Reforesting degraded hillslopes with exotic pines in Trinidad and Tobago: Infiltration, repellency and implications for runoff and recharge

Abstract

In the Caribbean, planting resilient, fast growing exotic pine trees have commonly been used to reforest degraded landscapes. However, little is currently understood about how these practices alter soil water movement and subsequently runoff and recharge. We monitored the surface soil water repellency, hydraulic conductivity, and subsurface soil moisture response to individual storm events in an exotic pine plantation and native forest in the dry and wet seasons. During the wet season, neither forest had a repellent soil surface, and the hydraulic conductivity in the exotic pines (48 mm h−1) were not significantly higher than the native forest (36 mm h−1). The speed and magnitude of the soil moisture response did not differ substantially between both forests in the wet season. However, during the mid dry season, a threshold soil moisture level was reached in the pines which triggered the development of repellent/hydrophobic conditions across all locations. Repellency reduced the surface hydraulic conductivity to 1.6 mm h−1, which in turn severely reduced the soil moisture response to rainfall. Our findings illustrate that throughout most of the year, the exotic pine plantation does not have a substantial negative impact on runoff or recharge. Only under the driest conditions do they negatively affect these processes. Our work also highlights the importance of characterising soil water repellency, hydraulic conductivity, and subsurface soil water responses over longer time periods to account for temporal variability.