Ecological Zones

Once upon a very distant epoch the great southern continent of Godwanaland broke apart, dispatching the Indian sub-continent north towards the Asian land mass. Eventually, inching its way across the oceans, riding the restless tectonic plate, the sub-continent crashed into Asia. At the point of direct impact the invading sub-continent punched against the plate on which Asia rested and forced up the land at its edge, creating the Himalaya Mountains. The land behind the mountains rose to become the Tibet-Qinghai Plateau. The land over a thousand kilometres to the east was also jolted. As a result of the crinkling of the land mountains rose throughout the area. Rivers that existed before the collision subsequently wore down trenches in these mountains, and formed valleys that separated the mountains into ranges, creating the isolated venues for human settlement many millenia later.

Yunnan is still a seismically active area. The entire northern half of the province particularly suffers the ravages of periodic earthquakes. In recent times alone a devastating quake struck the west, especially Dali, in 1924. Tonghai was nearly leveled in 1970. A major quake rocked Lijiang in 1996, while in 1998 earthquakes struck Ninglang in the northwest and Xuanwei in the northeast. The south is not exempt from this kind of disaster, for a quake killed scores in Lancang in 1992. It does seem, too, that most earthquakes occur in winter, forcing people into the open cold, just as nature takes away their protective shelter.

Yunnan's elevation is highest in the northwest, lowest in the southeast. The land between consists of successively lower plateaux, interspersed with mountain ranges. In the west these run north to south, but the province's two major ranges, Ailaoshan and Wuliangshan, run northwest to southeast. The valleys and plains,too, lie at progressively lower altitudes the further south they are.

The Tropic of Capricorn runs right through Mojiang, which is practically in the centre of the province. Thus the province lies in the geographical borderlands of the temperate and tropical zones. Consequently it has an enormous range of ecological niches, from the Arctic zone of the snow mountains' highest slopes to the fully tropical environment of most of Xishuangbanna.

The southeastern corner of the Tibetan Plateau is actually within the boundaries of Yunnan's Diqing Prefecture. At an average altitude of 3500 metres the elevated plain of Zhongdian County and the settlements of Deqin County are the highest inhabited zones in the province. The air is thinner, the sky clearer, the climate colder. Up on the mountain slopes, where herders take their yaks, winters are Arctic.

Yaks themselves are high-altitude animals, built to live in the thinner air. In fact, they cannot survive long in the low-altitude plains and are rarely seen below 2000 metres. On the Zhongdian County plateau the alveolar pressure, which measures the amount of oxygen that can reach the bloodstream, is about 60%. People and animals born at sea level have to adjust.Mountain-dwellers, on the other hand, have 20%-60% more blood in their systems, made up largely of hemoglobin-catching red corpuscles, enabling them to breathe normally in thinner air.

High mountain plants are stubbier, with thick leaves and long roots. Animals are bigger, with long hair or thick fur. Birds for the most part migrate out of the area with the coming of the frost in autumn.

Just off the Plateau the vegetation changes. Forests consist of the tree and animal species associated with the northern temperate zone. Deciduous trees compete with evergreens for space. These become mixed with tropical plants, trees and animals further south, until crowded out by the lianas, bamboos, vines and lush jungles of the southern lowlands.

The river valleys of central Yunnan already contain many tropical species, which flourish especially in the lands south of the Ailao Mountains. Broad stretches of central Yunnan, though, are virtually treeless, but blessed with rich coal deposits, enabling human settlement. Yunnan's mountains also impede the smooth progress of the annual monsoons, trapping and twisting the weather currents, so that some parts of the province are wetter than others. The northeast also gets the winter rains that spill over from Guizhou and Sichuan.

Sometimes the river valleys are broad enough to provide land for extensive cultivation, sometimes too dry, rocky and narrow for human habitation. The central lakes allow people to make a living by fishing and are thus part of a different ecology. Besides certain river valleys, scattered parts of Yunnan somehow miss most of the monsoon rains and are largely arid zones with sparse vegetation, nearly deserts. The ecological variety of Yunnan includes just about everything except a seaside.