Fine root dynamics in a loblolly pine forest exposed to FACE: An eight year minirhizotron study

Efforts to characterize carbon cycling among atmosphere, forest canopy, and soil carbon pools are hindered by poorly quantified fine root dynamics. We characterized the influence of free air CO₂-enrichment (ambient + 200 ppm) on fine roots for a period of 8 years (Autumn 1998 through Autumn 2006) in an 18 year old loblolly pine (Pinus taeda) plantation near Durham, NC (USA) using minirhizotrons.  Root production and mortality were synchronous processes that peaked most years during spring and early summer.  Seasonality of fine root production and mortality was not influenced by atmospheric CO₂ availability.  Averaged over all six years of the study, CO₂ enrichment increased average fine root standing crop (+23%), annual root length production (+25%), and annual root length mortality (+36%).   Larger increase in mortality compared to production with CO₂ enrichment is explained by shorter average fine root lifespans in elevated plots compared to controls.  The effects of CO₂-enrichment on fine root proliferation tended to shift from shallow (0-15 cm) to deeper soil depths (15-30) with increasing duration of the study.  Mycorrhizal root tips were more numerous in FACE plots, but only during the summer and in the deeper soil category.  Fine root survivorship is compared to published estimates of mean fine root C residence time determined using isotopic techniques.