Fire, hurricane and carbon dioxide: effects on net primary production of a subtropical woodland

Abstract

Summary Disturbance affects most terrestrial ecosystems and has the potential to shape their responses to chronic environmental change. Scrub‐oak vegetation regenerating from fire disturbance in subtropical Florida was exposed to experimentally elevated carbon dioxide (CO2) concentration (+350 μl l−1) using open‐top chambers for 11 yr, punctuated by hurricane disturbance in year 8. Here, we report the effects of elevated CO2 on aboveground and belowground net primary productivity (NPP) and nitrogen (N) cycling during this experiment. The stimulation of NPP and N uptake by elevated CO2 peaked within 2 yr after disturbance by fire and hurricane, when soil nutrient availability was high. The stimulation subsequently declined and disappeared, coincident with low soil nutrient availability and with a CO2‐induced reduction in the N concentration of oak stems. These findings show that strong growth responses to elevated CO2 can be transient, are consistent with a progressively limited response to elevated CO2 interrupted by disturbance, and illustrate the importance of biogeochemical responses to extreme events in modulating ecosystem responses to global environmental change.

Decadal biomass increment in early secondary succession woody ecosystems is increased by CO2 enrichment

“…Litterfall estimated as biomass multiplied by C turnover rate. Turnover rate measured using an isotopic tracer approach.for details seeTalhelm et al 35 Norby et al 13 McCarthy et al 15 Hungate et al 16,69 *Accounts for mortality such that mortality is not included in this estimate and the minimum NPP for this variable is zero…”

Mycorrhizal association as a primary control of the CO 2 fertilization effect

“…Moreover, because ECM fungi, unlike AM fungi, produce extracellular enzymes that degrade organic N compounds (27), increased allocation to ECM fungi under eCO 2 may supply host plants with the N needed to sustain their growth response to eCO 2 . This may explain why eCO 2 often stimulates priming effects in ECM-dominated ecosystems (28,29). Second, differences in litter quality between ECM and AM plants may influence how much N is available to be primed or decomposed.…”

Nitrogen inputs and losses in response to chronic CO<sub>2</sub> exposure in a subtropical oak woodland

“…3). This disturbance also preceded a large positive response of oak production to elevated CO 2 found both above-and belowground (Day et al, 2013;Hungate et al, 2013a), as well as the disappearance of the CO 2 suppression of G. elliottii growth and N 2 fixation (Fig. 1).…”

“…The absence of any significant effects of CO 2 on nonsymbiotic N 2 fixation (see below) provides some protection from the dangers of false inference caused by pseudoreplication (Hurlbert, 1984). We assessed effects of N 2 fixation as a function of time since disturbance by calculating time since disturbance as the number of years that had elapsed between the date of the measurement and the most recent disturbance, whether by fire at the beginning of the experiment or by hurricane in September 2004 (Hungate et al, 2013a). We expressed the effect of elevated CO 2 on N 2 fixation as the absolute difference between elevated and ambient CO 2 plots.…”