Coupling Fine Root Dynamics with Ecosystem Carbon Cycling in Black Spruce Forests of Interior Alaska

Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs, and mosses were measured relative to soil CO2fluxes. Fine root production, measured over a two-year period using minirhizotrons, varied from$0.004 \pm 0.001 ram\cdotcm^{-2}\cdot d^{-1}$over winter, to$0.051 \pm 0.015 mm\cdot cm^{-2}d^{-1}$during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. Monthly rates of production and mortality were somewhat asynchronous because mortality tended to peak during fall and be minimal during periods of peak production. Production and mortality were, however, positively correlated across all tubes and time periods. Annual fine root production averaged$2.45 \pm 0.31$,$8.01 \pm 1.39$, and$2.53 \pm 0.27 mm\cdot cm^{-2}\cdot yr{-1}$($means \pm 1 se$) among the three sites, when averaged across years. Fine root survival and decomposition were measured by tracking and analyzing the fate of individual fine roots using mark-recapture techniques. Fine root survival was greatest during periods of peak root growth, and least over winter ($\phi_{time}$). Roots first appearing in the middle of the growing season had higher survival rates than those first appearing early or late in the growing season, or over winter ($\phi _{cohort}$), and risk of mortality decreased with root age ($\phi_{age}$). Survival estimates translate to mean life spans of$108 \pm 4 d$during the growing season. While these values are in striking contrast to needle longevity and rates of aboveground litter decomposition, they are similar to many values found for temperate systems, supporting the notion that there are basic morphological and physiological traits of first-order roots that are common to most woody plant root systems. During the growing season, monthly fine root decomposition rates averaged$0.46 \pm 0.01$per mon