The Influence of Tree and Stand Age on Soil water movement in Theobroma cacao plantations

In many parts of the tropics, the increased demand for cocoa and its products has led to the development of new plantations. The soil properties in these young plantations may differ from older plantations or natural forests, which may affect soil water flow. As cocoa trees are very shallow rooted, the growth and survival of trees can be prone to changes in soil hydrology. We monitored the soil properties, soil water repellency and hydraulic conductivity in a 5, 12 and >30 year old cocoa plantation. During the dominant wet period, soil water repellency was absent in all stands while the hydraulic conductivity showed no significant differences among them. This suggests that water movement in the wet was not impacted by stand age. However, during the dry season, the water drop penetration times at the 5 (4.1 hours) and 12 (4.4 hours) year old stands were twice as long as the >30 year old plantation (2.1 hours). The extreme repellency in the younger stands were expected to reduce infiltration rates; however, higher rates were recorded in the 5 and 12 year old stands. We suggest that the higher infiltration rates in the younger stands are due to a combination of a highly repellent soil matrix and the presence of large, deep soil cracks which enhanced preferential flow. With the degree of repellency not being correlated with soil properties, we hypothesised that the high grass/sedge cover and high temperatures in the 5 and 12 year old stands enhanced it. While further research is needed to investigate the roll that grass and sedges play in developing repellent conditions and affecting soil water flow, managing their cover may prove beneficial for the growth and survival of young cocoa trees.