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International audience; The expansion of rubber cultivation into drought prone areas calls for innovative management to increase the drought resistance of the trees. The competition for water exerted by an intercrop in the upper soil layers will likely stimulate the growth of young rubber tree roots into deeper soil layers where water availability is more stable. This study examined the effects of a legume (Pueraria phaseoloides) and a grass (Vetiveria zizanoides) intercrop, on the fine root traits of young rubber trees (Hevea brasiliensis Müll. Arg.) established along a toposequence covering a range of soil depths in northeast Thailand. Two plots with and without the intercrops were set up in a 3-year-old rubber plantation. Tree girth, mortality rate, nutrient content in the leaves, predawn leaf water potential, and soil water content profiles were monitored over four successive years. Fine root length density, specific root length, fine root biomass, and fine root diameter of the rubber trees were measured in the fourth year. In shallow soils, the trees with the legume intercrop had a higher growth rate, a higher leaf nutrient content, and a higher fine root length density in the deepest soil layers than the controls, supporting the hypothesis of an adaptive root response, increasing drought resistance. However, the trees with the grass intercrop did not show this effect. In deep soils, specific root length was highest without the intercrops, and the soil water profile and predawn leaf water potential suggested that trees with intercrops benefited from increased water extraction below 110 cm depth. We showed, for the first time, that rubber tree root traits can be manipulated through intercropping to improve drought resistance. However, our results suggest intercropping might not reduce risks of tree mortality caused by drought in the shallowest soils of the subhumid area of northeast Thailand.

Rubber plantations (Hevea brasiliensis) are expanding in areas with low soil fertility, long dry seasons, and high risks of soil erosion in rainy seasons. The introduction of a N2 fixing legume cover crop in the interrows of the tree plantation might reduce runoff, soil erosion and increase the availability of nutrients and the growth of young trees. This study aimed at quantifying over a four year period (2007-2010), the impacts of a legume cover crop (Pueraria phaseoloides) to N nutrition, water status, and growth of young rubber trees planted along a toposequence with contrasted soil depths in north-east of Thailand. The biomass production and N released by the legume reached 8 Mg.ha-1y-1 and 240 kg N. ha-1y-1 respectively. The N2 fixation rates of the legume averaged 81% and N transfer from the legume to the rubber tree was also high; an average of 58% of tree leaf N was derived from atmosphere. Both variables were not significantly different along the toposequence. At the bottom of the toposequence, the combined improvement of nitrogen and water status of the trees in the rubber tree/cover crop system resulted in doubling the tree girth at seven-year age. At that position, the root profiles and soil water dynamic suggested that the cover crop allowed the rubber tree roots to tap water from deep soil layers during severe drought periods. Conversely, at the top of the toposequence where water was not available at depth, the legume had negative impacts on tree ability to survive intense drought. It is concluded that improving N nutrition of young rubber trees in marginal area could affect their resilience to drought. In a context of climate change, the questions of where and how the best trade-off between N and water nutrition of crops could be achieved would concern more and more areas. (Texte intégral)