## 3 Results
score = 2.30), and secondarily to image classification (unsupervised classification, rel. score = 2.09); for the phenology/climate (red) cluster, the only most relevant term was related to the geographical area of the analysis, i.e., North America (rel. score = 2.32). On the contrary, the most relevant terms in 2009-2018 were related to the phenology/climate cluster (red), e.g., ecosystem respiration (rel. score = 4.24), NEE - Net Ecosystem Exchange (rel. score = 3.20), and carbon uptake (rel. score = 2.91), followed by terms of the purely agricultural topic (blue): crop classification (rel. score = 2.85), and the use of red edge band (rel. score = 2.53) and SAR images (rel. score = 2.47). The most relevant terms belonging to the classification (orange) cluster was SVM-Support Vector Machine (rel. score = 2.34). Finally, it is to be noticed the presence in 2009-2018 of the term yield prediction (rel. score = 2.28). Finally, Figure 7 shows the temporal trend of the frequency of the ten most relevant terms identified in 2009-2018 (see Table 3) over the whole study period. The papers, dealing with the selected terms, passed from being 2% to 15% with respect to the total rs+pheno papers. In the early 1980s, only some of the most relevant terms were found, i.e., red edge, major crops, yield prediction. Overall, the majority of the terms were characterized by a gradually growing trend; on the contrary, yield prediction showed \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Term** & **Rel. Score** & **Occurrence** & **Cluster Color** \\ \hline  & Classification & 0.56 & 93 & blue \\  & Accuracy & 0.44 & 81 & blue \\  & Mapping & 0.51 & 77 & blue \\  & Temperature & 0.80 & 68 & red \\
1999-2008 & Day & 0.68 & 67 & red \\  & Reflectance & 0.62 & 60 & blue \\  & Start of & 1.16 & 55 & red \\  & Crops & 0.64 & 53 & blue \\  & Phenology & 0.41 & 53 & red \\  & stage & 0.71 & 50 & blue \\ \hline  & Image & 0.53 & 422 & blue \\  & Accuracy & 0.68 & 403 & blue \\  & Classification & 0.57 & 264 & blue \\  & Mapping & 0.57 & 261 & red \\
2009-2018 & Temperature & 0.89 & 253 & red \\  & Start of & 0.77 & 242 & blue \\  & Response & 0.67 & 229 & red \\  & Trend & 0.59 & 221 & blue \\  & Climate & 0.86 & 217 & red \\  & Climate & 0.75 & 211 & blue \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of the ten most occurment terms in 1999–2008 and in 2009–2018. a fast increase in the last decade (2009-2018). The terms with the highest frequency were red edge and SAR. The red edge term showed a quite constant trend until the beginning of the last decade when it showed a slight increase. Particularly interesting were the trends of SAR and SVM terms. The former presented a fast growth until its peak around 1995, and then a fast decreasing trend, with a moderate recovery in the last decade. The latter was characterized by an initial interest that ended completely at the end of the 1980s and arose again after the year 2000. It is to be noticed that some terms, like eddy covariance, carbon uptake, and ecosystem respiration, were not taken into consideration until the beginning of the 1990s. Figure 7: Temporal trend and proportion of the rs+pheno papers dealing with the ten most relevant terms identified in 2009–2018 (see Table 3) to the total rs+pheno papers over the period 1979–2018. A 5-year moving average was applied.