Changes in saturated hydraulic conductivity during forest regrowth over 10 years in the humid tropics

Landscapes in the humid tropics are undergoing change in land cover. Besides ongoing deforestation of old-growth forest there is also natural regrowth and active reforestation. These changes in land cover affect soil hydrological properties, eg. saturated hydraulic conductivity (Ks), and thus influence hydrological flow paths. While it has been well documented that removing forest in favour of pasture establishment frequently leads to soil compaction and hence increased occurrence of overland flow and erosion, the effect of reforestation on soil hydraulic properties is less studied, especially not in terms of longer time series of forest regrowth.

We monitored the development of Ks in three reforested catchments in the Panama Canal Watershed, two reforestation trials with native species and teak and a secondary succession, over the course of 10 years. We measured Ks on undisturbed soil cores from the depths of 0-6 and 6-12 cm, applying the constant-head method. We compare the results to a previous study based on a space-for-time substitution in the same area.

Our results show a marked increase in Ks variability in both depths after the first five years of measurement. This points to a non-uniform influence of vegetation development across the catchments. Median Ks values in the topsoil increased at all three reforestation sites over the course of the monitoring period and reached values that considerably exceeded those previously measured in 100 year-old forests in the region, which appears at odds with an assumed continuous increase of Ks with increasing forest age. Further comparisons with soil and vegetation characteristics will be used to explain this apparent contrast.