In a closed-forest community, if the gap occurs in the direction of the sun, direct light will be allowed to reach the forest floor. The amount of light now reaching the forest floor may be as much as 200 times that which was reaching the floor under a closed canopy. The exact amount is determined by size of the gap, time of day and also latitude. But, the amount of light that reaches the floor will still be roughly 10-15% less that at the canopy level. Also, the eastern side of gaps usually receives more radiation in the afternoon, when temperatures are high. The western side of gaps usually receives the opposite: higher radiation in the morning, when temperatures are lower.
In addition to direct light, diffuse light also reaches the forest floor following a gap. Blue sky and clouds are sources of this light. Diffuse light is different from direct light in spectral composition. Diffuse light is enhanced at the short wavelength (blue) end of the solar spectrum while direct light is not. Another factor caused by gaps is the increase in infrared, or heat, radiation. While light is important to plants for photosynthesis, the near-infrared radiation can not be used by plants but warms up leaves, soil, litter and whatever else there happens to be in the vicinity of the gap. The effect of this is a greater range of temperature of soils and plants within gaps.
Light is very important in determining the rate of photosynthesis in plants. To understand the growth of plants it is very helpful to know the amount of light that is intercepted by the canopy. This is useful because in many communities, plant growth rates are approximately proportional to the amount of light that is intercepted by the canopy. In the two different environments, North Stradbroke Island and Lamington National Park, photon flux densities were measured using a Quantum sensor. On North Stradbroke, readings were taken in the Eucalypt open-forest community. At Lamington, reading were taken in a closed canopy region near Duck Creek Road. At both locations readings of incident light, or full sunlight at the time, were also taken. Measurements of photon flux density (PFD) are in umole.m-2.s-1. Results were as follows.
Brown Lake on North Stradbroke Island. On Thursday, 17th October, 1996, at 1:55pm the incident light value was measured at 1650 umole.m 2.s-1. At 2:55pm measurements were made below the canopy toward the bottom of the hill and yielded PFD values of 212, 215 and 190 umole.m-2.s 1. These readings correspond to around 13%, 13% and 12% of incident PFD at this time of the day. Later at 4:05pm at the top of the hill, the incident PFD reading would have been around 867 umole.m-2.s-1. The reading under the canopy was 34.7 umole.m-2.s-1 which corresponds to around 4% transmission.
Near Duck Creek Road, Lamington National Park. On Wednesday, 30th October, 1996, at 11:00am, the incident PFD was measured at 2020 umole.m-2.s-1. Below the canopy at different locations, readings obtained were 5, 9, 20 and 30 umole.m-2.s-1. The average reading was around 9 umole.m-2.s-1. These readings correspond to around 0.24%, 0.4%, 1% and 1.5% of incident PFD respectively. The average percentage was 0.4%-0.5% transmission by the canopy. A reading was made on the driveway near the road to represent diffuse skylight. Around 2% of incident radiation was measured. A PFD measure of 50 umole.m-2.s-1 in a gap such as this represents around 2.5% of incident radiation.
COMPARE AND CONTRAST
The above calculations mainly support the notion that more light is getting to the forest floor in open-forest communities than in closed forest communities. The 4:05pm reading from Brown Lake was quite low, but possible explanation for this is due to the lateness in the day. The sun was lower in the sky; radiation was coming more from the east, rather than from directly overhead. As a result, radiation had to travel further through the canopy to reach the forest floor. But otherwise, in areas where the canopy is closed the percentage transmission is lower than that in areas where the canopy is open. The effect of gaps can be seen in the reading taken on the driveway near Duck Creek Road. The resulting percentage of incident radiation measured at this location was indeed higher than the average percentage measured under undisturbed canopy.
The difference in light amount and intensity reaching the forest floor can be used to make an analysis of the differences in photosynthetic rates in sun and shade plants. Plants under a closed canopy, like at Lamington, are physiologically different than the plants under an open canopy, as near Brown Lake. Although shade plants do not achieve high rates of photosynthesis, they do perform efficiently in low light situations. Similarly, sun plants growing in high light intensities exhibit high photosynthetic rates at a saturating light intensity, but if light conditions are of a lower intensity, the sun plants exhibit a lower net photosynthetic rate than shade plants at the same light intensity (Boardman, 1977).
Before discussing the morphological differences regarding sun and shade plants and their effect on photosynthesis, a look should be taken at the process of photosynthesis itself.
Sunlight is the main source of energy for all plant growth. Light is essential in stimulating such events as the differentiation of plant tissues and organs. Through the utilization of sunlight, plants synthesize carbohydrates and other sugars which are used to keep the plant alive. Other organic compounds synthesized by green plants through photosynthesis are used either directly or indirectly by every living organism on earth. All of the valuables that come out of the photosynthetic process are limited by light quantity and quality.
Respiration is the process by which energy gained in photosynthesis is released and distributed throughout the plant for maintenance and growth functions. Plants photosynthesize during the day in sunlight and respire at night in the dark. Decreasing light amount on a leaf from full sunlight to darkness leads to a change from the gain of energy (photosynthesis) to the loss, or utilization, of energy (dark respiration). The light compensation point is the term used to describe the level at which photosynthesis is balanced out by respiration. Above this point photosynthetic rate increases as the amount of light does, but only for a short period of time. The light saturated rate of photosynthesis is the value used to describe this limit. The steepness of the slope of the line showing this increase is known as the photosynthetic light use efficiency. Together, the dark respiration rate, light use efficiency and the light saturated rate of photosynthesis can be used as plant characterizations. Plants growing in low light situations like at Lamington, exhibit low rates of dark respiration. Therefore, when these plants are not able to photosynthesize they do not utilize much energy. However, in high light situations, such as that from a gap, these plants can not utilize large amounts of light because their photosynthetic light saturated rate is low. On the other hand, plants living in greater amounts of light like those at Brown Lake, have high dark respiration rates. They spend a lot of energy but can afford to because they are better able to make use of high light levels eg their photosynthetic light saturated rate is high. But, if these sun plants are shaded for an extended period of time, they will eventually die because they will use up more energy than they can produce. These are a few of the reasons why plants at Brown Lake living in 17% of incident light live just as successfully as plants near Duck Creek Road that are living in 0.4%-0.5% of incident light.
Just as photosynthetic rates vary between plants in closed-forest communities and open-forest communities, so do plant morphologies. The size of leaves can vary depending on exposure to sunlight. Leaves always in the shade are larger than leaves of the same species occupying direct sunlight in order to capture more of the available light. There are also adaptations leaves can undergo in response to changes in light availability or intensity. Chloroplasts can become spherical-shaped in low intensity light, whereas in bright light they can become flattened to minimize contact with light. Chloroplasts may also move within a cell from the illuminated face to the side walls depending on light intensity. Increases in temperature may bring about an increase in respiration rate and an increase in light intensity might change leaf shape. These are all considered tropisms, or plant "movements" in response to a stimulus such as sunlight availability or intensity. Another example is the growth of a plant shoot towards a light source, or positive phototropism. The magnitude of the response can be elicited in different ways. The same effect can be achieved by low intensity light over a long period of time and high intensity light over a very short period of time.
These are only a scarce few of the physical adaptations plants make to their environment in order to survive. To make this study complete there would have to be an in depth investigation and relation of plant structure and morphology of plants in open- and closed-forest communities. Similarly, an analysis would have to include the effects between different qualities of sunlight in addition to quantities.
HUMAN INDUCED PRESSURES
While gaps are a significant form of natural disturbance, the disturbance from man has been and continues to be much more significant.
North Stradbroke Island. Recently there have been dramatic expansions in the development of the coastal lowlands of the island. Some of these include mineral-sand mining, forestry plantations, pasture development, real estate development and recreation. Several of these are the results of increased population pressures. These all place stress on the island as a whole.
Lamington National Park. Agriculturally man has had a huge impact on the forest at Lamington. In the past, sites have been cleared for pasture and later abandoned. Sites have also been burned then reseeded as part of slash and burn agriculture. In the process there has been the removal of topsoil and plant nutrients. Logging has also been a major pressure on this environment. Additionally, another expanding pressure is that of tourism.
What is unfortunate in all these examples is that although some actions may have seemingly insignificant effects, this is usually not the case. Often disturbance or destruction of one element sets off a chain reaction of events terminally affecting larger areas and the organisms within them. Take for example mineral-sand mining on North Stradbroke Island. Heavy mineral deposits occur on present beaches as well as on former beaches in emerged heathlands and beneath paralleled dunes. To mine an area all vegetation must first be destroyed and removed. The topsoil is bulldozed, removing humus and any native species' seeds that may be growing there. This also interrupts habitats and results in the destruction of any unique species living in that environment. This in turn can disrupt feeding patterns of nearby organisms. There are rehabilitation efforts in many of these areas, but they are not always successful and if they are, it takes many years before the open-forest communities can be re-established. Species diversity is reduced in revegetated areas. Also, fertilization yields short, but not long time stabilization of mined areas. If any exotic species are planted they can interrupt the stability of the regeneration process. Additionally, there is always the threat of pollutants entering the environment from the mines. For example, the recent discovery of the leeching of pollutants that has been going on for years at a mining area on North Stradbroke Island. Unfortunately, when one part of a plant is affected by stress the entire plant suffers. Changes in the hormone system brought about by disturbance bring about morphological and metabolic processes designed to save the life of the plant. The redistribution of such things as nutrients and oxygen can sometimes lead to premature flowering and leaf abscission.
There are real estate pressures associated with population increase. These lead to development of areas once undisturbed. As pointed out earlier, this involves a destruction of natural vegetation, habitats and possibly organisms living there.
One of the most dramatic forms of disturbance in a forest is logging. It can be for any number of purposes, whether for farming and agriculture, development or just for the timber. Before any trees are even taken, destruction occurs in the building of roads and other means to transport heavy machinery required for clear-cutting. This can also result in the redirection of natural water courses and drainage lines. The removal of trees equals a removal of nutrients, habitats and organisms. Due to such high amounts of endemism in rainforests there is an irreplaceable loss of many plant and animal species. Clearing leaves large amounts of bare soil exposed and susceptible to high surface temperatures, rapid run-off of water and the formation of hard crusts. Since heavy machinery is used, soil is often compacted to the extent that it is rendered unsuitable for the germination of future seedlings. Any efforts to reforest the area would require specialized plants, not likely native to the area, to be able to survive in these conditions. Studies by Barry (1984) showed soil infertility caused by some logging situations to lead to the buildup of secondary compounds in plants potentially toxic to herbivores. Poor soils were also shown to cause lower primary productivites. Additionally, plants that colonize gaps are not necessarily the same ones that colonize elsewhere. Some species may undergo "population explosions" (Yates, 1996) affecting surrounding species. Weeds are one such example.
Areas cleared for logging, pasture or any other agricultural purpose are disturbances that introduce many forms of stress into the environment. One clear example is the introduction of light stress. Drastic changes in light levels can be very damaging to shade plants and can exceed their tolerance limits. Plants do have the capacity to adapt to different photon flux densities over time by adjusting the balance of their photosynthetic components, for example increasing the photosynthetic light use efficiency to comply with higher light situations. But, when there is a sudden and dramatic change in light intensity, this balance is disrupted and damage occurs.
With disturbances such as those mentioned above, it is often the case that more than one form of stress are introduced. Certain stresses individually may not result in the death of a plant, but in conjunction with other stresses will. For example, logging results in poorer soil. Water run-off of these soils can result in increased salt concentrations. Nearby plants now face not only light and temperature stress from the gap, but also water and salinity stress resulting from poor soils.
Another pressure in both forest communities though more so at Lamington National Park, is the pressure of tourism. People want to see preserved forest and all the beauty it holds. Not only is it aesthetically pleasing, but it also builds an appreciation of the grandeur of nature. But, to accommodate the demand, there has to be access to this beauty. This can be in the form of resorts, such as O'Reilly's, or simply means of getting in the forest such as designated paths and walkways. Similar problems already mentioned also apply here. In the creation of paths and walkways there is destruction of habitats. With the introduction of large numbers of people to the forest there is noise disturbance, potentially disrupting animals' nests and routines.
With all of these pressures and resulting negative effects, it might be simple to say a solution would be to stop logging, farming, mining and tourism in these areas. But this is a very naive approach for several reasons. For example, though many negative attributes are associated with logging and agricultural clearing, there are some species who exist only in gaps. Gaps are part of the natural succession of forests. To eliminate all disturbance would eliminate these species and alter forest development. Elimination of all agricultural actions would also be negative economically. Similarly, if tourism were to be eliminated there would be no knowledge and appreciation that is gained only from being in a forest. There is also a significant economic value tied to tourism.
The significance of sunlight in forests is remarkable. The characteristics of light in open- and closed-forest communities are very diversified but are all intricately connected with the environment. The role of man in this web of life is also very complex. There are pressures evident in the forest even without the influence of man. But, intervening human actions, though sometimes necessary, disrupt the delicate balance of nature. It is potentially dangerous to continue with activities such as slash and burn agriculture because we lose our forests as a result. But, it is also impossible and unrealistic to eliminate all forms of human impact. The difficult task that is faced is to find a medium between the two extremes, if one exists at all. This would be a means of gaining the benefits of nature while conserving the environment at the same time. Whether this is a Utopian idea or not, it is unsure, but this is the task that lies before us both presently and further along in the future.
Barry, S.J. 1984. Small Mammals in a South-East Queensland Rainforest: the Effects of Soil Fertility and Past Logging Disturbance. Australian Wildlife Research, 11, 31-9.
Boardman, N.K. 1977. Comparative Photosynthesis of Sun and Shade Plants. Annual Review of Plant Physiology, 28:355-77
Clifford, H.T. 1979. The Vegetation of North Stradbroke Island, Queensland. University of Queensland Press, St. Lucia.
Connor, D.J. and Clifford, H.T. 1971-72. The Vegetation near Brown Lake, North Stradbroke Island. Proceedings of the Royal Society of Queensland, 83(6):69-82.
Larcher, Walter. 1995. Physiological Plant Ecology. Springer Verlag, Berlin.
Liddy, John. 1996. Notes on North Stradbroke Island. Handout.
Osmond, C.B., Austin, M.P., Berry, J.A., Billings, W.D., Boyer, J.S., Dacey, J.W.H., Nobel, P.S., Smith, S.D., and Winner, W.E. BioScience, Volume 37 No.1.
Thompson, C.H., and Ward, W.T. 1975. Soil Landscapes of North Stradbroke Island. Proceedings of the Royal Society of Queensland, 86(3): 9-14.
Yates, D.J. 1996. How Plants Respond to Gaps in Forests. Handout.
Yates, D.J. 1996. Light in Tropical Forests. Handout.