There are multiple dialects in South China.
Strictly speaking, Mandarin is not a dialect. It is a language with multiple dialects. The national language of China, called Putonghua, is based on the Mandarin dialect of Beijing.
Mandarin dialects are roughly distributed over the northern, central and southwestern parts of China, corresponding more or less to the green patches on the map of China below. The Mandarin dialects co-evolved from Middle Chinese and share a large number of phonological, syntactical and lexical features because the flatter terrain in north China allows for easier diffusion of linguistic innovations.
On the other hand, linguistic diversity is much greater in Southern China, which is commonly defined as the area south of the Yangtze (or Changjiang in China) river. The more mountainous terrain in the south presents a more substantial barrier to communication between people living in different regions. Hence, in the south, there are multiple Chinese language families that have evolved independently from the ancestral Chinese language (Old Chinese or Middle Chinese) they share with Mandarin. Linguistic changes in the south tend to be more localized (unlike in the north).
These Chinese languages are for the most part not mutually intelligible. They include
- the Yue dialects in Guangdong,
- the Min languages in Fujian and Taiwan,
- the Wu dialects in Jiangsu and Zhejiang,
- the Gan dialects in Jiangxi
- the Xiang dialects in Hunan
There are also Mandarin dialects in the southwest of China (Yunnan, Guizhou and Sichuan) but they are largely the result of immigration from central China in recent centuries, as evident from their relatively low level of linguistic heterogeneity.