Climate and Weather

All biomes are universally affected by global conditions, such as climate, that ultimately shape each biome’s environment. First, it is important to distinguish between climate and weather. A common misconception about global climate change is that a specific weather event occurring in a particular region (for example, a very cool week in June in central Indiana) provides evidence of global climate change. However, a cold week in June is a weather-related event and not a climate-related one. These misconceptions often arise because of confusion over the terms climate and weather.

Climate refers to the long-term, predictable atmospheric conditions of a specific area. The climate of a biome is characterized by having consistent seasonal temperature and rainfall ranges. Climate does not address the amount of rain that fell on one particular day in a biome or the colder-than-average temperatures that occurred on one day. In contrast, weather refers to the conditions of the atmosphere during a short period of time. Weather forecasts are usually made for 48-hour cycles. Long-range weather forecasts are available but can be unreliable.

To better understand the difference between climate and weather, imagine that you are planning an outdoor event in northern Wisconsin. You would be thinking about climate when you plan the event in the summer rather than the winter because you have long-term knowledge that any given Saturday in the months of May to August would be a better choice for an outdoor event in Wisconsin than any given Saturday in January. However, you cannot determine the specific day that the event should be held on because it is difficult to accurately predict the weather on a specific day. Climate can be considered “average” weather that takes place over many years.

We can use climate to describe the long-term conditions of biomes. Several abiotic factors (e.g., temperature, precipitation, wind, humidity) interact to produce a biome’s unique climate which in turn, determines which flora and fauna are able to survive within that biome. 

Climate is primarily classified by both average annual precipitation and average annual temperature ranges. Climatograms (Fig 15.1) are a type of graph that show annual temperature ranges and precipitation totals for an average year for a given location. They often use a double-Y axis which plots both the average annual temperature and precipitation. A bar graph is often used to plot precipitation and is labeled on one of the the y-axes, while a line graph is used to show temperature and is labeled on the other y-axis. The x-axis contains the months of the year.

The patterns in a climograph describe not just a location’s climate but also provide evidence for that climate’s relative geographical location. For example, a climograph with a narrow range in temperature over the year might represent a location close to the equator, or alternatively a location adjacent to a large body of water exerting a moderating effect on the temperature range. Meanwhile, a wide range in annual temperature might suggest the opposite. We could also derive information about a site’s ecological conditions through a climograph. For example, if precipitation is consistently low year-round, we might suggest the location reflects a desert; if there is a noticeable seasonal pattern to the precipitation, we might suggest the location experiences a monsoon season. When combining the temperature and precipitation patterns together, we have even better clues as to the local conditions. Despite this, a number of local factors contribute to the patterns observed in a particular place; therefore, a climograph is not a foolproof tool that captures all the geographic variation that might exist.

Biomes

A biome is a large, distinctive complex of plant communities created and maintained by climate. The factors that primarily influence a region’s climate are the latitude, continentality, ocean currents, and elevation. Low latitudes (closer to the equator) receive higher total insolation (incoming solar radiation) year-round than higher latitudes (closer to the poles). So, locations at tropical latitudes show little temperature changes throughout the year compared to higher latitudes. Locations at middle and high latitudes show a wider range in temperatures because of the tilt of the Earth, which influences insolation amounts and daylight hours throughout the year. Continentality refers to whether a location is located along a coastline or is inland. Variations in climate are due in part to land-water contrasts. Land and water react differently to insolation. In general, land heats up and cools off faster than water does due to its specific heat. A location in the interior of a continent will be warmer in the summer and cooler in winter than a maritime (coastal) location at the same latitude. So, maritime regions have a lower annual temperature range than continental regions. Ocean currents also influence a location’s climate. The major surface ocean currents move warm water from equatorial regions toward polar regions and they bring cool water from the polar regions to equatorial regions. As a result, ocean circulation is a significant mechanism of global heat transfer. Elevation influences climate as temperature decreases as elevation increases. Typically, temperatures decrease between 6 and 10 degrees Celsius for every 1,000 meters that the elevation increases. Locations on the windward side of mountains receive precipitation and locations on the leeward side of mountains are in the rain shadow.

The Earth’s biomes are categorized into two major groups: terrestrial and aquatic. Terrestrial biomes are based on land, while aquatic biomes include both ocean and freshwater biomes. The eight major terrestrial biomes on Earth are each distinguished by characteristic temperatures and amount of precipitation. Comparing the annual totals of precipitation and fluctuations in precipitation from one biome to another provides clues as to the importance of abiotic factors in the distribution of biomes. Temperature variation on a daily and seasonal basis is also important for predicting the geographic distribution of the biome and the vegetation type in the biome. Biomes can be broken down into more categories by getting more specific, but at a high level, there are 7 common terrestrial biomes:

• Rainforest
• Desert
• Grassland
• Shrubland (also called scrubland or chaparral)
• Temperate deciduous forest
• Taiga (also called coniferous forest or boreal forest)
• Tundra

The distribution of these biomes shows that the same biome can occur in geographically distinct areas with similar climates (Figure 15.2).

Rainforest

Rainforests are the biomes that receive the highest amount of precipitation. Rainforests can be either tropical (near the equator) or temperate if they are farther from the equator and coastal (Fig 15.2). The vegetation is characterized by plants with broad leaves that fall and are replaced throughout the year. Unlike the trees of deciduous forests, the trees in this biome do not have a seasonal loss of leaves associated with variations in temperature and sunlight; these forests are “evergreen” year-round.

The temperature and sunlight profiles of tropical rainforests are very stable year-round in comparison to other terrestrial biomes., In tropical rainforests, temperatures range from 20 °C to 34 °C (68 °F to 93 °F). The lack of seasonal temperature variations leads to year-round plant growth, rather than the seasonal (spring, summer, and fall) growth seen in other more temperate biomes. In contrast to other ecosystems, tropical ecosystems do not have long days and short days during the yearly cycle. Instead, a constant daily amount of sunlight (11–12 hrs per day) provides more solar radiation, thereby, a longer period of time for plant growth.

The annual rainfall in tropical rainforests is typically over 200cm with monthly variation. While sunlight and temperature remain fairly consistent, annual rainfall is highly variable. Tropical rainforests typically have wet months in which there can be more than 30 cm (11–12 in) of precipitation, as well as dry months in which there are fewer than 10 cm (3.5 in) of rainfall. However, the driest month of a tropical wet forest still exceeds the annual rainfall of some other biomes, such as deserts.

Tropical rainforests have high net primary productivity (i.e., plant growth) because the high temperatures and precipitation are ideal for plant growth. Therefore, the extensive biomass present in the tropical rainforest leads to plant communities with very high species diversities (Figure 15.3). Tropical rainforests have more species of trees than any other biome; on average between 100 and 300 species of trees are present in a single hectare of South American Amazonian rainforest. One way to visualize this is to compare the distinctive horizontal layers within the tropical wet forest biome. On the forest floor is a sparse layer of plants and decaying plant matter. Above that is an understory of short shrubby foliage. A layer of trees rises above this understory and is topped by a closed upper canopy—the uppermost overhead layer of branches and leaves. Some additional trees emerge through this closed upper canopy. These layers provide diverse and complex habitats for the variety of plants, fungi, animals, and other organisms within the tropical wet forests. For example, epiphytes are plants that grow on other plants, which typically are not harmed. Epiphytes are found throughout tropical wet forest biomes. Many species of animals use the variety of plants and the complex structure of the tropical wet forests for food and shelter. Some organisms live several meters above ground and have adapted to this arboreal lifestyle.

Desert

Deserts are very dry; in some years, evaporation exceeds precipitation. Deserts are characterized by low annual precipitation of fewer than 30 cm (12 in) with little monthly variation and lack of predictability in rainfall. In some cases, the annual rainfall can be as low as 2 cm (0.8 in) in subtropical deserts located in central Australia (“the Outback”) and northern Africa. Due to the low humidity, temperatures can fluctuate dramatically. Deserts can have daytime soil surface temperatures above 60°C (140°F) and nighttime temperatures approaching 0°C (32°F). Subtropical deserts exist between 15° and 30° north and south latitude and are centered on the Tropics of Cancer and Capricorn (Figure 15.2).

The vegetation and low animal diversity of this biome is closely related to low and unpredictable precipitation. Very dry deserts lack perennial vegetation that lives from one year to the next; instead, many plants are annuals that grow quickly and reproduce when rainfall does occur, and then die. Many other plants in these areas are characterized by having a number of adaptations that conserve water, such as deep roots, reduced foliage, and water-storing stems (Figure 15.4). Seed plants in the desert produce seeds that can be in dormancy for extended periods between rains. Adaptations in desert animals include nocturnal behavior and burrowing.

Grassland

Grasslands get more precipitation than deserts to support robust grass growth, but have extensive dry seasons that hinder tree growth. Grasslands typically receive about 50-95cm of rain per year (compared to deserts with <30cm/yr and rainforests with >200cm/yr). Like rainforests, grasslands span a large range of latitudes and can be tropical or temperate (Fig 15.2). Tropical grasslands are called Savannas and temperate grasslands are called called prairies in North America or Steppes in Eurasia. 

Savannas are tropical grasslands with scattered trees, and they are located in Africa, South America, and northern Australia. Savannas are usually hot, tropical areas that have an extensive dry season; for this reason, forest trees do not grow as well as they do in the tropical rainforest (or other forest biomes). As a result, within the grasses and forbs (herbaceous flowering plants) that dominate the savanna, there are relatively few trees (Fig 15.5).

Temperate grasslands have pronounced annual fluctuations in temperature with hot summers and cold winters. The annual temperature variation produces specific growing seasons for plants. Plant growth is possible when temperatures are warm enough to sustain plant growth and when ample water is available, which occurs in the spring, summer, and fall. During much of the winter, temperatures are low, and water, which is stored in the form of ice, is not available for plant growth.

Fires, mainly caused by lightning, are a natural disturbance in grasslands. When fire is suppressed in grasslands, the vegetation eventually converts to scrub and sometimes dense forests with drought-tolerant tree species. Often, the restoration or management of temperate grasslands requires the use of controlled burns to suppress the growth of trees and maintain the grasses.

Shrubland

Shrublands are also called scrublands or chaparral. Shrublands have highly seasonal rainfall; they are characterized by hot and dry summers and cool and wet winters. Shrublands receive more rainfall than grasslands but less than deserts. The shrublands are made up of shrubs or short trees. Many shrubs thrive on steep, rocky slopes. There is usually not enough rain to support tall trees. Shrublands are usually fairly open so grasses and other short plants grow between the shrubs (Fig 15.6). Rainfall can be quite variable across shrublands, ranging 20cm to 100cm, and the majority of the rain falls in the winter. Summers are very dry and many shrubland plants are dormant during the summertime. Shrublands are predominantly temperate, located 30°-40° north and south latitude.

The vegetation in this biome is dominated by shrubs adapted to periodic fires, with some plants producing seeds that only germinate after a hot fire. The ashes left behind after a fire are rich in nutrients like nitrogen that fertilize the soil and promote plant regrowth.

Temperate Deciduous Forest

Temperate deciduous forests are most notable because they go through four seasons: Winter, Spring, Summer, and Fall. Leaves change color (or senesce) in autumn, fall off in the winter, and grow back in the spring; this adaptation allows plants to survive cold winters. The temperature varies widely from season to season with cold winters and hot, wet summers. Temperatures range between -30 °C and 30 °C (-22 °F to 86 °F) and drop to below freezing periodically during cold winters. These temperatures mean that temperate forests have defined growing seasons during the spring, summer, and early fall. Precipitation is relatively constant throughout the year and ranges between 75 cm and 150 cm (29.5–59 in). This biome is found throughout mid-latitude regions and is the most common biome in eastern North America, Western Europe, Eastern Asia, Chile, and New Zealand.

Because of the moderate annual rainfall and temperatures, deciduous trees are the dominant plant in this biome (Figure 15.7). Deciduous trees lose their leaves each fall and remain leafless in the winter. Thus, no photosynthesis occurs in the deciduous trees during the dormant winter period. Each spring, new leaves appear as the temperature increases. Because of the dormant period, the net primary productivity of temperate forests is less than that of tropical rainforests. In addition, temperate forests show less diversity of tree species than tropical rainforest biomes. The trees of the temperate forests leaf out and shade much of the ground; however, this biome is more open than tropical rainforests because most trees in the temperate forests do not grow as tall as the trees in tropical rainforests. The soils of the temperate forests are rich in inorganic and organic nutrients. This is due to the thick layer of leaf litter on forest floors, which does not develop in tropical rainforests. As this leaf litter decays, nutrients are returned to the soil. The leaf litter also protects soil from erosion, insulates the ground, and provides habitats for invertebrates and their predators.

Taiga

The taiga, also known as boreal forest or coniferous forest, is found south of the Arctic Circle and across most of Canada, Alaska, Russia, and northern Europe. This biome has cold, dry winters and short, cool, wet summers. The annual precipitation is from 40 cm to 100 cm (15.7–39 in) and usually takes the form of snow. Little evaporation occurs because of the cold temperatures.

The long and cold winters in the taiga have led to the predominance of cold-tolerant cone-bearing (coniferous) plants (Figure 15.8). These are evergreen coniferous trees like pines, spruce, and fir, which retain their needle-shaped leaves year-round. Evergreen trees can photosynthesize earlier in the spring than deciduous trees because less energy from the sun is required to warm a needle-like leaf than a broad leaf. This benefits evergreen trees, which grow faster than deciduous trees in the boreal forest. In addition, soils in boreal forest regions tend to be acidic with little available nitrogen. Leaves are a nitrogen-rich structure and deciduous trees must produce a new set of these nitrogen-rich structures each year. Therefore, coniferous trees that retain nitrogen-rich needles may have a competitive advantage over the broad-leafed deciduous trees.

The net primary productivity of boreal forests is lower than that of temperate forests and tropical wet forests. The above-ground biomass of boreal forests is high because these slow-growing tree species are long-lived and accumulate a large standing biomass over time. Plant species diversity is less than that seen in temperate forests and tropical rainforests. Boreal forests lack the pronounced elements of the layered forest structure seen in tropical rainforests. The structure of a boreal forest is often only a tree layer and a ground layer (Figure 15.8). When conifer needles are dropped, they decompose more slowly than broad leaves; therefore, fewer nutrients are returned to the soil to fuel plant growth.

Tundra

The Arctic tundra lies north of taigas and is located throughout the Arctic regions of the northern hemisphere (Figure 15.2). The average winter temperature is –34 °C (–29.2 °F) and the average summer temperature is from 3 °C to 12 °C (37 °F–52 °F). The annual precipitation of the Arctic tundra is very low with little annual variation in precipitation. And, as in the boreal forests, there is little evaporation due to the cold temperatures.

Plants in the arctic tundra have a very short growing season of approximately 10–12 weeks. However, during this time, there are almost 24 hours of daylight and plant growth is rapid. Plants in the tundra are generally low to the ground (Fig 15.9). There is little species diversity, low net primary productivity, and low above-ground biomass. The soils of the Arctic tundra may remain in a perennially frozen state referred to as permafrost. The permafrost makes it impossible for roots to penetrate deep into the soil and slows the decay of organic matter, which inhibits the release of nutrients from organic matter. During the growing season, the ground of the Arctic tundra can be completely covered with plants or lichens.

References

Adapted from

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Global Precipitation Measurement. (2022). Geographical Influences. Precipitation Education, NASA. Retrieved from https://gpm.nasa.gov/education/sites/default/files/lesson_plan_files/geographical%20influences/Geographical%20Influences%20-%20City%20Climatograms.pdf

Kimball, J. W. (2022) Biology. LibreTexts Biology. Retrieved from https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Biology_(Kimball)/17%3A_Ecology/17.01%3A_Energy_Flow_through_the_Biosphere/17.1C%3A_Biomes

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