International researchers from Finland, Germany, South Africa and Ethiopia report that deforestation over the past two decades has led to greater warming and a greater increase in cloud cover than climate change itself. This threatens biodiversity and water supplies in Africa’s mountain forests.

Often cloudy, wet and cold, montane forests are found on isolated mountains in Africa. They are rich in biodiversity and act as water towers by capturing water from mist and clouds, providing high-quality fresh water to millions of people in Africa’s lowlands.

Over the past two decades, 18% of Africa’s mountain forests have been lost to deforestation, causing warming and a rise in cloud cover twice as fast as climate change. Research shows that air temperatures have increased by 1.4°C, while cloud cover has increased by 230 metres over the past 20 years.

“It has serious consequences for water resources and biodiversity,” comments Prof. Dirk Zeuss from the University of Marburg.

The study was published in Nature communication.

Rising cloud cover reduces water extraction

The increase in cloud cover reduces water harvesting because when the cloud touches the forest canopy, the mist (water) is deposited on the plant and land surfaces. When the cloud cover is higher, this phenomenon does not occur, explains Prof. Petri Pellikka, director of the Taita research station.

This phenomenon also requires the forestation of mountain peaks because it increases the land area and allows water to be stored better in the trees and soil in the forest than in open land.

The study sites were in the highlands of Kenya, Tanzania, Ethiopia and South Africa. The study was one of the results of the Taita research station, which the University of Helsinki has maintained in southern Kenya since 2009. The University of Helsinki’s contribution to the study came from the Earth Change Observation Laboratory of the Faculty of Earth Sciences and Geography.

“During our research in the Taita Hills, we found that 20 percent more water falls to the ground each year on forested mountain tops than on open areas. This is due to the mist that settles on the trees and drips to the ground in the form of drops. This mist is in addition to the precipitation. When the clouds stay higher and do not touch the forests, this phenomenon no longer occurs,” says Pellikka.

There are still many small forested mountain peaks in the Taita Hills. The most important water towers in Kenya include Mount Kenya, the Mau Forest, the Aberdare Mountains, Mount Elgon, the Cherangani Hills and Kilimanjaro. Although Kilimanjaro is in Tanzania, it also supplies water to the Kenyan side.

“Around Africa’s highest mountain, Mount Kilimanjaro in Tanzania, 50 percent of the forest has been lost since 1880,” says Dr. Andreas Hemp from the University of Bayreuth. He has been researching Mount Kilimanjaro for 30 years.

Negative relationship between temperature and altitude

The study also found that the impact of warming from deforestation may decrease with increasing altitude due to the negative relationship between temperature and altitude. Nevertheless, large-scale deforestation (i.e. loss of tree cover of more than 70% in a 1 km x 1 km area) can offset the cooling effect of altitude, and comparable warming can be induced at higher elevations in African montane forests.

“The findings call for urgent action, as mountain deforestation caused by farmland expansion and logging poses a serious threat to biodiversity and ecosystem services such as water supply in Africa,” says Temesgen Abera, visiting scientist at the University of Helsinki and postdoctoral researcher at Philipps University Marburg (Germany).

The study used a data-driven approach based on satellite observations, reanalysis data, as well as ensemble learning, empirical methods, and independent field temperature and cloud base height measurements for validation.

The research group included scientists from the University of Helsinki, the University of Marburg and the University of Bayreuth in Germany, the Finnish Meteorological Institute, Addis Ababa University in Ethiopia and the North-West University in South Africa. The research was part of the ESSA project (Earth observation and environmental sensing for climate-smart and sustainable transformation of agricultural and pastoral ecosystems in East Africa).