# 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  
Traits and trait variations in FES display specific spectral responses in RS data that are caused by the taxonomy, phenology and phylogenetics of forest species and communities as well as by natural and anthropogenic stressors [16; 17]. The spectrum of responses ranges from additive reinforcing effects to contrary effects, reductive or even exclusive signals. To correctly interpret these spectral response signals in terms of disturbance or stress, as well as to gain sufficient information for calibrating and validating airborne and space-borne RS data, extensive close-range RS measurements (Figure 1, Table 2) as well as physically-based models (Section 5) are needed.  
In-situ sampling of forest canopy elements for measurement of leaf properties such as chlorophyll, protein, water content, leaf area index (LAI) or functional plant traits such as photosynthesis activity is difficult compared to sampling of ground vegetation. Recording this kind of information often requires tree climbers, lifting platforms or towers, Unmanned Arial Vehicle (UAV) with arms for cutting small branches or rifles or harpoons to shoot down leaf material (Figure 1).  
In addition to UAVs, aircrafts and space-borne RS applications, close-range RS approaches are increasingly being established to support the observation of FH indicators as shown in Figure 2. They range from spectral analyses of forest species to terrestrial wireless sensor networks as summarized in Figure 2 and Table 2.  
Figure 1: Methods and materials for sampling canopy materials in forest, (**a**) Tree climbers, photo by [PERSON]; (**b**) Cherry picker, photo by [PERSON]; (**c**) Crossbow; photos by [PERSON] and, (**d**) Unmanned Aerial Vehicle (UAV).  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Close-Range Measurement Approaches/References** & **Advantages/Applications** & **Disadvantages** \\ \hline \multirow{4}{*}{**Field Spectrometers**[21; 22; 23; 24]} & Basis for research on spectral characteristics of biochemical-biophysical, morphological traits. Spectral databases for classification and validation. Basis for research on taxonomic, phylogenetic, genetic, epigenetic or morphological-functional features. & Analysis at molecular level. Geometric, structural, distribution, population and community effects are not measurable. No standardized measurement protocols available \\ \hline \multirow{4}{*}{**Spectral laboratory** (Manual operation) [18; 25; 26; 27; 28; 29]. **Plant phenomics facilities (Fully automatic operation), Ecotrons (Controlled environmental facility), [30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48]. & Seasonal, annual, long-term. Biochemical-biophysical, structural variables in organs (roots, leaf, stem) and whole tree. Experimental stress analyses (drought, heavy metals, tropospheric ozone, flooding, nitrogen loads, etc). Extensive lab-based measurement program for biotic, abiotic, climate conditions. Comparative analyses can be conducted under natural or artificial conditions. Multi-sensor recording at specific plant development stages. Storage in spectral databases for validation and calibration. \\ \hline
**Tower** (eddy flux tower) with different non-invasive measuring technologies as well as RS technology (mobile, permanently installed), [49; 50; 51; 52]. & Long-term monitoring. International networks exist. Extensive multi sensor monitoring is possible for biotic and abiotic conditions (e.g., phenocams). Spectral measurements directly on canopy level. \\ \hline \multirow{4}{*}{**Wireless sensor networks (WSN) [53; 54; 55; 56; 57; 58; 59; 60; 61; 62]**} & Long-term high frequency monitoring. Extensive multi sensor measurement is possible. Measuring various biochemical-biophysical, structural variables in organs (roots, leaf, stem) and whole tree. Enables results over more extensive areas. Easy to install in remote areas. \\ \cline{1-1}  &