Rather than explain from scratch, I would refer new readers with such questions to the Basic Premise link at the top of the blog - or for a more recent iteration illustrated with pictures, this post, titled "Rude Awakening" and another with particularly good introductory links, here.
Regular readers are already familiar with the notion that the ability of foliage and needles to photosynthesize becomes impaired following exposure to background tropospheric ozone. Air pollution is thus the underlying, often hidden cause for a universal and widespread rapid decline of vegetation, whether trees, shrubs or annual agricultural crops. Damage generally mistaken as due to weather, drought, climate change - or diseases, fungal infection and insects - is thus frequently opportunistic, the so-called "sharks that smell blood in the water" as seen in the video at the end of this post. But first, below is a hauntingly evocative photograph from the UK Guardian, of caterpillars in the Netherlands...a species historically in harmony with its host, the oak, but which has suddenly run rampant across Northern Europe. This explosion of population is, like that of the bark beetle in the American and Canadian west, usually myopically attributed solely to rising temperatures from climate change rather than compromised immune functions due to volatile organic compounds traveling across the world, released from ever-increasing fossil and biofuel emissions: