GA3-mediated reforestation pioneering mechanism of actinorhizal Elaeagnus conferta Roxb. in the slashed and burnt shifting cultivation lands in India's megadiversity hotspot

Abstract

Forest slashing and burning is a key characteristic of shifting cultivation. Its intensification is a major anthropogenic cause of deforestation, as it reduces forest resilience. We analyzed the reforestation process in the regenerating shifting cultivation lands in India's Western Ghats, a megadiversity hotspot. An actinorhizal shrub, Elaeagnus conferta, was associated with the regenerating thickets' higher density and higher species diversity. Its presence increased the thicket density by >2.8-fold and the species diversity by >2.5-fold; these thickets showed >1.5-fold higher Shannon's diversity index than those without it. Moreover, the thickets containing E. conferta showed >7-fold more multi-species seedlings. Liquid chromatography-mass spectrometry analysis showed that rhizosphere soils of E. conferta containing thickets had >10-fold higher gibberellic acid (GA3) concentrations than the E. conferta-deplete thickets. When E. conferta was potted separately, its root nodules, roots, and rhizosphere soil showed about 3.6, 2.0, and 0.4 μg/g GA3, respectively. These E. conferta root and rhizosphere soil GA3 concentrations were >10-fold higher than those shown by other co-occurring species. These elevated GA3 levels were likely due to E. conferta's association with the rhizosymbiont Frankia elaeagni. Frankia elaeagni's GA3 synthesis may promote plant recruitment by stimulating seed germination and explain why we measured higher seedling density and plant diversity in E. conferta containing thickets. We hypothesize that E. conferta plays a two-stage pioneering role in the reforestation process by providing GA3 to facilitate seed germination and providing nitrogen for the developing plantlets. We recommend the inclusion of E. conferta in Western Ghats' reforestation programs.