Plantation-Driven Watershed Degradation and Intergenerational Erosion of Traditional Ecological Knowledge in the Chittagong Hill Tracts, Bangladesh: A Multi-Ethnic Comparative Study

Abstract

The Chittagong Hill Tracts (CHT) of Bangladesh harbors at least 13 indigenous communities whose livelihoods, cultural identities, and ecological knowledge systems are intimately embedded in a forested hill landscape now undergoing rapid transformation. This study investigates a theoretically proposed but empirically untested causal chain: that plantation expansion reduces spring water availability, which in turn severs the ecological contexts of traditional ecological knowledge (TEK) transmission, driving measurable intergenerational knowledge loss. Drawing on a mixed-methods design comprising multi-temporal Landsat LULC analysis (1993-2023), a structured household survey (n=600) using an adapted TEK Transmission Index (TEK-TI), and qualitative life-history interviews (n=60) across five ethnic communities (Chakma, Marma, Tripura, Mro, and Khasi) in all three CHT districts, the study tests this structural mechanism using piecewise structural equation modelling (piecewiseSEM) with bootstrap mediation analysis. Results reveal that dense forest cover declined by 6.23% and settlement/plantation area increased by 10.34% between 1993 and 2023, with rubber monoculture a primary driver. Spring Water Availability Index (SWAI) scores declined significantly across all communities, with communities in high-plantation-exposure zones reporting 47% lower SWAI than low-exposure communities. The intergenerational TEK-TI gap, the difference in mean ecological knowledge scores between elders (55+) and youth (18-29), averaged 28.4 points (on a 0-100 scale) across all communities, with the Mro community exhibiting the highest gap (34.7) and the Khasi the lowest (21.3). Piecewise SEM confirms that watershed degradation fully mediates the effect of plantation expansion on TEK erosion (indirect path coefficient: β = −0.41, 95% CI [−0.56, −0.27], p < 0.001), with the direct effect rendered non-significant once SWAI is controlled. Multi-group invariance testing reveals significant ethnic moderation, with land tenure security and cultural transmission institution strength moderating the plantation-watershed-TEK pathway. These findings provide the first empirical quantification of hydrologically mediated TEK erosion, with direct implications for CHT forest governance, indigenous land rights policy, and the integration of biocultural diversity conservation into the Bangladesh Delta Plan 2100.