Spatial distribution of mangroves in the Auckland Region (1940-2014) and implications for aboveground carbon stocks

Abstract

Temperate mangroves increase in area across several regions in the world, including New Zealand. However, knowledge of the underlying mechanisms and drivers of temperate mangrove cover change, and its effect on ecosystem services is limited but critical for future mangrove management. In this thesis an innovative approach was developed to map mangrove cover change of tall and dwarf mangroves with high accuracy (up to 95%). The new mapping approach was applied to quantify the change in mangrove area across 38 estuaries in the Auckland Region, New Zealand. The mangrove area has increased from 2,313 ha in 1940 to 10,483 ha in 2014 at an average rate of 3.2% yr-1 with the rate being higher in small estuaries (< 1000 ha) than in large estuaries (> 1000 ha). The increase in total mangrove area occurred primarily through gap filling, followed by expansion. The proportion of expansion-related increase was predominantly seaward (82%), but mangroves also expanded upriver (16%). Periods of cat |a chment forest clearing coincided with gains in overall mangrove forest area. Dwarf mangroves were more sensitive to changes in sea level, in particular over the past 25 years, when annual sea level rise exceeded sediment accumulation rates. Investigating the mangrove landscape pattern across the Auckland Region using spatial metrics analysis showed that the number of mangrove patches remained stable. In contrast, mangrove patch size increased significantly (75%), suggesting that the increase in total mangrove area between 1940 and 2014 was driven by an increase in patch size. The study also provided evidence that tall mangrove patches became more fragmented, through both reduction in connectivity (Mean Proximity Index in 2014 = 214, a decrease of 35% from 1940) and their shape becoming more irregular (Mean Shape Index in 2014 = 1.9, an increase of 24%). Patch complexity was mainly driven (up to 71%) by sediment accumulation rates whereas mangrove patch connectivity and configuration we |a re more sensitive to oceanic conditions. ii Aboveground carbon stock across the Auckland Region was determined by upscaling plot-level and LiDAR-derived estimates. Mean total aboveground carbon stock (including trees, seedlings, pneumatophores and woody debris) derived from field measurements was 40.2 Mg C ha-1 (= 148 Mg CO2e ha-1). Total aboveground carbon stock varied considerably with tall and riverine mangroves storing 86% more carbon than dwarf mangroves. While LiDAR proved to be a promising tool, the findings indicate that specific allometric equations need to be developed for tall and dwarf mangroves to reduce the uncertainty in aboveground carbon stock estimates. The findings of this thesis will facilitate decision-making concerning the sustainable management of temperate mangroves and the ecosystem function and services they provide.