CO2 Impacts Tropical Forest Resistance to Climate Change

by Leta Ames

It is well known that fire can play a crucial role in the reproduction and development of plant populations. The availability of water and CO2 also impact plant growth, especially of larger species. It is believed that the interactions of climate, fire, and CO2 greatly influence the shift between savanna and tropical forest ecosystems and their permanence thereafter. Previous research has relied on data collected from intact tropical forests, but although useful, these data only provide a snapshot of the impact of CO2, fire, and climate on these ecosystems. To gain a better understanding of what factors influence tropical ecosystems Shanahan et al. (2016) used the concentrations of carbon and hydrogen stable isotopes from sedimentary leaf wax n-alkanes (δ13Cwax and δDwax) and the frequency of charcoal layers from sediment obtained from Lake Bosumtwi in Ghana to construct a history of changes in vegetation and hydrology, as well as to estimate the annual fire frequency. Continue reading

Anthropogenic effects on greenhouse gas (CH4 and N2O) emissions in the Guadalete River Estuary (SW Spain)

by Rebecca Herrera

Burgos et al. (2014) discuss seasonal variations of the greenhouse gases methane (CH4) and nitrous oxide (N2O) in the Guadalete River Estuary ending in the Cadiz Bay of southwestern Spain. They found that greenhouse gas concentrations were higher in the more inland parts of the estuary compared to the mouth of the river. Concentrations of methane and nitrous oxide varied depending largely upon the seasonal precipitation regime. It was also observed that the Guadalete Estuary acted as a source, rather than a sink, of greenhouse gases throughout the entire year, as observed by measuring the fluxes of CH4 and N2O from the Estuary. Continue reading

Heat Stress and Low Humidity with Climate Change will be Hard on Midwestern Corn Crops

 

by Christina Whalen and Emil Morhardt

Maize (corn) production continues to be a very important source of food, feed, and fuel all around the world, but climate change has raised the concern about being able to maintain the yield rates. A negative relationship between extremely high temperatures (above 30˚C) and yield has already been observed in various regions. Previous studies have not been able to demonstrate which mechanism causes the correlation between extreme temperatures and yield, thus it is possible that the relationship reflects the influence of another variable, such as precipitation rates. There are other possible explanations for the observed relationships. This study explores the mechanisms used in other studies that document the importance of extreme heat on rainfed maize using the process-based Agricultural Production Systems Simulator (APSIM). The study asks three main questions: can APSIM reproduce the empirical relationships—what farmers are seeing on the ground?; if so, what does APSIM imply are the key processes that give rise to these relationships?; how much are these relationships affected by changes in atmospheric CO2? Continue reading

Elevated CO2 Affects Tropical Marine Fish Predator-Prey Interactions

by Jennifer Fields

Recent research has demonstrated that exposure to elevated CO2 affects how fish observe their environment, affecting behavioral and cognitive processes leading to increased prey mortality. However, it is unknown if increased prey mortality is caused by changes in kinematics of predator-prey interactions or from just increasing prey activity levels. Allan et al. (2013) studied the effect of anticipated end of this century CO2 concentrations on the predatory-prey interaction of two tropical marine fish. Both a predator and prey fish was exposed to present day and elevated CO2 levels in a cross-factored design. The authors investigated the changes in locomotion performance, prey reaction distance, and capture success of the interaction. Authors found that predators Continue reading

Species Most Vulnerable to Climate Change

by Elizabeth Medford

While it has been recognized in the past that climate change will have impacts on biodiversity, many approaches ignore the differences between species that will increase or reduce their vulnerability. Foden et al. (2013) chose to address three different aspects of climate change vulnerability to account for species’ biological traits: sensitivity, exposure, and adaptive capacity. In combining these traits with the modeled exposure to projected climate change, the authors assessed the species with the greatest relative vulnerability to climate change. These methods were applied to each of the world’s birds, amphibians, and corals. The authors also identified the geographic areas in which the most vulnerable species are concentrated. These included the Amazon basin for amphibians and birds, and the Indo-west Pacific for corals. The aim of Foden et al. is that Continue reading

A Review of CO2 Enrichment Studies: Does Enhanced Photosynthesis Enhance Growth?

Plants typically only convert 2 to 4% of available energy into actual growth and this natural inefficiency provides a reason for scientists to attempt to increase the efficiency of the process by increasing photosynthesis. One of the most common methods, other than genetic modification, to increase photosynthesis is to increase ambient CO2. Elevated CO2 can lead to growth increases ranging from 10 to 50%, depending on the plant’s carbon sink capacity and nutrient availability. Previous studies show that elevated CO2inevitably leads to increased growth, but the magnitude of the growth varies with the photosynthetic capacity of the plant. Photosynthesis is an inefficient process with a maximum of 8 to 10% of the energy in sunlight being converted into chemical energy. Realistically, only 2 to 4% of energy in sunlight is converted. In this paper, Kirschbaum examines previous studies and conducts experiments of his own in order to summarize the current knowledge on CO2enrichment studies, focusing on the ability of increased photosynthesis to ultimately increase plant growth. Kirschbaum studies the factors that affect plant growth under elevated CO2 in an attempt to determine if photosynthesis is the main factor increasing growth or if other factors are relatively more important.—Taylor Jones
Kirschbaum, M.U.F., 2011. Does Enhanced Photosynthesis Enhance Growth? Lessons Learned from CO2 Enrichment Studies.  Plant Physiology 155, 117-124.

          Kirschbaum first examines the photosynthetic response to increasing CO2concentrations and distinguishes between Rubisco-limited photosynthetic rates and ribose 1,5-bisphosphate (RuBP) regeneration-limited rates. For both photosynthetic rates, the relative responsiveness of increases in CO2concentration decreases as atmospheric CO2 continues to increase. Photosynthesis is limited by Rubisco-limited rates at low CO2concentrations and RuBP regeneration-limited rates at high concentrations, and scientists argue that the amounts of Rubisco plants have today is in excess of what is needed, so most plants experience RuBP regeneration-limited photosynthesis. Changes in plant photosynthesis are supported by previous studies, as 30 to 40% enhancements in photosynthesis were recently found in free-air CO2 enrichment experiments and a 58% increase was found in a controlled plot experiment. Kirschbaum notes that there are important limitations to any photosynthesis study as plants that experience less light and increased self-shading may have less enhancement of photosynthesis, while plants grown in high temperature conditions may have more. On average, increases in ambient CO2 lead to a 30% enhancement of photosynthesis, but does this translate to a 30% enhancement of growth?
          Studies show that the relative growth rate for plants is often similar among species and enhanced photosynthesis often leads to only a 10% increase in the relative growth rate. Kirschbaum suggests that previous studies show an increases in photosynthesis leads to 20% enhanced leaf area, but also a 6.5% increase in leaf weight due to increase amounts of carbohydrates and this leads to an ineffective transformation of increased photosynthetic rates to new growth. Extra amounts of carbon produced from photosynthesis can only be of use if the plant can utilize it through root growth, new foliage, or other carbon sinks. Also, carbon cannot be used efficiently if other vital resources, such as nitrogen, are lacking. Studies have shown that many plants show strong photosynthetic enhancement during the growth stages, reduced enhancement during the flowering stages and then increased enhancement during the fruiting stages. During the flowering stages, plants lost much of their potential carbon sink that exists in the growth phase and is regained through seed production in the flowering stage. Most plants show some increased growth response to elevated CO2, but the degree of this growth is determined by other limiting factors, such as carbon sink and nitrogen availability.
          Kirschbaum also notes that a large number of papers use biomass enhancement ratios to determine the effects of elevated CO2 on plant growth. Biomass enhancement ratios are often much greater than relative growth rates and also greater for single-plant studies and fast-growing plants. Under elevated CO2, plants often experience exponential growth in early stages, followed by average growth rates in intermediate stages. Plants that experience an overall relative growth rate of 10% can experience a biomass enhancement ratio of 50% in intermediate stages which eventually decreases to about 10% in later stages. This concept explains why fast-growing plants can have higher biomass enhancement ratios compared to slower-growing plants, but the same relative growth rate. Therefore, the length of an experiment is very important and should be considered when examining the biomass enhancement ratio of a plant to determine if real growth increases exist. The biomass enhancement ratio can often be a misleading value as it can be manipulated by varying the length of an experiment.
          Kirschbaum identifies other issues that may affect photosynthetic enhancement rates that need to be considered, such as natural competition and growth response in mixed-species communities. Also, some studies have shown a decrease in protein concentrations under elevated CO2. Plant herbivore interactions might also change as elevated CO2 usually leads to lower nutrient concentrations which reduces the rate of herbivores feeding on the plant and as a result, herbivores may attempt to consume more of the plant. All of these factors are important complicating issues and should be addressed further.
          In conclusion, photosynthetic enhancement due to elevated CO2 increases the carbon available to plants and whether or not this translates to growth depends on other colimiting factors, such as nutrient availability and carbon sink. Increases in carbon will exacerbate any other limitations. Plants are also subjected to genetic constraints and will only respond to increases in photosynthesis to levels within their genetic capability. By examining several CO2 enrichment experiments, Kirschbaum found that growth enhancements are modest and a 10% increase in relative growth rate can translate to a much higher relative growth rate in the early exponential phases of plant growth. Kirschbaum suggests that genetic manipulation of photosynthesis should include appropriate crop management and close examination of plant attributes to maximize photosynthetic enhancement.

The Effects of Increased CO2 on Biomass and Exogenous-Toxin Quantity in Transgenic Bt Cotton and Rice Crops

Transgenic crops have become an increasingly important component of modern agroecosystems, ideally providing environmentally friendly, disease resistant crops with combinations of multiple genes that improve productivity and agricultural yield. One of the most common types of transgenic crop is Bacillus thuringiensis (“Bt”), which is produced worldwide and exhibits a strong resistance to lepidopteran pests in multiple cropping environments. With CO2 levels expected to increase in the future, scientists question the ability of Bt crops to adapt to changing atmospheric conditions and some hypothesize that increasing CO2 will pose new ecological risks for Bt crops and possibly reduce their effectiveness against target pests. In this study, a series of open-top chamber (OTC) experiments were conducted to asses whether measured exogenous-toxin quantity is reduced in transgenic Bt cotton and rice due to increased plant biomass under elevated atmospheric CO2. This study also examines the effectiveness of Bt cotton and rice transgenes against H. armigera and C. suppressalislarvae respectively. The study showed that there are significant differences between the exogenous-toxin levels of Bt cotton and rice under increased CO2and both showed differences in toxin quantity among developmental stages. Also, the new properties of Bt crops under elevated CO2 significantly affected the performance of H. armigeraand C. suppressalis larvae, despite the adverse effects of Bt gene expression in elevated CO2conditions.—Taylor Jones
Chen, F., Wu, G., Ge, F., Parajulee, M. N., 2011. Relationships between exogenous-toxin quantity and increased biomass of transgenic Bt crops under elevated carbon dioxide. Ecotoxicology and Environmental Safety 74, 1074–1080.

          Chen and colleagues performed a series of OTC experiments with ambient (375μl/L) and elevated (750μl/L) CO2  conditions that were maintained via a continuous automatic control system. Thirty-six pots of transgenic Bt cotton and twenty-six pots of transgenic Bt rice were planted and their positions were randomized each day to limit positional effects in the OTC chambers. In select plots, cotton bollworm (H. armigera) and rice stem borer C. suppressaliswere added to examine the effects of pest larvae on Bt cotton and rice. Biomass index was used to determine the increased amount of biomass under elevated CO2conditions and plant tissues were tested for exogenous-toxin quantity. Chen et al. recognize the potential “dilution effect” in which percent biomass increase exceeds the percent increase in exogenous-toxin increase, resulting in decreased levels of exogenous-toxin.
          The results show that increasing CO2levels significantly increased leaf, petiole, shoot and total plant biomass production of 45-DAS (“days after seedling”) Bt cotton as well as increases in shoot and total plant biomass production in 90-DAS cotton. Similar trends resulted for Bt rice as root, above-ground, and total stem biomass increased in 50-DAS rice and root tissues increased in 100-DAS rice. Overall, elevated levels of CO2 led to increased biomass in Bt cotton and rice, as predicted by previous studies showing increased photosynthesis and growth rates.
          Elevated CO2 conditions significantly reduced exogenous-toxin content in both Bt cotton and rice tissues. However, the effect of CO2 level on exogenous-toxin amount varied among crops and their respective plant tissues. For example, increased CO2 significantly reduced exogenous-toxin content per plant in 45-DAS and 90-DAS Bt cotton, while simultaneously increasing exogenous-toxin content in the stems of 50-DAS Bt rice. This shows that the responses of transgenic Bt cotton and rice (relating to exogenous-toxin content) to increased ambient CO2 are different. Also, each plant exhibited different responses in different phases of development. Chen and colleagues compared percent changes in biomass and exogenous-toxin levels and concluded that a dilution effect exists in shoot and petiole tissues of 45-DAS Bt cotton and in root, above-ground, and total stem tissues of 50-DAS Bt rice as well as leaf sheaths of 100-DAS rice. This is probably related to increased plant nitrogen-use efficiency and the authors predict that increased plant carbohydrate concentration diluted Bt proteins. For other increases in exogenous-toxin levels, the authors conclude that the dilution effect is only partly responsible and the reduction is due to reduced expression of the Bt gene under increased CO2 conditions.
          The results also suggest that most H. armigera larvae preferred to feed on transgenic Bt cotton squares and bolls, and most C. suppressalis larvae preferred to feed upon leaf sheaths of transgenic Bt rice. These areas of the Bt crops correlate with decreases in exogenous-toxin production, as expected. Although feeding increased in certain areas, the study found overall decreases in larval survival rate and pupal weight of H. armigera and C. suppressalis, suggesting that the Bt cotton and rice in this study to not face serious risks of reduced efficiency against pests in increased CO2 conditions.