You don't get email notification for that. You can use the 'favorite discussions' feature to take you back to any thread quickly. To change any threads into your favorite discussions, click the little grey heart next to the thread title (the grey heart will change to orange), when you want to check updates on those 'favorite discussions', go to the bar on top of the forum topics list and click 'favorite discussions', all your favorite discussion threads will show up.
Another way to turn threads into your favorite discussions: In the forum topics list, find the thread topic you're interested in, scroll your cursor over the "#of posts" and the little grey heart will appear between the thread topic and "#of posts", you can again turn that thread into one of your favorite discussions by clicking the grey heart.
Hope I didn't make it too confusing for you!