Mangrove Above-Ground Biomass and Production Are Related to Forest Age at Low Isles, Great Barrier Reef

Above-ground biomass contributes a large proportion of mangrove carbon stock; however, spatio-temporal dynamics of biomass are poorly understood in carbonate settings of the Southern Hemisphere. This influences the capacity to accurately project the effects of accelerating sea-level rise on this important carbon store. Here, above-ground biomass and productivity dynamics were quantified across mangrove age zones dominated by Rhizophora stylosa, spanning a tidal gradient atop a reef platform at Low Isles, Great Barrier Reef, Australia. Above-ground biomass was extrapolated across the forest using field plot data, allometry, a canopy height model derived from remotely piloted aircraft (RPA) LiDAR, and regression analyses. Above-ground biomass production was calculated as mean annual biomass increments, and canopy production was determined using RPA-derived multispectral imagery and a Normalized Difference Vegetation Index. Mangrove above-ground biomass was estimated at 519.7 ± 3.11 t ha^-1 and increased with age up to the oldest forest (812.0 ± 12.9 t ha^-1), believed to be ~135 ± 40 years old. Above-ground biomass was explained by age and tidal position (r ² > 0.8), with a positive association between the two predictor variables. Above-ground biomass production peaked at the lowest intertidal position in the youngest forest aged < 11 years at 36.3 t ha^-1 yr.^-1, steadying thereafter, with a mean of 12.5 ± 5.4 t ha^-1 yr.^-1 across the island. Production in the canopy remained high until the oldest forest and was negatively associated with age and tidal position (r ² > 0.9). Declining production in the older zones corresponded to forest aging, tidal positions becoming suboptimal for growth, and increased exposure to prevailing winds and cyclones. By developing relationships between above-ground biomass accumulation and age and tidal position, this study informs parameterization of models of the response of biomass to sea-level rise but requires additional information about relationships between substrate evolution, forest development, and age.