Tropical Gardening: Colombia’s drylands expanding
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.
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-on 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 that 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
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 such as 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
can 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 such as the Bird of Paradise, autograph
tree, beach heliotrope and naupaka give the luxuriant look and are still
drought resistant. Many palms such as 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 such as macadamia and many others in the Protea group.
The use of mulches also will 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.
A series of dry years and increased pressure on water
supplies made us aware that water is an exhaustible resource. Limits on our
water resource mean we can sustain only a certain level of population.
Too many people can seriously threaten our water supply.
Limited water could mean a definite reduction in the quality of life in Hawaii.
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