Seasonal patterns of fine root production and turnover in a mature rubber tree (Hevea brasiliensis Müll. Arg.) stand- differentiation with soil depth and implications for soil carbon stocks

Fine root dynamics is a main driver of soil carbon stocks, particularly in tropical forests, yet major uncertainties still surround estimates of fine root production and turnover. This lack of knowledge is largely due to the fact that studying root dynamics in situ, particularly deep in the soil, remains highly challenging. We explored the interactions between fine root dynamics, soil depth, and rainfall in mature rubber trees (Hevea brasiliensis Müll. Arg.) exposed to sub-optimal edaphic and climatic conditions. A root observation access well was installed in northern Thailand to monitor root dynamics along a 4.5 m deep soil profile. Image-based measurements of root elongation and lifespan of individual roots were carried out at monthly intervals over 3 years. Soil depth was found to have a significant effect on root turnover. Surprisingly, root turnover increased with soil depth and root half-life was 16, 6–8, and only 4 months at 0.5, 1.0, 2.5, and 3.0 m deep, respectively (with the exception of roots at 4.5 m which had a half-life similar to that found between depths of 1.0 and 2.5 m). Within the first two meters of the soil profile, the highest rates of root emergence occurred about 3 months after the onset of the rainy season, while deeper in the soil, root emergence was not linked to the rainfall pattern. Root emergence was limited during leaf flushing (between March and May), particularly within the first two meters of the profile. Between soil depths of 0.5 and 2.0 m, root mortality appeared independent of variations in root emergence, but below 2.0 m, peaks in root emergence and death were synchronized. Shallow parts of the root system were more responsive to rainfall than their deeper counterparts. Increased root emergence in deep soil toward the onset of the dry season could correspond to a drought acclimation mechanism, with the relative importance of deep water capture increasing once rainfall ceased. The considerable soil depth regularly explored by fine roots, even though significantly less than in surface layers in terms of root length density and biomass, will impact strongly the evaluation of soil carbon stocks

Root-emitted volatile organic compounds: can they mediate belowground plant-plant interactions?

peer reviewedBackground Aboveground, plants release volatile organic compounds (VOCs) that act as chemical signals between neighbouring plants. It is now well documented that VOCs emitted by the roots in the plant rhizosphere also play important ecological roles in the soil ecosystem, notably in plant defence because they are involved in interactions between plants, phytophagous pests and organisms of the third trophic level. The roles played by root-emitted VOCs in between- and within-plant signalling, however, are still poorly documented in the scientific literature. Scope Given that (1) plants release volatile cues mediating plant-plant interactions aboveground, (2) roots can detect the chemical signals originating from their neighbours, and (3) roots release VOCs involved in biotic interactions belowground, the aim of this paper is to discuss the roles of VOCs in between- and within-plant signalling belowground. We also highlight the technical challenges associated with the analysis of root-emitted VOCs and the design of experiments targeting volatile-mediated root-root interactions. Conclusions We conclude that root-root interactions mediated by volatile cues deserve more research attention and that both the analytical tools and methods developed to study the ecological roles played by VOCs in interplant signalling aboveground can be adapted to focus on the roles played by root-emitted VOCs in between- and within-plant signalling

IJ_Rhizo : an open-source software to measure scanned images of root samples

This paper provides an overview of the measuring capabilities of IJ_Rhizo, an ImageJ macro that measures scanned images of washed root samples. IJ_Rhizo is open-source, platform-independent and offers a simple graphic user interface (GUI) for a main audience of non-programmer scientists. Being open-source based, it is also fully modifiable to accommodate the specific needs of the more computer-literate users. A comparison of IJ_Rhizo's performance with that of the widely used commercial package WinRHIZO (TM) is discussed. We compared IJ_Rhizo's performance with that of the commercial package WinRHIZO (TM) using two sets of images, one comprising test-line images, the second consisting of images of root samples collected in the field. IJ_Rhizo and WinRHIZO (TM) estimates were compared by means of correlation and regression analysis. IJ_Rhizo "Kimura" and WinRHIZO (TM) "Tennant" were the length estimates that were best linearly correlated with each other. Correlation between average root diameter estimates was weaker, due to the sensitivity of this parameter to thresholding and filtering of image background noise. Overall, IJ_Rhizo offers new opportunities for researchers who cannot afford the cost of commercial software packages to carry out automated measurement of scanned images of root samples, without sacrificing accuracy

Growth dynamics of fine Hevea brasiliensis roots along a 4.5-m soil profile

International audienceTo monitor root growth dynamics of rubber trees (RRIM 600) along a deep soil profile, a permanent access-well ~ 4.5 m deep was built and operated since the end of 2006. This facility allows the direct observation of root growth dynamics using so-called “root windows”, through which roots can be observed at regular time intervals. Using this access-well, we could estimate root length density and the decay of root tissues at various soil depths. Initial results were based on a 17-month observation. Some root incubations were also conducted for one year. Result from these experiments indicated that: A root growth peak occurred between May and June at a soil depths shallower than 150 cm, while it generally did not occur before November deeper in the soil profile; - Fine roots growing at all depths had a life expectancy of months rather than weeks and a large proportion survived for 12 to 17 months without showing any clear sign of senescence; - Fine root length density decreased by more than one order of magnitude from 5 to 50 cm while fine root biomass found below 100 cm still accounted for more than a third of the overall fine root biomass of the studied rubber trees; - The decay rate of dead root material at a soil depth of 400 cm was slow, with a half-life on the order of 21 months. Together with long fine roots' survival, this indicates that the residence time of carbon originating from fine rubber tree roots spans several years

Is impeded root growth related to the occurrence of rubber tree Trunk Phloem Necrosis (TPN)?: preliminary results from NE Thailand

Paper presented at IRRDB Annual Meeting, Siem Reap Cambodia, 12-16 November 2007This paper presents the results of a pilot project aimed at unravelling putative links between the occurrence of Trunk Phloem Necrosis (TPN) and impeded root system development in a small rubber tree (RRIM 600) plantation of NE Thailand. To investigate as many root parameters as possible, our methodology included a wide range of techniques: destructive sampling, root mapping, rhizolocation, architectural characterisation of coarse woody roots, and electrical measurements by capacitance and earth impedance. We found that under the environmental conditions that prevail in NE Thailand, there is no relationship between most of the root parameters we measured and TPN, at least within the first meter of the soil profile. This result contrasts with observations previously made in western Africa. However, we found that at a soil depth of ~25 cm where maximum soil bulk density is generally observed, Healthy (H) trees can grow roots of significantly higher Specific Root Length (SRL) than those affected by TPN. The next phase of this work which is part of a multidisciplinary programme on TPN, will therefore focus on root morphological descriptors such as SRL and on the analysis of the deeper parts of rubber tree root systems