The attitudes of GA architects about trees was something of a mixed bag.
Most all seemed to think that on wooded property, trees would look 'natural'. Hard to argue with that. (See Geo. Thomas)
What MacK says in S of SA about trees is interesting. Summarizing his argument, he first notes that trees were removed from older (read Victorian) inland courses because they were thought to give luck too big a role in outcomes. Hit a tree, and no telling what will happen...
One of MacK's main themes in S of SA, however, is that giving fluke a role in gca (or in golf generally) is ok. Heightens drama, fun, tests character, etc. So he offers a partial defense in S Of SA of the use of trees, suggesting that they can be used on a limited basis if kept in "groups" or "clumps". When all else fails, a "group of trees" might even be used for strategic purposes. (See pp. 81-83.)
So Mack was not dogmatic about trees. In moderation, they might serve important functions, including aesthetic functions. At ANGC, for example, Mack left many trees on a property that was originally majority wooded. The thick stands of pine along the left sides of 1, 2 and 8 were original to the property. Ditto the pines to the right of the LZ on 13 and the fw on 14. There are lots of other "groups" of trees on the course. But notwithstanding all the trees MacK let stand, ANGC was distinguished back in the day for its enormously wide playing corridors. Trees were there, however, and for really egregious misses they were a factor. (Bubba's shot to win the Masters this year was from out of a stand of pines original to the course.)
Back to my speculation that on treeless properties on which many UK courses were built in the 1920's. I'll stand by my thought that GA archies would not be pleased to see the "vast stands of hardwoods" that today ring every hole on so many of these courses. But I stand corrected in the sense that GA archies would have probably tolerated limited and discerning use of trees on those courses.
(To return to what I think is an interesting historical point. Virtually all inland Victorian courses I've seen pictures of were denuded of trees. There was a reason for that. Victorian gca's were all about hazards that were 'equitable', understood in the sense that the severity of a hazard should match with the severity of the miss it trapped. Trees failed that test. They were considered an inequitable or unfair hazards because the punishment they doled out was so flukey and unpredictable. One aspect of strategic golf design that its proponents believed distinguished it from older Victorian designs was a tolerance of fluke in gca. Hence MacK's defense of the limited use of trees was a small part of a much broader argument about the superiority of the principles of strategic gca.)
None of which answers the question of why circa 1960 so many clubs in the US and the UK rushed out - as if struck simultaneously with the same epiphany - to 'beautify' their courses by planting thousands of trees.
Bob