Enhancing soil water infiltration through tree-enrichment planting within an oil-palm dominated landscape in Jambi, Indonesia

1) Research Objectives
The proposed research work will provide baseline information for a Master’s thesis within the Biodiversity Enrichment Experiment (B11) of the Collaborative Research Centre 990: Ecological and Socioeconomic Functions of Tropical Lowland Rainforest Transformation Systems (Sumatra, Indonesia). The experiment is located in PT HUMUSINDO near Bungku village (Regency Batanghari, Jambi Province).

In this experiment, native trees have been planted in 52 islands of different sizes (5 m x 5 m to 40 m x 40 m) in an oil palm plantation. Different tree-diversity levels (0, 1, 2, 3 and 6 species) are represented for each island size. The tree species include three fruit trees (Parkia speciosa, Fabaceae; Archidendron pauciflorum, Fabaceae; Durio zibethinus, Malvaceae) and three other native species (Dyera costulata, Apocynaceae; Peronema canescens, Verbenaceae; Shorea leprosula, Dipterocarpaceae). About 40% of the oil palms have been removed from the islands in order to give space for the planted trees to establish. Three years after establishment of the experiment, some planted trees already reach oil palm height and understory vegetation (herb layer) develops. The project aims at understanding and assessing effects of tree planting in oil palm landscapes on biodiversity and ecosystem functions.

Four years after the establishment of the long-term biodiversity enrichment experiment, the vegetation cover is sufficient to analyse its impact on soil properties. High water infiltration capacities of soils are not only crucial for reducing soil degradation but also for filling up groundwater reservoirs. In oil palm plantations decreasing groundwater levels have been reported even though transpiration rates were comparable to those in forests (Merten et al. 2016). Furthermore, severe soil erosion has been observed after forest conversion. Hence, this study investigates potential soil restoration capabilities of tree islands located within oil palm plantations, specifically focusing on water infiltration properties. These results will then be compared to water infiltration capacities in Harapan Rainforest as well as in jungle rubber sites, rubber plantations and oil palm plantations near Harapan Rainforest.

Hypotheses

• H1: Water infiltration capacities vary between plots located in tree islands of different sizes and species compositions.
• H2: Water infiltration capacities are higher in larger tree islands than in smaller ones.
• H3: Water infiltration capacities are highest in natural forest, second highest in tree islands and lowest in oil palm plantations.

2) Methodology and concept

In each of the 52 tree islands as well as in the four control plots, subplots of 5 m x 5 m size were established to assess effects of the biodiversity enrichment experiment. Withing these subplots as well as in 16 core sites of the Harapan landscape water infiltration capacities will be determined using a Dual-Head Infiltrometer (Meter Group AG) which measures soil saturated hydraulic conductivity.