Checking out the fungi you see, enjoying how mushrooms are the flowering bodies of different large underground mycelial networks that are providing the trees with vital nutrients and water. Getting to know the earth around tree roots, where they grow and also what they look like when they aren’t in their full health.
The Forestry Commission has a good database of current tree diseases and ‘pests’. There is a national framework for reporting issues including but not limited to Ash Die-Back. Maybe you will discover ways of curing and boosting the immune systems that will allow the tree to ward off attacks?
From understanding the work of Dr Alan Rayner we have already discovered that much of tree health relates to their access to water. The flow of water up the tree trunk can be restricted and bubbles of air form in the trees pipes. This is the point where the fungal spores living inside the tree have the right conditions to start to grow and would be an example of when you might start to see heartwood rotting.
This makes it clear that the need to protect the water table from potential threats such as fracking and overuse is an issue that directly affects national tree health.
There is much evidence that trees in forests have increased immunity to infection because their underground network warns them of incoming attacks and they are able to create the right antibodies. Equally that the trees around them will support them with nutrients via the roots until they are strong again.
Maybe how we have farmed and also isolated trees is part of their current epidemic problems?
You might be able to halt the spread of disease, campaign for the cessation bring plants in across the borders without complete safety being ensured. There are two Forest Commission approved nurseries in the UK for native trees and unless you are absolutely sure that the tree has been grown in this country it might be best to stick to these.