# 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--Spectral Laboratory, Plant Phenomics Facilities and Ecotrons  
Close-range investigations are conducted to determine the spectral responses of FH traits and to derive meaningful models from them (see Table 2). Reactions of woody plants to stress are species specific [59; 63]. [PERSON] et al. [59] analysed different strategies of Brazilian forest tree species to cope with drought stress and maintain plant functioning. As reactions of woody plants 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 [27], the ability of different tree species to adapt to climate change is still not well understood [64]. With the help of close-range RS approaches, extensive long-term stress monitoring can be carried out that takes into account vegetation phenological cycles as well as inter-annual variations. To specifically investigate different stress factors, experimental settings are best suited as they provide reliable input for models and eliminate confounding factors. [PERSON] et al. [18] investigated the spectral response of ash trees (_Fraximus excelsior_ L.) to physiological stress from flooding over a 3-month period, whereas [PERSON] et al. [27] modelled the photosynthesis rate of young European beech trees under drought stress following a two-year water stress treatment by using close-range hyperspectral visible, near infrared and thermal sensors. Another common method to induce water stress is the girdling of trees with differing degrees of intensity [65; 66]. The Genotype comprises hereditary information about the DNA of a species while the phenotype represents the physiological, morphological, anatomical, development characteristics and the interactions of species with their environmental conditions, resource limitations and stress factors. Insights into genotype and phenotype interactions in plant stress physiology are not only gained from recording individual plant ST, but also by including the entire genotype-epigenetic-phenotypic-environment matrix [67]. This can be achieved, for example, by recording phenotypical plant traits in plant phenomic facilities [30; 34] or controlled environmental facilities--Ecotron [38]. The acquisition of phenotype plant traits in plant phenomics facilities has so far mainly been carried out for crop vegetation [33; 34; 68]. For woody plants, preliminary investigations have been restricted to fruit trees [36]. Phenotypical investigation of woody plants should therefore be augmented in the future, in order to be able to better understand the mechanisms and stress factor interactions involved when interpreting FH indicators observed using RS. It is these kinds of comparisons that support research on the effects of the Bidirectional Reflectance Distribution Function (BRDF), scale and different RS platforms as well as different sensor characteristics on model design [69]. The regulation of photosynthetic electron transport, as well as its feedback processes and the assessment of photosynthetic efficiency and stress in crops, grassland and forest canopies, have been investigated with sun-induced chlorophyll fluorescence methods at leaf [41], field, and regional levels [42; 43; 44; 45]. Spectroscopic techniques for the detection of chlorophyll fluorescence forms the basis for the European Space Agency (ESA) Fluorescence Explorer Sensors (FLEX, [45; 46; 70]) to be launched in 2018. With its very high spectral resolution across the 0.3-3.0 \(\upmu\)m range, FLEX will be the first satellite that is able to directly measure solar-induced chlorophyll fluorescence and thus forest stress indicators and other vegetation parameters. Disadvantages of close-range RS laboratory methods include the use of artificial light sources and the short distance between the sensor and object. Substantial image pre-processing and calibration is often required to compare lab and field measurements.