From diy_glossary.txt:
.Longbow. [effort:2] [phys:stance,arms] *BOWYER* /18h/ |2|
{Board} <Pine, Birch> [remove]
{Axe} <Carving axe> [wearpct:30]
{Knife} <Small knife>
{Bowstring} [remove] [noquality]
So pine or birch, doesn't matter which. Bowstring material or quality doesn't matter at all.