Species Mixing Increases Forest Growth and Soil Organic Matter in Coastal Mangrove Plantations in Bangladesh

Abstract

Coastal afforestation is critical for climate adaptation in the tropics, yet most planted mangrove area worldwide is monospecific. Whether mixing species improves the ecological performance of these plantations, and through which mechanisms, remains poorly quantified. We asked (Q1) whether mixed-species stands support higher levels of forest ecosystem properties and processes such as aboveground biomass (AGB) growth, soil organic matter, nutrient availability and structural diversity than monospecific stands along a plantation chronosequence, and (Q2) how stand type modulates the relationships linking these properties to stand structure and the physical setting. We used 36 permanent sample plots (25 mixed Sonneratia apetala - Excoecaria agallocha, 11 monospecific) spanning eight plantation age classes (10–45 year) and six locations in the Char Kukri-Mukri coastal area, Bangladesh, each inventoried in two consecutive years. We observed mixed stands had substantially higher AGB growth than monospecific stands (14.0 ± 4.2 vs. 9.3 ± 1.8 Mg ha⁻¹ yr⁻¹; P < 0.001), higher soil organic matter (SOM) (1.09 ± 0.32 vs. 0.82 ± 0.15%) and higher structural diversity. SOM increased with plantation age in both stand types (P = 0.016). Interactions between stand age and stand type were consistently positive for soil organic matter, structural diversity and nutrient availability but were not statistically significant, so we report a difference in level rather than a divergence with age. A piecewise structural equation model showed that mixing raised structural diversity strongly (β = +1.68) and retained a large direct effect on AGB growth beyond structure and soil properties, while soil salinity acted mainly through soil nitrogen. Because stand type covaried with plantation age and location in this operational plantation network. Mixed plantations nonetheless emerge as a promising option for raising the productivity and soil carbon return of coastal afforestation under saline, silting conditions.