13 Turn over 6 The photograph shows a glacier at the top of a mountain. Glaciers in many areas of the world are retreating (reducing in size). The line on the photograph shows the position of the front edge of the glacier in 1985. Front edge of glacier in 1985 Bare rock is exposed as the glacier retreats. Two hundred years ago, bare rock was exposed after a glacier retreated. A study has been made of the long-term changes in vegetation on that area of rock after the retreat of the glacier. 14 The graphs show the abundance of three species of tree since the bare rock was exposed as the glacier retreated. The abundance of each species was measured every ten years for 200 years. 1500 1200 900 600 300 0 20 40 60 80 100 120 140 160 180 200 Abundance / plants per hectare Time since bare rock exposed / years Alder 1500 1200 900 600 300 0 20 40 60 80 100 120 140 160 180 200 Abundance / plants per hectare Time since bare rock exposed / years Spruce 1500 1200 900 600 300 0 20 40 60 80 100 120 140 160 180 200 Abundance / plants per hectare Time since bare rock exposed / years Hemlock 15 Turn over (a) Comment on the changes in the abundance of these three species over 200 years. 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(b) Explain what happened to the bare rock to allow the growth of these trees. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 16 (c) Plants such as lupin are often found in the early stages of glacial retreat. Lupin plants have nodules on their roots containing nitrogen‑fixing bacteria that convert atmospheric nitrogen to ammonium ions. Plants can use ammonium ions as a source of nitrogen. Explain why lupin plants are able to grow in the early stages of glacial retreat. 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(Total for Question 6 =10 marks)