An in-depth knowledge of forest ecosystem functioning is essential for sustainable management of forest resources. A detailed description of forest vegetation 3D structure is required to provide relevant information on biodiversity, available biomass and stand growth rate. Such information enables the evaluation of the efficiency of management practices and the vulnerability to natural risks (e.g., forest fires, storms, insect epidemics,etc). It is also highly valuable information forassessing the role of forest biomass in climate mitigation, e.g. stands of young growing trees act as efficient atmospheric CO2 sinks. Moreover, forest structural characterization is usefulfor studies concerning the role of forests in air quality: e.g., air filtering of atmospheric pollutants and emissions of volatile organic compounds. These environmental and socio-economical issues require extensive and accurate characterizations of forest vegetation 3D structure.
In this context, the Full Waveform Technique for canopy lidars is suitable to fulfill such a task. Typically, the first and last significant lidar returns indicate the canopy top height and ground. Tree height estimates and vegetation profiles by lidar can be used to assess indirectly the stand volume and carbon stock of forest. Currently, national and local agencies contract commercial companies to operate airborne canopy lidar for forest inventory. Additionally, the relevance of lidar for global forest canopy survey has been recently demonstrated by using 60 m wide footprint observations of ICES at spaceborne lidar. Further improvements require the support of research airborne canopy lidar for optimizing footprint size, scanning and line-of-sight, range resolution and probing wavelength.
Fig. 1: (a) Three examples of lidar profiles: for bare ground (in light gray), ground with undergrowth vegetation (in black) and a single maritime pine with the ground return below (in dark gray). SR2 is the range-corrected lidar signal (in arb.unit). (b) Example of 6 consecutive lidar profiles describing the 2D structure of one single tree crown and undergrowth vegetation. The pictures show the LAUVAC (Lidar Aéroporté UltraViolet pour l’Atmosphère et la Canopée forestière) canopy lidar on board the ultra light airplane flying over the Landes forest (location of the Landes forest is indicated with the red square on the France map)
We have developed a new airborne UV lidar for the forest canopy and deployed it in the Landes forest (France) (Fig. 1). It is the first one that: (i) operates at 355 nm for emitting energetic pulses of 16 mJ at 20 Hz while fulfilling eye-safety regulations and (ii) is flown onboard an ultra-light airplane for enhanced flight flexibility. Laser footprints at ground level were 2.4 m wide for a flying altitude of 300 m. Three test areas of ~500 × 500 m2 with Maritime pines of different ages were investigated (Fig. 2). We used a threshold method adapted for this lidar to accurately extract from its waveforms detailed forest canopy vertical structure: canopy top, tree crown base and undergrowth heights. Good detection sensitivity enabled the observation of ground returns underneath the trees. Statistical and one-to-one comparisons with ground measurements by field foresters indicated a mean absolute accuracy of ~1 m. Sensitivity tests on detection threshold showed the importance of signal to noise ratio and footprint size for a proper detection of the canopy vertical structure. This UV-lidar is intended for future innovative applications of simultaneous observation of forest canopy, laser-induced vegetation fluorescence and atmospheric aerosols (Cuesta et al. 2010).
Fig. 2: Canopy structure of areas of ~500 × 500 m2 with Maritime pines of different ages as seen by the LAUVAC lidar