with periods of rapidity and slowness. Natural factors are related to changes in sea level rise and fall. During periods of high sea level, the advancement of the coastline slows down, while during periods of low sea level, it advances more slowly. quick. Man-made factors include building embankments to protect fields. For example, in the Song Dynasty, embankments were built to protect fields, which accelerated the sedimentation of sand fields downstream. In the Ming Dynasty, embankments were built to cultivate fields and reed planting to accumulate mud accelerated sand formation. Tidal action and upstream sand and water also have an impact. For example, the Humen and Yamen waterways accumulate very slowly because of the strong currents. The delta area between Humen and Yamen has intensified siltation.

(3) Tropical climate

The tropical climate of the Pearl River Delta is manifested in four unclear seasons, no snow in three winters, evergreen trees, evergreen fields, and frost that does not kill the green. According to Zhu Kezhen's book "Special Conditions", the tropics are a place where "summer is everywhere all the time, and autumn becomes autumn when it rains." He believes that south of Wuling is the tropics. Therefore, the Pearl River Delta is within the tropical range.

The main reason for the formation of tropical climate is the influence of geographical latitude. The Tropic of Cancer is the dividing line between the tropics and subtropics. Most of the Pearl River Delta is south of the Tropic of Cancer, that is, most of it belongs to the tropical zone. Here, the sun is shining on the zenith during the summer solstice. It is a place that has been called a "north-facing house" in ancient times, that is, the sun can shine into the house from the north. In winter, the sun is still high, so the delta still has enough heat. But the environment here is different from tropical environments around the world because of the influence of monsoons, with north winds blowing in winter and south winds blowing in summer. In winter, cold currents from the north can also blow into the delta, causing damage to tropical crops such as lychees, papayas, pineapples, and mangoes. During the summer solstice in the delta, the sun is shining directly on the zenith, and the sun's angle often reaches 87°, and it also reaches 43° during the winter solstice; the day is 14 hours long (summer solstice), and there are still 11 hours on the winter solstice. The sunshine hours reach 1900-2200 hours, and the total solar radiation The annual average temperature is 4541.6 MJ/m2 in Zhongshan City and 5404.9 MJ/m2 in Shenzhen, making the annual average temperature above 20°C, which has entered the tropical zone standard. In winter, the average temperature in Guangzhou in January is 13.3°C, and there are only three days when the temperature is lower than 5°C. Therefore, the Pearl River Delta can only be said to have "winter weather" but not "winter climate".

The number of hot days in Guangzhou is only 6 (>35°C), reflecting the maritime climate characteristics, that is, there is no intense heat in summer, which is different from the climate of the subtropical Yangtze River Delta.

The terrain of the delta is flat, and the rainfall is less than that of the surrounding hills. The average rainfall is about 1,600 mm, while in the outer areas it can reach 2,000-2,600 mm. Rainfall is concentrated in summer and less in winter. This rainy season and dry season are clearly the characteristics of a tropical climate. It is different from the equatorial zone where it is hot and rainy all year round. Therefore, equatorial crops such as rubber trees, coconuts, betel nut, cocoa, pepper, and durian cannot be introduced in the Pearl River Delta. For other crops, only tropical crops such as sisal, mango, pineapple, lemongrass and other drought-tolerant crops can be introduced. The stationary front in spring causes cloudy and rainy weather, and long-term rainy weather can cause early crops to rot. Summer is dominated by thunderstorms, similar to the equatorial zone. It rains for 2-3 hours every afternoon, becoming one of the peak rainfall peaks in the year. There are many typhoons and rains in summer and autumn, but they are not very damaging in the delta area because they are protected by hills and are helpful for autumn drought. This is also a characteristic of tropical climate, which often forms the second peak period of rainfall in the delta. In winter, when the morphic cold current passes through the East China Sea and enters the delta, there may also be light rain. Due to strong sunshine, evaporation is large. For example, the rainfall in Guangzhou is 1600-1700 mm, but the evaporation reaches 1715.5 mm, so during the dry season (October-February). Drought damage can still occur. There are only 1-2 typhoons that invade the delta every year, and each lasts only four days, but the rainfall they bring is just enough to alleviate the autumn drought. In addition, about 5-6 typhoons affect the delta area every year, so droughts are generally less severe than floods. If typhoons, cold waves, etc. are added, wind, cold, flood, and drought are all disastrous factors in the delta.

When a typhoon enters the country, we are most afraid of being supported by the tide. When the typhoon's increased water content and the high tide's support meet, a strong wind and wave climax is formed, which is called storm surge. The article states: "When the tide has not receded, hurricanes occur and the tide comes again, and then the waves overflow the shore, submerging people's houses, losing crops, and sinking boats. This is called a tidal wave in the south. It may occur every few decades. There is." Typhoons can increase and decrease water levels. Increased water levels combined with high tides can break through sea walls, causing disasters. The delta seawall is 1,935 kilometers long and needs urgent protection. Because the delta is located in the tropics, there are many tropical storms, and the mouth of the bay spreads out in a trumpet shape toward the southeast, so it has become a storm surge-prone area in my country. The measured highest tide level is more than 1.8 meters, mostly caused by storm surges. In the 100 years from 1848 to 1949, there were 60 storm surges in the Pearl River Estuary. The water increase time is only 4 hours, and the water level can increase up to 1 meter. If it reaches the peak of the flood, it will be a disaster. Taking into account that the waves can climb higher, the impact on the seawall will be greater, so special attention should be paid. Although storm surges do not occur often, there was a surge on the first day of July in the first year of Tongzhi (1862). "The sea water suddenly dried up like land" (water reduction), and "the level water was several feet deep" (water increase). As a result, the surface of the Guangzhou river " More than 80,000 corpses were recovered." The storm surge caused by Typhoon No. 9 (September 9) in 1983 crossed the sea wall at 2,129 places, with a total length of 47 kilometers, flooding 1.84 million acres of farmland, 70,000 acres of fish ponds, and causing a loss of 1.8 million acres of materials (ships, grain, fertilizer, cement). 100 million yuan, 23 people died and 172 were injured. In recent years, the sea level in the Pearl River Delta has risen by an average of 2.08mm per year, and storm surges are expected to be more intense in the future than in the past.

Because tropical crops and fruit trees are widely planted in the delta, cold damage is also a major problem. If the daily average temperature of the cold wave in early spring is less than 10℃ for 4 consecutive days, the seedlings will rot; if the temperature is lower than 15℃ for more than 5 consecutive days, the seedlings will die early and will turn green slowly; in late autumn, if the daily average temperature is lower than 20℃ for more than 3 consecutive days, it will be cold dew. The wind blows rice pollen, causing empty grains and reducing yields; tropical crops are damaged when the temperature is below 5°C in winter, which is called "dark frost".

Summer rainfall is caused by fronts, thunderstorms, convective rain, and typhoons. Heavy rains (daily rainfall greater than 50 mm) can cause disasters, especially the combination of the pre-flood season and the post-flood season, causing "dragon boat water" and When "Muxian Shui" is connected, the disaster will be greatest.

Droughts are mainly caused by stable high pressure. For example, from November to December 1964, there were 43 consecutive sunny days. Therefore, they are connected with the previous winter drought and early spring drought, forming a disaster.

In addition, tornadoes can also cause disasters, and the masses call them "Tailed Dragons". In March 1878, a dragon broke out in Baietan, destroying more than 1,500 houses and capsizing hundreds of people. There were not many foggy days. For example, Guangzhou had an average of 4.8 foggy days per year. This was due to the high temperature of the low-level air in the delta.

(4) Tropical animals and plants

The Pearl River Delta does not grow equatorial vegetation, but there are many tropical ones. Although artificial reclamation has caused the natural vegetation to disappear, in some areas In the "Feng Shui Forest" and "Miscellaneous Wood Forest" in front of and behind the village, remnants of tropical tree species can still be seen, such as Erythrophyleum fordii, Aquilaria sinensis, etc., indicating that the ancient hilly platform was a tropical monsoon rainforest vegetation distribution area. . Alocas (Alocasia macrorrhiza) and Musa balbisiana can also be found in clusters in the valley. Understory shrubs, vines, and herbaceous plants are also dominated by tropical species, each forming a dominant community. The widely distributed ficus, kapok, poinciana and poinciana are also tropical tree species. In the mangroves at the Pearl River Estuary, there are Kandelia candel (Kandelia candel), Bruguiera gymnorrhiza, etc., and on the beach there are Canavalia mariti-ma, Ipomca pescaprae, Cerbera manghas, salt-tolerant Plant mice (Acanthus ilicibolius) and others are distributed in Dongguan, Bao'an, Shenzhen, and Zhuhai. According to aerial photos in 1983, the area is more than 5,000 acres, consisting of 11 families and 13 species of mangrove plants.

There are more than 500 common species in the delta area, which also reflect tropical characteristics. They belong to more than 130 families and 373 genera (Chen Shupei, 1984), of which 42% are purely tropical genera, including pantropical genera. It reached 53%. Tropical genera account for 62% and tropical species account for 56% in Dinghu Mountain, that is, 1291 species. Among the 146 genera in Chongxuguan Fengshui Forest in Luofu Mountain, pantropical genera account for 91% (Zheng Zhiqing, 1987). The evergreen monsoon rainforest vegetation is mostly Fengshui forest and miscellaneous wood forest, which is mostly secondary. It is a type dominated by tropical flora (Wang Zhuhao, 1982) and has a small area. In the vast mesa and low hilly areas, there are mostly evergreen shrubs. Grassy slopes with mainly tropical components grow on brick-red soil and are prone to soil erosion. Carnivorous pitcher plants (Nepenthes mirabilis) are all tropical plants. The ecological characteristics of tropical plants are also fully reflected in common cultivated trees. For example, board roots, aerial roots, and pillar roots can be seen in banyan trees, which can form a forest on a single tree; flowers on old stems can be seen in star fruit and pineapple; year-round flowers can be seen in white orchids and hibiscus. ; The pincer leaves are found in mangoes; the declining leaves are found in mangos and lychees; the dripping leaves are found in bodhi trees. The ecological characteristics of these tropical plants are consistent with the distribution of tropical monsoon rainforest here. Among them, Baimuxiang (i.e. agarwood) was widely planted in Dongguan County in the Ming Dynasty and became a famous product in Dongguan, known as "Guanxiang". This all shows that the Pearl River Delta is a tropical delta. Correspondingly, there are also large tropical animals that inhabit the delta area, although they have been hunted by people today. For example, rhinos are recorded in the Dongguan hilly area.

The delta is a water network area, dominated by swamps, depressions and low plain forests. Therefore, it also inhabits many typical tropical megafauna, which also reflect the original tropical characteristics of the delta. For example, Asian elephants, Malayan crocodiles, peacocks, etc. have many descriptions in ancient books, or there are even fossils as evidence. For example, crocodiles still grew in Guangzhou during the Three Kingdoms period. The "Shui Jing Zhu" said: "There are thousands of rare and strange creatures, including turtles, crocodiles, and crocodiles. There are thousands of kinds of them, which are indescribable." Today, Pingzhou fishermen have also found crocodile skulls in the river. , indicating that there are crocodiles in Guangzhou. Several crocodile skeletons were unearthed in Shunde and Xinhui. The 14C value of Dalin (Tangxia) was 3020±80 years, and that of Leliu was 2540±105 years. There are also records of elephants damaging crops in Dongguan in the Southern Han Dynasty. The author also collected elephant tibias from the mud layer in the ancient West Lake area of ??Guangzhou. "Nanhai Zhi" (Yuan Dynasty) still records the survival of elephants. Peacocks were recorded in Kaiping during the Jin Dynasty and Xinhui during the Tang Dynasty. It can be seen that the Pearl River Delta has been a place where tropical animals have been distributed since ancient times, and ivory has been hunted and treated as a commodity. The "Bei Hu Lu" of the Tang Dynasty records: "The tusks are small and red, and they are worthy of being cut by Wat, and they are not imported."

The crocodile has been recorded as eating humans and animals. It is very different from the alligator in the Yangtze River, and is consistent with the Crocodilus porosus and Tomistoma sp. in the Southeast Asian Islands today. The elephant is the Asian elephant (Elephas maximus). Today, due to the continuous development of the delta, the increase in population and the destruction of forests, they have been hunted to extinction.

Summary

From the analysis of various elements of the geographical environment such as landforms, climate, hydrology, vegetation and animals, the Pearl River Delta is a tropical delta, different from the Yellow River and Yangtze River deltas. It is characterized by a good water network with multiple branches and numerous deep-water rivers; the climate is rich in heat and radiation; vegetation grows vigorously with many species and animals thrive, which is extremely beneficial to industrial and agricultural production. In terms of type, the Pearl River Delta is similar to tropical deltas such as the Red River and Mekong Delta. Given the relatively scarce tropical areas in my country, attention should be paid to giving full play to the tropical characteristics and potential of the Pearl River Delta.