A woodland ecosystem is described as being in a climax community. Explain why the net primary productivity (NPP) of a climax woodland community is lower than that of a young, rapidly growing woodland of the same gross primary productivity (GPP).
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ecologists studying woodland succession measured the gross primary productivity (GPP) of two woodland sites. Site A was a young, rapidly growing woodland established 10 years ago. Site B was a mature climax community of oak woodland. Both sites had identical GPP values, yet Site B had a significantly lower net primary productivity (NPP).
Model answer (4 marks)
NPP is the amount of carbon fixed by photosynthesis that is available for growth after subtracting the respiratory losses of the producers.
In a climax woodland the total biomass is much greater than in a young stand because many large trees are present.
The larger biomass requires more energy for maintenance respiration, so the respiratory losses are higher.
With the same GPP, the greater respiratory losses in the climax community mean that the NPP is lower than in the young woodland.
In a climax woodland the total biomass is much greater than in a young stand because many large trees are present.
The larger biomass requires more energy for maintenance respiration, so the respiratory losses are higher.
With the same GPP, the greater respiratory losses in the climax community mean that the NPP is lower than in the young woodland.
Examiner tips
- Define NPP as GPP minus producer respiration; mention maintenance respiration; link larger biomass to higher respiration; keep answer concise and use correct terminology
Common mistakes
- Confusing NPP with GPP; not mentioning maintenance respiration; failing to link larger biomass to higher respiratory losses
Mark scheme (4 marks)
- NPP is defined correctly as GPP minus respiratory losses by producers
- The climax community has a greater total biomass / larger number of large woody plants requiring more energy for maintenance respiration
- Greater biomass in the climax community means more energy is used in (maintenance) respiration
- Therefore, with the same GPP but greater respiratory losses, NPP is lower in the climax community
Key terms in this question
net primary productivity · gross primary productivity · climax community
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Ecosystems questions
- Explain why energy transfer between trophic levels in an ecosystem is never 100%…
- Explain why energy transfer between trophic levels in an ecosystem is never 100%…
- Explain why a woodland ecosystem with high biodiversity is likely to be more sta…
- Explain why energy transfer between trophic levels in an ecosystem is never 100%…