Jennifer Withington, Marc Goebel, Bartosz Bulaj, Jacek Oleksyn, Peter Reich, David Eissenstat

Long-term minirhizotron observations of absorptive fine roots provide insights into seasonal patterns of belowground absorptive fine root production and carbon dynamics. Our objective was to understand these dynamics over time and report about the phenology and growth patterns of absorptive fine root for 11, mature temperate trees species: five evergreen and six deciduous. We analyzed the timing and growth on absorptive, 1^st- and 2^nd-order roots in minirhizotron images down to a vertical depth of maximal 35 cm, as well as monthly and total annual length production, and related total annual precipitation of the actual and previous year of root production to investigate the impact of large fluctuations in total annual precipitation. Three years of our observation period had below-average rainfall. The main or largest peak of annual fine root production occurred between June and September for almost all species and years. In most years, when peaks occurred, the timing of peak root production was synchronized across all species. A linear mixed model revealed significant differences in monthly fine root length production across species in certain years (species x year, P < 0.0001), which was strongly influenced by three tree species. Total annual root production was much higher in 2000-2002, with above average rainfall the previous year, versus the data collected in 2005-2007, which followed years of lower than average rainfall (2003-2006): this latter period experienced a drop of 75% or more in average root production for all species. From our linear mixed model, total annual root length production was associated with the actual year’s (P = 0.003) and previous year’s precipitation (P < 0.001), but only the effect of previous year precipitation on RLP depended on species (interaction species * previous year annual precipitation, P < 0.014.