SummaryA basic understanding of nutrition effects on the mechanisms involved in tree response to drought is essential under a future drier climate.A large-scale throughfall exclusion experiment was set up in Brazil to gain an insight into the effects of potassium (K) and sodium (Na) nutrition on tree structural and physiological adjustments to water deficit.Regardless of the water supply, K and Na supply greatly increased growth and leaf area index (LAI) of Eucalyptus grandis trees over the first 3 yr after planting. Excluding 37% of throughfall reduced above-ground biomass accumulation in the third year after planting for K-supplied trees only. E. grandis trees were scarcely sensitive to drought as a result of the utilization of water stored in deep soil layers after clear-cutting the previous plantation. Trees coped with water restriction through stomatal closure (isohydrodynamic behavior), osmotic adjustment and decrease in LAI. Additionally, droughted trees showed higher phloem sap sugar concentrations.K and Na supply increased maximum stomatal conductance, and the high water requirements of fertilized trees increased water stress during dry periods. Fertilization regimes should be revisited in a future drier climate in order to find the right balance between improving tree growth and limiting water shortage.

Eucalyptus grandis (W. Hill ex Maiden) leaf traits and tree growth were studied over 3 years after the establishment of two adjacent complete randomized block designs in southern Brazil. In a nitrogen (N) input experiment, a treatment with the application of 120 kg N ha(-1) was compared to a control treatment without N addition, and in a potassium (K) input experiment a control treatment without K addition was compared to a treatment with the application of 116 kg K ha(-1). Young leaves were tagged 9 months after planting to estimate the effect of N and K fertilizations on leaf lifespan. Leaf mass, specific leaf area and nutrient concentrations were measured on a composite sample per plot every 28 days until the last tagged leaf fell. Successive inventories, destructive sampling of trees and leaf litter fall collection made it possible to assess the effect of N and K fertilization on the dynamics of biomass accumulation in above-ground tree components. Whilst the effects of N fertilization on tree growth only occurred in the first 24 months after planting, K fertilization increased the above-ground net primary production from 4478 to 8737 g m(-2) over the first 36 months after planting. The average lifespan of tagged leaves was not modified by N addition but it increased from 111 to 149 days with K fertilization. The peak of leaf production occurred in the second year after planting (about 800 g m(-2) year(-1)) and was not significantly modified (P < 0.05) by N and K fertilizations. By contrast, K addition significantly increased the maximum leaf standing biomass from 292 to 528 g m(-2), mainly as a consequence of the increase in leaf lifespan. Potassium fertilization increased the stand biomass mainly through the enhancement in leaf area index (LAI) since growth efficiency (defined as the ratio between woody biomass production and LAI) was not significantly modified. A better understanding of the physiological processes governing the leaf lifespan is necessary to improve process-based models currently used in Eucalyptus plantations.

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Resumo -O objetivo deste trabalho foi avaliar o crescimento inicial da parte aérea e do sistema radicular, a nutrição mineral e a fixação biológica de N 2 (FBN Growth, nutrition and biological fixation of nitrogen in mixed-species plantations of eucalypt with leguminous treesAbstract -The objective of this work was to assess interactions between species on the above and belowground growth, nitrogen uptake and biological nitrogen fixation (BNF) in mixed stands of Eucalyptus grandis and native leguminous N 2 -fixing trees. A complete randomized block design was installed with seven treatments and three blocks. Within the lines of the E. grandis seedlings, native leguminous N 2 -fixing trees -Peltophorum dubium, Inga sp., Mimosa scabrella, Acacia polyphylla, Mimosa caesalpiniaefolia -and one exotic leguminous plant, Acacia mangium, were intercropped. E. grandis was also solely planted. Mimosa scabrella and A. mangium were the legume trees that presented the highest growth. Although E. grandis showed a lower growth when combined with M. scabrella, this mixed-species stand exhibited the highest biomass accumulation. Eucalyptus grandis fine root densities (FRD) were 6-20 times higher than the FRD of the leguminous species in the upper soil layer (0-10 cm) 24 months after planting. The FRD of M. scabrella and M. caesalpiniaefolia in the 30-50 cm soil layer was higher than in the 10-30 cm layer. The δ 15N values of M. scabrella indicated that 90% of stocked N is derived from BNF.