It is well documented that forests are distributed in the areas with annual precipitation over 400 mm. Most practical experience has also demonstrated that trees cannot be successful afforested below this precipitation threshold. Liu (2019) used remotely sensed data to show that tree cover is mostly below 10% in areas with annual precipitation lower than 400 mm. A great deal of previous research has explored the afforestation areas in relation to precipitation and potential forest distribution, however, little is known about the altitudinal limitations of afforestation and suitable tree species across the altitudinal gradients.

Besides water limitations, temperature is another important factor that influences forest distribution in high altitudes, where the low temperature is the key limiting factor that constrains tree growth. Different mountain vegetation belt types are formed by different trees along the altitudinal gradients, but only certain treeline tree species can be established at the upper limit of the forests. Therefore, it is also necessary to investigate whether a particular tree species can grow in the upper limit of its distribution, especially if afforestation breaks through the upper limit of the tree species distribution. That is, afforestation needs to consider both the water demand and minimum temperature tolerance of the planted trees. Although some tree-like species above the timberlines, such as Rhododendron sp., can grow to over 10 m in height, these are multi-stem shrubs of alpine vegetation from a biological point of view, and thus beyond the scope of this paper.

The Qinghai-Tibetan Plateau is an important ecological security barrier in China. With the protection of natural forests and grasslands, some ecological projects such as grazing exclusion and restoration of degraded forests and grasslands have been conducted in the past several decades. Recently, afforestation has also been employed as an important approach to strengthen the ecological security barrier on the Plateau. For example, Tibet has been implementing landscape afforestation along Yarlung Tsangpo River, Nujiang River, and Shiquan River (Sengetsangbu). In practice, some afforestation plans have broken through the natural forest boundary, and they have even been implemented in alpine grasslands where the low temperature cannot support tree growth. For example, afforestation was carried out in Nakchu County at an altitude of more than 4500 m, and in Tsashi Island, Namco at an altitude of 4700 m. These afforestation efforts adopted a method of trial and error in order to find suitable species and demonstrate whether afforestation can be successful at high altitudes.

Determining which species are suitable and the highest altitudes for afforestation are the core questions to answer for high-altitude areas. We can only get the answers from the natural distribution of forests. Therefore, we can analyze the zonal distribution law of forest vegetation and determine the suitable tree species and the upper limits of afforestation. Based on an analysis of the horizontal distribution of forests and the vertical zonation of montane vegetation on the eastern Tibetan Plateau, we aim to: 1) Identify the highest altitudes of tree species at the climate tree line; 2) Determine the horizontal boundary of forest distribution and the altitudes of afforestation; 3) Summarize the experiences and lessons from the selection of tree species and whether our results are in line with the afforestation efforts in Nakchu County; 4) Provide practical recommendations for afforestation at high altitudes in the eastern part of the Tibetan Plateau.

The eastern Tibetan Plateau, affected by both the eastern Asian monsoon and Indian monsoon in summer, is characterized by a humid climate with abundant annual precipitation ranging from 600 to 1000 mm. It is one of the most humid natural zones on the Plateau, where diverse forests are distributed. Westerlies from the Pamir Plateau create a continental climate with features of being dry, sunny and cold on the Tibetan Plateau in winter. In summer, due to the heating of the plateau surface, a special plateau thermal depression is formed, which leads to a distinctly decreasing gradient of precipitation from east to west on the plateau. Because of the differences in precipitation patterns, the ecotones between forests and grasslands extend along the southeastern Tibetan Plateau from Kongpo Gyamda County to Lang County in the south, from the southeast of Soksan County and Bachen County to Tanggula Mountain in the north, and then from the east of Tongtian River along the Bayankala Mountains to the west of Maqu County at the junction areas of Sichuan and Gansu Provinces (Wu and Liu, 1998). The forests distributed in the above areas represent relatively narrow timberline ecotone vegetation on the Tibetan Plateau (Li, 1982). The tree species of the timberlines are mainly species of Abies, Picea, Larix, Juniperus and Betula (Shi, 1999; Guan, 2005), and the spatial distributions of these tree species at the timberlines represent species substitution from east to west (Guan, 2005). Among them, forests dominated by Abies and Picea are mainly distributed on the southeast of the Tibetan Plateau and the southern slope of the Himalayas, while treeline species of Larix, Juniperus, Betula and Quercus can form timberlines in the southeastern Tibetan Plateau and Northern Hengduan Mountains. Specifically, species of Abies are distributed in the humid timberline ecotones, mostly in shady slopes. On the southeastern edge of the plateau, there are a large number of treeline tree species from east to west, including Abies farbri, A. faxoniana, A. squamata, A. forrestii, A. georgei and A. georgei var. smithii. These species of Abies are distributed spanning from Abies farbri in the mountains around Sichuan Basin to A. georgei var. smithii in the eastern Tibetan Plateau, with A. squamata, A. forrestii and A. delavayi in the north of Hengduan Mountains, and A. spectabilis in the southern slopes of the Himalayas (Shi, 1999). Treeline tree species of Picea are distributed in Northwest Sichuan Province, southeastern Qinghai Province and eastern Tibet Autonomous Region, among which Picea balfouriana, P. crassifolia, P. purpurea and P. likiangesis can grow up to the timberlines, and P. balfouriana is the highest timberline species of Picea. Some species of Sabina (now Juniperus) reach the highest timberlines on the plateau. Juniperus species are among the cold- and drought-resistant vegetation, mainly distributed on the sunny slopes of northwestern Sichuan, eastern Qinghai Province and the southern Himalayas. Larix potaninii var. australis and Abies sp. often form timberlines in northwestern Yunnan, northwestern Sichuan and Southeastern Tibet (Sichuan Vegetation Editorial Group, 1980; Wu and Zhu, 1987).

On the Tibetan Plateau, forest vegetation is primarily distributed in the east of the Plateau, such as Tsona County, Metok County and Zayul County on the southern slope of eastern Himalaya (Fig. 1). In addition, it is distributed in the northern Hengduan Mountains, mainly in counties such as Lhorong, Rioche, Chamdo, Dayak, Gongjo and Markham. Forests also cover the low altitudes in the eastern Tibetan Autonomous Region, such as in the counties of Miling, Kongpo Gyamda, Nyingri, Pome and Zogong. It is worth noting that the upper limits of forests stretch from the eastern Tibetan Plateau to the mid-north of Tibet, such as in the east of Chali County and even some places in the south of Sokshan County (Fig. 1). The shrublands in the eastern part of the Tibetan Plateau are mainly distributed along southeast ern Lhodak County-northern Tsona County-Lhuntse County-Nang County-Miling County-Kongpo Gyamda County-Pome County-Palbar County-Tengchen County, and most of the counties in the east of the Hengduan Mountains. In addition, the shrubs are sporadically distributed in Bachen County, Sokshan County, Lhundup County, Gonggar County, Shigatse City and Namling County. The vegetation in the central and Northern Tibet areas are alpine rangelands, including alpine meadows, alpine steppes and desert steppes. For example, rangelands are extensively distributed in Nakchu County, Nyemo County, Shantsa County, Palgon County, Amdo County, and Nyerong County. Most of the counties, such as Chali, Dirl, Bachen and Sokshan, are also alpine grasslands.

Along the decreasing precipitation gradient from southeast to northwest, the vegetation distribution exhibits horizontal change patterns, i.e., from subalpine coniferous forests in the east, to alpine shrubs, alpine meadows, alpine steppe, and alpine deserts successively from the east to the west; and from tropical mountain forests, mountain shrubs- grasslands, alpine grasslands and alpine desert from the south to the north (Zheng, 1975; Zhang, 1978). The naturally zonal forests are mainly sclerophyllous Quercus forests, subalpine fir/spruce forests, juniper forests, larch forests, montane pine forests and their mixed forests. There are also evergreen broad-leaved forests, evergreen and deciduous broad- leaved mixed forests, and coniferous and broad-leaved mixed forests in the relatively low-altitude areas of the Southern Tibetan Plateau (Zhang et al., 2015). Moreover, the distribution of vegetation is also constrained by the absolute altitude. Once the elevation rises up to a certain level,

the climate, vegetation and natural landscapes change gradually with the altitude, forming a vertical zonation of vegetation patterns. There is apparent vertical zonation of the vegetation in the mountainous areas of the eastern Tibetan Plateau (Zhang, 1978). More specifically, the mountainous zones of forest vegetation change successively from tropical rain forests (ca. 1000 m a.s.l.), to subtropical evergreen broad-leaved forests (1000-2300 m), warm conifers and broad-leaved mixed forests (2000-2700 m), temperate montane pine forests (2700-3400 m), subalpine coniferous forests (3400-4300 m), and finally alpine shrublands and meadows, and alpine cushions (>4300 m). Therefore, the forest distribution patterns are constrained by both horizontal zonation and the altitudes of climatic timberlines. Generally speaking, there is no forest distribution in the vast area of the middle and northwestern Tibetan Plateau. The main vegetation types in the vertical natural zone of the Nakchu alpine shrub-meadow in the transition section from the northern edge of the plateau to the mound plateau are alpine shrublands, alpine meadows, alpine grasslands and alpine deserts. In addition, the forest lines of the Tibetan Plateau range from 3600 m to 4300 m, and even to 4500 m in some areas.

Figure 2 shows the changes in the average timberline elevations of the main treeline species in the eastern Tibetan Plateau. Juniperus tibetica can reach the highest timberline at about 4500 m. In addition, the timberlines of J. saltuaria are ca. 4400 m. P. likiangensis var. balfouriana and A. georgei var. smithii can reach timberlines at 4300-4400 m. The tree species with timberlines between 4100 m and 4300 m are Abies squamata and Larix potaninii var. australis, while A. georgei reaches timberlines ranging from 4100 m to 4300 m. The timberlines of Betula utilis and J. wallichiana are about 4000 m. The tree species with timberlines less than 4000 m include A. spectabilis, A. faxoniana and A. fabri, of which A. fabri has the lowest timberlines at about 3600 m.

We summarized the typical mountain vertical belts of vegetation zonation on the eastern Tibetan Plateau (Fig. 3). According to the altitudes, the mountain vertical zones can be divided into piedmont tropical montane rain forests, subtropical evergreen broad-leaved forests (1000-2300 m), warm temperate conifers and broad-leaved mixed forest zone (2000-3200 m), cool montane conifer forests (2700-3400 m), subalpine coniferous forests (3400-4500 m), alpine shrubs and meadows (4300-4700 m) and subnival scree vegetation (4500-5300 m). Due to the westward propagation of the Asian monsoon, some vegetation belts have higher altitudes on the western slopes than on the eastern slopes. The dominant species in the mountain vertical zones are also shown in Fig. 3. In the lower altitudes below 2300 m, there is a diversity of species in the tropical rain forests and subtropical evergreen broad-leaved forests. In the upper warm temperate conifers and broad-leaved mixed forests, the dominant species are Pinus armandii Franch., P. wallichiana A. B. Jackson, Quercus aquifolioides, Populus szechuanica C. K. Schneid. var. tibetica C. K. Schneid, Alnus nepalensis and Acer sp. The major species in cool montane conifers are Pinus densata Mast, Picea likiangensis var. linzhiensis, Larix griffithiana, Cupressus gigantea Cheng et L. K. Fu, Quercus Quercus aquifolioides Rehd. et Wils and Pinus yunnanensis. The treeline species in subalpine conifers include Abies georgei Orr var. smithii (Viguie et Gaussen) Cheng et L., Juniperus saltuaria and J. tibetica. The dominant alpine shrubs are Rhododendron and Salix, and genera such as Saussurea, Saxifraga and Rhodiola predominate in the subnival scree vegetation. On the western slopes, the piedmont belt hosts warm temperate conifers and broad-leaved mixed forests dominated by Pinus densata, Quercus aquifolioides and Picea likiangensis var. linzhiensis. The tree line species in the warm and cool mountain coniferous forest belt and subalpine coniferous forests are dominated by Abies georgei var. smithii, Juniperus saltuaria and Larix griffithiana and the former two species can reach up to the treelines. Similarly, the dominant species in alpine shrublands are Rhododendron and Salix.

Since the 1990s, many afforestation efforts have been made in Nakchu County, at an altitude of over 4500 m. In 2017, an attempt at afforestation with more than 20 species of trees and shrubs was implemented in a large area, but none of the afforestation has been successful so far. Tang et al. (2010) reported that evergreen conifers introduced into Nakchu could not grow normally, such as P. linzhiensis, Juniperus procumbens, Salix maizhokunggarensis, J. chinensis and J. vulgaris. Picea crassifolia survived, but grew very slowly during the growing season. In addition, most of the introduced deciduous trees died off in winter, and only a few deciduous trees such as Ulmus pumila, Populus szechuanica var. tibetica and P. beijingensis survived through one winter. However, the growth of these trees in the second year after overwintering was very poor, and they all died. Field investigations found that only small, creeping shrubs were successfully introduced and survived in Nakchu County. Among them, only cushion-like willow, Salix sclerophylla var. tibetica, Potentilla parvifolia, and Caragana versicolor Benth. survived, but they grew poorly.

The failures of afforestation in this alpine meadow should be attributed to the fact that Nakchu County is not a forest distribution area and the low temperature limitation could not support the growth of trees. The natural vegetation in Nakchu is alpine meadows dominated by Kobresia spp. Field investigations also found that low shrubs are very rare there. According to the long-term meteorological data of Nakchu County (4500 m a.s.l.), the thermal indices of annual biological temperature (ABT, 3 ℃), annual warmth index (AWI, 11.7 ℃ Month), mean temperature above 5 ℃ (MT₅, 8.0 ℃) and mean temperature of the warmest month (MT_July, 9.3 ℃) are lower than the corresponding values of world timberlines, which have ABT of 3.5 ℃, AWI of 14.2 ℃ Month, MT₅ of 8.2 ℃ and MT_July of 9.7 ℃ (Wang et al., 2004). In other words, the active accumulated temperature in Nakchu County is not enough to meet the needs of tree physiology from germination to xylem maturation and tree survival of the cold winter (Körner, 2003; Rossi et al., 2008).

Nakchu County is characterized by large daily ranges of temperature, with minimum usually below 0 ℃ and accompanied by frost, and thus leaf tissue is susceptible to freezing in the growing season (Sakai and Larcher, 1987; Zhuoga and Basang, 2020). As a consequence, trees cannot complete effective photosynthesis or normal vegetative growth. At the same time, tree growth consumes a large amount of stored carbohydrates, which makes it difficult for young shoots to survive (Stevens and Fox, 1991). For example, the introduced Populus szechuanica var. tibetica and Populus alba var. pyramidalis presented the phenomena of low photosynthetic efficiency, reduced carbohydrates, and abnormal growth. The reasons why it is difficult for deciduous trees to survive in Nakchu are mainly low leaf temperature, small leaf area index and short growing period (Tang et al., 2010).

Some species of Abies and Betula are more adaptive to a cold and humid climate, but no spruce is distributed up to 4500 m a.s.l. in the humid area of the eastern Tibetan Plateau. While P. balfouriana can grow up to 4500 m, it is commonly distributed on the drier sunny slope. The humid alpine climate of 4520 m in Nakchu County is not suitable for the growth of P. balfouriana. As far as we know, the treelines of Abies and Betula are not more than the altitude of 4500 m. For example, the tree line of Abies georgei var. smithii is about 4400 m, which is distributed in the Sygera Mountain of Nyingri County, while the treeline of Betula brachyphlla is distributed in Mila Mountain of Maizhokunggar County at about 4350 m, and some tree species of Juniperus can grow at higher altitudes of more than 4500 m. For example, Juniperus saltuaria is a tree species which grows in cold and drier conditions on sunny slopes. Although J. tibetica grows at altitudes over 4500 m in eastern Tibet, it is limited to the sunny slopes with a drier environment. Similarly, in the much drier environments, the treelines of J. tibetica can extend to 4600 m, 4700 m and 4900 m in Reting Monastery of Lhundup County, Sangding Gompa of Ngarzhag County and Basu County, respectively. However, J. tibetica has a limited capacity to adapt to the humid climate with more precipitation in Nakchu County (Miehe et al., 2007). Due to the limitations of the environment and the biological characteristics of tree species, Nakchu County is not a suitable place for afforestation at the altitudes above 4500 m.

Given the strong wind and low soil temperature to 6.5℃ in the growing season, planted trees grow very slowly and root systems cannot extend to the deeper layer in Nakchu County. The alpine meadow soil of cambisols with gravel is nutrient limited for tree growth. In addition, other stresses such as strong ultraviolet radiation, a greater daily temperature variation and almost no frost-free period impose frost damage which limits tree survival. Growth in the growing season is usually cancelled out by the inevitable shoot dieback in winter. Only some native deciduous shrubs with small leaves, such as Potentilla fruticosa var. pumila, Rhododendron nivale and Salix brachista, can adapt to the cold environment in Nakchu County, eastern Tibet.

To ensure successful afforestation at high altitudes, both the primary vegetation distribution and the native species which can tolerate the water and temperature limitations should be considered. In other words, the natural distribution of forests provides insight into finding suitable areas for afforestation. Afforestation in any areas should refer to locally zonal vegetation types around the afforested area. The upper limit of afforestation cannot exceed the altitudes of the climatic timberlines. In addition, in areas suitable for afforestation, the selection of tree species at different altitudes should follow the vertical distribution of vegetation along the altitudinal gradients. Native tree species in the corresponding vertical zone are the priorities for the selection of trees for afforestation. The natural forests in eastern Tibet are mainly distributed in the south of Chamdo County, Palbar County, Kongpogyamda County and the east of Miling County and Tsona County. The suitable afforestation area is to the east of this line. In the west of the shrubland distribution line, Tengchen County-Sokshan County- Kongpogyamda County-Lhundup County-Panam County -lhodak County is basically the alpine grassland distribution area, which is not suitable for afforestation. Between the forest line and the shrub line, shrub-meadows are distributed, in which only a few places can be afforested under the shelter of the terrain and the fragmented areas with low altitudes. In addition, the potential distribution areas of the main afforestation tree species in the vertical zone and the afforestation tree species for the upper limit of the altitude distribution are listed from the perspective of tree species distributions in the climate treelines (Fig. 3). For afforestation at different altitudes and slope directions, the selection of afforestation tree species is dependent on the corresponding altitudes.

Afforestation in high altitudes should follow the principle of vegetation zonal distribution and the temperature limitation for tree growth. Afforestation is feasible only in the forest distribution area and the area below the altitude of the climate treeline distribution. This paper demonstrates the potential spatial scope of afforestation based on the forest distribution pattern in the eastern Tibetan Plateau and the upper limit of the distribution altitude of the main climate treeline tree species, and it defines the possible afforestation tree species and their afforestation site conditions from the biological characteristics and climatic niches of the main afforestation tree species. We further addressed the problems with the existing afforestation tree species. Afforestation in the high altitudes of eastern Tibet needs to focus on the upper limit of altitude and the selection of suitable native tree species.