COS 31-6 - Response of tropical rainforest woody species to elevated CO2 using open-top chambers

Abstract: Anthropogenic activities have significantly increased atmospheric CO₂ over the last 170 years, driving global climate change. Understanding the response of plant communities to this elevated CO₂ (eCO₂) is of great importance because of the massive role that plants have in global carbon cycling, especially in tropical forests. Tropical forests store around 55% of the total aboveground carbon of all forests, yet there is great uncertainty around how these forests will respond to future CO₂ emissions. From experiments in temperate forests, we know the critical role soil microbial symbionts – mycorrhizal fungi and N-fixers ­– have in the response of plants to eCO₂. Plants associated with different microbial symbionts differ in their nutrient acquisition strategies and often respond differently to eCO₂, changing above and belowground biomass allocation. In future global change scenarios, species with a particular microbial symbionts may benefit disproportionately from elevated atmospheric CO₂, driving future changes in plant communities. To better understand how plant-microbe partnerships drive responses to eCO₂, we are evaluating the physiological and ecological responses of 6 woody species to eCO₂ using open-top chambers in the lowland tropical rainforest at La Selva Biological Station, Costa Rica. We built 4 chambers with ambient CO₂ and 4 eCO₂ chambers. During the day, CO₂ in the ambient chambers is on average 351±0.06 ppm, while CO₂ in elevated chambers is 654±0.17 ppm. The average temperature and humidity inside the chambers are 25.5 ºC and 88%, respectively, and there are no differences between CO₂ treatments. Of the physiological responses, stomatal conductance (g_s) was the variable that responded fastest to the treatments. After just 2 weeks of eCO₂, seedlings in the eCO₂ chambers showed a decrease, a pattern that is clearer for young leaves (control: 0.25±0.02 vs eCO₂: 0.11±0.01). We also observed that despite there being a slightly higher abundance of herbivorous insects in the eCO₂ treatment, there was a lower total percentage of herbivory, but a higher proportion of leaves with herbivore damage. These preliminary results are consistent with previous work that shows lower stomatal conductance and lower leaf quality for herbivores in response to eCO₂. Our results help to understand and improve predictions made about the response of tropical forests to future CO₂ emissions by providing a clearer emphasis on the role of microbial symbionts in this response.