Deforestation changes the water cycle: more rain in thinned areas during the wet season, less during the dry season. Recent research emphasizes that these changes threaten the survival of trees.
In an area of about 7 million square kilometers twice the size of India plus Egypt about 10 percent of the species known to humans live.
This is the Amazon rainforest, considered the largest tropical rainforest in the world and one of the most important and sensitive ecosystems on Earth.
A significant component of ecosystem survival is the water cycle, which in the Amazon changes seasonally: half the year is defined as the wet season (December to May) and half as the dry season (May to December).
During the wet season, the forest trees absorb large amounts of water up to 200 millimeters of rain per month while at the peak of the dry season they manage with about 50 millimeters per month.
The “famine” is long and harsh, but the trees are adapted to the arrival of the blessing rains.
However, environmental changes in the Amazon affect the delicate interactions between water balance, wind, and the flows of heat and radiation entering and leaving between vegetation and soil to the atmosphere.
These interactions affect the local climate, and therefore the rainfall cycle.
One of the most significant environmental changes in the last half century is deforestation cutting trees to convert forest areas for other uses, usually agricultural.
The deforestation rate is unprecedented in the state of Rondônia in Brazil.
For example, about a third of the 208,000 square kilometers of rainforest had already been cut down by the early 2000s.
Deforestation has global effects on the climate system and the carbon cycle, but a new study published in Nature focuses on its impact on the local rainfall cycle in the Amazon, showing that it also has devastating consequences.
Two Seasons
The researchers used forest cover maps of the Amazon for the years 2000 and 2020, between which about 60 percent of the Amazon area experienced a decrease in trees from partial thinning to complete deforestation.
These data were used to run climate models predicting rainfall amounts in the region during this period with deforestation and without allowing the isolation of the forest thinning effect on precipitation changes.
The study focused on the wettest and driest months, allowing the models to distinguish between changes at the peak of the dry season June to August and the peak of the wet season December to February.
They also distinguished between two different mechanisms: precipitation from local humidity changes versus precipitation from regional atmospheric humidity.
Researchers found that during the wet season, in areas where the forest is thinned, the ground heats up more compared to adjacent forested areas.
The heating causes rising air and the creation of a low pressure area at ground level, which draws moist air from the forested area into the thinned area.
This forms clouds and rain from moisture that is not local but atmospheric.
This phenomenon increases rainfall by about 6 millimeters per month in deforested areas, but its effect diminishes at distances over 40 kilometers from the thinned area, and in more distant regions a decrease in rainfall was observed.
The effect is more pronounced the greater the amount of deforestation.
The Dry Season Becomes Drier
During the dry season, the air is too dry for this phenomenon to be significant.
Here, the main impact of deforestation is a decrease in local humidity.
Without trees, water evaporation from the soil to the air decreases, and rainfall drops by about 1.5 millimeters per month.
Researchers found that deforestation during the dry season makes the season even drier.
The phenomenon is noticeable even up to 1,000 kilometers from the deforested areas, affecting the entire Amazon.
In summary, deforestation in the Amazon has opposite effects on the water cycle: it locally increases rainfall during the wet season but decreases it during the dry season.
Since trees experience water stress in the dry season, reduced rainfall may threaten their survival even in non deforested areas.
Meanwhile, the increase in rainfall during the wet season is not necessarily beneficial, as humidity is diverted from forested to thinned areas, reducing water available to the trees.
Researchers estimate that the trend of deforestation will continue, and rainfall will keep decreasing, leading to additional tree mortality.
A combination of deforestation and global warming could result in half of the vast forest being at risk of turning into a savanna like landscape by 2050.