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A level Biology Question

Hi, Could someone please explain to me why NPP decreases in this question? I thought that if more light is hitting the leaves of the trees than hitting the floor and being wasted, then more energy from light will be converted via photosynthesis? But it says it remains the same. Thanks.
(edited 1 year ago)
Reply 1
Because there are lots of trees, less light is reaching the floor - as you have identified. However, this means that the scrubs and grasses in the dunes are getting less light. So the NPP (Net Primary Productivity) of the plants near the ground will drop because they are photosynthesising less because they have limited access to light.

The Net Primary Productivity is the Gross Primary Productivity (basically how much the plant photosynthesises) minus the rate that energy is used up. So it doesn’t take energy that isn’t absorbed by the plant into account; hence here how much light hits the ground is irrelevant. All that matters is that plants near the bottom absorb less energy so have lower rates of photosynthesis so have lower GPP so have lower NPP.

Hope that makes sense! Let me know if you have any more questions about it.
Reply 2
Original post by Phoebe!
Because there are lots of trees, less light is reaching the floor - as you have identified. However, this means that the scrubs and grasses in the dunes are getting less light. So the NPP (Net Primary Productivity) of the plants near the ground will drop because they are photosynthesising less because they have limited access to light.

The Net Primary Productivity is the Gross Primary Productivity (basically how much the plant photosynthesises) minus the rate that energy is used up. So it doesn’t take energy that isn’t absorbed by the plant into account; hence here how much light hits the ground is irrelevant. All that matters is that plants near the bottom absorb less energy so have lower rates of photosynthesis so have lower GPP so have lower NPP.

Hope that makes sense! Let me know if you have any more questions about it.

Hi, thanks for your reply. That makes sense, but the way I thought about it was that all the light hitting the trees is being converted into chemical energy, whereas not all the light hitting the ground is hitting the shrubs because they do not have total coverage of the ground? Or maybe I am overthinking it? Thanks again.
Reply 3
Original post by Har6547
Hi, thanks for your reply. That makes sense, but the way I thought about it was that all the light hitting the trees is being converted into chemical energy, whereas not all the light hitting the ground is hitting the shrubs because they do not have total coverage of the ground? Or maybe I am overthinking it? Thanks again.


What you’re saying is correct but not relevant - because the light that isn’t absorbed is not included in the calculation of NPP. Only the energy released by photosynthesis and the energy used by the plant are important to the calculation. The percentage of light energy absorbed by the plants is not part of the calculation so will not affect NPP.

This is a tricky question! It threw me off at first. But hopefully that clears things up!
Reply 4
Original post by Phoebe!
What you’re saying is correct but not relevant - because the light that isn’t absorbed is not included in the calculation of NPP. Only the energy released by photosynthesis and the energy used by the plant are important to the calculation. The percentage of light energy absorbed by the plants is not part of the calculation so will not affect NPP.

This is a tricky question! It threw me off at first. But hopefully that clears things up!

Ohhh, I think I understand more now. Thank you.

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