## 4 Discussion
### Emerging Research Topics  
The term relevance analysis highlighted how some topics emerged as new research areas in rs+pheno studies, as well as how some relevant issues of the 1999-2008 decade lost their significance. In particular, the attention towards the vegetation growing season that characterized the first decade has increased through time to such an extent that it became a major, common topic and, hence, lost its relevance. On the contrary, some issues related to the climate change/carbon cycle linkage emerged in the 2009-2018 decade, for example, ecosystem respiration, net ecosystem exchange (NEE), and carbon uptake. These represent new research issues that scientists started to face at the beginning of the 1990s, and that in the last decade became the most relevant topic in rs+pheno studies, i.e., the most significant, with specific objectives and questions, mainly dealing with issues like forest ecophysiology [78; 79] and biomass estimation and anomalies [73; 80]. In this global energy balance context, special attention should be given to themes like ice cover dynamics and phytoplankton blooming. They gained a moderate relevance in the last decade, yet they are becoming increasingly significant, as demonstrated by several recent studies [81; 82; 83; 32]. In particular, as stated by [32], phytoplankton is an essential element of the ocean, influencing global biological, chemical, and physical environments in terms of water heat [81], net primary production, carbon cycling, and climate [81]. As a consequence, rs+pheno research focused on phytoplankton represents an emerging niche research area recognized as a key concern in the framework of the global climate observing system [32]. Some new topics came to light in the last decade, also, within the agricultural main domain. Actually, crop modeling and the use of SAR and red edge represent research issues that have gained some attention since the early 1980s and kept being studied throughout the 40 years. However, the initial interest was likely targeted towards several different research objectives, without a specific rs+pheno focus, to such an extent that these terms gained a high relevance only in the last decade. The use of new sensors and specific optical and radar bands [84; 85; 86; 87], their data fusion [55; 88], as well as the application of yield prediction and crop simulation models [89; 90; 91] may be, hence, considered as a new challenge for the next decade of rs+pheno studies. Finally, in the last decade, a new front appeared in rs+pheno research, linked with habitat and biodiversity conservation. According to our results, this front was connected with terms associated with anthropogenic activities and human presence. For example, the impacts of urbanization processes on vegetation health status and its seasonal variations have been mostly analyzed in terms of the phenology of urban environments [92; 93], effects of urban heat island [94; 95], and different phenological responses of rural vs. urban areas [96; 97]. At the same time, human management interventions, like deforestation, represent another issue faced in rs+pheno studies, mainly dealing with recolonization [98], mapping [99], and environmental damage estimation [100]. Biodiversity and related ecosystem services issues resulted as connected, also, to studies about environmental disturbance phenomena, like fire. Many papers in rs+pheno studies analyzed the role of wildfires' behavior both as a driver of vegetation recovery patterns [101; 102; 103] and as influenced by fuel phenology and flammability [104; 105]. In this latter case, a high potential exists for characterizing, classifying, and mapping fuel based on rs+pheno dynamics. Several studies have used variations in remotely sensed VIs profiles, such as the NDVI, as indicative of variations in moisture content and nutrient availability, which in turn are indicators of a marked susceptibility of vegetation to fire [106; 107].