Agroekologi dan Pengaruh Infestasi Asystasia gangetica (L.) terhadap Pertumbuhan dan Hasil Tanaman Kelapa Sawit

Abstract

This series of studies comprehensively examined the biological, ecological, and management aspects of Asystasia gangetica (L.) T. Anderson in oil palm (Elaeis guineensis Jacq.) plantations located in the tropical lowlands of North Sumatra. The research was motivated by the increasing dominance of A. gangetica across various growth stages of oil palm, both in immature (TBM) and mature (TM) phases, reflecting its high adaptive capacity and persistence under diverse environmental conditions and management practices. The main objective was to elucidate the ecological and biological factors supporting the persistence and dominance of this species and to develop effective and sustainable control strategies within oil palm agroecosystems. A series of field and laboratory experiments were conducted to investigate the effects of A. gangetica density on soil and plant nutrient status, the dynamics and vertical distribution of the soil seedbank, seed dormancy duration through burial experiments, the effects of light intensity on seed germination and early growth, and the efficacy of pre- and post-emergence herbicides. Each experiment was designed to explain the ecological mechanisms underlying the persistence of A. gangetica and to identify biological vulnerabilities that could be exploited for integrated weed management. The results revealed that A. gangetica exhibits complex survival strategies. Its seeds are highly viable and form a multilayered seedbank extending beyond 20 cm in soil depth, allowing rapid regeneration after physical or chemical disturbances. The seeds possess a long physiological dormancy and germinate readily under favorable conditions, particularly with adequate moisture and light availability. Light intensity trials confirmed that seed germination and early seedling growth of A. gangetica are strongly promoted by higher light exposure, explaining its dominance in open or recently weeded plantation areas. Density experiments demonstrated that increasing A. gangetica populations did not significantly affect the vegetative growth of oil palm but did alter nutrient dynamics and reduced production potential during the immature (TBM) phase. Nutrient analysis showed that nitrogen (N), phosphorus (P), and potassium (K) contents in oil palm tissues increased with higher weed density, while in the soil, only K increased; P and soil organic carbon decreased, and N remained stable. Despite the apparent nutrient increase in plant tissues, higher weed density was associated with reduced physiological efficiency in biomass allocation, leading to lower production potential in young palms. This indicates that A. gangetica primarily influences nutrient partitioning and uptake efficiency rather than morphological vegetative growth. Chemical control experiments showed that the pre-emergence herbicide indaziflam effectively suppressed seed germination down to a depth of approximately 20 cm by inhibiting cell division in germinating seeds. Post-emergence herbicides such as glyphosate and glufosinate were effective in controlling actively growing weeds, although their performance was influenced by environmental conditions, shading, and plant age. The combination of pre- and post-emergence herbicides provided the most consistent suppression of A. gangetica populations over time. Overall, this research emphasizes that successful control of A. gangetica cannot rely on a single approach. It requires an integrated management strategy that incorporates an understanding of seedbank dynamics, weed physiological and ecological traits, and the selective use of herbicides tailored to specific growth stages of oil palm. Such an approach will minimize ecological impact while maintaining soil and vegetation balance. This study provides important scientific insights into the ecology and management of A. gangetica as a dominant and persistent weed species in tropical oil palm plantations. The findings offer a foundation for developing sustainable weed management strategies that support plantation productivity, cost efficiency, and long-term ecological stability in tropical lowland agroecosystems.