Suggestions for the northern boundary of the tropical zone in China

The tropical region of China is generally recognized as the area on the northern edge of tropical Asia. Suggested demarcation lines for the northern boundary of the tropical zone in China are largely based on climatic and agroclimatic parameters or macro-level vegetation boundaries, and thus vary considerably. To clarify this controversial issue, a total of 202 regional floristic data that covered all of China were used to establish distribution-type frequency maps at the generic level, and the correlations between the floristic elements and climatic gradients were analysed. Furthermore, a total of 135 regional flora from southern China were used to reassess the extent and boundaries of the tropical zone. Results showed that the areas with tropical genera from the geographical elements of Chinese seed plants that accounted for more than 80% of total genera from regional flora were located south of 22°30'N in southern and southeastern China, which corresponds closely to the northern boundary of the tropical monsoon forest and rainforest in southeastern China; furthermore, south of the line, tropical rainforest occurred on lowlands and typical tropical families also existed. The line at 22°30' N is, therefore, suggested as the northern biogeographical boundary of the tropical zone in south and southeastern China. However, the northern boundary of the tropical area reached 24°30' N in western Yunnan and 29° N in southern Xizang(Tibet) disjunctly in deep valleys. This line is to the far north of the northern boundary of the marginal tropical climate(21°30'N, effective accumulative temperature of 8000℃ with > 10℃ daily mean temperature), implying that the tropical zone may have extended further north during geological history than it does at present. This conclusion supports the northward shift of tropical and subtropical broadleaved evergreen forest in eastern China during the mid-Holocene from paleoecological studies.