Water availability drives fine root dynamics in aEucalyptuswoodland under elevated atmospheric CO2concentration

Fine roots are a key component of carbon and nutrient dynamics in forest ecosystems. Rising atmospheric [CO2] (eCO(2)) is likely to alter the production and activity of fine roots, with important consequences for forest carbon storage. Yet empirical evidence of the role of eCO(2)in driving root dynamics is limited, particularly for grassy woodlands, an ecosystem type of global importance. We sampled fine roots across seasons over a 2-year period to examine the effects of eCO(2)on their biomass, production, turnover and functional traits in a native mature grassyEucalyptuswoodland in eastern Australia (EucFACE). Fine root biomass, production and turnover varied greatly through time, increasing as soil water content declined. Despite a lack of consistent effects of eCO(2)on fine root biomass, production or turnover across the 2-year sampling period, we found enhanced production pulses under eCO(2)between 10- and 30-cm soil depth. In addition, eCO(2)led to greater carbon and phosphorus concentrations in fine roots and increased root diameter, but no detectable effects on other morphological traits. Synthesis. We found minor quantitative effects of eCO(2)on fine root biomass dynamics that were largely driven by temporal variations in soil water availability. Our results suggest that in this mature grassy woodland, and perhaps also in other similar forested ecosystem types, eCO(2)effects are small and transient. This also implies a limited ability of these systems to mitigate climate change through below-ground mechanisms. A freePlain Language Summarycan be found within the Supporting Information of this article.