As Allarius said, there are several threads on this very subject already in existence in the Grove of Tales (the proper place for fluff and fluff-related questions...)
As well as the thread Allarius linked to, you might want to search for "Laurelorn" or "Lost Kindred" and you should find plenty of results.
To answer your second question (which is really unrelated to the first, and thus would probably warrant a separate thread), it depends on the type of forest spirit. Generally speaking, the difficulty of awakening gets harder the further up the "scale" you get although it also depends on the "personality" of the spirit involved; most Treeman are difficult to rouse at the best of times, for instance, whereas Dryads tend to be a lot more active overall and thus would be easier to awaken, or more likely not to sleep. Wild Riders are a general exception to the rule because their entire purpose for being is to accompany Orion when he's awake, and to protect him during winter - thus, they're active all year round. The likelihood of them participating in fighting during winter though is a different question, as their responsibility and duty during the winter months is to protect the Oak of Ages, not wander off fighting in the forest...
There is no "rule" that forces all forest spirits to fall asleep. They do so largely out of choice, because winter is a period where they are least active and most vulnerable. They hibernate largely to conserve their energy and to 'regenerate', just as animals do in the real world (and just as real trees do, in a sense). There is nothing preventing a forest spirit from remaining awake and active all year round, albeit with the considerations above taken into account (i.e. they'd be weaker, more lethargic and more vulnerable).