Tom,
Mass tree plantings did occur universally in the 1980's and 1990's. Ornamental tree planting committeees and beautification committees have left their conspicuous marks on golf courses most everywhere in the last twenty (20) years. They have made an absolute mess!
In the 1950's and 1960's, awareness of attrition escalated as the American elm perished. Replacement tree(s) became the rage.
Also, the wholesale installation of single row irrigation systems down the center of the fairway also served as a contributing factor.
The "PV Effect" of isolation and separation is alive today on a more limited basis, as I am aware of two clubs which have initiated tree planting efforts outlining holes. Their green's chairmen coincidentally are also members of PV.
Then of course trees are planted to defend par or attract benficial wildlife habitats in other isolated instances.
Tom, the fact of the matter is that people embrace trees on golf courses. Because of such, there will always be justifications for tree plantings, regardless of the decade, if we do not educate these people of their negative agronomic, strategic, and aesthetic impact.
You and Brad have done a great job in this respect.
But rankings do constitute a justification for tree plantings! Regardless of the feature, whether it is "length", "green speeds", "waterfalls", or "separation/frameworking" due to trees, the top ranked courses in the country serve as a model for all others. They will be emulated!!