Role of Legumes in Release of Successionally Arrested Grasslands inthe Central Hills of Sri Lanka

Background

Many cleared and abandoned forest areas have changed to successionally arrested grasslands, shrublands, or fernlands maintained by frequent fires and high herbivore populations. Many studies have shown that herbaceous legumes can protect soil surfaces, retain soil moisture, improve soil fertility, and retard ground fires. This study evaluates whether some of these species can satisfactorily establish in these arrested grassland areas to aid in the trajectory towards reforestation. 

Research goals & methods

The study evaluates the potential for four species of nitrogen-fixing legumes (Calapogonium mucunoides, Centrosema pubescens, Desmodium ovalifolium, and Pueraria phaseoloides) to establish on exposed soil within successionally arrested grasslands of Panicum maximum and Cymbopogon nardus in the central hills of Sri Lanka. Four different sites within rectangular grassland areas varying in soil properties were cleared of graminoids and sown with seed of each legume. Half of each planting was protected from browsing rabbits and porcupines, and half was not protected. Plantings were sampled after six months. Analyses of variance were performed to test for differences among sites, treatments, and species.

All three factors revealed differences, indicating that species must be matched to site. On sites with high amounts of herbivory, D. ovalifolium had the greatest dry biomass gain, possibly because of its relatively low nitrogen and moisture content. Where herbivory was absent, P. phaseoloides and C. muconoides had the greatest biomass gain. Biomass gain of all four legume ground covers was low on sites with lowest pH and nutrient concentrations. Though soil moisture availability was not measured, we speculate that these low fertility sites were also prone to drought.

Conclusions & takeaways

Findings support the establishment of legume species for purposes of reforestation and watershed protection in central Sri Lanka, with sites of higher nutrient concentrations and balanced pH more effective in establishment. This work is applicable to other regions particularly dominated by successionally arrested grasslands with similar circumstances in other parts of south and southeast Asia.