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

Abstract

Elevated atmospheric carbon dioxide (CO 2 ) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long-term CO 2 exposure. We investigated the effects of elevated CO 2 on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO 2 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 CO 2 would stimulate root growth; and (3) elevated CO 2 would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm À2 ) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO 2 significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO 2 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 CO 2 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 CO 2 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 CO 2 .

Root biomass and nutrient dynamics in a scrub-oak ecosystem under the influence of elevated atmospheric CO2

“…Our finding supports of Day et al (2006) and indicates that the root system may have reached the point where further investment in increased root length no longer conferred any advantage in increased resource acquisition. Day et al (2006) concluded that elevated CO 2 allowed the plant system to reach this point sooner than ambient CO 2 systems. Some other systems have exhibited more prolonged stimulation of CO 2 on root systems.…”

Environmental and biological controls on water and energy exchange in Florida scrub oak and pine flatwoods ecosystems

“…Roots below 0.5 m represent an important fraction in the 1 m root profile at the scrub oak site [ Dilustro et al , 2002; Day et al , 2006]. Deep roots explain the discrepancy between Et and water losses from the upper soil profile during the driest periods of the year in both ecosystems.…”

EFFECT OF ELEVATED CO2ON COARSE-ROOT BIOMASS IN FLORIDA SCRUB DETECTED BY GROUND-PENETRATING RADAR

“…However, many studies suggest changes in species dominance, resource availability and allocation, invasiveness, and, most importantly, alteration of ecosystem services (Ward and Strain 1999, Korner 2001, Bazzaz and Catovsky 2002, Poorter and Navas 2003. The allocation of carbon to the root system has implications for long-term sequestration and storage of excess atmospheric CO 2 in the rhizosphere (Pregitzer et al 1995, Johnston et al 2004, Day et al 2006). The allocation of carbon to the root system has implications for long-term sequestration and storage of excess atmospheric CO 2 in the rhizosphere (Pregitzer et al 1995, Johnston et al 2004, Day et al 2006).…”

“…However, as the community matured, the CO 2 treatment effect appears to have disappeared (Day et al 2006). In addition, we have suggested that this system has reached root closure (i.e., carrying capacity) in the fine root component of belowground biomass (Day et al 2006). We know little about effects on larger underground plant structures.…”