FAHM - Free Air Humidity Manipulation experiment
GENERAL BACKGROUND OF THE FAHM EXPERIMENT
Although research on global climate change already began two decades ago, it is a topic that still commands widespread attention among both scientists and the general public. Climate change studies have documented the rise of carbon dioxide levels in the atmosphere, as well as its impact on ecosystems. Scientists have also studied the effect of global warming and increased precipitation on plant communities. However, much less information is available on other aspects of global climate change, which also influence the functioning of ecosystems.
One of the most important greenhouse gases is water vapour, which absorbs the infrared radiation emitted by the Earth and thus affects the heat balance of the globe. Water vapour constitutes as much as 2% of the atmosphere and nearly two thirds of the possible greenhouse effect is in fact contributable to water vapour. Besides the global energy balance, changes in water vapour concentrations also depend on cyclonic flows in the atmosphere, varying widely between regions and being difficult to predict. If the water vapour concentration in the atmosphere increases due to climate warming, the climate will continue to warm at an even faster rate, which in turn feeds a further increase in water vapour concentration, since the atmosphere’s capacity to contain water vapour increases with temperature.
Whereas the effects of global climate change caused by high carbon dioxide concentrations have been studied relatively extensively and the respective experimental systems have been widely used (Free Air Carbon Enrichment (FACE) technologies), the effects of changes in atmospheric water vapour concentrations have received little attention. Without the respective core knowledge, however, it is impossible to predict the reaction of the ecosystem as a whole to the changes in climate conditions.
Air humidity can influence an ecosystem and its components in several ways, causing changes primarily in the intensity of evapotranspiration. Changes in evapotranspiration affect the balance of water and minerals in different parts of the ecosystem and consequently also the growth rate and competitiveness of plants and associated microorganisms. Since vascular plants possess the ability to adjust the intensity of transpiration (evaporation of water from the surface of plant leaves) via their stomata, it is difficult to predict the effects of changes in air humidity without conducting specific experimental studies. The difficulty here lies in the multitude of environmental factors (light, temperature, carbon dioxide concentration in the air, air humidity and soil moisture) influencing the regulation mechanisms of the stomata and in the species-specific nature of such regulation mechanisms. Besides the functioning of individual organisms, air humidity can also affect the biodiversity of the ecosystem (including its different trophic levels), causing changes in the functioning of the ecosystem.