Characterizing landscape patterns in changing mangrove ecosystems at high latitudes using spatial metrics

Abstract

Mangroves are coastal ecosystems and a detailed characterization of their spatial metrics is important to understand the underlying processes of mangrove change and to inform future mangrove management. The main objectives of this study were: (1) to determine the past (1940) and current (2014) spatial metrics of mangroves at high latitudes and (2) to assess the relationship between spatial metrics and land-based, climatic and oceanic factors in 2014. Mangroves were mapped into categories of tall and dwarf mangroves, as well as into delta, riverine and estuarine mangroves based on field surveys, satellite images and aerial photographs across 38 estuaries of varying size in the Auckland Region, New Zealand. Fragstats was applied to determine spatial metrics indices related to complexity, connectivity, and configuration of mangrove patches. While the number of mangrove patches remained stable, mangrove patch size increased significantly (75%) suggesting that the increase in total mangrove area in the Auckland Region is mainly driven by an increase in patch size. Tall mangroves became more fragmented, through both reduction in connectivity (Mean Proximity Index in 2014 = 214, decrease of 35%) and their shape becoming more irregular (Mean Shape Index in 2014 = 1.9, an increase of 24%). Catchment related factors (e.g., sediment accumulation rate) had a strong effect on the total mangrove area and number of patches. In contrast, mean patch size and patch connectivity were influenced by nitrogen concentration, mean wave height and tidal amplitude, which in combination explained up to 63% of the variability. These findings can be used to facilitate decision-making concerning the sustainable future management of mangroves.