Abstract
The intertidal habitat of mangroves is very complex due to the dynamic roles of land and sea drivers. Knowledge of mangrove phenology can help in understanding mangrove growth cycles and their responses to climate and environmental changes. Studies of phenology based on digital repeat photography, or phenocams, have been successful in many terrestrial forests and other ecosystems, however few phenocam studies in mangrove forests showing the influence and interactions of water color and tidal water levels have been performed in sub-tropical and equatorial environments. In this study, we investigated the diurnal and seasonal patterns of an equatorial mangrove forest area at an Andaman Sea site in Phuket province, Southern Thailand, using two phenocams placed at different elevations and with different view orientations, which continuously monitored vegetation and water dynamics from July 2015 to August 2016. The aims of this study were to investigate fine-resolution, in situ mangrove forest phenology and assess the influence and interactions of water color and tidal water levels on the mangrove–water canopy signal. Diurnal and seasonal patterns of red, green, and blue chromatic coordinate (RCC, GCC, and BCC) indices were analyzed over various mangrove forest and water regions of interest (ROI). GCC signals from the water background were found to positively track diurnal water levels, while RCC signals were negatively related with tidal water levels, hence lower water levels yielded higher RCC values, reflecting brownish water colors and increased soil and mud exposure. At seasonal scales, the GCC profiles of the mangrove forest peaked in the dry season and were negatively related with the water level, however the inclusion of the water background signal dampened this relationship. We also detected a strong lunar tidal water periodicity in seasonal GCC values that was not only present in the water background, but was also detected in the mangrove–water canopy and mangrove forest phenology profiles. This suggests significant interactions between mangrove forests and their water backgrounds (color and depth), which may need to be accounted for in upscaling and coupling with satellite-based mangrove monitoring.
Highlights
Phenology is a key factor explaining climate interactions between the biosphere and the environment [1,2,3]
normalized green–red difference index (NGRDI) profiles were relatively stable throughout the day and appeared to be the least affected by diurnal sun angle variations (Figure 3a)
As shown in this study, phenocams can be readily deployed in many different locations with minimal resources and their imagery can be spatially segmented into distinct regions of interest (ROIs) to evaluate the green leaf and water component signals of the highly dynamic mangrove–water canopy
Summary
Phenology is a key factor explaining climate interactions between the biosphere and the environment [1,2,3]. Mangrove forests have high potential to store carbon [7,8] and play an important role in coastal erosion, nutrient, and water quality issues in estuaries and coastal areas [9] They serve as a location for feeding and breeding of birds, fish, and crustaceans, such as crabs, shrimp, and prawns. The greenness index values the year corresponded very well with the apparent colors of the water observed in thepeaked at 10:00 to 11:00 in the July wet season period, and peak at 13:00 to 15:00 in the dry phenocam diurnal images (Figure 4). The greenness index values (GCC, NGRDI) peaked season (December), both of which matched the apparent “green water” color seen in the at 10:00 to 11:00 in the July wet season period, and peak at 13:00 to 15:00 in the dry phenocam images at these times (Figures 3c and 4a,b).
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