Tate Modern, LondonThe Sámi artist may use reindeer skulls and bones, but her spiralling wooden fences are so slight they fail to impose themselves on the venue’s vast space – or the imaginationThe Tate Turbine Hall, in case you didn’t know, is quite big. It gives an artist a unique opportunity to work on an epic scale and animate this colossal post-industrial space all the way from piazza floor to girdered roof. Artists have put the sun in here, built slides, opened a crack from one end of the floor to another. Yet Máret Ánne Sara seems either scared, repelled or just uninspired by it. She has built a little fort of sticks to hide away from the vastness. That’s the best I can say about her installation – that a small child might enjoy it as a pretend stockade. This easy-to-escape maze of trees actually reminds me of an upmarket adventure playground.It’s hard to understand why Tate Modern didn’t ask Sara for a bit more, well, art. She must have submitted drawings. Did these not suggest it was all going to be rather slight? You do know, they might have gently said, this is the space Ai Weiwei carpeted with sunflower seeds, and Rachel Whiteread filled with a simulacrum of Arctic ice?