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Assessing the impact of slash management on the Eucalyptus productivity : A modelling approach

Saint André L., Deleporte P., Gava J.L., Gonçalves J.L.M., Laclau J.P., Nzila J.D.D., Du Toit B., Ranger J.. 2004. In : by Borralho, N. ; Pereira, J.S. ; Marques, C. ; Coutinho, J. ; Madeira, M. and Tomé, M. (eds.), Proceedings o an IUFRO conference 11 a 15 de Outubro 2004, Aveiro, Portugal / IUFRO. Eucalyptus in a Changing World. s.l. : s.n., p. 274-282. IUFRO Congress on Eucalyptus in a Changing World, 2004-10-11/2004-10-15, Aveiro (Portugal).

Eucalyptus tropical plantations are often established on poor soils, characterised by low reserves of available nutrients. Intensive silviculture (very short rotations) is carried out, resulting in the removal of large quantities of biomass, and leading to high risks of soil nutrient deficiencies. Within this context, a Cifor's network has been established to evaluate the impact of soil and slash management practices on the productivity of plantations (Tiarks et al., 1998). The core treatments created a gradient in the nutrient capital of each site, ranging from low levels (where all above-ground organic residues were removed from the plot, SMTO), through various steps to high levels of nutrient capital (where additional organic residues were distributed on the ground, SMT3). The objective of this work was to consider the whole growth trend (and not only a single year) in the data analysis. The main idea was to quantify how the following relationships were modified by treatments: stand dominant height as a function of age, stand basal area increment as à function of stand dominant height increment, and tree height as a function of tree girth. The method consisted in fitting these models for each of the considered treatment and to test the parameter variations with the type of slash management. For the dominant height growth, the speed to reach the asymptote (parameter b) was the only varying parameter across treatments. The observed pattern suggests that the dominant height growth response is of type 1 (Snowdon 2002), indicating that the slash management regime can reduce the time required for the stand to reach a given stage of maturity. The stand basal area response differed between countries: slash management effects were found to be completely transmitted throughout the dominant height growth in the Brazilian site (medium fertility), whereas there was an additional effect to be modelled in the Congolese site (low fertility). A slight trend was found for the South-African site but it was nonsignificant. The height-girth relationship was sensitive to slash management treatments. For a given age, trees in SMTO treatment (all slash removed) were smaller and shorter than in the other treatments. An additional operation was performed on the Congolese site where we tested the influence of slash management on the biomass and nutrient allocation (allometric relationship between tree size and biomass/nutrient concentration of each compartment). A difference between treatments has been observed for leaves and dead-branches biomass equations. But, in both cases, this result is probably a dominance effect rather than a slash management effect. By contrast to the biomass equations, nutrient concentration relationships were in most cases different between treatments (significant effect of the slash management on model parameters). Treatment BLO showed, generally, a lower nutrient concentration whatever the considered compartment.

Mots-clés : eucalyptus; afrique du sud; congo; brésil

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