Land‐based and climatic stressors of mangrove cover change in the Auckland Region, New Zealand

Abstract

1. Changes in mangrove forest cover (increases and decreases) have been observed globally as a result of the influence of environmental changes and human impacts.Although mangrove forest increases are globally uncommon, in New Zealand mangroves are increasing in distribution at the expense of other intertidal estuarine habitats.2. This study quantified the change in mangrove area over a period of 74 years , and investigated the influence of land-based (e.g. land use conversion in the catchments), climatic, and oceanic factors on the change in temperate mangrove forest cover in the Auckland Region of New Zealand over that period.3. An improved mapping approach using aerial photographs and Landsat images was applied to quantify changes in tall and dwarf mangrove cover, and to identify the mechanism of change (expansion, gap filling, and loss), across 38 estuaries in the Auckland Region, New Zealand. 4. Mangrove area increased from 2313 ha in 1940 to 10 483 ha in 2014 (on average 3.2% yr −1 ), with higher rates in small estuaries (<1000 ha) than in large estuaries (>1000 ha). The area of the intertidal mudflat habitat, i.e. potential area for mangrove colonization, has doubled from 14 193 ha in 1974 to 28 764 ha in 2014.The increase in total mangrove area was explained primarily by gap filling followed by expansion. The proportion of expansion-related increase was predominantly seaward (82%) but mangroves also expanded upstream (16%). 5.Boosted regression tree analysis revealed that catchment forest cover, sea level, and mean annual air temperature explained around 85% of the variation in mangrove area. Periods of catchment forest clearing coincided with gains in mangrove forest area. Dwarf mangroves were more sensitive to changes in sea level, in particular over the past 25 years, when the annual sea-level rise exceeded the sediment accumulation rates. 6. Catchment land use, especially the clearing of forests, associated with increased sediment supply into estuaries is the strongest driver of mangrove forest gain.Thus, land-use management is a necessary aspect of the management of New Zealand estuaries to conserve the diversity of estuarine habitats (including mangroves) and the ecosystem services that each habitat provides.

Mountains‐to‐the‐sea conservation: An island perspective

“…Superimposed upon this risk are the more gradual effects of climate change, to which island states are particularly vulnerable, a recurring theme in several articles in this special issue (e.g. Augspurger & Closs, ; Moore et al, ; Suyadi et al, ). In particular, island environments are highly susceptible to the effects of sea‐level rise which may affect coastal reef systems, estuaries, and the extent of tidal incursion into lowland freshwater systems.…”

“…In more recent times, Suyadi et al () demonstrate how accelerating inputs of terrestrial sediment have had substantial impacts on the distribution and abundance of estuarine habitats. This study used remote sensing and field validation to assess changes in estuarine habitats over a 74‐year period, and correlated these changes with catchment land cover and climate variables.…”

“…This study also explored differences in rates of change for larger versus dwarf mangroves, and detailed much higher rates of expansion for dwarf mangroves that are likely to provide lesser contributions to ecosystem services such as coastal protection and carbon sequestration. These shorter morphological variants, which now comprise a larger proportion of the mangrove distributions in the Auckland region, are potentially more sensitive to sea‐level rise, reducing ecosystem resilience to climate change (Suyadi et al, ).…”

“…Increased sediment supply can also result in habitat change through accumulation of fine sediments that may favour different habitat‐forming species (e.g. mangrove; Suyadi, Gao, Lundquist, & Schwendenmann, ). Furthermore, rapid intensification of agriculture in some locations, particularly for dairying, has increased nitrogen yields and caused eutrophication of freshwater and coastal waters throughout New Zealand (Heggie & Savage, ).…”

Investigating changes in estuarine ecosystem functioning under future scenarios

“…Increased rates of sediment deposition can also result in changes in habitat types through changing sedimentary characteristics, facilitating colonization by vegetation such as mangroves (Suyadi et al. ). Eutrophication and the escalating addition of land‐derived nutrients is another common and diffuse‐source stressor to estuaries (Diaz and Rosenberg ).…”