Waring out the carbon: is net primary productivity proportional to gross primary productivity?

Gross primary productivity represents the total carbon fixed through photosynthesis in an ecosystem. Since plants also breakdown sugars produced through photosynthesis, releasing carbon and energy for metabolism, plant respiration uses up a substantial portion of gross primary production. Net primary productivity is the difference between these two processes and represents the carbon available to the ecosystem for plant growth and animal consumption. Understanding how entire forests and the biosphere at large will respond to climate change requires an understanding of net primary production. However, estimating net primary production can be difficult, so over 20 years ago, an assumption was introduced that net primary production was equal to approximately half of gross primary production. This may leave you wondering – is this assumption correct?

Cedrus deodara dominated forest in India. Controls on the proportion of carbon that forests store in plant biomass can complicate predictions of future carbon uptake in forests. Image: Paul Evans / Wikipedia

In Tree Physiology, Collalti and Prentice revisit this assumption for forests, looking at over 200 studies that previously addressed this question. In particular, they sought to understand whether forest age, structure, or climate could influence the ratio of net and gross primary productivity. They focused specifically on vegetation photosynthesis and respiration to understand how forests partition carbon that is fixed. While they found that the average ratio of net to gross primary productivity was close to the 20-year-old assumption, they found high variability in this number across different forest types, ranging from 0.22 (near the theoretical minimum) to 0.8. Looking into the causes of this variation, Collalti and Prentice found that forests with higher soil nutrient content (also known as soil fertility) had higher ratios than forests with nutrient-poor soils, while managed forests had higher ratios than unmanaged forests. Overall, this means that managed forests with fertile soils became more efficient for carbon uptake, using relatively more carbon for growth. These findings line up with predictions that greater nutrient availability stimulates photosynthesis, and how managing forests can maintain a light environment within the forest canopy favourable to growth.

What are the implications of these findings? Since the ratio of net and gross primary productivity is used for predicting future carbon uptake of ecosystems, Collalti and Prentice show that the simple assumption previously used does not capture the scope of biological processes at an ecosystem level. This will make predicting net primary productivity necessarily more complex, but also more accurate, by tailoring predictions to the prevailing environmental conditions of the ecosystem in question. However, the books are not yet closed on the topic: trees can store carbon in sugars and draw from them many years later. This means that there could be complex time lags where the net primary productivity to gross primary productivity ratio reflects the environmental conditions in previous years. It is clear that Collalti and Prentice’s findings are just the tip of the iceberg in understanding what controls this ratio.

Further reading

Collalti, A., & Prentice, I. C. (2019). Is NPP proportional to GPP? Waring’s hypothesis 20 years on. Tree Physiology. https://doi.org/10.1093/treephys/tpz034