Pedodiversity as a Key Element of Geological Heritage in the Shouf Jezzine Geopark Project Lebanon

Beneath the forest floor lies a world as complex and vital as the canopy above. In the study of the Earth's surface, scientists look at two interconnected layers: the rocks and minerals that form the ground, and the living skin of soil that develops on top of them. This living skin, known as soil, is not merely dirt; it is a dynamic mixture of minerals, water, air, and organic matter that supports all terrestrial life. When researchers examine the variety of these soils across a landscape, they are studying what is called pedodiversity. This concept recognizes that different soils, with their unique textures and chemical makeups, create the foundation for different types of plant and animal life. Understanding these variations is crucial because soil acts as a massive storage tank for carbon, a key element in regulating the planet's climate. By looking at how different trees interact with the ground beneath them, scientists can learn how nature manages resources and stores carbon, offering clues on how to protect forests and the climate they help sustain.

In the central southern region of Lebanon, a proposed area known as the Shouf-Jezzine Geopark project, researchers recently turned their attention to this hidden world. The landscape here is defined by limestone and dolomite rocks, which cover most of the country and give rise to soils rich in calcium carbonate. Within this rocky terrain, three distinct types of trees dominate the forests: Pinus brutia, Pinus pinea, and Quercus calliprinos. While these trees are common sights, their specific relationship with the soil beneath them had not been thoroughly mapped. A team of scientists set out to fill this gap, digging into the ground to see how the soil properties changed under each of these three species. Their goal was to understand the physical and chemical makeup of the earth under these forests, with a special focus on a specific layer of soil where calcium carbonate accumulates, known as a calcic horizon. This layer is significant because it forms over long periods and tells a story about the climate and the history of the land.

The team selected fifteen different spots across the geopark, ranging from the lower hills to higher mountain slopes, to examine the soil profiles. They carefully dug down through the layers, documenting the depth and color of each section before collecting samples. In total, they gathered thirty-eight samples from these sites, ensuring they captured the soil from the surface all the way down to the deeper layers. Back in the laboratory, they analyzed these samples for texture, the amount of organic matter, and the presence of various minerals and nutrients. They also measured the acidity of the soil and its ability to conduct electricity, which indicates the level of dissolved salts. The researchers were particularly interested in how the presence of the different tree species influenced the formation of the calcic horizon and how much carbon the soil was storing.

The investigation revealed a clear pattern in how these trees interact with the ground. The Pinus brutia and the Quercus calliprinos were found to grow over soils that had developed a thick, distinct layer of accumulated calcium carbonate. In these spots, the percentage of calcium carbonate in the soil was high, often exceeding fifteen percent, confirming the presence of a true calcic horizon. This accumulation happens when rainwater dissolves calcium from the surface and carries it down, where it settles and hardens in the deeper layers. In contrast, the soil under the Pinus pinea told a different story. The calcium carbonate content in these soils was much lower, rarely exceeding a few percent, indicating that a calcic horizon had not formed in the same way. The researchers noted that the Pinus pinea seemed to prefer conditions where this heavy carbonate buildup did not occur, suggesting that each tree species creates or selects for a specific type of soil environment.

The texture of the soil also played a key role in these findings. The scientists found that soils with a higher content of silt, which are fine, powdery particles, tended to hold more calcium carbonate. This is because the fine particles are better at trapping and holding the minerals as they move through the ground. Sandy soils, which are made of larger, coarser grains, allowed the minerals to wash away more easily, resulting in lower carbonate levels. This connection between soil texture and mineral accumulation helps explain why the different tree species are found where they are. The Pinus brutia and the Quercus calliprinos thrive in areas where the soil texture allows for this rich carbonate layer to build up, while the Pinus pinea is found in areas where the soil remains leaner in these minerals.

Beyond the minerals, the study looked at the organic life within the soil. Organic matter, which comes from decaying leaves and roots, is the engine of soil fertility. The researchers found that the amount of organic matter varied significantly depending on the tree species. The soils under the Pinus brutia contained the highest levels of organic matter, followed by the Quercus calliprinos, with the Pinus pinea having the lowest amounts. This organic matter is crucial because it acts like a sponge, holding onto essential nutrients like potassium, calcium, and magnesium, making them available to the trees. The data showed a strong link between the amount of organic matter and the availability of these nutrients, particularly in the soils under the Pinus brutia and the Quercus calliprinos. This suggests that the Pinus brutia is not just growing in the soil but is actively helping to build a rich, nutrient-dense environment that supports a healthy ecosystem.

One of the most significant findings of the study concerned the storage of carbon. Forests are known to absorb carbon dioxide from the atmosphere, but the soil beneath them is also a massive reservoir for this carbon. The researchers used satellite data to estimate how much organic carbon was stored in the soil at different sites. They discovered that the soil under the Pinus brutia held the highest amount of organic carbon, significantly more than the soils under the other two species. This finding highlights the Pinus brutia's unique role in the fight against climate change. By fostering soil conditions that allow for greater carbon storage, these trees help to lock away carbon that would otherwise remain in the atmosphere. The Quercus calliprinos also contributed to carbon storage, though to a lesser extent, while the Pinus pinea soils stored the least.

The study also examined the acidity and salinity of the soil, finding that these factors remained relatively stable regardless of the depth or the specific location. The soil pH, which measures acidity, was generally neutral to slightly alkaline, a condition typical for regions with limestone bedrock. The electrical conductivity, which indicates the presence of salts, was low across all sites, meaning the soil was not salty and was well-suited for plant growth. These consistent conditions suggest that the differences observed between the tree species were driven more by the biological interactions of the trees with the soil rather than by drastic changes in the underlying climate or geology.

In the end, this research paints a detailed picture of the Shouf-Jezzine Geopark as a place where geology and biology are deeply intertwined. The study confirms that the type of tree growing on a hillside is not just a matter of chance but is closely linked to the specific chemical and physical properties of the soil beneath it. The Pinus brutia and the Quercus calliprinos are shown to be key players in creating and maintaining soils rich in calcium carbonate and organic carbon, while the Pinus pinea occupies a different niche with soils that are lower in these elements. These findings underscore the importance of preserving these diverse forest types, as each contributes uniquely to the health of the land and the global effort to manage carbon. By understanding these relationships, conservationists can better protect the geological heritage of Lebanon and ensure that these forests continue to thrive as vital reservoirs of biodiversity and climate stability.