Why is the shrub-covered "Roof of the World" a "forbidden zone" for trees?

Friends who have traveled to the Qinghai-Tibet Plateau are deeply impressed by the fact that once you enter the "Roof of the World", you will find that there are vast grasslands and bushes, but trees are extremely scarce. Most areas seem to be "forbidden zones" for trees, and you can't see a single tree. So why don't most areas of the Qinghai-Tibet Plateau grow trees? Why can bushes and grasslands exist in high-altitude areas? Can trees be planted on the alpine grasslands of the northern Tibetan Plateau? What adverse consequences will it bring if trees are successfully planted on this land? Let's spend ten minutes to reveal the ecological mysteries of these phenomena by analyzing the ecological reasons behind them.

Image source: Pixabay

1. Why are there no trees in most parts of the Qinghai-Tibet Plateau?

The special geographical, climatic and soil conditions of the Qinghai-Tibet Plateau are the fundamental reasons why most places in the region are not suitable for growing trees.

The high altitude and harsh climate conditions restrict the average altitude of the Qinghai-Tibet Plateau to more than 4,000 meters, and even more than 5,000 meters in many places. The high altitude area of ​​4,000 meters above sea level has low temperatures, with the average temperature of the warmest month, July, below 10°C. The length of the growing season is less than 4 months, and the average temperature during the growing season is less than 6°C. The low temperature severely restricts the division of plant cells and the growth of trees, and severe frost damage occurs in winter. In addition, low temperatures and strong radiation will also limit the photosynthesis of trees and restrict the carbon balance of trees. Even in the Lhasa area at an altitude of 3,650 meters, the annual average temperature is only 8.1°C. Although the summer temperature is high, the growing season is only about 4 months, which is far from the long and warm growing season required for most trees to grow. Trees, especially large trees, usually require a warm climate and a long growth cycle, and the cold climate of the Qinghai-Tibet Plateau severely restricts the growth of trees.

Poor soil and wind erosion can also limit the growth of trees. Due to the low temperature, the organic soil layer in the Qinghai-Tibet Plateau is also very thin. The mineralization rate of soil nutrients is slow, and the soil is usually poor. Most of the plateau soils are poor mountain soils and grassland soils, which contain low organic matter and lack sufficient nutrients to support tree growth. In addition, due to the low vegetation coverage, strong wind erosion also exacerbates soil loss and restricts the growth of tree roots. Strong winds and strong radiation can also cause water evaporation, resulting in vegetation water shortage, thus hindering the growth of trees.

Most attempts at high-altitude afforestation have failed. Historically, Songtsen Gampo attempted to plant trees on a large scale in the Lhasa area to improve the local ecological environment. However, due to extreme climate and soil conditions, most trees failed to survive. According to historical documents, although the tree planting activities during the Songtsen Gampo era achieved some success, the overall effect was not lasting, and the difficulty of tree growth directly determined the scarcity of forests in the area. Recently, tree planting projects in Namtso and Nagqu have achieved little success despite the investment of a lot of manpower, material and financial resources.

2. Why are shrubs and grasslands distributed at higher altitudes than forests?

As the altitude increases, the climatic conditions on the Qinghai-Tibet Plateau become increasingly harsh, and the growth of trees is restricted. However, shrubs and grasslands have adapted to these changes and gradually replaced forests. In the mountain vertical zone, forests, shrubs and grasslands appear in order from low to high altitudes.

Image source: Pixabay

Adaptability of shrubs and grasslands Shrubs and grassland plants have stronger adaptability than trees. Trees are tall, and large trees attract wind and dissipate heat quickly, especially in high-altitude and low-temperature environments; shrubs and grassland plants are short in stature, and have an advantage over trees in accumulating heat in the plant canopy. In addition, the root systems of shrubs and grasslands are usually shallow, which allows them to adapt to extreme environments such as drought, cold, and strong winds. At high altitudes, the temperature is low, the climate is dry, and the soil contains less water. Shrubs and grassland plants can absorb moisture from the soil through their shallow root systems and complete the reproduction process through a shorter life cycle. Many alpine meadow and shrub plants can grow tenaciously in an environment with ever-changing seasons.

Differences in growth cycles Trees have a long growth cycle, requiring at least several decades to mature, and the cold severely limits the life history of trees; while the growth cycle of shrubs and herbaceous plants is relatively short, especially herbaceous plants, which can complete their life cycle within one growing season, allowing them to grow and reproduce in a short warm season. For example, on the northern Tibetan Plateau, there are a variety of grassland plants on the alpine grasslands, ranging from low-altitude Gobi grasslands to high-altitude meadows, all of which can adapt to local environmental conditions. The roots of grassland plants penetrate deep into the soil, making full use of the limited growing season and showing strong vitality.

3. Can trees be planted on the alpine grasslands of the northern Tibetan Plateau?

Image source: Pixabay

Most of the alpine grasslands on the northern Tibetan Plateau are located in extremely cold areas with extremely harsh climatic conditions, low annual precipitation, and scarce soil moisture. Especially in the case of permafrost, these natural conditions make it extremely difficult to plant trees on the northern Tibetan Plateau.

Severe climatic conditions: The temperature in the northern Tibetan Plateau is below 0 °C all year round, the winter climate is extremely cold, the summer warm period is usually less than 3 months, and the average temperature during the growing season is lower than the temperature threshold of 6 °C required by trees. Such climatic conditions directly restrict the growth of trees, especially for trees that require a long growing season, as the short growing season cannot meet their growth needs. According to climate data, the annual precipitation in the vast northern Tibetan Plateau west of Nagqu generally does not exceed 300 mm, and the water required by trees is much higher than that of grasslands and shrubs. Therefore, the possibility of trees growing here is very low.

Even if some cold-resistant tree species can barely grow in the short warm season, the soil conditions in the northern Tibetan Plateau are still not conducive to the growth of trees. Most of the soil in the area is frozen, with extremely low effective water content and shallow soil layers, making it difficult for tree roots to take root. According to research, frozen soil is very detrimental to the growth of trees, especially during the winter-spring transition season, when frost heave caused by soil freezing and thawing can cause the root system of trees with deep roots to die, posing an extremely severe challenge to the survival of trees.

Historical Cases and Failed Attempts There have been similar attempts to plant trees on the northern Tibetan Plateau. For example, some areas have tried to plant cold-resistant tree species such as sea buckthorn. Although they are relatively drought-resistant and cold-resistant, these trees did not survive due to extreme climate and soil conditions. Even in areas with slightly better conditions, the growth of trees still faces huge difficulties, and the final result often ends in failure.

4. Ecological consequences of planting trees on grasslands

Although there are many natural obstacles to planting trees on the northern Tibetan Plateau, forcibly planting trees on grasslands may lead to a series of adverse ecological consequences.

Destroying the ecological balance of grasslands Grasslands are a unique ecosystem on the northern Tibetan Plateau and a habitat for a variety of wild animals and plants. Afforestation may destroy the natural ecological structure of grasslands, reduce the diversity of grassland plants, and even affect the habitat of grassland animals. According to ecological research, the stability of an ecosystem often depends on species diversity, and planting trees on grasslands may cause the grassland ecosystem to become unbalanced.

Competition for water resources Trees require a lot of water, especially in relatively dry alpine grassland areas. Planting trees may lead to excessive consumption of water resources, affecting the growth of grassland plants. In some areas, excessive tree planting attempts have caused the groundwater level to drop, affecting the water supply of grassland plants and even causing grassland degradation, which in turn affects the biodiversity of the grassland.

Conclusion: Respect nature and protect ecology

The ecosystem of the Qinghai-Tibet Plateau is extremely complex and unique. The scarcity of trees is not accidental, but an inevitable result of the natural environment and ecological laws. We must respect the particularity of the alpine ecosystem, follow the laws of nature, protect nature, and avoid excessive interference with the natural ecosystem. We must learn the laws of nature from nature, draw valuable experience on ecological protection, promote sustainable ecological restoration and protection, and ensure that this mysterious "Roof of the World" can exist forever in the future.

Source: Chongqing Science Writers Association

Author: Shi Peili, Research Fellow at the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences; Zou Jingbo, Chief Technician at Mr. Zou's Science Garden

Audit expert: Li Hanbin

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