this is a great and healthy treat! not the ones sold at the petstores!
below a list of some of the brenches we can give them: (I add "Arbutus unedo", Corbezzolo in italian, cane apple/strawberry tree in english)
This is the source:
Guinea Pig food list
Branches and leaves
Alder Provide in small amounts only
Apple Can be provided in lage amounts
Beech, european beech Provide in small amounts only. Rich in oxialic acid
Birch Rich in tannic acid. Provide rarely
Cherry
Currant Can be provided in lage amounts
Fir Provide rarely. Not really liked by all Guinea Pigs.
Hazelnut Can be provided in lage amounts
Lime
Maple Feed rarely
Pear Can be provided in lage amounts
Pine Provide rarely. Not really liked by all Guinea Pigs.
Plum
Spruce Provide rarely. Not really liked by all Guinea Pigs.
Willow, corkscrew willow