## 1 Introduction  
In late 2015, the European Space Agency (ESA) selected the FLuorescence EXplorer (FLEX) as the Earth Explorer 8 mission for launch in 2022 [1; 2]. FLEX will be the first satellite mission to be specifically designed to retrieve chlorophyll fluorescence across the full fluorescence emission spectrum (650-800 nm) along with reflectance, and the observations will be made globally over land at an ecologically relevant spatial scale of 300 m approximately monthly. FLEX will be a tandem mission, paired with ESA's Sentinel-3B, which will provide additional reflectance bands and thermal information [1]. The local equatorial overpass time for this polar orbiting pair will be in mid-morning, at 10 a.m., but local overpass times will vary from solar noon to early morning at different latitudes. In preparation for FLEX, the _HyPlant_ system was developed as the airborne simulator for the FLEX mission (PI, [PERSON], Forschungszentrum, Julich, Germany) and flown in campaigns conducted in Europe since 2012 and in the collaborative ESA-NASA airborne campaign conducted in the USA, and described in this paper.  
Airborne remote sensing observations can provide timely, spatially explicit information for monitoring terrestrial ecosystems. The joint ESA/NASA FLEX [1; 2] airborne campaign conducted in the fall of 2013 in North Carolina (NC), USA (called FLEX-US) was co-led by NASA/GSFC ([PERSON], [PERSON]) and Forschungszentrum Julich ([PERSON]) and brought together scientists from Germany, Italy, and the USA. Data processing/analysis was further assisted by ESA scientists from Spain and Switzerland. The primary objective of this airborne campaign was to co-manifest two unique instrument packages for vegetation studies of carbon cycle dynamics and ecosystem health, to investigate the potential of this sensor combination for acquiring new remote sensing information for a well-studied and managed coniferous forest.  
These instrument packages are: (i) the _HyPlant_ system (PI, [PERSON], Forschungszentrum, Julich, Germany), the first high spectral resolution imaging system to obtain airborne observations of vegetation solar-induced chlorophyll fluorescence (SIF) emissions in conjunction with a state-of-the-art visible to shortwave infrared (VSWIR) imaging spectrometer to acquire hyperspectral surface reflectance [3]; and (ii) the G-LiHT (Goddard's Lidar, Hyperspectral, and Thermal) airborne system (PI, [PERSON], NASA/GSFC, Greenbelt, MD, USA) that combines a commercial off-the-shelf scanning LiDAR [4] with two commercially available imagers; a visible to near infrared (VNIR) hyperspectral system for reflectance and a thermal sensor. Together, this unique combination of the _HyPlant_ and G-LIHT systems enabled simultaneous and comprehensive airborne measurements of vegetation structure, fluorescence, canopy hyperspectral reflectance spectra, and surface temperatures to describe physiological responses to environmental conditions at ecologically relevant spatial scales.  
The FLEX-US campaign examined two different NC forests, the Parker Tract Loblolly Pine plantation addressed in this work, and the mixed deciduous forest at the Duke Forest (Durham, NC, USA). Preliminary reports were produced for the two study sites: Parker Tract study [5], Duke Forest study [6], and the full campaign [7]. Here, we demonstrate the importance of diurnal observations to address the dynamic physiologic responses of the pine forest at Parker Tract, made possible by integrating spectral information from chlorophyll fluorescence and reflectance spectroscopy, surface temperature,and canopy structure variables retrieved with LiDAR, all from airborne instruments. In the future, this remote sensing approach using multiple high resolution instruments together is expected to support retrievals of photosynthesis and energy budget parameters from aircraft and satellite data [1; 2; 3].