Divergent rainfall delivery, runoff response, and retained-material composition across contrasting Andean coffee systems
This study demonstrates that in Andean coffee agroforestry systems, canopy-mediated rainfall interception significantly reduces soil water delivery and alters the composition of retained materials, indicating that lower absolute runoff in shaded systems reflects reduced rainfall input rather than superior surface water regulation.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
On the steep, misty slopes of the Colombian Andes, coffee farmers often plant their crops under the cover of other trees. This practice, known as agroforestry, is widely praised for creating a cooler microclimate, protecting soil, and supporting biodiversity. A key promise of these shaded systems is that they act as natural sponges, absorbing heavy rains and preventing water from rushing down the hillsides as destructive runoff. The logic seems straightforward: more trees mean more leaves to catch rain, which should mean less water hitting the ground and flowing away. However, the relationship between the canopy above and the soil below is rarely that simple. Rain does not fall uniformly; it is intercepted by leaves, funneled down branches, or dripped from specific points, creating a patchwork of wet and dry spots on the forest floor. To truly understand how these systems protect the land, scientists must distinguish between how much rain actually reaches the soil and how the soil itself reacts once that water arrives. If a shaded plot shows less runoff, is it because the trees are regulating the water better, or simply because they caught most of it before it could reach the ground?
A team of researchers set out to untangle these layers in the Santa Rita district of Antioquia, Colombia, where coffee is grown on hillsides with slopes as steep as 52 percent. They monitored three distinct types of coffee arrangements over twenty-one weeks during the rainy season. The first was open-sun coffee, growing without any overhead trees. The second was coffee planted alongside plantain plants, which have large, broad leaves. The third was coffee shaded by avocado trees, creating a dense, multi-layered canopy. The researchers placed small, contained plots in each system to catch every drop of water that fell within their boundaries, as well as the water that flowed off the surface as runoff. They also collected the solid material carried by that runoff, sorting it to see how much was organic matter, like decaying leaves, and how much was mineral soil.
The results revealed a surprising disconnect between what happened in the air and what happened on the ground. The trees and plants above did indeed intercept a massive amount of rain. In the open-sun plots, the soil received nearly 88 percent of the total rainfall that fell in the area. In contrast, the coffee under avocado trees received only about 40 percent of the rain, and the coffee with plantains received roughly 47 percent. The vegetation had effectively blocked or redirected more than half the water before it ever touched the soil. Consequently, the total amount of water flowing out of the shaded plots was lower in absolute terms, simply because less water had arrived to begin with.
However, when the researchers looked at how efficiently the soil surface turned the water it did receive into runoff, the story changed. The open-sun coffee plots, which received the most rain, actually converted the smallest percentage of that water into runoff. The shaded plots, despite receiving far less water, converted a higher proportion of their local rainfall into flowing water. This suggests that the presence of trees did not necessarily make the soil surface better at absorbing water; in fact, the soil under the trees seemed to shed a higher fraction of the water that managed to reach it. The lower total runoff in the shaded systems was therefore a result of the canopy acting as a filter, not necessarily a result of the ground being more effective at holding water.
The composition of the material washed away by the runoff offered a third, independent clue about what was happening. The water flowing from the avocado-shaded plots carried the highest amount of organic material, nearly 60 percent of the total mass, likely consisting of leaf litter and decomposing plant matter. The plantain plots, on the other hand, carried the highest proportion of mineral soil, almost 50 percent, suggesting that the water there was more effective at picking up bare earth or mineral aggregates. The open-sun plots fell somewhere in between. This means that the type of material being eroded did not follow the same pattern as the amount of water flowing. The system with the most water did not necessarily carry the most soil, nor did the system with the least water carry the least soil.
These findings challenge the common assumption that simply observing less runoff in a shaded coffee farm proves that the system is regulating water better. The study shows that lower runoff can be a side effect of the trees catching the rain, rather than a sign of improved soil performance. Furthermore, the nature of the soil being lost—whether it is rich in organic matter or heavy with mineral soil—depends on factors that are not directly tied to the volume of water flowing. To truly assess the health of these mountain farms, scientists and farmers need to look beyond the total amount of water leaving the field. They must measure how much rain actually reaches the ground, how the soil surface responds to that specific amount, and what kind of material is being transported. Only by separating these different stages can we understand whether the trees are truly protecting the land or merely changing the way the water falls.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.