JUSTIN BIEBER IT'S ALL DOWNHILL FROM HERE ... With Model Gal Pal

Justin Bieber is in a race to the

 bottom ... but at least now he's

 got some attractive company.

Bieber took a 1-on-1 hike with

 his model friend, Ashley Moore,

 who seemed down to get 

physical with him in a friendly 

competition running on a canyon trail. 

If there's one thing we know 

about Bieber ... he tends to open 

up in the wilderness. Bodes well 

for Ashley. Plus they've hung out

 before -- couple years ago at a

 Clippers game -- so, seems like

 they've got a connection.