After trees die: quantities and determinants of necromass across Amazonia

Abstract

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 west to 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 across Amazonian forests. Field-based and literature data were used to find relationships between necromass and possible determinants. Furthermore, a simple model was applied to estimate and extrapolate 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 the density of each decay class was estimated. Forest structure and historical mortality data were used to determine the factors controlling necromass. Necromass is greater in forests with low stem mortality rates (northeast) rather than in forests with high stem mortality rates (northwest) (58.5±10.6 and 27.3±3.2 Mg ha −1 , respectively). Using all published necromass values, we find that necromass across terra firma forests in Amazonia is positively related to both forest dynamics (mortality mass inputs and a surrogate for decomposition rate (average wood density of living trees)) and forest structure (biomass), but is better explained by forest dynamics. We propose an improved method to estimate necromass for plots where necromass has not been measured. The estimates, together with other actual meaCorrespondence to: K.-J. Chao (kjungchao@dragon.nchu.edu.tw) surements 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.

Soil-induced impacts on forest structure drive coarse woody debris stocks across central Amazonia

“…Our current understanding suggests that CWD stocks generally decrease from north-eastern to south-western Amazonia (Baker et al 2007;Chao et al 2009a). Spatial variation in CWD stocks across the landscape may respond both to short-term climatic disturbances (e.g.…”

“…Kissing and Powers (2010), working in secondary forests in Costa Rica, showed strong positive correlations between stand age and the amount of CWD. Chao et al (2009a) working in mature forests in Amazonia showed that there was a relationship between forest structure and CWD, in particular with regard to biomass, wood density of living trees and mass of individual dead stems. Although these studies successfully associated CWD stocks with forest structure and dynamics, to our knowledge there has been no analysis of a potential effect of edaphic properties on CWD stocks.…”

“…Within tropical forests, CWD accounts for 6 to 25% of total above-ground carbon stocks (Nascimento and Laurance 2002; Rice et al 2004;Baker et al 2007; Palace et al 2012), implying a total pan-Amazon CWD carbon stock of ca. 10 Pg (Chao et al 2009a). The variation in CWD stocks across the Amazon Basin is thought to be modulated by 2012), implying a total pan-Amazon CWD carbon stock of ca.…”

“…Spatial variation in CWD stocks across the landscape may respond both to short-term climatic disturbances (e.g. Phillips et al 2009;Negrón-Juárez et al 2010) and to long-term differences in forest dynamics in response to environmental characteristics (Keller et al 2004;Malhi et al 2006;Chao et al 2009a). Soils represent an important environmental gradient in Amazonia, with a wide variety of soil types across the Basin and with diverse chemical and physical conditions (Quesada et al 2010;.…”

“…The variation in CWD stocks across the Amazon Basin is thought to be modulated by 2012), implying a total pan-Amazon CWD carbon stock of ca. 10 Pg (Chao et al 2009a). The variation in CWD stocks across the Amazon Basin is thought to be modulated by environmental factors, such as hydrology and soils, and by forest biomass itself (Rice et al 2004;Baker et al 2007;Chao et al 2009).…”