# Technological Advancement in Tower-Based Canopy Reflectance Monitoring: The AMSPEC-III System
## 4 Discussion and Conclusions  
Upscaling of spatially discrete observations to landscape and global scales is crucial to developing better insights into the carbon cycle. However, due to the different nature of tower-based EC and satellite systems, this task remains challenging. The high spatial, spectral and temporal resolution of the tower-based AMSPEC systems (_cf._[7, 8]) allows to observe vegetation canopy under different view and sun angles for a comprehensive analysis of spectral reflectance at the stand level (e.g., [43, 44]), of fundamental importance for the interpretation of satellite observations. Initial findings show a good correspondence between UDC and JC irradiance and radiance, with R\({}^{2}\) comprised between 0.75 and 0.85 (Figures 3 and 4), with JC reflectance spectra noisier than UDC (Figure 5) due to the different spectral resolution (_cf._ Table 1; [45]). This suggests that JC can successfully be used to substitute more expensive measurement units such as the UDC, although extreme regions of the spectral range of each instrument may not be reliable (e.g., Figure 5). Figure 6 does not show a strong regression fit between half hourly EC-measured LUE and PRI measured by neither UDC and JC; however, the linear regression between PRI measured with the two spectro-radiometers shows a very good correspondence (R\({}^{2}\) = 0.89). By design, AMSPEC-III allows to characterize the BRDF, an essential requirement to scale measurements across different view and sun angles (e.g., Figure 2) when detecting physiologically induced changes in spectra. The similarity of the BRDF models (Figure 7) highlight the potential of JC to derive year-round estimates multi-directional PRI measurements from AMSPEC III instrumentation (e.g., Figure 7). In addition, the webcam implemented in the AMSPEC-III system will help to further analyze the impact of phenological changes on vegetation canopy spectral reflectance and the correspondent stand-level photosynthesis. Based the results presented in this work, we believe that AMSPEC-III equipped with JC is a powerful tool for investigating these stand-level relationships and detecting photosynthesis and canopy spectra. Despite JC is more sensitive to differences in temperature and requires a temperature-controlled housing and therefore increases also the energy requirements for the system, the majority of the AMSPEC-III system costs is associated with the UDC spectro-radiometer (Table 2). However, the performance obtained by JC and presented in this study justify the choice of JC over UDC for network of sensors for monitoring of vegetation physiology. Interoperability of different tower based instrumentation is an important prerequisite of tower based spectral networks [21], especially when composed of instrumentation provided by multiple investigators and funding sources. Our study has demonstrated the interoperability of the UDC spectro-radiometer and the more cost effective JC used in an AMSPEC-III system. On the other hand, simpler low cost instruments are available as well. For example, QuadPod is capable to quantify NDVI and PRI [46] at a fraction of the cost of AMSPEC. However, these highly specialized sensors are dedicated to the measurement of specific indices and cannot be implemented in a multi-angular setup. In addition, the collection of spectra over multiple bands will allow to analyze various vegetation indices. Cost effective solutions, such as the JC instrumentation used in this study, could make important contributions to tower based remote sensing networks and therefore to scaling forest ecosystem productivity from stand to satellite and global scales.