[MUSIC] The unique challenges posed by mountain climates strongly influence the types of organism and vegetation that can exist in these places. One of the most striking features of high alpine environments is the absence of trees. Beyond a certain elevation, environmental constraints prevent tree growth and the dominant vegetation shifts to low statured plants, such as shrubs and herbs and grasses. This transitional zone is called the alpine treeline. The formation of the alpine treeline is a global phenomenon primarily caused by temperature constraints. Surprisingly, it's not the extreme cold in the winter that restricts tree growth at high evaluations, but rather low temperatures during their growing season. More specifically, trees require a minimum growing season of three months, during which the mean air temperature reaches at least six degrees Celsius. At lower elevations, higher trees can more effectively capture light resources, giving them a competitive advantage over the shorter growth forms. However, the height of trees becomes a disadvantage at higher elevations. Here, they're exposed to high winds and have little protection from cold, relative to shorter growth forms. As a result, near the upper limits of their distribution, trees become crooked and stunted, and are only able to survive by growing horizontally. This type of vegetation is called krummholz. A common variation of krummholz formations is flagging. Where tree limbs on the windward side of the mountain are unable to survive the high winds, so branches only for in alignment with the prevailing winds. In addition to elevation treeline, like climate, is controlled by latitude and slope because these factors influence the amount of solar radiation that's received on mountain surfaces. Treelines on mountains are higher at lower latitudes. Compared to higher latitudes because they receive greater amounts of solar radiation. Similarly, in the northern hemisphere the southern slopes of mountains and mountain ranges receive more solar radiation than the northern slopes. Therefore, treelines are usually higher. The influence of continentality on treeline may be less obvious than that of latitude or slope aspect. However, like these other factors, continentality influences the amount of solar radiation that a mountain receives. Mountains closer to continental interiors tend to have higher treelines for two reasons. First, mountains near continental interiors have less cloud cover so higher levels of solar radiation reach their surfaces. And second, the mountain mass effect means that continental mountains that are part of a larger mass of mountains are better able to retain heat than those at the periphery of the range. High elevations are also better protected from the wind in continental interiors. We've identified how global drivers that influence mountain climates also control alpine treelines. However, if you inspect treelines up close, you'll find that they're rarely sharp boundaries but instead are fragmented and spatially variable. Such transitional zones between biomes are called ecotones. The transition zone between forests and alpine tundra is referred to as the alpine treeline ecotone. Several factors influence treeline at the local scale. For example, treeline may not coincide with the growing temperature limits of trees if trees are prevented from colonizing higher elevations, because they lack adequate soil. Alternatively, the region could simply lack species that could potentially disperse to higher elevations and would otherwise be able to tolerate the local conditions. Local disturbances that cause mechanical tress can also account for their absence from higher elevations. These disturbances can include both natural events like avalanches, fires or insect outbreaks and those that occur as a result of human activity, like deforestation and grazing. Through the creation of microclimates, local topography can also influence treeline. For example, at high altitudes, snow accumulates in depressions and valleys. And snow melt occurs later, which can prevent the establishment of seedlings. As a result, treeline is lower. In contrast, ridges at high elevations are often snow free, enabling the growth of some species of trees. In some places, cold air drainage onto valley floors, or inversions, as we discussed earlier, may create what is known as inverted treeline. For example, in parts of the Australian Alps, cold air tends to accumulate in valleys at night, producing a large differential in air temperature between valley bottom and ridge top. In such situations, plants growing in valleys encounter more frost than those growing high up on the slope. The relative tolerance of high mountain plants to frequent frost is unknown, but it is clear that snow gums, or eucalyptus pauciflora trees and some shrubs are intolerant, at least in their juvenile stages. This creates the inverted tree lines in the Australian Alps. With taller trees on the upper slopes and ridges, and with grassland and low heath communities occurring in the valley bottom. Let's summarize the key points of this lesson. At a global scale, climate in the mountains is affected greatly by latitude, elevation, and continentality. These large scale earth system processes have been studied for a couple of centuries and are generally predictable. However, climate is also affected at a local scale, primarily as result of slope, aspect, and topography. These influences can be much more variable. Mountain climates are often characterized by harsh conditions with extremes in temperatures, precipitation, wind, and radiation. And have a large influence on the plants and animals that live in these environments. Later on in the course, we'll spend more time looking at how plants and animals have adapted to living in these variable environments. Next though, we'll explore some of the challenges of living, working, or visiting high altitude environments and the different physiological and genetic adaptations that humans have evolved to cope with these conditions. Once again, we visited some new ranges and individual mountains. Now it's time to return to your own mountain world and try to identify the places we visited. Matt Peter has a great tech tip on backpacks and the sorts of things you might want to consider taking on a day hike in the mountains. And don't forget your end of lesson quiz. I'll see you next time for a lesson on human physiology.
