When models meet data: Limits to detecting CO₂ effects in tropical forests

Photosynthetic theory predicts that rising atmospheric CO₂ should enhance photosynthesis in tropical trees, potentially increasing stem growth and strengthening the tropical forest carbon sink. However, consistent positive CO₂ effects on stem growth are rarely detected in observational studies. Here, we investigate whether tree light exposure, climatic variability, and statistical limitations can explain this apparent discrepancy.

We used a previously parameterised and tested forest model to simulate tropical tree populations under fixed and rising historical CO₂ and climate representative for SE Asian lowland tropical forest. To represent realistic variation in light availability, trees were simulated in gaps of different sizes, explicitly resolving height-dependent light gradients, constraints on maximum canopy size, and dynamic changes in light conditions as trees grow. Simulations were conducted under source-limited conditions.

Across simulations, CO₂ effects on growth were weak compared to the effects of climate and light availability. The simulated CO₂ response was comparable in magnitude to effects reported in temperate forest FACE experiments, but substantially stronger than those typically inferred from tree-ring studies. CO₂ effects were amplified in cooler years but showed little sensitivity to precipitation variability.

Using the simulated data, we then evaluated whether recommended statistical approaches for the detection of CO₂ effects in tree rings could recover the CO₂ signal obtained from the model simulations. We found that, despite its relatively strong magnitude, the CO₂ effect was difficult to detect reliably. Two out of four tried methods detected a CO₂ effect, but its presence and strength were strongly dependent on the statistical model assumptions.  

These results highlight the challenges of attributing CO₂ effects on tree growth in real-world observational data, which are subject to substantial noise and may exhibit weaker responses. Progress in detecting CO₂ effects may benefit from closer integration of simulation experiments and statistical inference to guide study design and interpretation.