Application of vertical skyward wide-angle photography and airborne video data for phenological studies of beech forests in the German Alps
Vertical skyward wide-angle photography, photosynthetically active radiation (PAR) measurements and analysis of leaf parameters were used as ground data for the assessment of phenological change of montane beech forest using airborne video data in the German Alps. The main objectives of the study were: (1) to test the feasibility of wide-angle photographs as ground data for remotely sensed data, (2) to evaluate the feasibility of airborne video data in a change detection study, and (3) to study the phenology of beech forests in different elevation zones to acquire information for regional photosynthesis and evapotranspiration studies. The results showed an increase in canopy closure from April to early June, no changes during the summer months, and a slow decrease in autumn. The results also showed a strong correlation between canopy closure estimation using the photographic method, PAR measurements, and leaf variables. The adjusted r2 values between photographic canopy closure estimation and PAR ratio ranged from 0.72 to 0.88 and between canopy closure estimation and leaf size from 0.77 to 0.88. The problem of using photographic canopy closure estimation in phenological studies is that canopy closure is difficult to compare with spectral information of remote sensing data. The estimations had high correlation (r2=0.71) only with the red band of the airborne data, while with green and the near-infrared (NIR) band the correlations were very weak.