Carbon and Nitrogen Stocks and Below-Ground Allometry in Temperate Mangroves

Abstract

Mangroves play an important role in the storage of carbon (C) and nitrogen (N) within estuarine systems, yet are being lost at an alarming rate throughout the tropics. In contrast, temperate mangroves have increased in area at many locations in recent decades. Field surveys, sediment sampling, allometry, and C and N analysis were used to determine total C and N stocks in five temperate Avicennia marina subsp. australasica forests in New Zealand. This is the first study developing allometric functions to estimate root biomass C and N stocks for A. marina. A. marina forests stored 117.1 ± 16.8 t C ha −1 and 15.4 ± 1.0 t N ha −1 in above and below-ground biomass and sediment to 100 cm depth. Below-ground biomass and sediment C and N stocks contributed 88 ± 3 and 99 ± 0.4% to total C and N stocks, respectively, emphasizing the importance of below-ground biomass and sediment in mangrove ecosystems. The results of this study can be used to inform management decisions for estuarine and coastal ecosystems, currently undergoing rapid changes in mangrove area.

The Dynamics of Expanding Mangroves in New Zealand

“…High rates of primary production in combination with unique sediment conditions, such as anoxia which slows the breakdown of carbon (Figure 10), result in carbon burial rates in New Zealand mangroves of 0.67 tonnes C ha -1 yr -1 (Pérez et al 2017). The accumulated carbon stocks within New Zealand mangrove ecosystems are estimated at 117 tonnes C ha -1 , including above and belowground biomass, and carbon within the sediment up to 1 m below the surface (Bulmer et al 2016b).…”

“…However, it is important to note that when carbon losses associated with the CO2 efflux are subtracted from carbon stocks stored within tree biomass or buried within the sediment, expanding mangrove ecosystems still provide a net gain in carbon stocks (Pérez et al 2017). Taking account of these factors, expansion of New Zealand mangroves has been estimated to result in an increase in carbon stocks by 79 tonnes C per ha -1 (Bulmer et al 2016b). This gain in carbon stocks following mangrove expansion is not immediate, rather it is estimated to occur over decade to century time scales, based on carbon accrual rates in sediments of Avicennia marina forests of 0.67 tonnes C ha -1 yr -1 (Pérez et al 2017).…”

“…It is estimated that mangrove deforestation generates as much as 10% of carbon emissions from deforestation globally, yet mangrove ecosystems account for only 0.7% of the deforested area (Donato et al 2011). Not only does mangrove clearance result in a loss of mangrove primary production, estimated at 4 to 10 tonnes carbon per year for New Zealand (derived from Bulmer et al 2016a, b;Gladstone-Gallagher et al 2014;Woodroffe 1982)), mangrove clearance also results in a loss of estuarine carbon stocks, estimated at 79 tonnes carbon per hectare of mangrove cleared (Bulmer et al 2016b). Rather than a rapid loss of carbon immediately following mangrove clearance, this loss is expected to occur over a number of years to decades following clearance due to the slow decomposition of organic matter within New Zealand mangrove sediments (Bulmer et al 2015;Bulmer et al 2017b;Gladstone-Gallagher et al 2014).…”

“…As the majority (>90%) of carbon stocks within mangrove ecosystems are stored belowground in the sediment (Bulmer et al 2016b), one of the primary pathways for carbon export from mangrove ecosystems following mangrove clearance is via sediment CO2 efflux . The average rate of sediment CO2 efflux from mangrove clearance sites throughout New Zealand is 133.9 ± 37.2 mmol m -2 d -1 , which is comparable to rates observed in cleared mangrove sites in the tropics (Bulmer et al 2015).…”

“…Moreover, in the face of expected sea-level rise, accelerated sedimentation in mangrove forests could provide coastal zones with a natural resilience that engineered structures are lacking (Alongi 2008;Barbier 2014;Borsje et al 2011;Bouma et al 2014;Stagg et al 2016;Temmerman et al 2013). Another benefit of mangrove expansion is the potential for carbon burial, combating increasing carbon emissions that are aggravating the severity and impacts of climate change Bulmer et al 2016b;Donato et al 2011;Friess et al 2016b;Twilley et al 1992).…”