A review of remote sensing for mangrove forests: 1956-2018

Mangrove forests are highly productive ecosystems that typically dominate the intertidal zone of tropical and subtropical coastlines. The history of mangrove remote sensing (RS) can be traced back to 1956. Over the last six decades, hot spot topics in the field of mangrove RS have evolved from mangrove distribution mapping, biophysical parameters inversion, to ecosystem process characterization. Although several review articles have been published to summarize the progress in this field, none of them highlighted the key milestones of historical developments pertinent to major research topics or key drivers that stimulate such milestones. In this review, we aim to identify key milestones in mangrove RS by associating the emergence of major research topics with the occurrence of new sensors in four historical phases, i.e. before 1989, 1990-1999, 2000-2009, and 2010-2018. For each identified research topic, an in-depth theoretical understanding was achieved by analyses of both the first published article and the most-cited article. Based on the analyses, the current state of knowledge as well as existing limitations were summarized. In addition, in order to gain insights on driving forces for emergence of new research topics, we compared the chronological evolution of mangrove RS with that of terrestrial forest RS. Interestingly, we found that key research topics in mangrove RS replicated those of forest RS yet with varying time lags. This can be attributed to the following two facts: 1) mangrove forests often appear as more elongated patches than terrestrial forests; 2) field work is more challenging in mangrove habitat. Along with the RS sensors' advancement, various topics that had been studied in terrestrial forests were later transformed to mangrove studies. Based on the projected growth of foreseeing earth observation capacity, insights on future research directions in mangrove RS were also presented.