Botany

Plant Adaptations in Varied Biomes on Earth



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Plants grow in many different habitats. Ideal conditions for most are probably to be found in the tropical rainforest. There, warm temperatures and abundant rainfall promote rapid growth. Moving away from this ideal, plants adapted to a wide range of more challenging habitats. When rainfall is less, plants have evolved adaptations to locate, store, and conserve the limited and unreliable supplies of water, culminating highly specialized desert plants. Where temperatures are lower, different adaptations have come into play with equally specialized results in the high mountains and Arctic tundra. Some plants have adapted to living with fires, to toxic soils, and to an excess of water. Some insectivorous plants have even turned the tables on smaller animals by trapping and digesting them. Plants have evolved means of preventing or reducing browsing damage from animals, and some exact a service for the food they provide. The most notable benefit provided by animals are pollinating flowers and dispersing fruit and seeds. Some plants make use of other plants. Vines climb trees to get to good light, and for the same reason epiphytes grow in tree crowns. Parasitic plants behave like animals and obtain some or all of their nutrients from their cousins. Animals can move to where conditions are most suitable, and in case of migratory birds this can involve journeys of thousands of miles. Seeds germinate where they land, if conditions are not entirely suitable, they struggle, and if conditions are totally unsuitable, they die. So plants produce large numbers of seeds and spores, to ensure that at least a small proportion will land in a place where they can grow and reproduce. As animals ourselves, it is easy to identify with other animals. But plants, too, have many remarkable and fascinating aspects that need to be explored.
The forests of the world, especially in the tropics, are probably the most complex of all the communities of plants and animals. In tropical forests the trees often support cable like vines on their trunks, gardens of orchids and other epiphytes on their branches, and may also have parasitic plants plugged into the living tissue of their trunks or roots. Epiphytes, such as the Birds Nest Bromeliad (Bromeliad Nidularium regeliodes)get the full light they need by germinating on the branches, or sometimes the trunks of trees. The species presents the pollination syndrome of chiropterophily, and it is visited by the small bats and birds. Most plants opened a single flower per night, either every day or at one-day intervals during the flowering period. Bromeliads, whose leaf bases overlap to form water tanks in which a high diversity of animals live, produce bright red sticky fruits that are distributed to tree branches by perching birds. When more species are present, the formation of coevolutionary partnerships, such as pollination and dispersal associations, is more frequent. Layering of the forest, with tall canopy trees, several layers of shorter trees or herbs, and many types of plants that grow on them, are yet another reason tropical rainforests are so diverse. The layering, known as stratification, provides a wide range of temperature, humidity, light and nutrient conditions and thus many more niches than in less complex ecosystems. Stratification is an adaptive response of the tropical forest ecosystem to competition among plants for light, which arises when plant height is not constrained by water availability. The equatorial position of tropical forests provides them with a high level of insolation throughout the year. The mean temperature is about 18C throughout the year (sometimes much higher), so plants are not subjected to extended cold periods and are thus physiologically active year round. There are some downsides, however, to plants being continually exposed to high temperatures. Nighttime respiration rates are high, which consumes carbon fixed during the day and thus lowers net productivity. In contrast, cooler nighttime temperatures of boreal and temperate forests help plants conserve fixed carbon. High tropical temperatures also favor the reproduction of insect pests and disease microbes, which attack tropical plants throughout the year, not just seasonally as in temperate or boreal forests. On the reverse side, many plants have formed symbiotic relationships with certain insect species, providing the insects with shelter while the insects provide defense. There are numerous evergreen species in the tropical rainforest, such as the Madagascar Dragon Tree (Agavacaea Dracaena marginata) which have adapted leaves that are not only thin, elongate, and linear to reduce surface area for repiration through stomatal openings; the leaves also do not change color seasonally and persist for very long times. Tropical rainforests are known for their amazingly high biodiversity, which results from great age, rapid evolution, and complex structure. Plants have adapted many, many forms of remarkable defense, survival, and reproduction strategies each designed for its niche and dependant on the staggering interconnectivity of this ecosystem.
The Boreal coniferous forest or taiga ecosystem in the Northern hemisphere encircles the entire Earth across northern Eurasia, Alaska, and Canada. The taiga parallels the tundra, with no sharp boundaries between the two environments. The boundary where trees disappear is called the tree line or timberline. The climate of the taiga is less sever than that of the tundra but it is still one of low temperatures, low precipitation, and a short growing season of from 90 to 120 days. Annual precipitation is around 40 to 50 cm, but the landscape is dotted with lakes and bogs because evaporation is low and soil drainage is poor. Glaciers across arctic Canada leveled the landscape, so surface relief is low. Plant species diversity is low. Trees at the tree line are short and stubby but grow taller as you move south. Coniferous forests have four layers of vegetation: the canopy of coniferous trees, the layer of shrubs, an herb layer, and a surface layer of mosses on wet sites or lichens on dry sites. Boreal forest species must adapt to short growing seasons, snowfall, low temperatures, and limited moisture. Most conifers, such as the Eastern Larch (Pinaceae Larix Americana), are evergreen, retaining their needles throughout the year. Leaf retention allows them to begin photosynthesis as soon as temperatures rise in the spring, without waiting to generate new leaves. Conifers are typically cone shaped and have flexible branches, features that allow snow to slide off the branches without breaking them. Conifer leaves are adapted to survive dry conditions in winter by having thick cuticle, a tough epidermis, and sunken stomata, a feature that prevents water loss. Conifers also enter a period of dormancy in the winter, reducing their respiration to very low levels. Conifers have shallow roots that can quickly absorb surface moisture as it becomes available in the spring. Many conifers are associated with mycorrhizal fungi, which enlarge the root system to pick up nutrients and break down organic matter to make more nutrients available to the roots. The presence of these fungal relationship is especially important in a soil where low temperatures slow down decomposition. As we can see, with a marked change in climactic conditions diversity tends to be lowered to a few hardy evolutionary soldiers when we encounter the extremes of cold and dry, each with their own set of armaments against a less than bountiful world. What can we observe if the environment lent itself to dry yet extremely hot conditions?
Deserts occur on the Earths surface wherever there is either low rainfall or high evaporation from land surfaces together with high transpiration from plants, or a combination of both under present climactic conditions. One or more of four physical factors are responsible for their locations; subtropical high pressure zones, continentality, cold ocean currents, and rain shadows. Subtropical high pressure zones result from airflow patterns called Hadley cells. Air masses circulating in the Hadley cells sink back to the surface at around latitudes 30N and 30S after releasing their stores of moisture over the tropics. As they sink they warm and take up moisture from the land thus drying it out beneath them. If there were no topographical barriers against the Hadley cells they would generate desert conditions above 30 latitudes encircling the entire earth. African deserts fit this pattern, but other physical factors alter it. Continentality means that some land masses can be located so deep in the interior that moist ocean winds cannot reach them, as in central Asia. Cold ocean currents moving from the poles to the tropics experience much less surface evaporation thereby increasing the aridity of coastal deserts. As air masses move up over mountain ranges they cool and condense into rain on the windward side of the mountains. Passing over, the now cold dry air descends and draws moisture from the land in the rain shadow. In deserts the main limiting resource is, of course, water. Most plants have adapted ways to get water in the sporadic brief periods when its available, and to conserve it when its scarce. There are two main sources of water in the desert, surface water and the deep water table. Desert trees send long roots into the water table. Many algae, mosses, lichens, annuals, and perennials use surface waters when available. Desert succulents conserve water by having low surface to volume ratios, reduced numbers of stomata, and CAM photosynthesis. Cacti are very tolerant of high temperatures, and they can rapidly take up and store large volumes of water after rains. Not all biomes are in the extreme ranges of wet, cold, dry, and hot and in the blurred edges of biomes called ecotones other less drastic but equally adapted forms of plant life struggle to survive.
The tropical savanna is large expanses of grassland, interrupted by trees and shrubs. This is a transitional biome between tropical forests, semi arid highlands and deserts. The savanna biome also includes treeless tracts of grasslands, and in very dry savannas, grasses grow patchily in clumps, with bare ground between. The trees of savanna woodlands are generally flat topped, in response to light and moisture requirements. There is one special savanna on the globe that gets its environment through a partial rain shadow effect caused by central highlands which don't allow the warm moist trade winds to reach it. This savanna biome is located on the Western and Southern sides of the island of Madagascar. The west coast is drier than either the east coast or the central highlands because the trade winds lose their humidity by the time they reach this region. The southwest and the extreme south are semidesert; as little as one-third of a meter of rain falls annually at Toliara. Savannas are typically home to large grazing land mammals. The Madagascar Palm (Apocynacea Pachypodium lamerei) is one plant that resides in the incredibly hot and dry savanna lowlands and thorn forests of Southwestern Madagacar. This plant retains that characteristic short flat topped shape with an extensive network of thorns from ground to apical meristem which protects it from grazing herbivores. The Crown of Thorns (Euphorbiacea Euphorbi milii tulerensis) is another species of plant that grows close to the ground to protect it from high drying winds; its leaves have thick waxy cuticles and reduced stomata on the shaded undersides of its leaves. This plant has also developed an extensive network of thorns on its stem to protect it from grazers and rodents. Both of these species of plants are somewhat xerophytic with various adaptations to protect them from the dryness: small, thick leaves, rough bark, and leaf surfaces that are waxy. Plants in temperate and tropical deserts and savannas show remarkable adaptations to high temperatures and water stress, including tolerance and avoidance, succulence, and crassulacean acid metabolism.
Human activities affect the world's environment through the production of acid rain, soil erosion, and depletion of nutrients, clearing of forests, pollution of freshwaters and coastal ocean zones, overfishing, and the transformation and degradation of ecosystems, with a corresponding loss of species diversity. The rapid increase in human population has resulted in global scale transformation of natural ecosystems into agricultural and other human modified environments. Transformed and degraded ecosystems experience loss of biodiversity and therefore reduction in the functioning of essential ecosystem services. These environmental services are performed by Earths organisms and help maintain the stability of Earths environment. High biodiversity is essential for the maintenance of these ecosystem services. At present, species loss is occurring about 1000 times faster than evolution can replace them. Modern humans appeared around 40,000 years ago and in the blink of an eye, with respect to geologic time, we are destroying the exquisite complexity of our home we have explored in this paper. Today, more than 1 in 10 plants are on the brink of extinction, and 34,000 plant species are at risk, according to the Threatened Plants Report from the World Conservation Union. The climate has determined the space and time and form of these complex systems, yet what also must be realized is that the interruption of these delicate systems will in return, change the climate and threaten the existence of the human race itself.

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