WEEK 6 - SAMPLE LAB EXAM QUESTION
- QUESTION:
- On the forest walk this week, you noticed that the lower branches of many large evergreen trees, such as western hemlocks, lack needles. During the winter months, diciduous trees, such as maples, lack leaves altogether. How do changes in light availability and the cost of water loss help to explain these observations?
- If evergreen trees like red cedar and western hemlock grow bigger, live longer and dominate mature coastal forests, why are red alder still abundant, when they are not well adapted to the conditions characteristic of a mature coastal forest?
- ANSWER:
- Transpirational water loss in leaves carries a cost to the trees. If leaves are not capturing enough energy through photosynthesis to outweigh the cost, selection may favor the loss of leaves.
The total surface of area of leaves is important in determining how costly they are to retain. Large leaves like those of maple are well adapted for capturing lots of sunlight, but are poorly adapted for prevention of water loss through transpiration.
Reduced levels of sunlight during the winter and at lower levels in the canopy may make it too energetically costly to retain leaves.
- Red alder might not be well adapted to the conditions of the mature coastal forest, but they are well adapted to the conditions found in disturbed coastal forest. They successfully invade and grow in disturbed (burned or logged) areas. Habitat conditions in forests are not static but change as the forest matures. Many plant species are adapted to do well in these transitional stages.
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