# Understanding Forest Health with Remote Sensing-Part II--A Review of Approaches and Data Models
## 2 Trends in Close-Range RS Approaches for Assessing FH
### Close-Range RS Approaches--Towers  
In order to upscale findings from experimental laboratory settings, which are often for leaf scale analysis, close-range RS can be implemented using mobile towers [20] or permanently installed towers [49] equipped with RS sensors to analyze spectral effects on the canopy level. (Table 4, Figure 2). Such systems may include automated digital time-lapse cameras (e.g., phenocams [51]), multispectral and thermal sensors, and non-imaging spectral sensors to monitor the status and changes of FH indicators over long periods of time at regular intervals. The US National Ecological Observatory Network (NEON) and the European Union's Integrated Carbon Observation System (ICOS) are continental-scale ecological research networks that promote the international implementation, use, standardization as well as analysis of phenocam data [51]. Flux towers generally include integrated sampling of different ecosystem variables such as carbon dioxide, water vapour and energy fluxes. They are often coupled with sensor technologies such \begin{table}
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**Application** & **Example Studies** & **Main Findings** \\ \hline Genotype-epigenetic and phenotype interactions & [30; 34] & Qualitative, quantitative and spectroscopic recording of plant species phenotypes for better understanding of the link between the genotype and the phenotype. \\ \cline{2-3} Genotype-epigenetic and phenotype interactions & [67] & Understanding the impacts and resilience to stress, disturbance or resourcelimitations of forest species and ecosystems is crucial for understanding the genotype-epigenetic-phenotypic-environment matrix. \\ \hline Goal of the Plant Phenotyping Network & [32; 71] & International Plant Phenotyping Network [72]: (1) Innovative non-invasive techniques such as stereo systems, hyperspectral, RGB, thermal, fluorescence cameras, laser scanners or X-ray tomography; (2) Continuous, very high temporal resolution acquisition of phenotypical traits that provides important reference information for RS approaches; (3) ST/STv are saved in databases; (4) Data can be used for calibration and validation of air- and spaceborne RS data. \\ \hline Spectral traits of leaves & [27] & Laboratory-based imaging spectroscopy combined with the inversion of a radiative transfer model is able to derive biochemical spectral traits (N, chlorophyll content, carotenoids, brown pigments, water content, dry mass) at the leaf level on the sub-millimeter scale. \\ \hline Developmental and testing of new close-range RS technologies & [44; 45; 46; 47] & Investigations of sun-induced chlorophyll fluorescence methods at the leaf level, field level, and the regional level are only possible through fundamental research in Plant Phenomics Facilities and Eotrons. \\ \hline Development and testing of new close-range RS technologies & [46; 47; 48] & Evaluated the effects of different plant stresses on photosynthetic performance. This research on chlorophyll fluorescence and its acquisition using spectroscopic techniques forms the basis for developing the European Space Agency (ESA) Fluorescence Explorer Sensors (FLEX, [45; 46; 70]). \\ \hline  & [47] & Development of 3-D digital imaging and a portable terrestrial laser scanner for detecting seasonal change within broad-leaved forest. \\ \cline{2-3}  & [73] & Development of a canopy leaf area density profile. \\ \cline{2-3}  & [48] & 3D-imaging techniques for monitoring the spatio-temporal effects of herbicides on plants. \\ \hline Monitoring of stress to woody plants & [27] & Reactions to stress factors like drought, can often only be observed years later in the form of biochemical, physiological or geometrical changes to woody plant traits (e.g., in tree rings observed from cross cutting). Age and development stages of the trees are a limiting factor (often only trees up to the age of 5 years can be recorded). \\ \hline \hline \end{tabular}
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