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

Abstract

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.

Intercrops improve the drought resistance of young rubber trees

“…The higher nutrient availability with the N fixing Pueraria intercrop reduced the need for explorative fine roots and accounted for the relatively low SRL, in agreement with other studies (Olsthoorn et al 1991;Ostonen et al 2007;Wang et al 2017). FRLD and SRL were similar to those for mature rubber trees grown in field conditions of northeast Thailand (Maeght et al 2015).…”

“…Jessy et al (2013) reported higher fine root production, turnover and carbon and nutrient recycling through fine roots when the trees were subject to water stress and nutrient stress. Maeght et al (2015) by monitoring the root system up to a depth of 4.5 m, showed that deep roots of rubber trees only extended when rainfall became scarce, which was interpreted as an adaptive response of trees to water stress. These results suggest that rubber tree root systems would have enough plasticity to develop deep roots in response to the competition for resources.…”

“…The fine root biomass (FRB) and fine root length density (FRLD) are the most common root traits measured in agronomical and ecological root studies (Hendrick and Pregitzer 1993). Specific root length (SRL), calculated as the ratio of root length to dry biomass and fine root diameter (FRD), are generally used as proxies for potential fine root growth (Espeleta and Donovan 2002;Wang et al 2014) and to characterize the trade-off in root construction at a community level (Roumet et al 2016); Quite a few studies have been carried out on the evaluation of fine root traits of rubber trees in field conditions (Devakumar et al 1999;Jessy et al 2010;Jessy et al 2013;Soong 1976;Maeght et al 2015). However, there is a need for more study of the potential for manipulating young rubber tree root systems through intercrop interactions to improve their drought resistance.…”

Stable carbon in soils under rubber tree (Hevea brasiliensis) agroforestry systems in the south of Bahia, Brazil

“…Thus, we can suggest that 20 Mg ha −1 of C represented rubber tree roots' contribution in C input to soil during the 35 years of this system's implantation and can explain partially the difference between the secondary forest in SOC stock up to 100 cm. A rubber tree has a well-developed root system with a high turnover rate, which is a significant source of soil organic matter [26]. The C-C 4 contribution in the rubber tree plantation, as a result of grasses in their understory, was relevant up to the first 40 cm depth and represented approximately 9% of SOC stocked up to 100 cm depth, and in pasture, this contribution was around 15%.…”

Small-Scale Forest Structure Influences Spatial Variability of Belowground Carbon Fluxes in a Mature Mediterranean Beech Forest

“…Despite the importance of fine roots in the ecological processes, our understanding on their dynamics is still limited [13,14]. Studies were mainly focused on the role of environmental drivers on FRP [15,16], evidencing the impact of FRP on SR, both within and among ecosystems [11,17,18]. Therefore, a better identification of drivers regulating belowground processes through FRP is essential for a correct estimation of the ecosystem C budget [19].…”