Normally the main problem in using a vehicle satnav outdoors is the fact that they aren't waterproof, however you won't have this problem with the Zumo as it is designed for motorcycles and is waterproof, another potential problem is the screen not being bright enough, but again the Zumo is designed for outdoor use, so the only problem I can see is battery life, the Zumo would normally get it's power from the motorcycle, so you may only get a hour or so from the internal battery, you could connect a USB battery bank to exend running time
As for tree cover satnavs aren't the best but it should still work most of the time