Coupling fine root dynamics with ecosystem carbon cycling in black spruce forests of interior Alaska

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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 CO2 fluxes. Fine root production, measured over a two-year period using minirhizotrons, varied from 0.004 ± 0.001 mm·cm -2·d-1 over winter, to 0.051 ± 0.015 mm·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 ± 0.31, 8.01 ± 1.39, and 2.53 ± 0.27 mm·cm-2·yr-1 (means ± 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 (Φtime). 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 (Φcohort), and risk of mortality decreased with root age (Φage). Survival estimates translate to mean life spans of 108 ± 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 ± 0.01 per month, while decomposition rates over winter averaged 0.73 ± 0.01 per winter. These growing season estimates translate to 49 ± 2 d from the time a root was first observed as dead, to the time it disappeared. For roots that decomposed during the growing season, those with longer life spans decomposed more slowly after death. Comparing these results with other minirhizotron studies suggests that life-history traits of black spruce first-order roots are similar to those from temperate (and perhaps most) forest ecosystems. Annual production of fine roots averaged 228 ± 75 g biomass·m-2·yr-1, constituting ∼56% of total stand production. Aboveground production of trees (50 ± 14 g biomass·m-2·yr-1, 13%) and shrubs (40 ± 2 g biomass·m-2·yr -1, 11%) contributed similarly to total production, while mosses (73 ± 14 g biomass·m-2·yr-1, 20%) accounted for the largest component of aboveground production. Soil temperature had a strong control over both soil respiration (Q10 = 2.21 ± 0.31) and root respiration (Q10 = 2.30 ± 0.37). During the growing season (15 May to 15 September), ∼56% of soil CO2 efflux (580 ± 40 g C/m2) was derived from fine root respiration (329 ± 54 g C/m2). Although apparent rates of heterotrophic respiration (May through September) and total production did not differ, definitive estimates of net ecosystem production are impossible given the potentially large, unmeasured components of NPP (net primary production), such as root exudation and mycorrhizal production. Nevertheless, rates of fine root production, mortality, and decomposition indicate that in these black spruce ecosystems, fine roots are much more dynamic than would be predicted from patterns of aboveground processes, and that carbon, and presumably nutrients, are cycling through fine roots at rates several orders of magnitude faster than through aboveground tissues.

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Ecological Monographs