Ecologically relevant canopy openness from hemispherical photographs

Canopy openness determines forest biodiversity, radiation regime and understorey microclimate. Traditionally, canopy openness was calculated from hemispherical photographs with a full 180 degrees angle of view, but photographic equipment providing only narrower angles of view is increasingly used. Despite that, it is unknown how well canopy openness estimated from restricted angles of view predicts different forest characteristics. To fill this gap, we explored how different canopy openness estimates predicts vascular plant species richness and composition, in-situ measured air and soil temperature and photosynthetically active radiation across different types of European deciduous broadleaved forests. We further benchmarked performance of canopy openness against radiation site factors and evaluated accuracy of canopy openness calculation using artifitial calibration targets. Canopy openness calculated from angles of view < 180 degrees was a better predictor of all measured variables than canopy openness from the whole hemispherical photograph. Openness from 80 degrees - 120 degrees angle of view best explained plant species richness, community composition, in-situ measured air and soil temperatures, and the photosynthetically active radiation. Moreover, canopy openness from a 100 degrees angle of view predicted all variables except maximum air temperature better than the total radiation site factor. Finally, canopy openness calculated from angles of view < 120 degrees was less affected by image quality and therefore more comparable between different photographic equipment including smartphones with an external fisheye lens. We found that canopy openness from a complete hemisphere is not required for many ecological applications. Therefore, many consumer-grade cameras and smartphones with an external fisheye lens can provide ecologically relevant canopy openness. Nevertheless, specialized cameras and fisheye lenses should still be preferred. Finally, researchers can predict plant assemblages, forest microclimate and understorey radiation regimes better if they calculate canopy openness from similar to 100 degrees angle of view rather than from the whole hemispherical photograph.