Two independent estimations of stand-level root respiration on clonal Eucalyptus stands in Congo: up scaling of direct measurements on roots versus the trenched-plot technique.

Root respiration at the level of a forest stand, an important component of ecosystem carbon balance, has been estimated in the past using various methods, most of them being indirect and relying on soil respiration measurements. On a 3-yr-old Eucalyptus stand in Congo-Brazzaville, a method involving the upscaling of direct measurements made on roots in situ, was compared with an independent approach using soil respiration measurements conducted on control and trenched plots (i.e. without living roots). The first estimation was based on the knowledge of root-diameter distribution and on a relationship between root diameter and specific respiration rates. The direct technique involving the upscaling of direct measurements on roots resulted in an estimation of 1.53 micromol m(-2) s(-1), c. 50% higher than the mean estimation obtained with the indirect technique (1.05 micromol m(-2) s(-1)). Monte-Carlo simulations showed that the results carried high uncertainty, but this uncertainty was no higher for the direct method than for the trenched-plot method. The reduction of the uncertainties on upscaled results requires more extensive knowledge of temperature sensitivity and more confidence and precision on the respiration rates and biomasses of fine roots.

The function of the superficial root mat in the biogeochemical cycles of nutrients in congolese eucalyptus plantations.

BACKGROUND AND AIMS: The importance of superficial root mats inside the forest floor for the nutrition of Amazonian rain forests has been extensively investigated. The present study was aimed at assessing the function of a root mat adherent to decomposing organic material observed in Eucalyptus plantations. METHODS: The development of the root mat was studied through micromorphological observations of thin litter sections, and the influence of soil microtopography and soil water repellency on root mat biomass was assessed in situ on an area of 5 m2. In addition, input-output budgets of nutrients within the forest floor were established from measurements of litterfall, dissolved nutrients in gravitational solutions, and forest floor nutrient contents. KEY FINDINGS: The amounts of nutrients released during litter decay in this ecosystem during the period of study were, on average, 46, 3, 4, 19 and 17 kg ha-1 year-1 for N, P, K, Ca and Mg, respectively. The simultaneous measurements of the chemical composition of throughfall solutions and leachates beneath the forest floor showed a very quick uptake of nutrients by the root mat during the decomposition processes. Indeed, the solutions did not become noticeably enriched in nutrients during their passage through the holorganic layer, despite large amounts of elements being released during litter decay. The root mat biomass decreased significantly during the dry season, and a preferential development in microdepressions at the soil surface was observed. A strong water repellency observed in these depressions might enhance the ability of the roots to take up water and nutrients during the dry periods. CONCLUSIONS: The root mat was active throughout the year to catch the flux of nutrients from the biodegradation of the forest floor, preventing the transfer of dissolved nutrients toward deeper soil horizons. This mechanism is involved in the successful adaptation of this Eucalyptus hybrid in areas covered by 'climacic' savannas in Congo.