## 3 Discussion
### Effect of Stand Ages  
Stand age significantly affected the total fine root biomass and nutrients (except N) under young stands (12 years old) of rubber monoculture and rubber-_F. macrophylla_ intercropping system compared to the mature stands (15 and 24 years old). Similarly, [PERSON]'s [41] findings supported our results that the increased fine root biomass under rubber monoculture decreases with stand ages. In contrast, young rubber plantation stands use more water than mature plantation stands, thus producing higher fine root biomass. [PERSON] et al. [42] consistently found that fine root biomass increased within the age of 7-year-old and 17-year-old stands and then decreased in the 25-year-old Chinese Fir plantation. In contrast, [PERSON] et al. [43] found that fine root biomass increased with stand age in high species evenness in a mixture of plantations dominated by two boreal tree species. In the present study, higher fine root biomass under young stand age (Figure 1) can be attributed to increasing N and altered species composition and biomass across stand age [25; 44]. A more significant proportion of fine roots in the young stand is an efficient strategy to enhance the absorption of available nutrients by exploring large soil volumes and enabling stand recovery [45; 46]. The significant effect of stand age and seasons suggest that environmental variability between different ages and seasons creates distinct effects on the soil N, P, and K concentrations. Soil N contents did not differ significantly between cropping systems. However, N contents significantly varied between stand ages, i.e., higher N under the mature stands than in young stands (Table 3). The previous inconsistent study [47] also found high N concentration under mature stand (20 years old) of _Gliricidia_-based intercropping systems. Consequently, lower fine root biomass is needed for nutrient uptake in mature age than in young age, and the above-ground/below-ground biomass ratio decreases with tree age [48]. On the other hand, decreasing P and K concentrations under mature stands of intercropping systems could be attributed to a high amount of P loss from consecutive latex tapping of mature rubber trees along with soil runoff and erosions over time. Soilavailable K is mostly easily leached by runoff; hence, K's accessible by fine roots is lower than the N or P [49; 50]. These results suggest that total P and K gradually deplete with plantation stand age. Thus, P and K fertilizers might be necessary to maintain rubber latex productivity under mature rubber stands [29]. Generally, the reason for lower fine root biomass under mature rubber stands could be the lower soil fertility (available P and K), and rubber trees may compromise the capability to sustain biomass during the dry season. We found a clear declining trend in the soil available P and K with the increasing stand age; however, we did not find any apparent trends in available N across stand ages. In contrast, previous studies observed increasing soil N concentrations with the increase in stand age [46; 51]. The fine root biomass was greater during the dry season (Figure 1), although peaking was expected during the rainy season when rubber trees were physiologically active in capturing nutrients [52]. This contrasts with many other previous observations from northern savannah, Australia [53], California, USA [54], and boreal forest [8] where the maximum biomass was found during the rainy season. Our result is similar to temperate forest studies, wherein standing root biomass peaked during the dry season when roots grew throughout the winter in the absence of leaves when the trees are physiologically inactive [55]. The maximum fine root biomass was observed during the dry season in some tropical forests, such as central Sulawesi, Indonesia, and the eastern Amazon, Brazil [56]. Rubber-_F. macrophilla_ intercropping systems will be enormously beneficial under N-limited soil like rubber-grown soil, reducing the need for N fertilizers. This is consistent with the study of [PERSON] et al.