Date | May 2016 | Marks available | 2 | Reference code | 16M.3.HL.TZ0.16 |
Level | Higher level | Paper | Paper 3 | Time zone | TZ0 |
Command term | Explain | Question number | 16 | Adapted from | N/A |
Question
The graph is a model showing biomass and respiration levels in a field where farming stops at time zero and the abandoned land develops into forest.
Describe the change in biomass over the 100 year period.
Outline the evidence from the graph that the area had plentiful rainfall.
Explain the changes in biomass.
Explain why biomass continues to increase after the respiration levels plateau.
Markscheme
Increases
At a greater rate early on
Seems to plateau/levels off
Forest developed
Increase in biomass «over years» requires rainfall
Water is a limiting factor to photosynthesis
Initially there is low competition for sunlight/resources
Photosynthesis allows accumulation of biomass
Biomass rises as larger plants replace smaller plants
«In later stages» biomass increase limited due to competition for resources
OR
biomass stabilizes as climax community reached
Development of mature trees requires xylem/wood
Xylem/wood contributes to biomass but not respiration
Photosynthesis/production greater than respiration
Photosynthesis/production continues to accumulate biomass
OR
accumulated biomass remains/increases
Examiners report
Many candidates received the full 2 marks for seeing both the increase in biomass as well as the change in rate of increase as well.
Many candidates were able to score one mark for indicating that the increasing biomass was evidence for plentiful rainfall but seldom was a second point made. A few noted that a forest could not develop without rainfall or that water is a limiting factor for photosynthesis.
A few candidates realized that succession was occurring and the biomass increased as larger plants replaced smaller ones. Carrying capacity was sometimes mentioned when climax community is what they should have been referring to. Others seemed to have no idea as to why biomass was increasing.
Candidates did not seem to understand what was being asked in this question so marks for this were seldom seen.