Well I'd say a small hand axe at least . Trees are , well , very rarely nice and square and an axe makes adjusting timber to fit branches etc a relatively easy job. Plus I'm surprised no one has suggested a bog standard hand saw . Power tools are great ,and I'd agree with the suggestions made here , but there will be times when an adjustment is required and it 's so much easier to use a handsaw than dragging leads up a tree and if you drop it it'll be much more forgiving than dropping a nice shined electric or cordless tool.
