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

• Published in: Global change biology 2006-06, Vol.12 (6), p.1047-1053
• Main Authors: DAY, FRANK P, STOVER, DANIEL B, PAGEL, ALISHA L, HUNGATE, BRUCE A, DILUSTRO, JOHN J, HERBERT, BRANDON T, DRAKE, BERT G, HINKLE, CHARLES R
• Format: Article
• Language: English
• Subjects: Atmospheric pressure; Carbon dioxide; carbon dioxide enrichment; Ecosystems; fine roots; fire; minirhizotrons; mortality; nutrients; Plant growth; root closure; root growth; root length; root systems; shrublands; soil
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/j.1365-2486.2006.01148.x

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.