Pinus - "hard" [yellow] pine is a species discriminator, as opposed to "soft" [white] pine, nothing to do with the density or strength of different ages or cuts of the species "yellow pine" .... I suspect the use of both category names together ['hard' and 'yellow'] was an industry [timber and automotive] advertising ploy back in the day
consider the wood categories "softwood" and "hardwood", any type of pine is in the softwood category, and any type of 'softwood' will rot fairly quickly outdoors if unprotected
Bill
