Philippe Deleporte

Is the simple auger coring method reliable for below-ground standing biomass estimation in Eucalyptus forest plantations?

BACKGROUND AND AIMS Despite their importance for plant production, estimations of below-ground biomass and its distribution in the soil are still difficult and time consuming, and no single reliable methodology is available for different root types. To identify the best method for root biomass estimations, four different methods, with labour requirements, were tested at the same location. METHODS The four methods, applied in a 6-year-old Eucalyptus plantation in Congo, were based on different soil sampling volumes: auger (8 cm in diameter), monolith (25 × 25 cm quadrate), half Voronoi trench (1·5 m(3)) and a full Voronoi trench (3 m(3)), chosen as the reference method. KEY RESULTS With the reference method (0-1m deep), fine-root biomass (FRB, diameter <2 mm) was estimated at 1·8 t ha(-1), medium-root biomass (MRB diameter 2-10 mm) at 2·0 t ha(-1), coarse-root biomass (CRB, diameter >10 mm) at 5·6 t ha(-1) and stump biomass at 6·8 t ha(-1). Total below-ground biomass was estimated at 16·2 t ha(-1) (root : shoot ratio equal to 0·23) for this 800 tree ha(-1) eucalypt plantation density. The density of FRB was very high (0·56 t ha(-1)) in the top soil horizon (0-3 cm layer) and decreased greatly (0·3 t ha(-1)) with depth (50-100 cm). Without labour requirement considerations, no significant differences were found between the four methods for FRB and MRB; however, CRB was better estimated by the half and full Voronoi trenches. When labour requirements were considered, the most effective method was auger coring for FRB, whereas the half and full Voronoi trenches were the most appropriate methods for MRB and CRB, respectively. CONCLUSIONS As CRB combined with stumps amounted to 78 % of total below-ground biomass, a full Voronoi trench is strongly recommended when estimating total standing root biomass. Conversely, for FRB estimation, auger coring is recommended with a design pattern accounting for the spatial variability of fine-root distribution.

Root elongation in tropical Eucalyptus plantations: effect of soil water content

Abstract• Sustainability of Eucalyptus plantations is often questioned in resource-limited environments, especially in areas characterized by soils with poor nutrient and water holding capacities. Yet, field-based observations of fine root dynamics in relation with the seasonality of rainfall are lacking.• This study was undertaken on two Eucalyptus stands planted in the Kouilou Region (south-western Congo), which is characterized by a four-month-long dry season. Fine root (less than 2 mm in diameter) dynamics were studied using rhizotron observations of root elongation in the field.• Fine root elongation rates displayed a seasonal variation in the two stands, with higher elongation rates during the rainy season than during the dry season. Positive and significant correlations were found between fine root elongation rates and soil water content at all depths, but a better correlation was found with soil water content in the deep soil horizon than in the superficial horizons.• These results suggest that the temporal variations in fine root elongation were related to the seasonality of rainfall, and they were probably associated with seasonal changes in tree water status, carbon assimilation and belowground allocation.Résumé• La question de la durabilité des plantations d’Eucalyptus est souvent posée dans les environnements où les ressources sont limitées, en particulier les zones où les sols ont une faible capacité à retenir l’eau et les nutriments et où la saison sèche est longue. Pourtant, les observations in situ de la dynamique racinaire en relation avec la saisonnalité des pluies sont inexistantes.• Cette étude a été réalisée dans deux plantations d’Eucalyptus de la région du Kouilou dans le sud ouest du Congo, qui est caractérisée par quatre mois de saison sèche. La dynamique des racines fines (moins de 2 mm de diamètre) a été étudiée à l’aide de rhizotrons permettant d’observer l’élongation racinaire au champ.• La vitesse d’élongation des racines fines montrait une variation saisonnière dans les deux plantations, avec des vitesses plus élevées en saison des pluies qu’en saison sèche. Des corrélations positives et significatives ont été trouvées entre la vitesse d’élongation des racines fines et la teneur en eau du sol à toutes les profondeurs, mais les meilleures corrélations ont été observées avec la teneur en eau des horizons profonds.• Cela suggère que les variations temporelles de l’élongation des racines fines sont reliées à la saisonnalité des précipitations, et qu’elles sont associées aux changements saisonniers d’état hydrique des arbres, d’assimilation carbonée et d’allocation vers les parties souterraines.

What factors influence the stem taper of Eucalyptus: growth, environmental conditions, or genetics?

Abstract• IntroductionStem taper equations have been widely used for volume estimation to varied top diameter limits or for biomass calculations. However, their main drawback is that specific calibration is often necessary for each species or clone, and accounting for genetic and environmental effects is often a challenge.• MethodsIn order to investigate this point, we decided to study the stem shape of several clones growing in Congo and to build an equation which should (1) be simple and have a good predictive quality, (2) have explicit parameters, and (3) be generic enough so as to be transposable to other species or to other eucalyptus clones. A sample of 1,623 trees, representing 16 genetically different clones selected for their contrasting growth patterns, was used.• ResultsEven though these clones had different stem shapes, we successfully developed a single equation using the triplet (height/diameter at breast height/age) for all the clones.• ConclusionsThe study also indicates that both growth environmental conditions and genetics have an impact on the stem shape, but that the genetic effect was fully realized through tree growth. The root mean square error for the over-bark diameter was 7 mm, allowing its use over the whole plantation area.

24-h variation in soil respiration after a long dry season in a Sudano-Sahelian region

Soil respiration is a major component of the global carbon cycle which links ecosystems and the atmosphere. To evaluate the reaction of soil respiration after wetting, during a dry period, soil respiration and associated environmental factors were measured over a 24-h period, during the dry season in North Cameroon after wetting the soil. Over 24-h, soil respiration rates followed a quadratic curve during the day coming close to linear at night, while soil temperature and moisture together explained at least 73 % of the variations during the 24-h observed. These soil respiration rates increased during the morning, peaked between 11h00 and 13h00 and then decreased gradually to the minimum around 06h00. These observations were used to propose a method for estimating mean daytime and nighttime soil respiration after wetting the soil. The method proposed in this study has the advantage of being based on a small number of measurements and is, therefore, easier to implement for monitoring 24-h soil respiration after the first rains following a long dry period.