LAUSR

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. This study quantified the change in mangrove area over a period of 74 years (1940–2014), 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. 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. 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). 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%). 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. 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.