Question

Norby et al. (1992) grew yellow poplar (Liriodendron tulipifera) trees for three years at three levels of \(\mathrm{CO}_{2}\) enrichment. Photosynthetic rate nearly doubled at high \(\mathrm{CO}_{2}\) levels, but no difference in aboveground biomass of trees grown at normal or elevated \(\mathrm{CO}_{2}\) levels was found after three years. How is this possible?

Short answer

Increased CO2 uptake raises photosynthesis but doesn't help growth if nutrients or water limit it, or if growth resources go elsewhere.

Step-by-step solution

Understanding Photosynthesis Rate Increase

At higher CO2 levels, the photosynthesis rate increases, which generally means the plant can produce more glucose and oxygen through the process of converting carbon dioxide and water with the help of sunlight.

Exploring Biomass Outcome

The biomass is an indicator of growth and development typically measured by the accumulation of organic material in a plant. Despite the increased rate of photosynthesis, biomass did not increase for yellow poplar trees even with elevated CO2.

Examining Resource Limitation

The potential reason for no increase in biomass, despite doubling photosynthesis, could be due to other limiting factors such as nutrient availability, water supply, or genetic growth constraints that restrict the plant's growth regardless of photosynthetic capacity.

Considering Allocation of Resources

Alongside nutrient limitations, trees may allocate the additional resources from increased photosynthesis to roots or other non-aboveground parts, rather than increasing the aboveground biomass such as leaves and stems.

Key concepts

CO2 Enrichment

Plant growth and development can be significantly influenced by the levels of carbon dioxide (CO2) in their environment. In the process of photosynthesis, plants use CO2 along with water and sunlight to produce glucose, which serves as a source of energy, and oxygen, a byproduct released into the atmosphere.

Enriching the air with higher concentrations of CO2 can substantially boost the rate of photosynthesis. This happens because plants have more CO2 available to convert into energy, theoretically supporting more vigorous growth. For instance, yellow poplar trees in elevated CO2 environments have shown an increase in their photosynthetic activity, sometimes nearly doubling.

The increased availability of CO2 allows plants to make more glucose, supporting enhanced growth under optimal conditions. This might lead one to assume that higher CO2 levels would automatically equate to a larger plant size or mass, known as biomass. However, as we delve deeper, we'll find that this isn't always the case.

Biomass

When discussing plant growth, the term 'biomass' often comes up. Biomass refers to the total mass of all the organic material that can be harvested from plants. This includes both aboveground parts like leaves and stems, and sometimes the belowground parts such as roots.

Interestingly, even with increased photosynthetic rates in the presence of elevated CO2, studies, like those by Norby et al., have not always shown a direct increase in aboveground biomass. One might find it surprising that yellow poplar trees did not show a noticeable difference in biomass after being subjected to higher CO2 levels for three years.

This indicates that while photosynthesis might be happening at a higher rate, it doesn't necessarily mean more biomass is created. This is a crucial point to understand, as it stresses the importance of how plants allocate their resources and respond to other existing environmental conditions.

Resource Limitation

Plants require more than just carbon dioxide to grow and thrive; they also need nutrients, water, light, and suitable temperatures. Despite higher photosynthesis rates due to CO2 enrichment, growth can still be limited by a scarcity of these essential resources.

For example, if nutrients like nitrogen or phosphorus are not sufficiently available in the soil, it can restrict growth, preventing plants from producing more biomass despite higher CO2 levels. Similarly, insufficient water availability can limit the plant's ability to utilize the extra glucose produced efficiently.

Moreover, trees like the yellow poplar might choose to allocate resources towards non-visible parts like roots instead of aboveground biomass. This is a survival strategy enabling them to search for nutrients or store energy. This complex allocation strategy highlights that other environmental or internal factors can significantly influence plant growth responses to elevated CO2, beyond just boosting photosynthesis.