Gielinor, the high fantasy virtual world where Gods are real and we are the Time Guardian/World Guardian, of course.
Of course, high fantasy world shouldn't be "natural", or else it will just be garbage. That's why we can talk to cats, camels, crocodiles, ghosts and such. That's why we can respawn from death. So why should snow and trees in Gielinor be garbage just like in the natural world? Imagine if trees are natural... uh oh, the tree gnomes will be homeless and we Time/World Guardian will have to walk instead using the Evil Tree teleport system.
There are some trees that keep their leaves during the winter. They are known as Marcescent trees. Oak, beech, ironwood, hornbeam, witch hazel, and frailejones trees keep most of their leaves during the winter. In early spring is when this type of tree loses all of its old leaves.
There are oak trees in the game so some of the trees are correct.
I have to agree that the trees look unrealistic without snow being present in other parts of the environment. In Catherby, for example, there are evergreens frosted with snow right next to the beach, on what mostly appears to be a warm, sunny day. Putting some snow on rooftops and on the ground would help make things more consistent, as would updating the basic tree models to have the old "dead tree" appearance. As Zeggy pointed out, oak trees and of course evergreens could stay as they are.
It would also be nice to have default skyboxes updated seasonally to reflect weather changes, for the main areas. Although I remember this getting some pushback because of differences in location and time zones among players, I don't think having skyboxes change seasonally would make things less realistic for those living in discrepant locales. So long as the RuneScape environment is believable, I think it's okay for it to not necessarily line up perfectly with real world conditions at any given location.
Spirit of Forinthry
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Runecrafting: Magic Armour
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F2P Extensions
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Dragon Slayer improvements
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New Player Experience