Since the decades following the industrial revolution, carbon dioxide levels in the atmosphere have been on the rise. The increasing concentration of CO2 in the air now presents a great ecological question about human effects on chemical cycles that shape the temperature, water level and weather patterns of the entire planet. Although a few individual countries contribute a significant proportion of atmospheric CO2 via pollution, through emissions of burning fossil fuels, the atmosphere is a global commons and changes in CO2 level effect those who are polluting and those who are not without discretion.

When examining global CO2 rates on a timeline, it becomes shockingly clear that human use of fossil fuel burning technology (cars, industry, heating/cooling systems) has had a major impact on the rise of CO2 levels in our environment. "Given the current and projected extent of urbanization, ecologists cannot afford to ignore the existence and impacts of Homo sapiens in urban and suburban landscapes"(McDonnell and Pickett, 1990). The concentration of CO2 in parts per million (ppm) has rapidly increased since 1960. From 4000 B.C. to 1700 A.D. CO2 concentrations fluctuated between 260-280ppm, in 1959 the level had risen to 316 ppm, now in 2001 the level has risen to 370ppm (Ricklefs 2001). This greenhouse gas that is naturally a part of the earth’s chemical balance has been skyrocketing in concentrations by more than 30% since the industrial revolution. What does rapid change in CO2 levels mean for the environment?

The increase in ppm of CO2 is contributing to the greenhouse effect. This heating of the earth is changing environments and habitat communities and has unknown long-term effects on biodiversity, population dispersal and fitness. Serious consequences of the greenhouse effect have the potential to radically change our earth’s ecosystem. Polar ice caps are melting and increasing the ocean level. Temperature is changing gradually all over the world. As the temperature range for our planet shifts, all organisms have to acclimate and eventually adapt to a new environment. As a major influence on the eco-system, we as humans must try to understand how we are changing the environment and what these changes may mean for biodiversity and balance in the natural world.

Already there are organisms that must adapt to higher temperatures because of increased carbon dioxide concentration; all those that live within urban areas for instance. City temperatures have shown to be 6-8 degrees higher than surrounding regions. This phenomenon is often referred to as the urban heat island effect. "Urban heat islands arise because buildings and pavement absorb so much heat. Moreover, fewer trees in cities reduce shading and evapotranspiration, in which plants use heat up by sweating" (May 2001). How are plants reacting to this environmental change of increased temperature and CO2 levels? Is global climate change going to change the growth rate of flora? What does this mean for the rest of the trophic chain? Specifically, how is this going to affect Cottonwood Trees.

Cottonwood Trees, Populus Salicaceae, are a common, fast-growing tree found near stream and riverbeds all across North America. Cottonwoods are deciduous, sexually dimorphic trees that reach heights of over 100 feet and have root distances of up to 200 feet. These trees are common throughout Colorado along riverbeds in the lower elevations. They grow in and out of urban environments alike and are native to this region.

It is our hypothesis that increased CO2 levels within an urban area will increase the growth rate of cottonwoods within that same environment, compared to lower CO2 levels outside the city with slower growth rate of cottonwoods in that area. Several questions are involved in testing this hypothesis: Is there a variance in CO2 levels inside and outside the city? Is there a difference in temperature inside and outside the city? Is there a significant difference in growth rates between trees inside and outside the city?

Both sites are on the banks of Monument Creek with little to no slope and neither had any blocking agents to the sun. Nine trees were selected from each site along a _ mile plot. Trees were selected to be adult trees, ranging in diameter at breast height from 35-50 inches.

From each tree we took a CO2 sample, from the air, above head height. We measured diameter at breast height of the Cottonwood at 4 and a half feet from the ground, soil and air temperature and also took a tree core sample to measure growth rings. Once our field tests and samples had been taken, we returned to the lab to analyze our data. We measured our CO2 samples in a gaschromatograph, which revealed CO2 and CH4 (methane) levels from the air surrounding each of our trees. Each tree core was mounted and sanded before being measured for the distance between growth rings in centimeters for the most recent ten and twenty years.

After collecting the measurements from all of our data, we calculated averages and compared numbers to see, mathematically, what conclusions we could draw from our samples.

As stated in the hypothesis we predicted that higher CO2 levels would cause warmer temperatures and higher growth rates. Since our urban site was exposed to more CO2 via human expulsion our prediction was that CO2 levels would be notably higher at the urban site as opposed to the non-urban site. After analysis of our data we found that our predictions proved to be true. After analyzing the samples from the urban and non-urban sites through the gaschromatograph, the average level of CO2 at the urban site was calculated at 674.37 ppm, compared to 638.21 ppm, the average level of CO2 from the non-urban site. Although the statistical significance between these two sets of data was not very high (P=.628), there was a definite pattern that average CO2 levels were higher inside the urban site (Graph 1).

Graph 1

Levels of atmospheric CO2 compared between two test cites

We then compared the CO2 levels to Cottonwood growth rates and statistically found that the results were not conclusive, yet again, a pattern could be seen. (Graph 2). Graph 2

CO2 levels and growth rates from individual trees at both test sites

Based on previous experiments and the overall trend of our data comparison of CO2 to Cottonwood growth rate, we decided that CO2 levels do have an affect on growth rates of trees.

Before gathering our data for our growth rings we predicted that the Cottonwoods inside the urban site would have a faster growth rate because of the warmer temperatures and higher levels of CO2. We found this to be true in our data of growth rings. The average growth rate of the nine trees sampled within the urban site was 5.39 cm/10yrs opposed to the 3.86 cm/10yrs average of the nine trees at the non-urban site (Graph 3).

Graph 3

There was a definite statistical significance between the two growth rate averages (P=.057), thus showing that it was more than 94% likely that the correlation between these two sets of data did not happen by chance.

When we formatted our project and hypothesis we never predicted the appearance of methane, or the effect it would have on plant growth. The reason we did not account for methane was because methane usually occurs in or near marshlands or waste disposal sites, neither of which was near either of our sites. However, our non-urban site was near several agricultural fields, which possibly could account for the higher methane levels. The average level of CH4, which were taken at the same time as the CO2 samples, inside the urban site was 1.58 ppm, whereas the average level of CH4 in the non-urban site was 92.63 ppm. We then wanted to see the affect methane levels had on tree growth. We took the samples from the trees with the nine highest CH4 levels and their related growth rates and compared them to the trees with the nine lowest CH4 levels and their related growth rates. The sets of data were then graphed (Graph 3). Although statistical significance was not very high, there is an intriguing pattern to be noticed here. The trees with the higher methane levels had an average growth rate that was more than one centimeter higher than the samples with lower methane levels.

Graph 3

This experiment was a brief glimpse at a question that must be explored further in considerable detail. Although the CO2 and tree growth data was not totally conclusive in supporting our hypothesis, we have reason to believe from the support of past studies and from the correlation we could see that our hypothesis was indeed correct and that elevated atmospheric CO2 levels increase tree growth. CO2 sampling in the open environment is subject to many uncontrollable variables, such as passing traffic, human exhalation, wind direction and speed, which all can alter CO2 ppm measured in the atmosphere. The range of trees available for coring was also skewed because in the urban site, the riverbanks had been heavily impacted by beavers and the selection of trees left standing was a young adult population of small numbers. Interestingly enough, this section of the creek seemed to be grazed the heaviest by beavers, despite its location in the heart of downtown, sandwiched between the freeway and a massive coal-burning power plant.

Despite these confounding factors, our data did show a correlation, despite it’s low statistical significance, between the CO2 levels inside and outside the city and tree growth rates inside and outside the city. In a current study done by the Lamont-Doherty Earth Observatory of Columbia University, scientists found that plants exposed to elevated atmospheric CO2 produced "significant fine structural changes in major cellular organelles. These changes may reflect a major shift in plant metabolism and energy balance that may help to explain enhanced plant productivity in response to elevated atmospheric CO2 concentrations" (Griffin et al, 2001).

In another study attempting to understand the interaction between elevated CO2 levels and plant growth Taub, Seemann and Coleman of the Division of Earth and Ecosystem Sciences, Desert Research Institute, examined how growth of plants in elevated CO2 protects photosynthesis against high temperature damage. They found that plant growth at elevated atmospheric CO2 levels does indeed increase the high-temperature tolerance of photosynthesis in a wide variety of plant species under both greenhouse and field conditions. This enhanced thermotolerance was found in both woody and herbaceous species and in both monocots and dicots. "Given that photosynthesis is considered to be the physiological process most sensitive to high-temperature damage and that rising atmospheric CO2 content will drive temperature increases in many already stressful environments, this CO2-induced increase in high temperature tolerance may have a substantial impact on both the productivity and distribution of many plants species in the 21^st century" (Taub, Seemann and Coleman, 2000). These two studies show that elevated atmospheric CO2 levels cause accelerated plant growth via warmer temperatures and increased rates of photosynthesis.

As demonstrated by the CO2 samples taken, the levels of CO2 present in our urban air currently are not only soaring above historical figures, but also are significantly above the present global average. Although we did have a contrast between sites in an out of an urban area, both sites were affected by their proximity to mass CO2 expulsion. The out of town site was still affected by the footprint of the nearest city, Colorado Springs. In order to show even more effectively the effect of elevated atmospheric CO2 on plants, a third site would need to be tested that was not affected by the footprint of any nearby cities. In addition, to develop this experiment, more data must be collected over a longer period of time. We suggest that CO2 levels be measured twice daily at the exact same time in the three separate locations every day for a year. The source of the unusually high atmospheric methane in the non-urban area must also be investigated. Temperature data could also be collected at the CO2 stations to determine if there was a significant correlation between CO2 level, soil and air temperature and growth rate.

We did obtain temperature data from Colorado Springs Utilities for two urban sites, one at The Colorado College and another in the northern section of Colorado Springs at Woodmen road, and one non-urban site, in close proximity to our sample site, south near Fountain at Nixon base. However, because this data was incomplete, we could not draw statistics from it and consequently did not have temperature data to compare in our results.

Another possible development of this experiment is related to a study done by Lindroth, Roth and Nordheim, concerning genotypic variation in response of quaking aspen to atmospheric CO2 enrichment. These scientists from the Department of Entomology at the University of Wisconsin expanded on the knowledge that quality and quantity of plant production changes with exposure to elevated levels of atmospheric CO2. Their question was how such effects vary among plant genotypes. They used six types of aspen (Populus tremuloides) genotypes as their subjects. They determined that "projected atmospheric CO2 increases are likely to alter the genetic structures and evolutionary trajectories of aspen populations" because the changes in the plants seen were growth and chemical characteristics directly related to the biological fitness of the aspen (Lindroth, Roth, Nordheim, 2001).

This study provides a good example of laboratory manipulations that could explore the effect of elevated CO2 on genotypic change. This would be an aspect of the experiment that would connect current ecosystem trends to future possibilities of ecosystem adaptation to a changing global climate.

The connection between increased atmospheric CO2 levels and increased rate of plant growth is important information for ecologists who may need to predict environmental changes triggered by a global rise in CO2 concentration in the atmosphere. Changes that could occur are global temperature increase, species migration, extinction or genotypic change in flora and fauna. Increases in concentrations of atmospheric CO2, contributing to the greenhouse effect, not only causes temperature increase but unpredictable environmental shifts, such as changing precipitation rates, desertification, increased sea level and global weather patterns, (El Nino, El Nina). Changes in local and global ecosystems cannot be predicted, but better understood with knowledge about the correlation between elevated CO2 and flora response. Because plants are primary producers, the changes that occur in flora populations will affect the rest of the trophic chain and the greater ecosystem balance.

Understanding the relationship between urban areas and increased atmospheric pollutants, such as CO2, helps to understand the greater effects of urbanization and suburbanization. Development trends in the United States, and clearly in Colorado Springs, in the last decade, have shown suburban areas developing most rapidly, which extends urban footprints into non-urban areas. This creates a more even distribution of pollutants as opposed to a patchy dispersal, solely concentrated around isolated urban centers. The urban-rural gradient theory of atmospheric pollutants, as proposed by McDonell and Pickett (1990) is a way of measuring footprint effects into surrounding areas. In addition, this gradient helps to explain the urban heat island effect. The urban-rural gradient helps to explain this, because as CO2 levels rise, towards the center of the city and the concentration of fossil fuel burning, temperatures also rise and as CO2 levels decline towards the outside of the urban footprint, temperature also declines to match the surrounding non-urban area temperatures.

As urbanization spreads throughout the developing nations and suburbanization spreads across the developed world, footprints will begin to overlap more, leaving no pockets of unaffected land or air. Politicians and government bodies look at global averages of CO2 when discussing environmental protocols, such as the Kyoto Protocol, however CO2 levels in urban areas, where most of the worlds population lives is much higher than the global average. This urban-rural gradient and an understanding of the urban heat island effect should heighten awareness and hasten policy for control of CO2 emissions, however, we fear that until global averages reach the height of urban concentrations, the attention of policy makers may not be drawn to the severity of the ecological ramifications of the elevation of atmospheric CO2 and the greenhouse effect.

As the field of ecology becomes more explored through scientific research, understanding human disturbance and interaction in the global ecosystem becomes more pertinent and crucial to understanding the future of our ecosystem as a whole. Understanding the impact of elevated CO2 levels on plant growth is a positive first step in understanding the great domino effect of humans and our pollutants on the ecosystem that we are a part of, for better of for worse.