Why do trees prefer their sunlight a bit diffused?
The studies are very complex and varied. There is evidence that strong omnidirectional light can get through to the understory of dense foliage more effectively.
Here is a Dutch study for crops in dutch greenhouses using light-diffusing rooves which maintain the quantity of light while making it omnidirectional. They report higher growth from diffuse light. https://www.researchgate.net/publication/40803021_The_Effect_of_Diffuse_Light_on_Crops
Studies of tree rings after volcanic eruptions have also found an overall higher growth for the periods following 5 major eruptions.
quotes: Under the presence of clouds, some studies have reported increases in both photosynthetic carbon gain (due to light levels closer to optimal) and water use efficiency, with less impact of photoinhibition due to excessive sunlight (Germino and Smith 2000; Gu et al. 2002)
This research suggests higher growth from direct sunlight in open forests and higher growth in dense, shaded forest under cloudy condition (research)