Water availability drives fine root dynamics in a Eucalyptus woodland under elevated atmospheric CO2 concentration

Fine roots are a key component of carbon and nutrient dynamics in forest ecosystems. Rising atmospheric [CO2] (eCO2) 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 eCO2 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 eCO2 on their biomass, production, turnover and functional traits in a native mature grassy Eucalyptus woodland 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 eCO2 on fine root biomass, production or turnover across the 2-year sampling period, we found enhanced production pulses under eCO2 between 10- and 30-cm soil depth. In addition, eCO2 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 eCO2 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, eCO2 effects are small and transient. This also implies a limited ability of these systems to mitigate climate change through below-ground mechanisms. A free Plain Language Summary can be found within the Supporting Information of this article.