The Collaborative International Dictionary of English v.0.48:

Subalpine \Sub*al"pine\, a. [L. subalpinus.] (Bot. & Zool.)
   Inhabiting the somewhat high slopes and summits of mountains,
   but considerably below the snow line.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subalpine
    adj 1: growing at high altitudes [syn: alpestrine,
           subalpine]