Here is a final attempt to shed light on this difficult question.
We have noticed perhaps that those exotic broad-leaves that were planted in parkland tend to be planted near structures and buildings, or might have been used as nurses for the slower-growing natives. If we turn now to the pleasure ground we will find by contrast an abundant use of floriferous exotics, and it might be possible to show that Capability Brown’s hand lay behind their purchase.
Consider the Magnolia for example which the nurseryman John Williamson (who seems often to have supplied the clients of that workman worth his hire, Capability Brown, with plants) provided to Burton Constable in 1759. One route to judgement on Brown’s ability as a plantsman is to look at the coincidence between collections of unusual trees and sites attributed to him. This that certain genera, such as Magnolia indeed, and certain species, such as Willow-leaved Oak (Quercus phellos), crop up more often on sites attributed to Brown than elsewhere. This may simply be because Brown’s clients were rich men and rich men were buying expensive exotics, but it tends to encourage the conclusions that Brown was not barred by ignorance from planting exotics in parkland, that he was perfectly happy to use them in pleasure grounds (though constrained by their expense and lack of hardiness) and, in conclusion, that he had some reason for not using them a great deal in parkland.
With which, I feel I must bring this difficult subject to a close for the present.