ESTIMATES OF FACTORS DIRECTLY RELATED TO FINE ROOT LONGEVITY USING A HIERARCHICAL BAYESIAN MODEL

Fine root longevity, measured using minirhizotrons, range from days to years (Hendrick & Pregitzer, 1992; Eissenstat et al., 2000). Although there are several hypotheses that relate to root tissue lifespan (Ryser, 1996), very few long-term studies have examined the factors that may be directly related to survivorship of individual roots. It is known that atmospheric CO2, which is the major greenhouse gas, directly affects plant photosynthesis and water use. As an important plant tissue that acquires water and nutrients, fine roots may limit the forest productivity by limiting plant absorptive capacity under the enriched atmospheric CO2 concentration. Moreover, the turnover time of fine roots, which are the major component of carbon input to the soil carbon pool, may respond to this enriched CO2 effect and thus have impact on belowground carbon balance. Free air CO2 enrichment (FACE) facilities enable research on the effects of elevated atmospheric CO2 concentrations over extended periods of time at ecosystem scale. By using a hierarchical Bayesian model with covariance variable estimates, we were able to identify this CO2 effect as well as several other covariates that correlate with fine root persistence. According to our result, enriched CO2 did not have an immediate effect on fine-root longevity; rather, it increased longevity with time over the 8.2-year study period. Furthermore, fine root longevity increased with soil depth, yet the effects of CO2-enrichment on longevity decreased with increasing depth. Coarser roots and roots grown in plots with higher N-mineralization rate had longer life spans. Nitrogen fertilization enhanced fine-root lifespan only in CO2-enriched plots.