[MUSIC] Above treeline, plants have adapted to harsh alpine environments with specialized adaptations. Alpine plant communities consist of a variety of low stature plants, including wildflowers, grasses, yeasts, mosses and succulents. We're going to discuss some of the key adaptations that allows these plants to persist and thrive in alpine habitats. In a previous lesson, we discussed how temperature is a limiting factor that prevents the growth of trees above a certain elevation. So clearly, alpine plants must have developed some unique traits that helped them to survive the cold. [SOUND] Alpine plants stay warm using two pathways. They either increase the amount of heat that they absorb from the Sun called radiative heat gain or, they decrease the amount of heat that's loss from wind called convective cooling. Their coloring, which absorbs more heat than lighter pigments is one way that alpine plants have increased radiative heat gains. Some plants also orient their surfaces, so they're perpendicular to the Sun to receive the greatest amount of heating. To reduce convective cooling, plants may find refuge from the wind by growing in sheltered microclimates. For example, plants that grow close to a boulder that blocks the wind or provides shade from intense Sun may be more likely to survive and reproduce. Many alpine plants also have a dense hairy surface called pubescence. These hairs trap a thin layer of air above the surface of the plant called the boundary layer. By reducing air movement over leaves, boundary layers reduce convective heat loss and stabilize the microclimate against temperature fluctuations. Growing close to the ground provides protection from harsh winds and it's a strategy employed by many alpine plants. A compact growth form also increases the likelihood that plants will be entirely covered by snow during the winter. Intuitively, it may seem that snow accumulation would crush alpine plants and detrimentally effect their survival. Instead, snow provides a protective blanket that insulates plants against fluctuating air temperatures, shields them from high winds and radiation, and provides a source of moisture. This subnivian space below the snow is critical for many alpine species. A cushion growth form is one strategy that has been highly successful in alpine environments. Cushions are tightly packed clusters of many smaller stems. The cushion growth form is a highly efficient way to stay warm as it both increases radiative heat gains and restricts air movement through the low canopy. These adaptations allow cushions to create favorable microclimates. Temperatures can be up to 15 degrees Celsius warmer than the surrounding air temperature. The warm microclimate is not only beneficial for the cushion species, but also for other species that take advantage of the shelter provided by the cushion, including other plants, microorganisms, spiders and insects. Dead plant matter within the cushion also promotes nutrients recycling and encourages further plant growth. In this sense, cushions can be thought of as ecosystem engineers, organisms that modulate the availability of resources to other species through habitat modification. The dead leaves that are retained on the stem act as insulators, buffering against temperature fluctuations. A low growth form reduces exposure to drying winds. The erect flowering stems of the rosette growth form are obviously more exposed, but this is also an adaptation for seed dispersal and attracting pollinators. There are many other variations on these growth strategies that plants have evolved to cope with extreme alpine conditions. For example, Espeletia schultzii is a giant rosette species common above the tree line in the Venezuelan Andes. The trunk is thick with succulent hairy leaves arranged in a dense spiral pattern. Marcescent leaves that senesce, but do not fall off the plant provide protection from the cold. Paradoxically, individuals of this plant increase in stem height with increasing elevation. This is partly explained by the longevity of this plant at high elevations when water freezes it expands and plant cells can be damaged or even burst. So beyond their morphological, structural or phenological adaptations, alpine plants have developed three physiological or functional adaptations to help prevent their tissues from freezing. First, a process called freezing-point depression allows plants to increase the concentration of soluble sugars in their tissues in order to reduce the temperature at which they will freeze. This is similar to the concept that is applied when roads are salted in the winter to prevent ice from forming. Second, water inside the plants can cool below its freezing-point of zero degrees Celsius in a process called supercooling. Normally, ice forms around a seed crystal or nucleus. However, plants can achieve supercooling by segregating water into cells in the absence of other particles and prevent ice formation. Finally, plants can move water to the otherwise empty spaces outside of their cells where it will not damage the tissue if it freezes. This approach dehydrates the plant. But fortunately, they're adapted to cope with dry conditions. Plants lose water through a process called transpiration. Transpiration involves both water transport within a plant and the loss of water from the plant to the atmosphere through evaporation. Pumping water upwards from the roots to leaves functions to distribute essential nutrients within the plant while evaporation is an inevitable consequence of opening the leaves to the atmosphere to take in carbon dioxide required for photosynthesis. Carbon dioxide diffuses into leaves as water diffuses out, primarily through specialized pores called stomata. The transpiration process is driven by the water-potential gradient. Higher moisture within the leaf relative to the surrounding air causes net movement of water out of the leaf. However, boundary layers can lessen the water-potential gradient between the inside and the outside of the plant which reduces transpiration. Transpiration increases at high elevation, making it challenging for plants to retain moisture. Lower atmospheric pressure means that less water is held in the surrounding air. This increases the water potential gradient between the inside and outside of the plant, resulting in more rapid diffusion of water out of the plants. High winds in alpine environments remove the protective boundary layer around the plants and can increase water stress even when water doesn't appear to be limiting. Hairy, fuzzy and succulent leaves can reduce transpiration rates which helps plants cope with dry and windy conditions. Like the needles on coniferous trees, alpine plant leaves tend to have cuticles that seal in moisture. Alpine plants tend to have greater control of their stomatal apertures than do plants in less extreme environments and they may be able to reduce water loss by closing their stomata. However, closing their stomata to conserve water comes at the cost of reduced photosynthesis, because it also prevents the diffusion of carbon dioxide into the plant. However, most alpine plants are able to maintain efficient photosynthesis at low temperatures and have other adaptations to sustain high grow rates during summer. Many alpine plants have deep root systems. Taproot systems have a large main root with smaller roots branching off the side. Deep root systems are an adaptation to the thin soils at high elevation. This feature helps stabilize the plants and places where soil is constantly on the move. Taproots allow plants to exploit deeper soil moisture and reach more nutrients. They also provide anchoring in mountain regions, which prevents them from being uprooted in highly unstable soils by harsh winds and other mechanical disturbances. Allocating resources into root systems and other below ground storage structures is another adaptation for coping with variable conditions, and short growing seasons. A high root-to-shoot ratio enables storage of water and nutrients which is beneficial, because having stored water and nutrients allows plants to grow immediately as temperature is increased in the spring. Examples of deeply rooted alpine species include pasqueflowers, gentians and oxytropes. Lichens are desecration tolerant, non-flowering organisms with truly remarkable adaptations to arid, low-nutrient environments. They thrive in extreme alpine places where they may be found on nearly all rock surfaces. Lichens are technically not plants. They are partnership or symbiosis between an algae or bacterial species and a fungus organism that could not otherwise survive alone in the alpine region. The algae or bacteria photosynthesizes to produced food energy that it shares with the fungus while the fungus provides shelter and a site on which the algae, or bacteria can grow. This type of relationship that's beneficial to both organisms is called a mutualistic association. Incredibly, lichens do not have a root system at all. Instead, they collect nutrients and water from the atmosphere. Lichens also obtain soluble nutrients from the surface they live on by releasing unique biochemical enzymes that decompose their substrate even if it's a rock. Lichens are able to capture nutrients in unique ways which helps them thrive in alpine environments. While lichens lack adaptations like cuticles that prevent alpine plants from drying out, they tolerate severe desiccation or dehydration and enter dormancy until wetter conditions return. When moisture does return, brittle lichens rapidly absorb water to become soft and fleshy. In fact, the fungal layers of lichens can soak up more water than their own weight. Lichens are also well-adapted to cold environments, because they can photosynthesize at any temperature above zero degrees Celsius.
