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.