what sorta wood would yall recommend for the following?

southern england, so temperature ranges from very cold (near -10/5 rarely in winter) to, occasionally, 28 degrees in summer

am trying to build quite a large treehouse attached to bridge-like 'branches between trees'

weather ranges from very rainy (would cover with tarps, though bridges may be left exposed to very sunny

can wood be left exposed in the rain? i have a lot of questions! thanks guys : )