The Effects of Elevated CO2 on Trees - Assignment Example

The affects of elevated CO2 on trees One of the effects of the global warming phenomena is the introduction into the atmosphere of a large amount of additional carbon dioxide (CO2). This is caused by the continued burning of fossil fuels as well as the effects of deforestation taking place in many areas of the world. While many trees may benefit greatly from this increased CO2, as it remains a primary agent in the photosynthesis process, the long term effects are unknown. Numerous studies have been conducted, but what they’ve managed to prove so far has only proven that not enough is known about how these changes in the atmosphere might affect the various components that contribute to a healthy forest system. Before understanding how elevated CO2 ­affects longleaf pine systems, it is first necessary to understand how the various trees of these systems differ from each other in their fundamental photosynthesis processes. Generally, trees can be divided into either C3 or C4 plants. “In both groups, net CO2 assimilation occurs in the chloroplast through the reductive pentose phosphate (RPP) pathway, or the Calvin Cycle” (Monson, Edwards & Ku, 1984: 563). In C3 plants, the process, which completely takes place in the mesophyll cells, begins when O2 reacts with the enzyme Rubisco, linking the photosynthetic RPP pathway with the photorespiratory glycolate pathway. This linkage allows the carbon atoms in the compounds of the RPP pathway to enter the glycolate pathway, but typically leads to increased photorespiratory CO2 loss and less efficient CO2 assimilation. This process is referred to as the C3 process because the initial products of the Rubisco reaction are two molecules of the compound 3-phosphoglycerate, a three carbon compound. “When, catalyzed by Rubisco, ribulose-1, 5-biphosphate (RuBP) reacts with oxygen, the photorespiratory process begins. The products of this reaction are 3-phosphoglycerate (as with CO2 assimilation) and the two-carbon compound phosphoglycolate. The phosphoglycolate is converted to glycolate, which is further metabolized in the peroxisome and mitochondrion through the glycolate pathway” (Monson, Edwards & Ku, 1984: 563). O2 inhibits this process by first competing with the CO2 for the reaction with RuBP and then through CO2 loss in the glycolate metabolism. In the C4 reaction, however, the process takes place in the cooperative action of two cell types, the mesophyll and the bundle sheath cells. “C4 plants fix atmospheric CO2 through phosphoenolpyruvate (PEP) carboxylase in the mesophyll cells to form oxaloacetate, which is subsequently converted to the four-carbon dicarboxylic acids malate and/or aspartate” (Monson, Edwards & Ku, 1984: 563). These four-carbon acids are then transferred into the bundle sheath cells where they release the CO2 as well as a C3 compound. The C3 compound is transferred back into the mesophyll cells while the CO2, now concentrated, is fixed through Rubisco. “The C4 plants are also distinguished from C3 plants by several other specific physiological and anatomical features such as leaf anatomy, organelle structure, low photorespiration rates, high photosynthetic efficency, and reduced discrimination of 13C” (Kennedy & Laetsch, 1974: 1087). To help facilitate this process, the bundle sheaths of the C4 plants contain numerous, well-developed chloroplasts, which are not present in C3 plants. Further distinguishing these cells from those found in C3 plants, the choloroplasts found in the bundle sheath cells of C4 plants “are centripetally located and much larger than those of mesophyll cells” (Kennedy & Laetsch, 1974: 1088). C4 plants also contain a peripheral reticulum, which is an anastomizing system of tubules, that is not present in C3 plants. For these reasons, and others, it is generally believed that the C3 plants are ancestral to C4 plants, but intermediate plants have been difficult to identify, most displaying processes that fall into either C3 or C4 categories even when aspects of the other type are present. It is generally accepted that C4 plants are able to photosynthesize faster under high light intensity and higher temperatures because of the fact that the CO2 is delivered directly to the Rubisco instead of grabbing the O2 and undergoing photorespirtation. C4 plants also are more water efficient because of the method by which it brings in CO2, decreasing the amount of time the stomata must remain open. There are also some significant differences in the way in which these different types of plants respond to normal and introduced weather conditions. This is demonstrated in a report comparing the seasonal trends in photosynthetic rates in loblolly pines as compared to white pines (McGregor & Kramer, 1963). In this study, both types of trees slowly increased rates of photosynthesis between the months of February and April, rose to a peak during the summer months and then declined throughout the fall and winter. However, the maximum production periods differed from one species to the other. While the loblolly pine was seen to peak in mid-September, experiencing a sharp decline in production by November, the white pine peaked through the period from mid-July through mid-September and experienced a much more gradual decline in production through the fall months. Taking growth rates into account did not sufficiently explain these peaks. “Although stem elongation had begun on all seedlings by April 9, no new foliage had appeared. Therefore, this early increase in photosynthesis resulted from an increased photosynthetic capacity of needles already present. Likewise, the decrease after the midseason peak in both species was not caused by loss of needles, but rather resulted from a decrease in photosynthetic capacity of the existing needles” (McGregor & Kramer, 1963: 761). The needles of the loblolly pine were shown to attain their maximum rate of photosynthesis in May and retain this rate through September while the white pine didn’t reach its maximum rate until July, sustaining this activity through September as well. The researchers suggested that a possible cause of these variations was a seasonal change in resistance to CO2 uptake. “This might be related to changes in the mesophyll cells or to stomatal behavior” (McGregor & Kramer, 1963: 761). In a study conducted by Robert Wright (1970), the rate of CO2 exchange was examined in three different species of pine as they reacted to higher temperatures and drought conditions. The trees included in the study were knobcone pine, coulter pine and sugar pine and were compared at several times throughout the year primarily to determine the causes for their elevational differences, but fluctuations in CO2 intake were noticed as well with differences in water conditions. “It appears that all three species reach a condition under drought stress in which high temperature produces immediate depression of CO2 fixation; they are distinct from one another in the critical temperature and the degree of depression of CO2 fixation” (Wright, 1970: 325). Sugar pine in particular demonstrated a significant reaction to drought conditions, with CO2 fixation ceasing altogether in these conditions. Other studies are cited that indicate sugar pine is also more susceptible to these conditions than other pines such as ponderosa pine, incense cedar, Douglas fir or grand fir, perhaps because of deeper root structures of these species. “Mooney et al (1966) found that the sensitivity of CO2 fixation to drought in sagebrush was greater than the same sensitivity of bristlecone pine when mature field specimens were studied” (Wright, 1970: 326). The study also cites several other studies that indicate a long season of effective photosynthesis observed throughout the rest of the year observed during this and other studies may be a built-in mechanism for avoidance of drought in many of these species. Tree species are also significantly affected by the actions of insect populations within the forest. “Studies of insect-plant interactions have led to three general conclusions: 1) insect feeding, development and survival are often inhibited by host plant defenses … 2) insect herbivores change plant competitive relationships through selective herbivory, often accelerating the rate of ecological succession … and 3) insect herbivores influence the rate and direction of nutrient transfer between vegetation and litter” (Schowalter, 1981: 126). Although they can sometimes get out of hand, exploding in population numbers sufficient to overcome the natural defense mechanisms of host plants and causing significant devastation in a forested area before being brought back into manageable numbers, Schowalter argues that insects play an essential role in the distribution of the necessary nutrients required for these plants to survive. There are several ways in which insects accomplish this service. One way is by grazing plants on a moderate level which stimulates primary productivity and nutrient uptake. They also increase the translocation of nutrients to those areas where insects graze and increase the mass and nutrient content of litterfall throughout the growing season. Chewed leaves facilitate the leaching of foliar nutrients. Insects also help the forest by “stimulating (via nutrient-rich leachate, litterfall and insect feces) nitrification, nitrogen-fixation, litter decomposition and/or plant root growth rates within the litter/soil complex, and altering long and short-term nutrient cycling pathways through changes in the relative biomass of canopy and subcanopy plant species” (Schowalter, 1981: 127). In addition to this, Schowalter illustrates the ways in which insects contribute to the natural succession of plant species from early growth to late growth forests, and that the lack of these insects may have a detrimental effect upon these healthy changes to the forest habitat. An example of an insect/host plant interaction that isn’t necessarily beneficial for the individual tree is examined in Raffa and Barryman’s study on the bark-beetle. These insects rely upon the tree to survive, but must kill the tree, or that portion of the tree in which it lives, to do so. This is because the trees themselves have developed defense mechanisms that can be fatal to the beetle. Because of this, the beetle must find weakened trees in which to breed. “Bark beetle populations often remain at low densities for long periods of time, during which they colonize only highly stressed trees. Droughts, windstorms, and other environmental disturbances, however, can suddenly increase the availability of weakened hosts, causing beetle populations to rise dramatically. Following the exhaustion of this breeding material, some species may then invade healthy trees. When this occurs, the beetle population expands rapidly and declines only after most of the host population has been exploited” (Raffa & Barryman, 1987: 236). By tracing the changes and adaptations that the trees and the beetles have made to protect themselves, it is possible for researchers to prove some of the important roles insects play in the evolution of the forest as well as the ways in which they work to redistribute the plant resources. Another factor that significantly affects plant and tree growth is the level of CO2 available to them for the photosynthetic process. “The amount of CO2 in Earth’s atmosphere is growing by ~3 PG C per year, due largely to fossil fuel emissions and deforestation” (Allen et al, 2000: 437). With proper additional resources, such as light, water and soil nutrients, are available, this increased CO2 contributes to significant growth processes thanks to increased photosynthetic ability. Logically, this increased productivity may require either increased uptake of nitrogen from the soil to support this growth, or better efficiency in using the available nitrogen supply. While it is expected that lower levels of nutrients in the soil might limit the growth abilities of those plants exposed to increased CO2 levels in the atmosphere, it is also suggested that increased carbon stores in the trees themselves might contribute to greater production of fine root systems, which could facilitate the greater nutrient uptake required to support this additional growth. Other studies have suggested that increased CO2 levels do little to increase the decomposition of litter, which could result in a shift of the microbe-plant balance to favor the microbes. As a result of these conflicting studies, Allen and others conclude that the result of increased CO2 levels on the availability and necessity of additional nitrogen may depend greatly upon the type of forest being studied. A subsequent study into the effects of higher levels of CO2 on a loblolly pine forest revealed the trees had indeed increased their fine root systems and leaf litter, but no significant adverse effects or nutrient deficiencies as a result of the increased growth rates. The concept that forests of different types might be affected differently by the increased levels of CO2 and limited soil nutrients were supported in another study conducted comparing loblolly pine growth with ponderosa pines. In this study, researchers Griffin, Winner & Strain (1995) worked to determine any differences in the growth rates of these two species when exposed to differing levels of CO2 and soil nitrogen. Seedlings were exposed to low and high CO2 levels during a season’s worth of growth period under controlled conditions. Because loblolly pines are a fast-growing tree, it was expected that the effects of increased CO2 and nitrogen availability would result in significant differences between the two species, which surprisingly did not materialize. However, “elevated CO2 resulted in significant increases in total dry matter only when nitrogen availability was high” (Griffin, Winner & Strain, 1995: 553). It was also found during this study that the loblolly pine only responded to increased levels of nitrogen when exposed to increased levels of CO2, but the healthiest trees were those that were given high CO2 levels and lower nitrogen. “In contrast, ponderosa pine growth was nearly N-saturated in this experiment since there were no significant increases in seedling growth with elevated N availability” (Griffin, Winner & Strain, 1995: 553). In addition, the ponderosa pines demonstrated a high mortality rate among the seedlings exposed to high levels of nitrogen. This variability in responses according to species indicates a high degree of unpredictability in terms of how increased levels of CO2 will affect the surrounding forest structure and the function of elements within the forest. As has been suggested in the literature, tree response to the higher levels of CO2 can be variably affected depending upon the level of other necessary resources in the soil, or the ability of the forest to recycle these resources in accelerated time to maintain increased growth. In addition, the increased levels of carbon stored in the trees or in the soil can lead to unknown developments in overpopulation of currently beneficial microbes. As was demonstrated in studies regarding the effects of insects upon the forests, overpopulation of beneficial or slightly damaging herbivorous creatures such as these can lead to highly destructive behaviors if not kept in check. “As far as the direct effects of CO2 are concerned, we are less able to comment on the effects of an increase in CO2 concentration on processes of regeneration, competition and species composition because of the lack of data. We know of no experimental studies on relevant species in which the interference between species has been investigated at elevated CO2 in either field or laboratory” (Shugart, Antonovsky, Jarvis & Sandford, n.d.). Concerns have been focused upon the idea that C3 plants may replace C4 plants as this type of photosynthesis is most able to take advantage of the increased biomass, but several studies have indicated this is not the case. “C4 plants appear to be more responsive to CO2 than C3 plants in terms of their ability to enhance their water and nutrient acquisition capabilities as the air’s CO2 content rises, possibly as a consequence of the greater water and nutrient gathering effectiveness of mycorrihizal fungal associations with C4 plants as opposed to C3 plants (Center for the Study of Carbon Dioxide and Global Change, 2006). The increases in growth rates of trees effected by increased CO2 levels could have the effect of crowding out smaller vegetation as a result of decreased light reaching down to the forest floor as the upper story spreads. However, these effects have also been studied, indicating that the growth rates required for this to happen will not occur without the increased nutrients necessary. “In reviewing 15 previously published studies, for example, Kerstiens (1998) found that shade-tolerant trees were to two to three times more responsive to atmospheric CO2 enrichment than were sun-loving trees. Hence, even if the sunlight transmitted trhough the upper-canopy foliage of a forest ecosystem were to be dramatically reduced, the growth-enhancing effects of the rise in atmospheric CO2 concentration would likely more than compensate for the reduced intensity of the transmitted solar radiation, thereby enabling the full complement of understory species to maintain viable niches in the forest ecosystem” (Center for the Study of Carbon Dioxide and Global Change, 2006). While there have been numerous studies investigating the effects of higher CO2 levels on C3 and C4 plants of various types, including various types of trees and forests, the fact remains that not enough is known of how these atmospheric changes will truly affect the future of these forests. The fact that different trees react to these changes in different ways indicates that studies done on one species may not be transferable to other species, even when closely related. Symbiotic relationships between insects and trees, as well as other organisms such as microbes are known to have very destructive potential when thrown out of balance and certain microbes could become overproductive with the increased levels of stored carbon. Yet none of these effects are certain, or even tested to satisfactory levels. Concerns of overgrowth could be limited by the lack of nitrogen enrichment in certain areas while other areas might become somewhat overproductive as trees in nitrogen-rich soils respond to the increased CO2. The long-term effects on the forest systems of the world are still too variable to predict with any great deal of accuracy. Works Cited Allen, A.S.; Andrews, J.A.; Finzi, A.C.; Matamala, R.; Richter, D.D.; & Schlesinger, W.H. “Effects of Free-Air CO2 Enrichment (FACE) on Belowground Processes in a Pinus taeda Forest.” Ecological Applications. Vol. 10, N. 2, (April 2000), pp. 437-448. Center for the Study of Carbon Dioxide and Global Change. “Biodiversity: Summary.” CO2 Science. (2006). November 11, 2006 < http://www.co2science.org/scripts/CO2ScienceB2C/subject/b/summaries/biodiversity.jsp> Griffin, Kevin L.; Winner, William E. & Strain, Boyd R. “Growth and Dry Matter Partitioning in Loblolly and Ponderosa Pine Seedlings in Response to Carbon and Nitrogen Availability.” New Phytologist. Vol. 129, N. 4, (April 1995), pp. 547-556. Kennedy, R.A. & Laetsch, W.M. “Plant Species Intermediate for C3-C4 Photosynthesis.” Science. New Series. Vol. 184, N. 4141, (June 7, 1974), pp. 1087-1089. McGregor, William H. Davis & Kramer, Paul J. “Seasonal Trends in Rates of Photosynthesis and Respiration of Loblolly Pine and White Pine Seedlings.” American Journal of Botany. Vol. 50, N. 8, (September 1963), pp. 760-765. Monson, Russell K.; Edwards, Gerald E.; Ku, Maurice S.B. “C3-C4 Intermediate Photosynthesis in Plants.” Bioscience. Vol. 34, N. 9, (October 1984), pp. 563-566 + 577-574. Raffa, Kenneth F. & Berryman, Alan A. “Interacting Selective Pressures in Conifer-Bark Beetle Systems: A Basis for Reciprocal Adaptations?” The American Naturalist. Vol. 129, N. 2, (February 1987), pp. 234-262. Schowalter, Timothy D. “Insect Herbivore Relationship to the State of the Host Plant: Biotic Regulation of Ecosystem Nutrient Cycling through Ecological Succession.” Oikos. Vol. 37, N. 1, (July 1981), pp. 126-130. Shugart, H.H.; Antonovsky, M.Ya.; Jarvis, P.G.; & Sandford, A.P. “CO2, Climactic Change and Forest Ecosystems.” Scope 29: The Greenhouse Effect, Climactic Change and Ecosystem. N.d. November 11, 2006 < http://www.icsu-scope.org/downloadpubs/scope29/chapter10.html#t10.3> Wright, Robert D. “Seasonal Course of CO2 Exchange in the Field as Related to Lower Elevational Limits of Pines.” American Midland Naturalist. Vol. 83, N. 2, (April 1970), pp. 321-329. Read More