How would you explain this sentence?
In the court of Norton-farmhouse, a manor farm to the north-west of the village, on the white maims, stood within these twenty years a broad-leaved elm, or wych hazel, ulmus folio latissimo scabro of Ray, which, though it had lost a considerable leading bough in the great storm in the year 1703, equal to a moderate tree, yet, when felled, contained eight loads of timber; and, being too bulky for a carriage, was sawn off at seven feet above the butt, where it measured near eight feet in the diameter. This elm I mention to show to what a bulk planted elms may attain; as this tree must certainly have been such from its situation.
How would you explain this sentence: This elm I mention to show to what a bulk planted elms may attain? Would you analyze the grammatical structure of this sentence? In my opinion, “this elm” is the subject. If so, what is the predicate?
PS: this extract is taken from The Natural History of Selborne.