# Phenology and Seasonal Ecosystem Productivity in an Amazonian Floodplain Forest
## 1 Introduction  
Tropical forests are known for their high productivity, accounting for 33% of terrestrial net primary production, which regulates carbon-climate feedbacks [1]. In this context, the Amazon forest represents a sink of 0.42 to 0.65 Pg C per year of total carbon assimilated, making it the most significant carbon sink on the planet [2]. However, extreme droughts have demonstrated the sensitivity of this ecosystem to environmental changes. Previous studies have reported aboveground biomass loss, increased tree mortality [3; 4] and a long-term decreasing trend of carbon accumulation [5], which could lead to the possible shift of forest functioning from a sink to a source of CO\({}_{2}\) during long-term droughts [6; 7]. Although detailed knowledge about the tropical forest carbon cycle is still lacking, previous studies have sought to understand the mechanisms of seasonal photosynthesis control, such as water and light availability [8; 9] and phenological cycles [10; 11; 12]. To understand these seasonal patterns on a larger scale, several studies have correlated seasonal environmental drivers with remote sensing-based multispectral vegetation indices in the Amazon basin. Some of them have suggested the Amazon rainforest is resilient to seasonal droughts [13] and extreme droughts [14]. Other studies, however, reported that an apparent greening of the forest canopy could be attributed to abnormal leaf flushing or as an effect of sun position in relation to the sensor, increasing near infrared reflectance, and thus not reflecting a true resilience from the forest [15; 16; 17; 18; 19; 20]. Part of these uncertainties has been attributed to the high atmospheric contamination of the MODIS (Moderate Resolution Imaging Spectroradiometer) sensor surface reflectance data, the most commonly used sensor for phenological studies in the tropics [21; 22]. Given the reported uncertainties related to sun-sensor geometry effects and atmospheric contamination, progress has been made towards the development of more robust atmospheric correction methods and data normalization procedures for MODIS data, such as the multi-angle implementation correction (MAIAC) algorithm [23] implementation of the bidirectional reflectance distribution function (BRDF). Enhanced vegetation index (EVI) values extracted from this improved MODIS product (MODIS-MAIAC) have shown a correlation between phenology and productivity of the Amazon forest [11; 24; 25; 26; 27]. Apart from these advances, a remaining limitation is that the majority of studied forest plots in the Amazon are located in _terra firme_ (upland) forests, even though c.a. 14% of the basin is covered by flooded forests [28], which remain understudied. These ecosystems are subjected to seasonal, long-lasting and monomodal flood pulses [29], with studies reporting adaptation mechanisms developed by species in these environments in response to the excess or lack of water. During the flooded period, some species can increase root porosity, release biological volatile organic compounds into the atmosphere, enrich the rhizosphere with oxygen, reduce photosynthesis and exhibit anaerobic metabolism [30; 31]. During the dry season, decreases in leaf water potential, foliar surface, and xylem flow have been observed, which reduce water loss through transpiration [32]. Floodplain areas are widely recognized for the ecosystem services they provide, playing an important role in water and climate regulation services [29; 33]. Different studies have pointed to a higher resilience of Amazonian floodplain forests to seasonal droughts [31; 34; 35], as during the terrestrial phase, these forests had higher rates of stem growth [34] and greater rates of CO2 assimilation as a result of new leaves and soil aeration [36; 32]. Conversely, a period of dormancy has been suggested to be a survival strategy during the aquatic phase [34].