From High to Low Latitudes

Typically, snowline and timberline, just as other altitudinal belt limits, ascend from high to low latitudes, but reach their summit at 30° northern latitude and 20° southern latitude, and between these latitudes are relatively low value areas, forming a saddle-shaped global distribution pattern. So, there is no uniform relationship between tree line altitudinal position and latitude. Usually, tree lines decline/ascend steeply between temperate and boreal latitudes but are relatively constant between ~30°N and 20°S. And the pattern is not symmetrical around the equator.^[3,4]

From Coastal to Inland Areas of Continents

From coastal to inland areas of any continent, the limits of altitudinal belts ascend gradually, usually by 1000–2000 m or even larger. The 2000 m amplitude seen within a narrow latitudinal belt in the temperate zone illustrates that latitude is a very imprecise predictor of tree-line elevation,^[2] or the inland position and relief are as significant factors as latitude for the elevation of altitudinal belts. In addition, compared to mountains at similar latitudes in continental areas, tree line on oceanic islands is substantially lower than the continental tree line,^[5] and the forest line on the islands is found at 1000–2000 m lower elevations.^[6]

From the Outer Margins to the Interior of Mountain Massifs

More than 100 years ago, a tendency was observed for temperature-related parameters, such as tree line and snowline, to occur at higher elevations in the central Alps than on their outer margins.^[7] This is more evident in the Tibetan Plateau, where the timberline ascends from 3500 m in the southeastern rim to 4700 m in the interior and the snowline from 5000 m to 6200 m. In the European Pyrenees, the timberline is at 1500 m in the rim and 2300–2400 m in the interior. In the comparatively high and dry western cordillera of Bolivia, for example, the upper limit of the occurrences of Polylepis tarapacana (P. tomentella)is about 1000 m higher than the upper limit of P. tarapacana in the humid and lower eastern cordillera.^[8] Polylepis climbs even higher, to 5200 m, and even to 5400 m, making the highest tree line in the world.^[9]

Difference between Shady and Sunny Slopes and between Windward and Leeward Slopes

Usually, a certain type of altitudinal belt appears at a relatively higher position in sunny slopes than in shady slopes. In the Catalina Mts. of Arizona, the fir forest, ponderosa pine forest, oak pine woodland, and dry woodland and Chaparral belts are all at a higher elevation in the south-facing slope than in the north-facing slope.^[10] In Mt. Kenya of Africa, the orotropical cloud forest lies between 3100 and 3200 m in the humid south slope and 2650 and 3000 m in the arid north slope.^[11] Extremes are not present on the sunny slopes of certain belts (e.g., montane forest), as are present in shady slopes, as in the northern Tien Shan Mountains of northwest arid China. Some east–west extending ranges often serve as significant geographic demarcation (e.g., the Qinling Mts. in central China, as the transition from subtropical to warm-temperate zone), and different spectra of altitudinal belts develop in the southern and northern flanks. On the other hand, altitudinal belts show differently in windward and leeward slopes, often with a higher position in the leeward slope. If the sunny slope is the windward slope at the same time, a complicated situation occurs—the altitudinal belt in question may be at a higher position in the shady slope, depending on the degree of aridity resulting from the interplay of so many climatic factors.

The Upward Shifting of Altitudinal Belts with Global Warming

Theoretically, an increase of mean temperature by 6°C would mean an upward shift of timberline by 1200 m.^[12] However, the fluctuation of altitudinal belts are related with many other factors (e.g., topography, tree species, etc.), also including the importance of site history to the present timberline dynamics.^[13,14] With global warming, some altitudinal belts may disappear. In arid and semiarid areas, the future dynamics of the present montane forest stands at timberline might be controlled by moisture supply rather than by higher temperatures. In addition, fluctuations of climates and timberline were not synchronous and occurred also at different regional and local intensities. In case of a sudden rise of temperature as has been predicted by many models, mountain timberlines would not respond spontaneously, but rather with a time lag of several decades or even centuries. The general trend is being modified by regional, local, and temporal variations and thus is different as to its extent, intensity, and process of change. Consequently, it seems difficult to make generally acceptable statements without restricting them at the same time in view of the many regional and local peculiarities.

Geographical Factors

Ecological Factors

Many temperature and aridity factors have been found to closely relate to the elevation of altitudinal belts, including the 10°C isotherm for the warmest-month, mean annual temperature, annual range of temperature, growing-season temperature, soil temperature, aridity, mean annual precipitation, the existence of well-adapted tree-line species, competition between communities or species, etc. In total, 14 ecological factors have been used to separately explain the altitudinal belts. In exploring the “ultimate cause” of altitudinal tree line, it was found that low mean temperatures in the rooting zone during the growing season or all-year-round (at tropical tree lines) is the critical factor controlling directly worldwide tree line position.^[21–24] The latest research shows that the current natural high-elevation tree line is associated with a growing season that is at least three months long and during which the mean air temperature reaches at least 6.4°C.^[2] With it, in addition to some necessary fine tuning that accounts for regional aridity and snow pack effects, the global pattern of natural climate-driven tree-line positions can be modeled with great confidence. The obtained error term of ± 0.7 K corresponds to a mean uncertainty of ±130 m of elevation, sufficiently robust at a global scale to encourage a discussion of the likely causes of the global tree-line isotherm. Our most recent study shows that four factors (mean temperature of the warmest month, mean temperature of coldest month, continentality, and annual precipitation) together could almost perfectly explain the global-scale distribution of timberline (R² ≈ 0.94).

Physiological Factors

UV-B (280–315 nm) is damaging and mutagenic to living organisms.^[25] It is known to be particularly high in the alpine zone of tropical mountains. This involves the occurrence of dwarf forest at anomalously low altitudes, especially for isolated peaks in tropical mountains. It has also been suggested that temperature could operate via the soil,^[26] lower temperatures leading to a greater accumulation of organic matter and to changes in nutrient status. The increasingly poor supply of nitrogen and phosphorus depends on decrease in temperature and the increase in frequency of fog is likely to be important not only in governing the distribution of forest types but also in influencing their biomass and physiognomy. This also leads to relatively lower elevation of forest belts in tropical mountains and islands.