## 1 Introduction  
Tropical rainforests have high gross primary production (GPP) and huge biomass [1, 2]; therefore, they play an important role on carbon cycling at a global scale. Higher annual temperatures in tropical area leads to higher GPP; yet, higher net primary production (NPP) is not always observed [1]. Annu fine root production (FRP, g m\({}^{-2}\) year\({}^{-1}\)) contributes considerably to carbon cycling, representing10-60% total net primary production in tropical forests [2]. For instance, [PERSON] et al. [3] showed that the FRP in tropical forests is significantly higher than that in boreal forests. However, the authors found no clear relationship between FRP and mean annual temperature in forest ecosystems. Thus, FRP might not always be as high as expected in the tropics, with other factors, such as nutrient availability and forest structure impacting FRP. Alternatively, there might be an issue with the method used to estimate FRP. Many studies in tropical regions estimate FRP using the ingrowth core methods [4; 5]; however, this method underestimates FRP [6; 7; 8]. Therefore, we need to examine accuracy of this method in tropical forests. The ingrowth core method might underestimate FRP because of the way decomposed fine roots are treated in the cores during incubation period. Part of the fine roots that grow inside the cores die and decompose during the incubation period. Yet, most studies do not properly integrate these decomposed fine roots into FRP estimates. Recently, a new approach for estimating FRP using the ingrowth core method was proposed by [PERSON] and [PERSON] [9]. The authors suggest that fine root decomposition between times \(i\) and \(j\) may be obtained by assuming that the fine roots die continuously, with constant instantaneous mortality and disappear at an instantaneous decomposition, with a constant decomposition rate. The authors evaluated the decomposition rate of fine roots using litter-bag experiments in a temperate coniferous forest. They found that fine root production was 14% lower when excluding decomposed fine roots than the new method. [PERSON] et al. [10] also estimated FRP using the Osawa method and found that decomposed fine roots contributed to 33.3% of total FRP in a secondary Mongolian oak forest. Neglecting decomposed fine roots causes considerable underestimates of FRP in tropical rainforests, where high temperature and humidity cause high decomposition rates of root [11]. Thus, here, we set out to examine how decomposed fine roots contribute to FRP estimates in tropical forests. FRP is also underestimated because of the slow growth rate of root-free soil in ingrowth cores [7; 12]. For example, fine root biomass inside the cores was half that of the outside biomass after 3 years of installation in a boreal forest [7]. Thus, ingrowth cores should be set in the soil so that sufficient fine roots grow inside the cores. Furthermore, ingrowth cores should be sampled after more than 1 year of installation [7; 12]. Yet, the sampling period of some previous studies in tropical forests was much shorter, in the range of several months [4; 5; 13]. [PERSON] et al. [13] observed comparable root biomass inside the cores with that outside after 3 months of installation in a tropical plantation; thus, several months might be enough for fine roots to grow inside the cores in tropical rainforests. However, no studies have examined how long ingrowth cores should be installed. Thus, it is necessary to compare FRP estimates at different sampling intervals. FRP is often estimated as the difference in total fine root biomass inside the core 1 year after installation from that 2 years after installation in boreal and temperate forests, to eliminate the disturbance effect of installation [6; 7]. This method was called \"the differential method\" in the present study. [PERSON] and [PERSON] [5] also estimated FRP using the differential method. A different calculation method, termed \"the simple method\" in the present study, is often used in tropical rainforests [4; 5].