Rapid root closure after fire limits fine root responses to elevated atmospheric CO2 in a scrub oak ecosystem in central Florida, USA

Elevated atmospheric carbon dioxide (CO₂) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long-term CO₂ exposure. We investigated the effects of elevated CO₂ on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO₂ enrichment began immediately after a controlled burn, which simulated the natural disturbance that occurs in this system every 10–15 years. We hypothesized that (1) root abundance would increase in both treatments as the system recovered from fire; (2) elevated CO₂ would stimulate root growth; and (3) elevated CO₂ would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm⁻²) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO₂ significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO₂ stimulation was initially more pronounced in the top 10 cm and 38–49 cm below the soil surface. However, these responses completely disappeared during the third year of experimental treatment: elevated CO₂ had no effect on root abundance or on the depth distribution of fine roots during years 3–7. The results suggest that, within a few years following fire, fine roots in this scrub oak ecosystem reach closure, defined here as a dynamic equilibrium between production and mortality. These results further suggest that elevated CO₂ hastens root closure but does not affect maximum root abundance. Limitation of fine root growth by belowground resources – particularly nutrients in this nutrient-poor soil – may explain the transient response to elevated CO₂.