Aspen produce new plants by both seed and by sucker growth, in Glenveagh there are 3 Aspen all connected with underground roots. There is talk of this being the oldest tree in Glenveagh, and may be a 1000 years old....
I'm not too familiar with Aspen, even in Scotland its quite a rare tree (see Scottish Aspen here)....it is however a tree I plan to plant a good few of this coming winter during bare-root season.