1) May have had a distinct meaning from 'hedge', based on the trees being placed an intervals.
Although one example of this word is on record from the Old English period, the other references in the OED date only from 1577, although ‘hedge’ was frequent throughout the intervening period. Yorkshire examples from before 1577 seem to throw new light on its meaning: c.1540 Also ther are oks and esshes growynge in hedgerows upon theste parte of the house which is worthe to be solde by estymacon Ł10, Hampole
1546 Item, certen okes and other underwoodes growing upon the hedgerows of the parkes, parockes and the oxcloses valued at iiijli, Well
a.1568 within the said groundes in hedgerawes there one hundrith okes Scrubbes worthe to be solde every tree iid, Goathland
1672 Except Eightye trees to be marked … to be preserved & alsoe Except all the hedgrowes & trees therein growing, Tong. The inference is that the emphasis is on the timber trees which were placed at intervals throughout the length of a hedge: 1619 ‘to plant, set, preserve and cherish twelve oak plants and twelve ash plants in the hedges’, Austby. The authors of the book Hedges thought there was no ‘useful distinction to be made between hedgerow and hedge’ but this evidence suggests that hedgerow developed its meaning when hedges became a source of great timber: it defined that extension of their function.