Trees are ecosystems
Biodiversity works in harmony and balance to create a tree’s healthy biome, that is then part of the larger macro-organism that we call a forest.
Left alone and in optimum conditions this balance occurs naturally and It’s only a lack of balance that causes the conditions for disease and illness - nature's clean up crew of what is not of benefit, senescent or out of place.
A tree's own ecosystem is diverse and complex and ultimately made up of ecosystems within ecosystems - all made to tick and run via interacting biodiversity.
One word to describe this interaction is a 'holon' - the endemic micro ecosystems of the tree and its immediate surrounding area.
It's said the holon contains all the biodiversity and plant life needed for healthy growth and life, it's really just the plants and organisms that grow naturally in symbiotic relationships when left to be. In the case of trees this can have taken millions of years of adaptation and partnering up.
Normally a negative change in the holon or ecosystem of a tree causes disease and even issues such as drought stress. Pathogenic fungi levels being out of balance is a prime example. Equally lack of natural tree strata layers that recycle precipitation to the forest floor is another example.
It's always good to take a walk in native, ideally old growth forest to see what grows with what in these mini ecosystems.