What's Stomata with Our Climate?

Stomata have the ability to shed light on past environments and describe how physical and biological processes have been altered in the face of climate change. Could what we have learnt from the past aid our understanding and predictions for the future?

What are stomata?

Stomata are tiny pores located on the surfaces of leaves and stems.

They are bordered by a pair of guard cells which carefully dictate the exchange of gases – primarily water vapour and carbon dioxide (CO2) between the interior of the plant and the atmosphere. The aim of the game is to maximise CO2 uptake whilst minimising water loss. Measuring anything from a tenth to several hundredths of a millimetre across, stomata may be small but they are far from insignificant.

Stomatal sensitivity to atmospheric CO2

The general consensus is that as atmospheric CO2 increases, the number of stomata on a leaf (stomatal density) decreases. This is because in a CO2 rich atmosphere, the plant will be able to receive its essential ‘dose’ of CO2 in a shorter period of time, with fewer stomata needed to do so, resulting in less water being lost from the plant – a potential win-win situation you might think. Such a feedback is of global significance past, present and future.

Historical stomatal influence on vegetation

During the Phanerozoic eon (approximately (~) the last 540 million years), atmospheric CO2 concentration was generally high and stomatal densities low, subsequently affecting the evolution of land plants during that time. This significant point in Earth’s history marks the emergence of new plant groups, such as ferns (during the Devonian Period, approximately 410 to 360 Million years ago (Mya)) and seed ferns (during the Carboniferous Period ~ 360 to 286 Mya).

Approximately 200 Mya, there was a mass extinction event, known as the Triassic/Jurassic (Tr/Jr) boundary. During this time life was experiencing a high CO2 world, resulting in low stomatal densities across the global flora. Plants were able to harness the abundance of CO2 and lose less water however; the rewards were not without their costs. Temperature control of a leaf is also governed by stomata. It a bid to cool off a plant will release water through its stomata in a process called evapotranspiration; it works in a similar way to how we perspire to cool down.

Due to the reduction in stomatal densities during the Tr/Jr boundary, plants were unable to release sufficient water to cool down thus causing the leaf to ‘cook’, as its internal temperature was higher than that of the surrounding air. This drove the preferential extinction of large leaved species (e.g. Ginkgoites obovate) and the evolution of highly dissected leaved species that were better adapted for effective convective cooling (e.g. Ginkgoites hermelini).

Atmospheric CO2 concentration is therefore capable of driving significant changes in plant structure and steering the evolutionary course of land plants throughout the Earth’s history. With atmospheric CO2 concentration having such a profound impact on stomata, it is important to look at climate change and how elevated levels of CO2 could affect stomatal functioning, affecting vegetation dynamics and the Earth system as a whole.

Climate Change

Climate change is the long term shift in weather patterns and average temperatures across the Earth. Throughout the Earth’s 4.5 billion year history it has experienced, on numerous occasions, anything from sweltering tropical climates to bitter ice ages. A shift in the Earth’s climate is perfectly natural, it’s shaped the landscape we see today and driven the diversification of life on Earth; nevertheless the rate at which it has been changing since the industrial revolution (around 100 years ago) is cause for concern. CO2 has been accumulating in the atmosphere at an accelerating pace, spurred on by the burning of fossil fuels. Today in 2015, the concentration of atmospheric CO2 is around 400 ppm (parts per million), approximately 40% higher than at any time during the last 20 million years. It is important to understand stomatal response to this rapid elevation in atmospheric CO2 concentration, for stomata are key players in the Earth system, affecting the diversification and spread of vegetation which in turn influences landscape processes and ultimately the climate itself.

Stomatal influence on the hydrological cycle:

The acquisition of stomata by terrestrial plants around 400 million years ago, coincided with the evolution of trees with advanced rooting systems, capable of retrieving water from deeper in the soil. This meant that the trees were able to transpire more water, leading to an increase in precipitation over the continents. Now the once dry, desolate interiors of continents were given the gift of water, something they would cherish for millions of years to come. Plant life steadily clambered into these new environments, gaining valuable footholds across the globe that would inevitably lead to the spread and diversity of terrestrial life we see today. Remarkably the emergence of plants with deeper rooting systems also drove significant changes in river systems. The plant roots secured the soil, stabilising banks of river and streams. These more durable banks were eroded less easily by the water, leading to the rise of the meandering river systems we recognise today.

Stomatal response to CO2 also affects the hydrological cycle on a global scale. By observing stomatal transpiration in the context of the hydrological cycle, we can see that double the amount of water passes through stomata per year (32×1015 kg yr-1), than is held in the atmosphere (15×1015 kg yr-1), stomata therefore have the potential to significantly affect the hydrological cycle. An example of which comes from research by Nicola Gedney and colleagues of the Hadley Centre for Climate Prediction and Research (2006), they found that the reduction in stomatal densities as a response to increasing atmospheric CO2 has increased surface runoff, accounting for a 3% rise in annual river flow!

Stomatal influence on land warming

Decreased stomatal density results in less evapotranspiration by the plant; we have already seen this cause extinction of large leaved species during the Tr/Jr boundary. However this lack of a cooling effect will not only have detrimental effects on the individuals concerned but also the land as a whole. Remarkably, research suggests that 25 percent of land warming in North America and Asia can be attributed to stomatal response to increased atmospheric CO2. Stomata l response to atmospheric CO2 therefore has the ability to significantly impact preferential species extinction and global land warming.

In Summary

Stomata are not only key to unlocking and understanding the underlying mechanisms that have shaped out planets biological and physical cycles in the past, but are also essential for predicting how the Earth will respond to the new chapter in its life, 21st century climate change.

Further Reading

Cao, L., Bala, G., Caldeira, K., Nemani, R., Ban-Weiss, G., 2010. Importance of carbon dioxide physiological forcing to future climate change. Proc. Natl. Acad. Sci. 107 (21), 9513–9518.

Gedney, N., Cox, P. M., Betts, R. A., Boucher, O., Huntingford, C., & Stott, P. A. (2006). Detection of a direct carbon dioxide effect in continental river runoff records. Nature, 439 (7078), 835-838.

Hetherington, A. M., & Woodward, F. I. (2003). The role of stomata in sensing and driving environmental change. Nature, 424(6951), 901-908.