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19. List and understand the role of important abiotic features of the Hawaiian environment which impact plant growth:
a. The effect of solar radiation (temperature and light factors) on day length, latitude, air movement, the oceanic factor and elevation.b. The movement of trade and seasonal winds, patterns of orographic rainfall, and rain shadow.
c. Fresh and salt water sources, erosion, volcanism, micro climates and tropical soils.
Suggested Reading: Chapter 2 in Carlquist's Hawaii: A Natural History.
I. HAWAI'I'S UNIQUE CLIMATE AND RELATED FEATURES.
A. We think of temperature as being mild and unchanging in Hawaii with plentiful rainfall. That is more the tourist image. It is true for most of the areas in which people live on the islands but NOT TRUE if you look island-wide.B. Hawaii has dry deserts, warm and low, like the Ka'u desert of the Big Island and the cold deserts, like that found in the Haleakala Crater. On Mauna Kea there are freezing temperatures and snow. These are related to the elevation and geologic features of the islands.
C. In reality, Hawaii is a mixture of some of the most equitable temperatures in the world with some of the most extreme.
1. As a result, of the 13 major life zones found in the world, 11 are found in Hawaii. That is the greatest number found in the smallest area in the world.2. Life zones are areas of distinctive plant and animal forms: like deserts, grasslands, and tropical rain forests.
D. Hawaii has many micro climates, small places or regions that exist side by side with strikingly different climatic conditions. Special topographic and other features contribute to their formation.
II. Factors which Determine Hawaiian Island Temperatures.
A. Temperature and latitude.1. The islands are located in the tropical and subtropical zones: about 20 degrees North latitude above the equator - the same as Calcutta, India and lower Egypt.
2. Fortunately in Hawaii, we do not have as hot a climate as these places for reasons given below. Also, the islands unlike a number in the Pacific, are north of the equatorial zone where there are warmer temperatures.
B. The oceanic effect on temperature.
1. A major factor for generally widespread mild temperatures of the Hawaiian Islands is the proximity the land to the ocean.2. Given the same amount of temperature variation, large bodies of water, like the ocean, increase and decrease in temperature much less than a similar area of land.
As a result, any land near a large body of water remains milder than land farther away from the water.This results in what is called the "Mediterranean climate" or is called "the oceanic effect."3. Since the Hawaiian islands are small in relation to the surrounding ocean, their surface temperatures are moderated by the ocean presence as well as by the cooling trade winds.
C. Day length and average temperatures.
1.There is relatively little variation in day length: about 13 1/2 hours during the longest day to about 11 hours for the shortest. That is only a 2 1/2 hour variation from the shortest to longest day length.
2. The result is a small amount of seasonal variation in temperatures because of the amount of incoming solar energy varies little throughout the year.
3. For instance, in Honolulu, the warmest month is August with average of 79 degrees and coolest in January and February with averages of 72 degrees F - only a 7 degree difference!
4. Trade winds also moderate temperatures. Wherever trades are blocked from areas of the islands by the mountains, like at southern coasts, some of the warmest areas of the islands are found: Honolulu and Wai'anae, Oahu and Lahaina, Maui.
D. Impact of elevation on temperatures.
1. As elevation increases, temperature decreases and the higher it is, the cooler it is. You've experienced this, for example, when you drive up the Pali or any mountains, it is much cooler than at the beach.2. Elevation influences the impact of rainfall. The air holds less moisture at lower temperatures; therefore, it forms precipitation as it cools or rises.
3. Elevation also has an effect on plants and where they can grow. Cool-adapted plants grow at higher elevations. The cooler temperature often prevent warm-loving plants growing at the higher elevations.
III. TRADE WINDS AND RAINFALL
A. The trade winds.1. The "Trades," are global patterns of air movement due to the rotation of the earth and other factors.2. The Hawaiian Islands are in the belt of the trade winds. These winds were "harnessed" by the sailing ships of the early European traders to go across oceans and engage in trading. The trade winds move steadily for most of the year from north east to the south west. (See Figure 19-A)
3. The side of the island which they first impact is called the WINDWARD side and the opposite side is the LEEWARD side.
B. The KONA storms. (See Figure 19-B)
1. These winter-time storms, from November to April, are in the wettest period of the year, called the Hoo'ilo season by the Hawaiians.2. The trade winds from the northeast slacken during this time, allowing storms from the south to more easily approach the islands. Hence the name "kona," which means the leeward direction, indicates the direction from which these storms come. The kona winds are generally warmer carrying moisture which is dropped as rain over the whole island, more or less evenly.
3. The lower elevations and southern, drier side of the islands get most of their rainfall at this time. It is about 25-30 inches each season. The low Leeward Islands of the northwest chain get almost all their rain this way.
C. The KAU season
1. In the summer months, the trade winds are the major source of rainfall for parts of the islands.2. The trade wind belt passes directly over the islands in the summer bringing summer showers and pleasant relief from heat.
3. These winds are heavily laden with moisture, obtained as they pass over the broad expanse of the ocean. When the warm moist air encounters the mountains on the windward side of the islands, the winds are forced to rise.
4. The rising air is cooled and no longer can hold as much moisture. This moisture is dropped as precipitation or rain. Heavy rainfall develops on the windward slopes, heavier still at the top, some over the top and down the leeward slope. (See Fig19-C)
D. Orographic Rainfall, Temperature Inversion Zone, and Rain shadow.
The three phenomena above are factors in the amount and pattern of rainfall due to the positions of mountains. Most plants, have fairly specific water requirements and amounts and patterns of rainfall are major determinates of where plants can or cannot grow.
1. Orographic rainfall
a. In the term OROGRAPHIC, " ORO" - means mountains and "graphic" means form. Therefore, it has the meaning of mountains "forming" or creating rainfall.b. OROGRAPHIC rainfall is caused by moist air that is intercepted by mountains causing it to form rain as it cools and rises. In the islands, it is often due to the interaction of the trade wind patterns and the mountains. It's as if the mountains "pull" moisture out of the clouds.
c. What is the contribution of the orographic rainfall to the overall yearly accumulation? The annual rainfall for O'ahu is about 45 inches island-wide. 25-30 inches of that rainfall comes from the winter Kona storms which usually cover the island evenly. The other 15 to 20 inches comes from orographic rainfall. It provides about a third of the annual rainfall!
Islands with little elevation will not have mountains resulting in a much lower annual rainfall because of the lack of the supplementary rainfall generated by orographic sources.
(See Figures 19-B and figure 19-E)
Rainfall figures for Hawaii are misleading, when compared to similar figures for continental areas. For example, 30 inches a year rainfall in a temperate climate may support trees. In Hawaii, only grasses and dry adapted plants will grow under such amounts of rainfall per year.
This is due to several factors: soil porosity, constant warm temperatures, and winds. Because of these factors, water is more rapidly lost from the soil and therefore is not available to plants as compared with areas in temperature climates with similar yearly rainfall figures.
d. The orographic rainfall is not distributed evenly. See Fig.19-C for the primarily pattern of orographic rainfall and variations due to different topography of the islands.
Be able to explain each of these patterns based on the discussions above.
e. Look at Figure 19-E showing rainfall pattern on O'ahu and compare with the vegetation zones for O'ahu in your text by Sohmer and Gustafson on page 41.
What is very striking about these two patterns? Are they similar or different? Explain. (The different vegetation zones follow rain map lines in Figure 19-E. For example, rain forest is found to a large extent where rainfall is heaviest. Similarly, dryland shrub and forest, as well as coastal vegetation zones are largest at lower elevations and the leeward side of the island where rainfall is low.)
2. The TEMPERATURE INVERSION ZONE. (See Figure 19-C-3.)
a. The temperature inversion zone is formed when hot air, which normally rises without restriction, is trapped by cooler air above. This tends to happen at 5,000 to 7,000 foot elevations and above land masses.b. The inversion layer develops mostly in the summer months, when the air above the land mass becomes warmer. Moisture is forced out of the rising trade winds at the inversion elevation where they are trapped beneath the zone. Orographic rainfall can be result.
c. If the mountain rises above the inversion zone, the dry air now devoid of moisture released below, may rise to the top of the mountain creating desert-like conditions above the inversion zone.
d. On the Hawaiian Islands, there are three mountains high enough for temperature inversion zones to develop: Mauna Kea and Mauna Loa on the Big Island and Haleakala on Maui.
e. In recent years, temperature inversion zones have plagued cities surrounded by high mountains, like Los Angeles and Mexico City, where noxious air is caught under the inversion layer causing serious air pollution problems.
3. The RAIN SHADOW. (See Figure 19-D)
a. The rainshadow is an area of lower rainfall because it is "shadowed" or blocked from rain clouds by a mountain. In the case of Hawai'i, the mountain range is usually blocking the path of incoming trade winds which carry much rain causing moisture.A regular shadow is created by something blocking the light. Rain shadow is caused when some barrier prevents rain in an area.
b. For example, on O'ahu there is a rain shadow on most of the Wai'anae Mountains due to the Ko'olau Mountains. There is even a double rain shadow in the Wai'anae region along the coast due to two mountain ranges standing in the way of the trade winds! That helps explain why is it the hottest and driest part of O'ahu.
c. One area which escapes the rain shadow effect is the highest point of the Wai'anae range, Mount Ka'ala. It is higher than any Ko'olau peak by 1,000 feet. As a result, the trade winds and rain reach the Ka'ala peak. (See illustration of this in Figure 19-D)d. Another example is the islands ofKaho'olawe and Lana'i which are in the rain shadow of the mountains on Maui. They receive 30 or fewer inches per year which is very dry in our climate.
IV. FRESH AND SALT WATER ON THE ISLANDS.
A. Lava rock is very porous, unlike most continental rock. There are very few permanent lakes or rivers in the Hawaiian Islands because of this porous rock. The permanent river, Waimea on Kaua'i, is an exception because is it being fed by the world's rainiest spot on Wai'aleale mountain.
B. At the top of the large young mountains of the Big Island, the rock is so porous that the water table is literally out of reach for use of people living at the higher elevations. Most of them have water catchment systems where they catch rainfall and store it in large cisterns beside their homes.
C. Rain water rapidly sinks into center of island and collects just above sea level in an area called a fresh water lens. It floats on the base of the island, which is saturated with salt water. Fresh water floats on salt water because it is less dense. This is called the Ghyben-Herzberg Principle. Fortunately for us, the fresh water does not readily mix or sink into the salt water. (See Figure 19-F)
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D. Two physical features of the islands help hold internally an increased amount of fresh water, especially above sea level. These are caprock and dikes.1. Caprock forms at edge of islands made up of old coral beds and sedimentary deposits. They take time to form, so the island of O'ahu has many, while the younger Big Island has few. They form below the shoreline.Dikes and caprock formations help to create a large lens of fresh water on the island of O'ahu. In constrast, the Big Island, loses much of its fresh water which percolates to the base of the island drains off into the ocean.
2. Dikes are formed by still active volcanoes when splits or cracks form and later eruptions force magma up into cracks under pressure forming a dense basalt. They can be at high elevations inside mountains. This dense basalt is fairly water-impermeable barrier trapping a great deal of water behind them. (See Figure 19-G)
3. Waihe'e pumping station Windward has a large dike which holds the water at a high elevation. This makes it ideal for delivery at lower elevations because less pumping is needed.
E. . Fortunately for us on O'ahu, we have a large fresh lens because of dike and caprock formations. Unfortunately, because we are wasteful in water use, we will face water limitations in the near future.
A. Often in the Hawaiian islands often there are sharp environmental changes from one location to another, thus forming small adjacent areas of widely different climatic conditions. These are called microclimates.As a result, there are very different plant and animals living quite close to each other under these different conditions.
B. One example can be seen in adjacent ridges and valleys. Ridges get more sunlight and wind compared to the immediate downslopes on either side and into the valleys where it is moister and cooler. The valleys are even moister, cooler and less windy.
C. Another example is caused by the physical difference between different types of lava.A'a lava formation which has crumbly texture, which catches and holds organic particles and water more rapidly than than a surface covered with pahoehoe. ,. A'a lava then will be colonized by new plant growth more rapidly than the surface covered with pahoehoe.
D. Kipukas.
1. Another special microclimate found on the newer islands, especially the Big Island, is the kipuka, sometimes called "an island within an island".
Kipuka
2. These are regions of older lava flow surrounded by newer flows. The more recent lava flow surface is hotter and drier than the nearby older flow surface. Also it has not developed as much soil and plant cover.
E. Augmented Precipitation due to cloud cover.
1. Another interesting phenomenon is precipitation due to the leaves of forest plants which can condense a considerable amount of water from clouds.2. At the summit of the island of Lana'i, the rainfall is only 30 inches annually which alone cannot support the forest that grows there. However the region does have cloud cover believed to provide another 20 inches of water resulting from leaf condensation!
3. How does the leaf condense moisture from the air? The thin leaf surface is slightly cooler than the surrounding air. When the slightly warmer air contacts it, the moisture in the air, especially if it is a cloud, will condense on the leaf surface and drip to the ground.
VI. VOLCANISM.
