Forests in Water Cycles

The role of forests in water cycles from rainfall to cloud formation, was a topic that highly attired my attention in the Ecology course at university. I´ve already heard of some other Ecosystem Services forests provide like storing Carbon or providing Oxygen. However, I haven´t thought about, how trees can influence rainfalls on a global and local scale. So, I became interested in investigating this topic further.

A waterdrops way from the ground to the atmosphere

Since I was doing an Erasmus Semester in Guadeloupe, I could tell that it rains a lot in the tropics in summer. High temperature is causing water to escape into the atmosphere. Further, the transpiration occurring in forests, also plays an important role, in getting water molecules up into the atmosphere. But how does this happen exactly?

In the Paper Synergy between small- and large-scale feedbacks of vegetation on the water cycle ¹ I read about a phenomenon, called the hydraulic lift. The expression describes the transfer of deep groundwater into dryer soil layers through trees with a vast root system. This water can become accessible for plants with shallower roots, situated closely to the reaching plant. ¹

On a larger scale, the trees also provide water for plants further located from the source, through transpiration. The solar energy absorbed by them is used to rise water vapor into the atmosphere- a process the authors describe as atmospheric recycling. “As a result […] the local climate may be much rainier than if forests would be absent” (Scheffer et al., 2005) ¹. ¹

Building of rainclouds

Furthermore, forests play a part in cloud formation. Terrestrial vegetation in general produces volatile organic compounds (VOCs). These compounds react to secondary organic aerosols (SOAs) through oxidation. SOAs in turn function as cloud-condensing nuclei and are therefore essential for cloud formation. ² In other words: Forests produce compounds that transition into molecules that are important for cloud condensing.

How the reflexivity of the ground influences rainfall 

Most astonishingly the grounds´ reflexivity, also called albedo, can also influence rainfall. A high albedo does mean that much light is being absorbed not reflected. A dark body absorbs more light than a bright one, therefore more light is absorbed not reflected. Concluding from this, the albedo of unvegetated land is higher than the albedo of a forest. This leads to heat loss of unvegetated dry land. This surprised me at first. I´ve been thinking through everyday experience, in unvegetated land the heat accumulates, which is not the case according to the albedo. But I´ll refer to this again later on. Nevertheless, the higher surface temperature leads to a movement of the hot air upwards and therefore to more rain. ¹

However, I wondered why does deforestation lead to regional warming, when there is a lower albedo? Obviously, the shadowing of trees is cooling the surrounding area. However, also the cooling transpiration of trees reduces heat. When water transitions from a liquid into a gas during transpiration the surrounding air is cooled. This results in an energy loss in the air that needs to occur to transform a liquid into a gas. ³

Conclusion

To summarize what we´ve learned in this post about the role of forests in water cycles. Forests contribute to weathering which can lead to rainfall. Trees can drag up the water of deep soil layers and rise it into the atmosphere through transpiration. Further, vegetation produces organic compounds which can form condensing nuclei. In this sense, forests are influential for cloud formation. Last but not least forests absorb much solar radiation due to their high albedo. This favors an air motion upwards and favors rainfall. So what do you think after reading this post? Should we thank the forest for the last rainfall?

References:

1. Scheffer, M., Holmgren, M., Brovkin, V. and Claussen, M., 2005. Synergy between small- and large-scale feedbacks of vegetation on the water cycle. Global Change Biology, Volume 11, Issue 7, 1003–1012.
2. Zhao, D.F., Buchholz, A., Tillmann, R., Kleist, E., Wu, C., Rubach, F., Kiendler-Scharr, A., Rudich, Y., Wildt, J. and Mentel, Th. F., 2017. Environmental conditions regulate the impact of plants on cloud formation. Nature communications, Volume 8, 14067-14067.
3. Trees for Energy conservation., 2019. How do trees cool the air? Available at: https://trees-energy-conservation.extension.org/how-do-trees-cool-the-air/ [15.10.2022].