Planting trees in Sudan’s drylands changed the soil in ways that became clearer as the plantations grew older. Acacia Senegal gradually built up more organic carbon and improved the soil’s ability to hold water for plants. A 2018 study titled ‘Changes in soil hydraulic properties, soil moisture and water balance in Acacia senegal plantations of varying age in Sudan’ compared plantations of different ages with nearby grasslands at two sites in Sudan. Despite the gains under older trees, the grasslands still held more soil moisture overall, showing that planting more trees does not always make dryland soils wetter.
What happened to soil carbon as Acacia senegal plantations aged
The study looked at two sites in North Kordofan state. One was El Demokeya forest reserve, an experimental site used for gum arabic research, while the other was El Hemaira forest, which was managed by farmers for gum arabic production.The researchers examined Acacia senegal plantations of different ages and compared them with nearby grasslands. The plantations ranged from younger stands to much older ones, allowing the researchers to see how changes developed over time instead of treating all planted areas as the same.One of the clearest patterns was found in soil organic carbon, often used as an indicator of organic matter in soil. The study found that carbon levels increased with plantation age. Although the overall carbon content remained low, the older plantations had accumulated more carbon than the grasslands.This mattered because carbon in soil can influence how much water the soil is able to retain. As the plantations became older and their soil carbon increased, the soil developed a greater capacity to hold water that could potentially be used by plants.
The older plantations still have less soil moisture
The increase in the soil’s ability to hold water did not mean that the plantations actually contained more water than the grasslands. Measurements taken by the researchers showed that soil moisture was higher in the grasslands.The study measured soil moisture during 2011 and 2012 and found that moisture increased with plantation age. This followed the same general pattern as soil organic carbon and the soil’s capacity to hold plant-available water. Older plantations therefore performed better than younger plantations in terms of soil water retention.However, even with this improvement, the grasslands remained wetter. The researchers linked this difference to the way water moved through the two types of land. Trees use water through evapotranspiration, the combined loss of water from the soil and plants into the atmosphere. The plantations had greater evapotranspiration than the grasslands, meaning more of the available water was used or returned to the atmosphere.The grasslands, by contrast, had lower evapotranspiration. This helped them maintain higher soil moisture and also allowed more water to move down through the soil as drainage.
How did the plantations change the movement of rainfall
The study did not find that tree planting simply reduced the amount of water available in the soil system. Instead, it showed that the plantations changed how rainfall moved after reaching the ground.Compared with the grasslands, the Acacia plantations produced less surface runoff. More rainfall entered the soil through infiltration, which means water was able to move from the surface into the ground more effectively.This was an important difference because drylands receive limited and often irregular rainfall. Keeping more rain within the soil rather than allowing it to run across the surface can reduce immediate water losses.At the same time, the plantations had greater evapotranspiration. More water entered the soil, but a larger share was subsequently used by the vegetation and returned to the atmosphere. The study’s water-balance modelling therefore showed lower drainage and lower overall soil moisture in the plantations.The findings demonstrate that infiltration alone does not determine how much moisture remains in the soil.
Grasslands remained better at retaining soil moisture
The higher soil moisture found in the grasslands was one of the study’s most notable findings. It challenged the expectation that increasing tree cover would necessarily make more water available in the soil.The researchers had expected greater tree biomass and cover, together with increased soil carbon, to reduce runoff and improve infiltration, plant-available water and soil moisture. Some of those expected changes did occur. Older plantations had higher soil carbon, greater water-supplying capacity and lower runoff.But the measurements showed that the grasslands still retained more moisture. The difference was largely associated with lower evapotranspiration in the grasslands. Their higher soil moisture also produced greater drainage compared with the plantations.This means the wetter grassland soils were not necessarily evidence that more rainfall was entering them. Instead, less water was being removed, leaving more moisture in the soil. They emphasised on the need to look beyond a single measure when assessing dryland plantations. The success of tree planting in drylands cannot be judged simply by how much vegetation grows or how much carbon accumulates. Water availability remains a separate and important part of the picture.
