THE IMPORTANCE OF THE SUN TO LIFE AND THE FUNCTION OF THE ECOSYSTEM

-by William Moir

-edited by C. Wayne Cook

This is a Colorado State University Range Science Review…..The verbage was prepared by Dr. William Moir of the Range Science Department at Colorado State University in 1976…..Topics to be presented include the principles of canopy architecture, photosynthesis, and primary productivity on rangeland ecosystems…..

The amount of light energy falling on an acre of temperate rangeland during a clear summer day is a staggering 22 trillion calories. For the agronomist, this quantity of energy can represent an energy transfer of about 4% to the shoot portions of his crop, or a day's yield of about 150 pounds of dry herbage. The harvest of range forage however, almost never approaches this yield. Why not?

The agronomist has had about 50 generations of genetic selection to give plant varieties incredible production efficiency in his cultivated fields. However, in native grasses, forbs, and shrubs of arid rangelands, natural selection has been operative for as long as 30-50 million years toward much the same end; that is, an efficient command of available energy in the ecosystem. What, then, are some causes for the production discrepancy between fields of the agronomists' "green revolution" and the vast acreage of western rangelands where natural selection for survival and adaptation has been going on for thousands of plant generations?

We are aware, of course, that about 45% of western rangelands have a mean annual precipitation of less than 15 inches per year. To realize higher crop yields in these arid range areas requires luxurious applications of water – something the range manager cannot provide or afford. We also suspect, although records are not altogether clear, that grazing animals of the western range were a periodic influence in the pre-white man environment of natural selection and adaptation. The large herbivores of the Great Plains, for example, were migratory and with the coming of domestic herbivores, the frequency and intensity of grazing may have substantially changed.

In the book, "The Great Plains," Walter Prescott Webb mentions three important tools: railroads, barbed wire, and windmills. These drastically changed the environment of mid-continental North America. Barbed wire did away with erratic migratory influences of the grazing animal and permitted sustained cropping of range plants for the first time. Under this evolutionarily new range environment, the herded animal became man's principal means of manipulating vegetation composition and structure of the ecosystem. In this presentation, we would like to explore some of the possible methods and principles by which vegetation structure can be improved to increase the amount of the 22 trillion calories per acre that reach range plants through solar energy.

In a review on the effects of light energy on a number of grass and pasture species, it was found that most species grow best under maximum light conditions. Highest yields were attained under full sunlight conditions, but were realizable only when other environmental factors were not limiting. Subsequent studies on the photosynthetic capability of leaves have shown that not all green leaves of the same plant are equally productive. As leaves age, they lose considerable capacity to fix solar energy into chemical energy. Young leaves, generally have the greatest photosynthetic ability.

Another principle that has been effectively exploited by agronomists is to distribute green leaves in as dense a manner as possible over the land. The trick here is to minimize mutual shading by green leaves, but at the same time, pack leaves so closely together that a light beam may pass through as many as 5 or more leaf layer equivalents before reaching the ground. Impossible, you say? Not at all. The natural growth form of many native plants, particularly grasses and small leafed shrubs, permits an astonishing display of leaf surface area over the small units of land area in which these plants are growing. The trick is to get these plants to grow close together. The more bare ground we see on rangeland the more solar energy is wasted.

Finally we must acknowledge that rangeland greenness is a seasonal event. The longer we can maintain the green season, the more solar energy we can harvest by photosynthesis. In native rangelands a mixture of plant species grow together. Some of these are usually dormant, while others are green and active. Thus, we speak of cool and warm season grasses, spring and summer forbs, and so forth. Each plant population may exploit certain resources of its range environment that are available to it, and not to other plant populations. This may occur for a limited time only. The more diversity in the makeup of range plant composition, the more extended is the overall growing season, and the more completely are the total resources of the range environment exploited.

Now, let us examine the four principles of effective light capture on rangeland acreage.

• First, green leaves should be exposed to full sunlight conditions; that is, little or no shading should exist, since shading reduces photosynthetic capacity of the leaf and plant.
• Second, green leaves should be young, and physiologically vigorous.
• Third, leaves should be spaced as closely together as possible, in a manner that minimizes the possibility of the light beam striking the bare ground.
• Fourth, the longer the duration of active photosynthetic greenness over the entire potential growing season, the greater the quantity of solar energy converted.

The next breakthrough in our quest for increasing the productivity in range ecosystems may be in ascertaining in detail the ideal architectural arrangements of foliage in the multitude of range vegetation types. Then the important task will be learning the necessary techniques for such achievements.

Granted, that an inherent limitation in rangeland photosynthesis is the availability of soil water, are we nevertheless applying the four principles of effective light capture to range vegetation during those seasons when soil moisture is available for plant growth? At the moment the answer, in general, seems to be no. Look, for example, at our first guideline -- green leaves should be exposed to full sunlight, or full skylight. On many rangelands, our first impression that the vegetation canopy is flooded under intense solar radiation is faulty. Closer examination of the leaf environment will reveal that litter and mulch, the dead inflorescences of the previous growing season, and other non-green parts of the canopy architecture are exerting considerable shading influence. Young foliage at the start of the growing season may be exploring its way through the shaded tangle of the previous year's growth. One means of removing this dead material is fire. When reviewing the role of fire in grasslands, Daubenmire reported in 1968 in Adv. Ecol. Res. that many grassland ranges, especially those in more humid climates, were stimulated toward increased growth if fires occurred during dormancy. This is especially true, for example, in tall grass prairies. Now fire, of course, produces many changes in the microenvironment of range grasses, and one of these is the increased solar radiation intensity at the leaf and ground level. Reduced shading of young leaves in burned grassland is probably one of the factors that enhances production.

In some rangelands, however, fire can be wasteful of accumulated photosynthetic energy in the form of dead plant parts. Burning converts the chemical and nutrient quality of plant biomass to gaseous and inorganic forms. This is the exact reversal of what photosynthesis accomplishes. The grazing herbivore can also be used, especially on winter ranges, to reduce the quantity of litter and old growth that might shade next season's new foliage.

It is useful here to recognize two pathways of energy flow in rangelands. These are the consumer and the detritus pathways. Both result in the disappearance of organic matter, but the detritus pathway is almost always the slower of the two. Energy flow in detritus food chains is mostly the consequence of microbial and micro-arthropod activity. By regulation of the kind and number of domestic and wild grazing animals on rangelands we can control energy flow through detritus and consumer pathways and the amount of non-green material in the aboveground biomass of vegetation.

We can also regulate canopy architecture and the proportion of green and non-green plant parts with grazing intensity during the active growing season. As photosynthetic products build up in well-illuminated canopies, physiological changes in plants may cause a shift from vegetative to reproductive growth. Further leaf growth may cease, and flowering and seed production may ensue. A great deal of the plant's chemical energy then channels into the production of coarse flowering stalks. This plant material is high in cellulose and comparatively low in other nutrients. When the herbage dries with maturity it is often unpalatable. The culms also decompose slowly, and may contribute heavily to litter and dry growth. As leaves are removed by grazing, grasses often continue to produce new leaves rather than shift into flower and seed production. Thus the green canopy perpetuates itself undiluted, so to speak, by non-green parts of the canopy mass.

One problem of effective grazing regulations is the determination of just how much defoliation can occur in order to remove only "excess" photosynthetic foliage and still maintain normal foliage efficiency and plant vigor. Here, our second principle of effective light capture reenters the system. Occasional removal by grazing at proper developmental stages in the plant's development can result in excessive removal of photosynthetically active young leaves. This, in turn, will lead to a decline in future range productivity.

Let us look at the third guideline in efficient energy capture of sunlight by green range plants: leaves should be spaced together as closely as possible in a manner that minimizes the possibility of the light beam striking bare ground. Again, many range practices seem to fall short on this score. Farmers wouldn't dream of planting corn at three-foot intervals. Yet how often do we see rangelands with large bare areas???? We can pick plants very close together, agronomists have learned, if green leaves can be oriented in an upward rather than a horizontal direction. This upward -- or erectophile -- orientation minimizes the shading of lower leaves by upper ones of the plant. It may be no mere coincidence that many grasses and other plants of rangelands have primarily erectophile orientation.

Another trick of evolution in plant spacing concerns what we might call canopy roughness. The more variation there is in the height of leaf-bearing plant structures, the more overall surface area a canopy presents to the sun. If the upper canopy surface presents a flat, monotonous plane, then that canopy volume cannot maintain many leaves at high photosynthetic efficiency. On the other hand, a rough, undulant canopy consisting of plants of different heights and life form, has greater light capture efficiency because more leaf mass can be supported per unit of ground area.

Sward growth, in relation to patterns of defoliation, relates to the quantity of leafy growth that could be harvested by different clipping patterns affecting the heterogeneity of the crop height. A harvest pattern that produces alternate high and low canopy tops generally gives greater regrowth of hay grasses than harvests with uniformly high or uniformly low cuttings. We cannot, of course, dictate to a cow just how high it should clip a plant. However, by varying the proportions of grasses of different sizes in a pasture, we can space plants closer together and still maintain high light conditions around individual leaves.

The yield, canopy structure, and light interception of varieties of grasses in mixed culture and monoculture shows that mixed swards give superior yields under conditions of high fertilization and frequent defoliation than a single variety growing by itself. The fact that a greater percentage of the intercepted light energy was distributed over the most photosynthetically vigorous young tissue may explain the higher productions of the mixture when compared to the monocultures. Canopy structural diversity may be one of the means whereby native range plants can maintain high foliar densities and efficient light capture during the growing season. If each of the herb and shrub could be harvested in some useful, sustained-yield program, then this is probably true. More research is needed in this area, but arguments on the basis of canopy geometry show that, indeed, the mixed life form presents a more effective energy trapping foliar surface.

The seasonal duration of plant greenness depends on many factors in arid lands: temperature, availability of soil moisture, soil nutrients, and the composition and life form of range plants. However, for any single plant population on a given range there is usually a comparatively brief season of foliage display, flowering, and seed dispersal. This brief sequence of plant activity is usually followed by a comparatively long season of dormancy. In extending foliar greenness over longer seasons, we can take advantage of the fact that different plant populations have offsetting periods of seasonal activity.

Again, we are reminded, for example, of our concept of warm- and cool-season grasses, or a combination of such species that have varying periods of growth, maturity, and dormancy. The potential growing season for most pastures and ranges may greatly exceed the growing season of any particular range site. But taken in their entirety, we might carefully regulate through seeding or grazing the plant composition in a way that can produce green growth at any time over the potential growing season. In doing this we must capitalize on our knowledge of growing requirements for diverse range plants. When particular combinations of limiting factors such as temperature and soil moisture are not restrictive for a particular plant population during some portion of the growing season, then the range manager can see to it that the green plant population is, indeed, well represented in the total vegetation composition. The maxim is to "minimize the duration of range dormancy" by making use of the staggered phenologies of diverse plants. In practice, this is often difficult -- especially on depleted rangelands. But how frustrating it must be at times when soils are moist and temperatures optimal for growth to have barren brown rangelands because the appropriate plant populations for that season are lacking!!!!! And how many of those 22 trillion calories are merely transformed to heat or reflected off the soil or straw colored plant surfaces!!!!!

Let us conclude with an example from an important range type in western North America that illustrates three points concerning canopy architecture, photosynthesis, and primary productivity. Many range types can well illustrate our points.

It has been observed many times that any shading, whatsoever, by trees and shrubs reduced dry matter production of lower growing forbs and grasses. Observations show that most lower growing plants grew best only when maximum possible light conditions prevailed in the leaf environment. All understory growth achieved maximum photosynthetic capacity when other factors, such as soil fertility was not limiting.

The second point is that green leaves should be young and physiologically vigorous. Unfortunately, there is scant published data on age and photosynthetic capacity for herbaceous plants as in undergrowth.

Our final point about seasonal periodicity is well illustrated in native pine-bunchgrass rangelands. In the Pacific Northwest, bunchgrasses, such as sandberg bluegrass, green up early in the growing season. Their root systems make use of soil water at shallow depths. Later, grasses, such as fescues, stipas, and a shrub (bitterbrush), green up as somewhat deeper rooting plants make use of soil water deeper in the soil profile. These deeper rooted plants present a rougher canopy cover to capture more solar energy. In the central and southern Rocky Mountains, summer-season grasses include mountain muhly, blue grama, and others. With late autumn rains and lower late-season temperatures, bluegrasses, sedges, and winter annuals will provide end-of-season greenness and renewed photosynthetic activity. In the northern Great Plains, a combination of blue grama, stipas, wheatgrasses, and andropogons present a combination of warm- and cool-weather grasses that grow and regrow at different intervals and with varying rates.

We have no final answers on how light interception and canopy architecture can be best brought about in terms of the guidelines in this discussion. But, with these guidelines in mind, we can be well on the way toward a "green revolution" of rangelands. And though we might never realistically expect to approach agronomy's crop yields, we can at least gather in a far greater fraction of the light energy that is presently wasted in many of our ranges.

Intensive management of the rangeland ecosystems will someday consider the configuration of the plant canopy for greater efficiency of converting solar energy into plant biomass. In turn, the chemical energy of the plant will be passed on to increasing numbers of grazing animals. These animals, both domestic and wild, will be carefully chosen by the range manager to bring about a sustained and efficient canopy architecture. The increase in energy flow efficiency through the rangeland ecosystem will be of enormous long-term benefit to the people of this good earth.