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After trees die: quantities and determinants of necromass across Amazonia

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Abstract:

The Amazon basin, one of the most substantial biomass carbon pools on earth, is characterised by strong macroecological gradients in biomass, mortality rates, and wood density from the west to the east. These gradients could affect necromass stocks, but this has not yet been tested. This study aims to assess the stocks and determinants of necromass patterns across Amazonian forests. Field-based and literature data were used to find relationships between necromass and possible determinants. The final regression result was used to estimate and extrapolate the necromass stocks across terra firma Amazonian forests. In eight northwestern and three northeastern Amazonian permanent plots, volumes of coarse woody debris (≥10 cm diameter) were measured in the field and density of each decay class was estimated. Forest structure and historical mortality data were used to determine controlling factors of necromass. Necromass is greater in forests with low stem mortality rates (northeast) rather than forest with high stem mortality rates (northwest) (58.5±10.6 and 27.3±3.2 Mg ha 1, respectively). After integrating all published necromass values, we find that necromass across terra firma forests in Amazonia is positively related to stand biomass, mortality mass input, and average wood density of live trees (ρBA j). We applied these relationships to estimate necromass for plots where necromass has not been measured. The estimates, together with other actual measurements of necromass, were scaled-up to project a total Amazonian necromass of 9.6±1.0 Pg C. The ratio of necromass (on average weighted by forest region) to coarse aboveground biomass is 0.127. Overall, we find (1) a strong spatial trend in necromass in parallel with other macroecological gradients and (2) that necromass is a substantial component of the carbon pool in the Amazon.

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