Leaf area, photosynthesis, respiration and NAR
The photosynthetic capacity of the plant is mainly determined by the leaf area of the plant. An increase in the leaf area will cause an increase in photosynthesis, but if the leaves get too dense and start to shade each other, this increase will tend to disappear.
Opposite to photosynthesis is the process of respiration which is more or less in relation to the size of the plant. The Net Assimilation Rate (NAR) is the result of the balance between photosynthesis and respiration. For example, if the rate of photosynthesis is high and the respiration rate is low, the NAR will be high and the plant will grow fast. The reverse is also true.
Defoliation decreases the leaf area and the photosynthetic capacity of the plant, and at the same time increases the respiration rate. Defoliation will therefore cause a lower NAR which means a slower growth rate.
A vigorous plant will, however, soon recover from defoliation and will often be just as vigorous as an undefoliated plant if given time to recover. There are several reasons for this:
- No defoliation may cause leaves to shade each other, thereby decreasing the efficiency of the leaves. This seldom happens in sparse veld like the Karoo.
- No defoliation sometimes causes the accumulation of a lot of old leaves which have a lower efficiency than young leaves.
- No defoliation causes less stimulation of auxiliary buds and less development of new young leaves.
These three points again stress the fact that defoliation is not an unnatural phenomenon. It is clear that although moderate defoliation may be detrimental to the plant for short periods, it also has certain advantages in the long run. Too severe defoliation, however, has more disadvantages than advantages.
The higher the intensity of defoliation the bigger will be the reduction in the leaf area, photosynthetic capacity, etc, and the longer it will take the plant to recover. This is why many researchers (Americans) feel that at least half of the leaves of the plant must be left after grazing (take half, leave half).
The higher the frequency of defoliation, the less time the plant will have to recover from the defoliation. It is also clear that the frequency (or intensity) of defoliation becomes more important as the intensity (or frequency) increases. This is also true for most of the other processes influenced by defoliation, such as root development, plant reserves, DM production, etc. The season of defoliation has no special influence on these processes except that defoliation in the dormant season of summer grasses will have very little effect on these processes.
Root development
The root system supplies the plant with water, and mineral salts and also serves as a storage place for some of the plant reserves. A well-developed root system is especially important in dry areas where the root system has to ‘explore’ the soil in search of moisture.
Besides trapping light energy, green leaves act as pumps to help the plant extract water from relatively dry soil. A severely defoliated plant could therefore suffer from induced drought. Root systems are restricted if plants are repeatedly defoliated too heavily. All else being equal, proportionately less water and mineral nutrients will be taken up by such plants competing with others having larger root systems.
The activities of the root system are dependent on food substances produced by the leaves of the plant. Defoliation decreases the production of compounds necessary for root growth. Single moderate defoliation (+/-50% or less) usually causes a delay or stoppage of root growth for several days. The higher the intensity of defoliation, the bigger will be the reaction on the roots. Single high-intensity defoliation (80 – 90%) can stop root growth for several weeks. Successive high-intensity defoliations can even lead to the starvation of some roots and die-back from the tip because there is not enough time for the plant to recover.
The plant always tries to keep a balance between the size of the root system and the size of the aerial portion. After severe defoliation, the plant will have a relatively smaller aerial portion. In an attempt to correct this, the plant will neglect the root system for some time. The result is that the influence of defoliation is first seen in the roots and is more noticeable in the root system than in the aerial portion of the plant.
Effects of defoliation on root growth
As a result of the delay and stoppage of root growth caused by severe defoliation, the following characteristics of the root system can also be influenced by defoliation:
- Depth of root penetration;
- Root branching and root distribution;
- Absorption capacity;
- The lifetime of roots.
Moderate defoliation will not necessarily be detrimental to the abovementioned characteristics but severe defoliations (overgrazing) will be extremely detrimental.
Plant reserves
Plant reserves are organic compounds manufactured by the plant and stored in the more permanent parts of the plant during certain times, to be used by the plant at a later stage as a source of energy or structural material.
Reserves consist mainly of carbohydrate compounds (sugars and starch) as well as non-carbohydrate compounds (lipids and proteins). Reserves are stored mainly in the roots, rhizomes and stems (bases) of plants.
Small amounts of reserves are also found in the leaves where they are manufactured, but these are of little importance because they are subjected to removal and big fluctuations. The kind of reserves and the place they are stored will differ among the different plant species.
The accumulation of reserves can only take place if the number of compounds produced by photosynthesis (during the day) exceeded the number of compounds used by respiration and the growing processes.
The accumulation of reserves is of bigger importance in the case of summer grasses which become dormant during the winter, than in the case of Karoo bushes and shrubs which stay green throughout the year.
The biggest accumulation of reserves takes place during periods of decreasing top growth. For most plants, this is from March to May.
The condition of the plant with regard to the reserve supply is known as the reserve status of the plant. The reserve status of the plant is the mass of available reserves in relation to the dry mass of the plant. (The concentration [%] means very little as it gives little indication of the amount.)
The importance of reserves in the plant centres around two main phenomena, namely:
- Defoliation usually causes a decrease in the number of reserves because the plant utilises some of the reserves to start the new growth;
- The vitality of the plant is correlated with the reserve status of the plant.
A plant usually utilises some of its reserves under the following conditions:
- To start new growth after the winter or after a severe drought when there are no leaves to photosynthesize;
- After defoliation, especially when more than 60% of the plant is defoliated;
- During seed production because the demand for compounds for seed formation may become bigger than the plant can supply. In a case like this, some of the reserves may be used to supplement the shortage.
The higher the intensity of defoliation, the more reserves the plant will use for regrowth and the longer it will take the plant to restore its reserves. In the case of some grasses, very high intensity of defoliation can cause a direct physical removal of the reserves in the stem bases. The higher the frequency of defoliation, the less time the plant will have to restore its reserves. If this cycle is continued in such a manner that the plant is unable to restore its reserves, root dieback will occur and eventually lead to total death of the plant.
Under favourable conditions reserves can be restored within a month.
The plant will only utilise its reserves if photosynthesis cannot supply the demand. After severe defoliation (+/-90%) the reserves will initially play an important role in the provision of energy for regrowth. As more and more green leaves are formed, and the rate of photosynthesis increases, fewer reserves will be used until the plant will start to replenish the reserves again.
A decrease in the reserve status of the plant is also correlated with a poorer root system so it is often very difficult to distinguish between the exact function of the reserves and the influence of the poorer root system.
Growing point damage, shoot growth, growth form and seed production
Until recently it was believed that the reactions of plants to different defoliation treatments could be attributed mainly to the physiological reactions in the plant. It has recently been shown that the physical damage or removal of growing points can have an important influence on the reaction of (grasses) plants.
Defoliation will also alter the above-ground growth habit. If plants are grazed at flowering time, productive growth points will be removed and lateral growth buds will be released from dormancy. The plants will thus tend to grow vegetatively. Lateral tillering and branching will be encouraged if plants are closely cropped during the vegetative stage. This can result in a more prostrate growth habit which is desirable in a grazed pasture. One can also encourage seed production by defoliating at the right time.
Some pastures, (e.g. annual medics) must be closely cropped in the vegetative stage to encourage branching. The more branching that takes place, the higher the seed yield, all else being equal.
Defoliation can be used to control the composition of a pasture. The arrangement of leaves differs widely amongst pasture species and some reach the critical leaf area index (the ratio between leaf and ground area at which about 90% light interception takes place) at a much lower height than others. Plants with horizontally arranged leaves (most clovers) may reach this stage before they have grown to a height of 20cm while plants with vertically arranged leaves (many types of grass) may not reach this stage at all in a season of growth. Taller plants will also be in a better position to compete for light than shorter plants and lenient defoliation will favour tall plants and those with vertical leaves, whereas severe defoliation will favour shorter plants and those with horizontally arranged leaves.
Grasses
Diagram of a grass plant and its different parts.
In the case of grasses, many of the growing points are normally located near the ground level and are referred to as tillers. These growing points are normally not removed or damaged by defoliation. These low-lying growing points are normally vegetative which means that they produce leaves. At a certain stage, some of these growing points become reproductive. With the change from the vegetative to the reproductive stage, the growing point stops producing leaves and starts to produce an inflorescence. At the same time, the growing point starts to lengthen and becomes vulnerable to grazing.
If the main aim of a rest period is to stimulate seed production of grasses, care should be taken not to remove these reproductive growing points. It is also important to remember that these growing points are vulnerable to grazing long before the inflorescences become visible.
The reproductive growing points of valuable perennial grasses like red grass can be high enough early in spring to be removed by grazing.
A full season’s rest is therefore usually recommended for maximum seed production of climax grasses. Sub-climax grasses like Eragrostis species do not need such a long period to produce seed. A rest period during autumn is sometimes also recommended for maximum seed production during the next season because it assures a physiologically healthy plant in the next spring.
The presence of many reproductive growing points on a grass tuft suppresses vegetative growth to some extent. The removal of reproductive growing points will stimulate the development of auxiliary buds and vegetative growth. If maximum dry-material production is the main aim of a farmer, it might be a good practice to graze grasses early in the reproductive stage. This will prevent excessive seed production and stimulate vegetative growth.
Below is a detailed explanation of the effect of ‘defoliation’ or cutting on a Themeda triandra grass plant and the difference the defoliation length makes.
Diagrammatic representation of the growth after clipping of a Themeda triandra tiller.
Diagrammatic representation of a vegetative Themeda triandra tiller before clipping (A), immediately after moderate clipping (B), 2 weeks after clipping (C), and 4 to 6 weeks after clipping (D).
Four typical stages of the defoliation process are illustrated:
a) A vegetative Themeda triandra tiller before clipping. Two of the living leaves have emergent ligules; while the three youngest emerged leaves have their ligules protected within the older leaf sheaths.
b) The same tiller immediately after moderate clipping. The two youngest leaves that had emerged before clipping no longer have any portion of their leaf blades emerged.
c) The same tiller approximately 2 weeks after clipping. The two oldest leaves which had ligules that had already emerged at the time of clipping have not expanded, while the younger clipped leaves have continued expanding. The degree of expansion increases with diminishing leaf age. The oldest leaf, with an unmerged ligule at the time of clipping, is now fully expanded.
d) The tiller approximately 4-6 weeks after clipping. The two oldest leaves have senesced and the three leaves that were still expanding at the time of clipping are now fully expanded (ligule emerged). Two new unclipped leaves have emerged, but their ligules are protected within the older leaf sheaths. The tiller has five emergent green leaves, the same number it had before clipping.
Karoo bushes and shrubs
Growth points of shrubs and bushes.
In contrast to grasses, the growing points (terminal buds) of Karoo bushes and shrubs are carried mostly on the outside of the plant or at the ends of tree shoots. It is therefore almost impossible to graze veld without removing many of these growing points.
These growing points can also change from a vegetative to a reproductive stage at certain times (the influence of the stage of the growing points at the time of defoliation has not been investigated yet).
Observations have shown that moderate defoliation of Karoo bushes and shrubs leads to the stimulation of auxiliary buds and side stems. This means stimulation of vegetative growth which can prevent Karoo bushes from becoming too woody and less palatable.
On the other hand, it must be kept in mind that the buds and flowers of Karoo bushes and shrubs are usually the most palatable and most easily attainable parts of the plant. Even very light grazing can therefore have a very great effect on the seed production of shrubs and Karoo bushes.
Summary
- No grazing for long periods.
- Poor vegetative growth for both types of grass and Karoo bushes (tiller development, shoot growth, auxiliary bud stimulation, etc).
- Moderate to good seed production for both types of grass and Karoo bushes.
- Over utilisation.
- Poor vegetative growth because of physiological damage to the plant.
- Poor seed production for one of the following reasons:
- The direct removal of reproductive growing points, inflorescences, flowers and seeds by the grazing animal.
- A shortage of nutrients for seed production because of a too-small leaf area and physiological damage.
- Moderate grazing with sufficient rest periods.
- Good vegetative growth.
- Good seed production.
DM production and the quality of the defoliated material
The DM production and the quality of herbage, discussed earlier are two interrelated facets that must not be ignored since they will determine to a large extent the quantity and quality of marketable animal products.
The general ecosystem
It is known that there is a delicate equilibrium between the climate, the vegetation and the soil and that plants are always in competition with each other for light, nutrients and water.
Plants are not equally resistant to defoliation, not equally palatable, etc, and defoliation or grazing can therefore cause big botanical changes. Apart from the changes in the botanical composition, the following factors may also be influenced by grazing or defoliation.
- The production and vigour of the vegetation;
- The percentage of plant cover;
- The infiltration, run-off, evaporation and transpiration of water;
- Water and wind erosion of the soil;
- Temperature variations in the soil.
Our knowledge is insufficient to deal with each of these factors individually. Further, many of these factors are influenced by the presence of animals, apart from their defoliation effect.
There is an incessant (continual) change in the ecosystem towards a new equilibrium. In this process, the plants best adapted to the soil, climate, and grazing treatment will tend to dominate the vegetation.