TRANSPIRATION INTRODUCTION

 

 

TRANSPIRATION BY TREES

Posted on March 9, 2011 by tedfloyd

 

Transpiration by Trees

Introduction
Water is absorbed by plant roots from soils and flows up stems to the leaves. Transpiration occurs when water vapour flows from leaves into the atmoshere.

Water absorbed from the soil, flowing up stems, carries nutrients essential for plant growth. The sun is the driving force for transpiration.

Transpiration is an important part of the water cycle. A large proportion of rain falling on land is transpired by plants back into the atmosphere.

In natural bushland surrounding Sydney, most rainfall enters the soil and 60% of the total rainfall is returned into the atmosphere by plant transpiration. In the suburbs there are not as many trees and only 10% of rainfall is transpired into the air and most rain lands on streets and houses then flows swiftly down gutters.

In Sydney a large gum tree transpires up to 200 litres of water a day. A well maintained garden in Sydney will transpire nearly twice the volume of water as the total rainfall.
Transpiration and water storage in garden soils
Plants in gardens will transpire up to 2,000 litres of water from one square metre every year (equal to 2,000 mm rain). The total average rainfall in Sydney is 1200 mm/year.

Home gardens planted with native species can be grown with a minimum amount of fertilizers and watering.

In many gardens vigorous plant growth is achieved by watering with valuable tap water. In times of drought this is an expensive and silly habit.

Water runoff from a house roof can be absorbed by a garden soil in an area equal in area to the house and transpired by plants back into the atmosphere.

Water runoff from of a house roof can be absorbed by a garden soil and transpired by plants back into the atmosphere.

The big trick is to be able to store water in the garden after heavy rain and to encourage all water to enter soils rapidly. A dry garden soil can store water equal to one third the total soil volume. A storm of 100 mm can be stored in 300 mm of soil. All water in heavy storms may not enter the soil in the short time period of a storm.

When suburbs are built many trees are cut down. Porous soils are covered by roads, paving and buildings. Rain falling on an impermeable surface immediately flows over the land into drains.

Surface runoff and flash flooding is increased when trees are cut down and soils covered by impermeable surfaces.

In Sydney a large gum tree transpires nearly 200 litres of water a day.

Native Australian trees have the advantage of having deep roots and a degree of drought resistance. Often the roots of gum trees reach down to the water table and are able to absorb water for plant growth when the surface soil is dry. Gum tree roots are often over 10 metres deep while the annual lawn grass Poa may only be 0.15 m deep. Poa lawns need to be regularly watered in the summer while gum trees surive for long periods between rain.

Gardeners should aim to harvest all the rain falling on the home block. Deep porous soils are needed to store large volumes of water and vigorous plant growth to transpire water into the atmosphere. A garden equal in area to the house is needed to achieve this plan. Water tanks will help to store water and tankwater can be used in toilets and for other suitable household needs.

All rainwater harvested on the home block reduces flooding and water pollution which is a serious problem in flood waters.
How Transpiration Works
Transpiration is the flow of water vapour from leaves into the atmosphere.

The driving force of transpiration is radiation from the sun heating the interior of leaves. Evaporation occurs at the surface of cells and water vapour flows through stomates into the drier atmosphere.

The loss of water at the leaf surface creates a suction pulling water up through the xylem tubes in the stem and roots and water is sucked out of the soil through root hairs Water movement by transpiration pull is assisted by osmotic pressure and capillary rise.

 The major factors affecting the rate of transpiration is the strength of solar radiation and the presence of available soil water. Transpiration is greatest in the middle of the day when maximum solar radiation occurs and nearly ceases during the middle of the night.

The rate of transpiration is increased by winds, low relative humid y and low atmospheric pressure.

Photosynthesis requires, carbon dioxide to enter through open stomates and oxygen to flow out. Water vapour also flows out through open stomates into the atmosphere. Stomates are small breathing pores mainly found in the lower leaf surface. Plants regulate water loss through transpiration by opening and closing stomates.

Normally stomates are open in the day and closed in night. Cacti and some desert plants conserve water by closing stomates in day and opening them in the night.

In different plant species the number, size and location of stomates helps to control transpiration rate. Often plants close their stomates during high temperatures and water shortages.

In the winter transpiration is slow and when deciduous trees lose their leaves nearly stops.

Higher leaf area in plants increases transpiration and at the top of a canopy, transpiration is greater than close to ground level. An open canopy facil ates transpiration compared to a dense, closed canopy. A tree standing alone has a higher transpiration rate than a tree in a forest surrounded by many tall trees.

There needs to be sufficient available soil water for transpiration. When a water shortage occurs, leaves will droop and wilting occurs.

An evaporimeter measures evaporation directly from a water surface. In Sydney the annual evaporation is 1800mm and is higher than the rainfall of 1200. The ratio of P/E (precip ation/evaporation) indicates when soil water is available for plant growth. In Sydney the annual P/E is 0.67 and in January is 0.33.

Normally transpiration can not exceed evaporation and under ideal cond ions transpiration may reach 95% of evaporation. In the middle of winter transpiration from deciduous trees when they have lost their leaves is less than 15% of evaporation.

Evapotranspiration is the sum of evaporation from the soil surface plus transpiration from plants.

———————————————————————————————————–

Plants in gardens will transpire up to 2,000 litres of water from one square metre every year (equal to 2,000 mm rain). The total average rainfall in Sydney is 1200 mm/year.

Home gardens planted with native species can be grown with a minimum amount of fertilizers and watering.

In many gardens vigorous plant growth is achieved by watering with valuable tap water. In times of drought this is an expensive and silly habit.

Water runoff from a house roof can be absorbed by a garden soil in an area equal in area to the house and transpired by plants back into the atmosphere.

Water runoff from of a house roof can be absorbed by a garden soil and transpired by plants back into the atmosphere.

The big trick is to be able to store water in the garden after heavy rain and to encourage all water to enter soils rapidly. A dry garden soil can store water equal to one third the total soil volume. A storm of 100 mm can be stored in 300 mm of soil. All water in heavy storms may not enter the soil in the short time period of a storm.

When suburbs are built many trees are cut down. Porous soils are covered by roads, paving and buildings. Rain falling on an impermeable surface immediately flows over the land into drains.

Surface runoff and flash flooding is increased when trees are cut down and soils covered by impermeable surfaces.

In Sydney a large gum tree transpires nearly 200 litres of water a day.

Native Australian trees have the advantage of having deep roots and a degree of drought resistance. Often the roots of gum trees reach down to the water table and are able to absorb water for plant growth when the surface soil is dry. Gum tree roots are often over 10 metres deep while the annual lawn grass Poa may only be 0.15 m deep. Poa lawns need to be regularly watered in the summer while gum trees surive for long periods between rain.

Gardeners should aim to harvest all the rain falling on the home block. Deep porous soils are needed to store large volumes of water and vigorous plant growth to transpire water into the atmosphere. A garden equal in area to the house is needed to achieve this plan. Water tanks will help to store water and tankwater can be used in toilets and for other suitable household needs.

All rainwater harvested on the home block reduces flooding and water pollution which is a serious problem in flood waters.

 

 

 

About tedfloyd

born 18th Aug 1946 at summer hill, sydney australia school north junee primary yanco agricultural high school leaving certificate 1963 sydney university facultary of agriculture major soil science 1964 to 1968 BSc Agr web. http://www.ramin.com.au/creekcare

 

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