Insights from 15 Years of Free-Air CO2 Enrichment (FACE) Studies

For the past fifteen years, researchers have used free-air CO2 enrichment (FACE) experiments to study the impact of elevated atmospheric CO2 levels on plants and ecosystems under natural conditions^[1]. Let’s dive into what we've learned about photosynthesis, plant growth, and how different plants respond to increased CO2.

What is FACE?

FACE stands for Free-Air CO2 Enrichment, a technique that enriches the air around plants with higher levels of CO2 to mimic future atmospheric conditions without enclosing them in chambers. This method allows plants to grow in their natural environment, interacting with real-world variables like wind and light^[1]. The main objective of FACE experiments is to understand how rising CO2 levels affect plant physiology and productivity.

Major Findings

Increased Photosynthesis

Studies show that elevated CO2 boosts several key aspects of photosynthesis. For example, light-saturated carbon uptake (Asat) increased by 31%, and diurnal carbon assimilation (A’) grew by 28%. Additionally, maximum quantum yield was enhanced by 12%, and stomatal conductance (gs) decreased by 20%, indicating improved water-use efficiency^[1].

Differences Among Plant Types

C3 vs. C4 Species

C3 plants, including most trees and many crops, are significantly more responsive to elevated CO2 than C4 plants. The photosynthetic rate in C3 plants increased by 33%, whereas C4 plants only showed an 11% increase^[1]. This disparity confirms theoretical expectations that C4 plants, which already have a CO2-concentrating mechanism, benefit less from higher atmospheric CO2 levels.

Functional Groups

Trees exhibited the highest increase in photosynthesis at 47%, followed by crops with high nitrogen conditions at 36%, and C3 grasses at 36%. Shrubs and legumes showed 21% and 15% increases, respectively^[1].

Acclimation of Photosynthesis

Photosynthetic acclimation is a plant’s adjustment to sustain a balance in nutrient allocation in response to higher CO2 levels. FACE studies indicated a 13% reduction in maximum carboxylation rate (Vc,max) and a 5% reduction in the maximum electron transport rate (Jmax). Additionally, there was a minor reduction in nitrogen content, largely accounted for by decreased Rubisco, the enzyme responsible for CO2 fixation^[1].

Function-Specific Responses

• Trees and Legumes: Trees had minimal reduction in Vc,max and no significant change in Jmax and leaf nitrogen content, suggesting a more stable acclimation to elevated CO2. Legumes, benefiting from nitrogen fixation, also showed a less pronounced acclimation compared to C3 grasses^[1].

Influence of Stress and Environmental Conditions

Stress factors like nutrient deficiency and drought significantly altered the plants' responses. For instance, under low nitrogen conditions, Vc,max decreased more significantly (22%) compared to conditions with adequate nitrogen (12%)^[1].

Impact on Growth and Biomass Production

CO2 enrichment resulted in overall plant growth and structural changes. Plant height and stem diameter increased by 14% and 9%, respectively, with more branching and leaf numbers^[1].

Differences by Plant Type and Stress Conditions

• Trees: Exhibited a 28% increase in above-ground dry matter production, the highest among functional groups.

• C4 species: Had minimal response in terms of biomass production, aligning with their lower photosynthetic responsiveness.

• Crop Yield: On average, crop yield increased by only 17%, less than anticipated from chamber studies which projected 28-35% increases. Notably, only cotton showed a significant yield boost of 42%^[1].

Limitations and Future Directions

While FACE provides valuable insights, it has limitations. The technology often doesn't elevate CO2 at night, potentially missing effects on dark respiration. Additionally, larger-scale experiments involving diverse biomes like tropical and boreal forests are necessary to capture a more comprehensive understanding of global vegetation responses to elevated CO2^[1].

Key Takeaways

• Elevated CO2 increases photosynthesis, growth, and water-use efficiency in many plants, especially trees.

• C3 plants generally benefit more from increased CO2 than C4 plants.

• Acclimation processes, particularly reductions in Vc,max, suggest complex responses to sustained high CO2 levels.

• Despite positive impacts on growth, crop yields have not increased as much as earlier enclosure studies predicted, indicating the need for reevaluation of future food supply projections based on FACE data.

FACE experiments have reshaped our understanding of plant responses to increased CO2, highlighting the necessity for continuous research and adaptation of agricultural practices in the face of climate change.