Amazonian understory response to elevated CO2

The response of plants to increasing atmospheric CO₂ concentration depends on several factors such as life history of specific species, availability of water, nutrients and light, and the ecological context that the plants are found. Although several experiments with elevated CO₂ (eCO₂) have been done worldwide, none was performed in the Amazon forest understory focusing in a community growing naturally. The understory of the central Amazon is limited by both light and phosphorus. Understanding how such ecosystem responds to eCO₂ is important to foresee how the forest will function in the future. Also, quantifying the response of this forest compartment helps to constrain Ecosystem Models that compute carbon and water fluxes.

For this study, we used the open-top chamber (OTC) approach, with a CO₂ enrichment of +250 ppm above the ambient concentration. Eight OTC were installed (4 with ambient CO₂ and another 4 with eCO₂) in the understory of a natural forest in the Central Amazon, approximately 70 km from Manaus city. The eCO2 experiment started in November 2019 and, after 120 days, we quantified the average community response of the following photosynthetic parameters: light saturated carbon assimilation rate (A_sat), stomatal conductance (g_s), transpiration rate (E), intrinsic water use efficiency (iWUE), apparent quantum yield (Φ), light compensation point (LCP), maximum carboxylation capacity (V_cmax), maximum electron transport rate (J_max). After 240 days of treatment, we quantified mean individual leaf production and accumulated leaf production, leaf area (Lf_area). After 300 days, we quantified the increment in base diameter (BD), height (H_t) and relative growth rate (RGR).

Under eCO₂, we observed increases in A_sat (67%), J_max (19%), Φ (56%), and iWUE (78%), in agreement with the hypothesis that plants near the light compensation point respond strongly to eCO₂. We also detected an increase in Lf_area (51%) and BD (65%), indicating that the extra primary productivity was not allocated to growth in height, but to supporting more light intercepting organs (leaf and conducting tissues). No detectable changes were observed for the other variables.

Apart from the expected increase in assimilation rates, understory plants in Central Amazon responded positively to eCO₂ by increasing their ability to capture and use light (leaf size, Φ, and J_max). The increment in leaf area while maintaining E rates signifies that this forest compartment will increase its contribution to the whole forest water fluxes to the atmosphere. That might be related to the prevailing acquisitive strategy necessary for competing for phosphorus brought by water flow through plants. As a possible consequence, this forest might be less resistant to extreme drought associated with El Niño years.

Funding: Coordination for the Improvement of Higher Education Personnel - CAPES (grants 312589/2022-0) and São Paulo Research Foundation-FAPESP processes numbers (2022/07735-5) and (2015/02537-7).