Desertification is nothing new to the world, but global warming combined with human activities is accelerating this phenomenon. The old saying “Rain follows the forest and desert follows man” is rather depressing but it doesn’t have to be if we understand how deserts are created and how we can reverse the process. The Brazilian Amazon forests covered almost twice the size they do now prior to when the first Europeans entered the scene. Now vast grasslands that are prone to fires put more pressure on the remaining forests.
It is difficult to do much about another country, but we can learn from mistakes made in other places to make sure we don’t repeat them. Luckily we live on a relatively big island. Our population is small and we have a sense of aloha aina, so let us focus on our own gardens.
A garden planted with no thought given to dry spells will do well in rainy periods but deteriorates without irrigation in dry periods, even in East Hawaii. Fortunately, many garden plants in Hawaii are fairly hardy when it comes to short water supply, so we have a long list to call upon. It’s important to vegetate these areas so that our islands don’t look like Death Valley in years to come.
There are two factors that make these plants able to survive moisture stress.
First, some plants are notably resistant to drought. This quality is centered largely in the cellular structure and has a bearing on the economy with which the plant functions. Some plants have the ability to carry through extended dry periods because of a happy faculty of closing the pores of the leaf against transpiration, or turn the leaf back or edge, to the sun. Others root deeply to tap, and have available for dry periods, any accumulated moisture of sub soil. Our native acacia koa is an example.
The garden environment is the other critical factor. Water use is a process controlled by energy. The source of that energy is the sun. To move water out of the soil directly or through the plant and away into the atmosphere requires energy. The amount of energy available and the nature of the conducting medium which is the soil-plant-atmosphere complex determine how much water will be used in a given time.
Consider the amount of energy available on a piece of the landscape. The total available is the solar radiation which reaches the earth’s surface, plus the heat in heated-air radiation which reaches the earth’s surface by wind. The amount of energy reaching the earth’s surface is limited by the cloud cover and the dust in the atmosphere.
Air that is heated in another and drier part of the landscape and moves across the area of land in which we have our plants growing and also adds heat. The result is a larger amount of water evaporated than we would predict purely on the basis of solar radiation.
This is why the more shade and wind protection from trees we have in the garden, the less water is required to keep moisture levels up. And, conversely, the more asphalt and concrete to heat up, the more rapidly our planted area dries up, even in high rainfall areas like Hilo.
It helps us understand the reason for the common observation that an inch of general rainfall is much more useful and long lasting than an inch of irrigation water.
In effect, when we irrigate a small area, we are creating an “oasis.” If we have low, relative humidity and enough wind to move hot air across our irrigated surface, we can have losses of water nearly double that we would expect from direct solar radiation.
Besides the wetness of the soil and the plant, the nature of the plant itself has considerable effect on the amount of water lost into the air. The height of the plant and the roughness of the surface have an effect on the wind movement and mixing of air across the surface of the vegetation. A rough surface will cause more water loss than a smooth surface.
The amount of water conducted away from the soil and the plant surface depends on wind movement, wind speed, temperature of the air and the vapor pressure or relative humidity of the atmosphere. If water is conducted away very rapidly by rapid wind movement or low relative humidity, we may have high rates of water use.
Plants that are tolerant of salty beach conditions often use less water than many soft, luxuriant jungle plants because they are streamlined for water conservation. However, plants like the bird of paradise, autograph tree, beach heliotrope and naupaka give the luxuriant look and are still drought resistant. Many palms like the date and coconut also have this quality.
The overall conditions for a high rate of water use are rapid movement, high temperature and a low humidity and bright sun with no cloud cover or shade.
What can we do in managing the soil to take advantage of our knowledge of the factors affecting water-use rates? First of all, we can irrigate only when the soil-water becomes low and plants begin to show evidence of wilt during the hottest part of the day. This forces deep rooting. Daily watering tends to promote shallow roots.
We can understand that we will have to irrigate sooner following a previous irrigation than following a general rainfall. And we can provide soil with good physical and chemical properties for deep rooting of plants.
Proper fertilization will help accomplish this. Also, poor soils should be improved with the necessary amendments to help the plants develop good root systems. Addition of well rotted organic matter or compost often helps increase moisture and nutrient holding capacity. In many Hawaiian soils, available phosphorus is lacking. This is essential to root growth, so addition of this element is particularly important with many plants. Exceptions to this are plants that evolved in low phosphorus soils like Macadamia and many others in the Protea group.
The use mulches will also help conserve soil moisture.
Proper planning and maintenance of our gardens will help in the short run, but we must do something about the future of the islands as well. Reforesting some of our Hawaiian grasslands will help restore the rainfall patterns, sequester carbon and supply oxygen. Our forests are the lungs of the planet.
We can mitigate global warming just a little bit by planting a tree.
