For many years, selective thinning of forests, both private and public, has been an accepted method for maintaining healthy, disease free trees. Tree diseases, and insects, are the two largest causes of tree loss for both private landowners and and also public forest managers. It was (is still) believed that removing any competing trees from around the root zone of a desirable tree is helping it, by allowing it to receive more sunshine, which results in more photosynthesis, giving us healthier trees with less tree diseases.


New scientific knowledge shows that nature is more complicated than that. Experts at the University of Turin have shown that trees are capable of connecting with other trees of the same species, via root networks that are facilitated by fungi that assist in the transference of nutrients (sugars). In other words, when one tree is sick, either with a tree disease or insect attack, neighboring trees help the sick tree, like a helpful community. Until very recently, we did not have the knowledge to understand how trees help each other. Selective thinning may increase the growth rate of trees by increasing photosynthesis, but fast growth does not necessarily mean healthy growth, without tree diseases.

When trees grow too fast,  they may not store enough nutrients to generate defensive compounds, to defend themselves against attacks by insects and tree diseases. So, why fast growth may give you trees that are ready to harvest faster (100 years is the usual harvest age),  those forests are going to have a higher mortality from tree diseases and insects.
Forests that are allowed to evolve naturally as they did for thousands of years, prior to just being planted with the goal of being cut down in 100 years, grow to be much healthier forests, with larger, healthier trees, that are more resistant to tree diseases and damaging insects.

Another disadvantage to selective thinning other than removing the social network the trees desire, is that you make it make it much harder for the trees to cool themselves in the summer, which can lead to drought stress and also allow wind to get down into the forest where in a natural forest the trees form a natural barrier against the wind. Without this barrier the tree are much more susceptible to wind throw.
Whether or not foresters managing large populations of trees will change practices due to this new knowledge remains to be seen, but I for one will not be doing any selective thinning on the few acres of beautiful ponderosa pine that I live on. I will let them live and thrive together.