CO2 levels in Earth’s atmosphere have just reached 400ppm. This is more than a third higher than what it was in pre-industrial times (about 270-280ppm). It is clear that CO2 has an impact on climate and it is more than just the mean values that will change, but also greater variability around the mean. Also, it is more than just the impact that CO2 has on warming the atmosphere that is interesting. There are numerous direct impacts on other parts of the ecosphere. One area of interest to me is the direct influence that elevated levels of CO2 have on plants. The initial instinct is to assume that because ‘plants eat CO2‘, elevated levels of CO2 will have positive effects on plant growth – this is called CO2 fertilization. There are numerous greenhouse studies with short-lived herbaceous plants that support the CO2 fertilization hypothesis. However, when different systems have been looked at the results vary. Körner (2003) provides a nice review of the impacts that elevated CO2 has on plants. The paper is presented in three themes: (1) the interaction between CO2 enrichment and nutrients; (2) water and CO2 enrichment and (3) how CO2 enrichment influences plant-animal interactions.
1. CO2 and nutrients
The biomass of plants is made up of predominantly carbon based compounds. Therefore, as I mentioned before we might imagine that more CO2 means more growth. But, we also know that photosynthesis (the process that fixes carbon) requires several other components. That means that regardless of how much CO2 increases, growth rates can be limited by other factors. This is nicely demonstrated in studies in natural systems (forests and grasslands) that have experimentally increased CO2 in conjunction with fertilizer additions versus those that have just used CO2 enrichment: productivity is typically increased significantly in the CO2 plus fertilizer studies but, either increased marginally or not all in the CO2 only studies. This suggests that many systems are not CO2 limited and that it is other factors that are limiting to growth. This might include nitrogen, phosphorous and water. One thing to take note of is that the Körner (2003) paper talks almost exclusively about direct CO2 effects of plants. If we consider indirect effects through changing climate the picture is a bit different. For example, one study that looked at long terms changes in tree growth in tropical forest in relation to CO2 increases and climatic variation found growth was influenced by dry-season rainfall and night-time temperature but not by annual CO2 levels (Clarke et al. 2010).
2. CO2 and water
Plants are the highway for CO2 entering and water leaving terrestrial systems. As much as 70% of water that enters the atmosphere and almost all the carbon in terrestrial systems passes through trees. At the centre of this are the little pores (stomata) on the leaves of plants, which I like to call the gatekeepers. What some studies have found is that under elevated levels of CO2 stomata reduce their aperture. What this does is allow the same amount of CO2 in but less water out. That means that trees are taking up less water from the soil which can result in water logging. The follow on effect of this is that the water logged soils can favour the competitive abilities of some species over others. In grasslands, and other herbaceous communities with short turnover times, the community composition can change remarkably fast. Interestingly, stomatal closure varies among species with some species not responding at all. Therefore, it is difficult to make general predictions about the effect of CO2 on water dynamics of different systems.
3. CO2 and plant-animal interactions
Trees fix carbon as structural compounds for building biomass (cellulose and lignin) and for use in energetic metabolic tasks. In addition, plants typically contain storage compounds, often called non-structural carbohydrates (NSCs). In times of growth, NSCs are depleted from the storage pool and in times when growth is limited (e.g. water or nutrient shortage) NSCs accumulate in the plant tissues. This happens because carbon supply from photosynthesis and carbon demand for growth is not always in sync. What has been found in CO2 enrichment studies is that carbohydrate concentration often increases in tissues but protein concentration decreases. This change in ratio of carbohydrate to protein can then change the feeding behaviour of herbivores (e.g. caterpillars). And again, as with species responses in stomatal closure, herbivore species have been found to respond differently to food plants grown in elevated CO2. This makes general predictions difficult to make given our current knowledge.
So what do we learn from Körner (2003)? Elevated CO2 has direct impacts on plants. These impacts are not as intuitive as we may think they are. Different plant species respond differently to elevated levels of CO2. The responses have broad reaching follow on effects on whole communities of plants and animals. We also get the picture that the CO2 issue is a complex one and difficult to make generalizations on – at least at the moment and so more work needs to be done.
I have just read a paper (Jasechko et al. 2013) that looked at transpiration vs total evapotranspiration (the combination of transpiration and evaporation) and found that transpiration contributes as much as 90% of evapotranspiration. That mean that in order to understand water cycles effort should be concentrated on the biotic factors (plants), rather than the physical factors (evaporation). I really nice strong justification for my work!
Clark, D. B., Clark, D. A., & Oberbauer, S. F. (2010). Annual wood production in a tropical rain forest in NE Costa Rica linked to climatic variation but not to increasing CO2. Global Change Biology, 16(2), 747-759. doi: 10.1111/j.1365-2486.2009.02004.x
Jasechko, S., Sharp, Z. D., Gibson J. J., S. Jean Birks, S. J., Yi Y., & Peter J. Fawcett, P. J. (2013). Terrestrial water fluxes dominated by transpiration. Nature 497, 341-451.
Körner, C. (2003). Ecological impacts of atmospheric CO2 enrichment on terrestrial ecosystems. Philosophical Transactions of the Royal Society of London A, 361(1810), 2023-2041.