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Fine roots (FR) play a major role in the water and nutrient uptake of plants and contribute significantly to the carbon and nutrient cycles of ecosystems through their annual production and turnover. FR growth dynamics were studied to understand the endogenous and exogenous factors driving these processes in a 14-year-old plantation of rubber trees located in eastern Thailand. FR dynamics were observed using field rhizotrons from October 2007 to October 2009. This period covered two complete dry seasons (November to March) and two complete rainy seasons (April to October), allowing us to study the effect of rainfall seasonality on FR dynamics. Rainfall and its distribution during the two successive years showed strong differences with 1500 and 950 mm in 2008 and 2009, respectively. FR production (FRP) completely stopped during the dry seasons and resumed quickly after the first rains. During the rainy seasons, FRP and the daily root elongation rate (RER) were highly variable and exhibited strong annual variations with a total FRP of 139.8 and 40.4 mm-2 and an average RER of 0.16 and 0.12 cm day-1 in 2008 and 2009, respectively. The significant positive correlations found between FRP, RER, the appearance of new roots, and rainfall at monthly intervals revealed the impact of rainfall seasonality on FR dynamics. However, the rainfall patterns failed to explain the weekly variations of FR dynamics observed particularly during the rainy seasons. At this time step, FRP, RER, and the appearance of new FR were negatively correlated to the average soil matric potential measured at a depth of between 30 and 60 cm. In addition, our study revealed a significant negative correlation between FR dynamics and the monthly production of dry rubber. Consequently, latex harvesting might disturb carbon dynamics in the whole tree, far beyond the trunk where the tapping was performed. These results exhibit the impact of climatic conditions and tapping system in the carbon budget of rubber plantations. [ABSTRACT FROM AUTHOR]

Rubber tree (Hevea brasiliensis Müll. Arg.) radial growth dynamics were monitored with displacement sensors, together with latex production, to investigate three aspects of the dual production of latex and wood: (1) the usefulness of fine-scale dendrometric measurements as a physiological tool to detect water shortage through radial growth; (2) the dynamic aspects, both at the seasonal and at the multi-year scale, of the competition between latex and wood production; and (3) the spatial distribution of radial growth rates around the tapping cut. Radial growth of untapped control trees started with the onset of the rainy season and lasted until the onset of the dry season, ceasing completely during the driest period. Displacement sensors provided a sensitive means of detecting water shortage, with a clear correlation between diameter variations and changes in water availability (both daily evapotranspiration and monthly rainfall) over the whole annual cycle. However, the correlation was significantly disturbed in tapped trees. After resumption of tapping, the radial growth rate dropped sharply within two weeks and the effect persisted throughout the whole season, so that the cumulative growth of tapped trees was about half that of untapped trees, with the cumulative growth deficit reaching 80% for the period from mid-June to November. This long-known negative impact of tapping on growth was much stronger in the second year of tapping than in the first, whereas latex production increased significantly between the first and second year of tapping. The increased latex production, which could not be ascribed to climatic conditions, shows that the establishment of an artificial latex sink is a progressive, long-term process likely involving many aspects of metabolism. As expected, ethylene significantly increased latex production in both years; however, ethylene had no effect on the growth rates of tapped trees. Radial growth was differentially affected at different locations around the tapping cut, with growth rates significantly lower in the tapped panel than in the untapped panel, and higher above the cut than below the cut. Thus, caution is needed when deriving whole stem wood production from girth measurements at one location on the stem, especially from girth measurements made close to the tapping cut. This also provides new evidence for the location of the latex regeneration area in the tapped panel, below the cut. [ABSTRACT FROM PUBLISHER]