# Monitoring Forest Phenology in a Changing World
## 3 Emerging Technological Advancements
### Beyond the Visible Light Spectrum  
Remote sensing, be it far (satellite), intermediate (drone or plane) or near ground (fixed camera), is not restricted to imaging in the visible spectrum alone (380-740 nm). The use of multispectral (380-2500 nm) and hyperspectral (10-12,500 nm) sensors can capture additional phenological variables related to plant chemistry and health that are reflected in the infrared and ultraviolet spectra [30]. Newly launched satellites such as GOSAT (greenhouse gases observing Satellite), for example, can measure very weak, beyond visible leaf spectral changes such as solar-induced chlorophyll fluorescence (SIF), a by-product of photosynthesis [95; 96]. [PERSON] et al., [97] showed the use of a ground-based method to correlate SIF with Gross Primary Productivity (GPP) measures, which can in turn be used to monitor phenology change. Passive microwave sensors (0.01-31 cm) onboard satellites have been used to monitor phenology by calculating vegetation optical depth (VOD), which varies with canopy density and structure and correlates with phenological change [98]. Furthermore, Synthetic Aperture Radar (SAR) (\(\sim\)5.5 cm) onboard the Sentinel-1 has recently been gaining traction as a method for monitoring phenology, having successfully mapped the phenology of crops [99].  
As sensors become smaller and more affordable, UAVs are becoming a viable alternatives to satellites and planes when it comes to spectral range [174]. Miniature multispectral, hyperspectral or narrowband cameras attached to UAVs are now being used to capture phenological variables related to plant chemistry and health [175; 176; 177; 174]. For example, [PERSON] et al., [175] used multispectral cameras on-board UAVs to estimate leaf area index, chlorophyll content, leaf water stress, and canopy temperature of crops. Similarly, [PERSON], [PERSON] and [PERSON] [176] used hyperspectral and thermal imaging cameras on-board UAVs to examine water stress in agricultural crops. To date most of these studies have focused on agricultural crops but, as the cost of sensors decreases, we will likely see them used to assess the phenology of natural habitats.  
Techniques that use a wider array of spectra have potential to provide a new avenue for phenological monitoring. They will allow indirect measurement of the chemical quantities within the plants, as opposed to relying on indirect measures of these quantities through plant colour. For example, shifts in Gcc values of individual leaves are an indirect measure of the altered concentrations of chlorophyll in new vs. old leaves [178], so we predict that indirect measurement of leaf chlorophyll will improve the accuracy of leaf phenology data beyond what is achievable through a reliance on leaf colour changes alone. Hyperspectral imagery is beginning to be used for mapping leaf traits and chemistry over large forest areas [95; 179] and for automated mapping of species distributions [180], but to date this approach has not been used to monitor phenology. We expect this will be an exciting new direction of research in the future, especially with the recent and upcoming launches of new hyperspectral imaging satellites in the EnMAP, PRISMA, and NASA HyspIRI missions [181; 182; 183; 184].