## 5 Conclusions  
The present study elucidated the effect of stand age on the seasonal and vertical distribution (0-60 cm) pattern of fine root biomass and soil available nutrients in rubber monoculture and rubber-_F. macrophilla_intercropping systems. The results showed that stand age, season, soil depth, and cropping systems strongly influenced fine root biomass and soil nutrient availability. Fine root biomass was higher under rubber monoculture and lowered under rubber-_F. macrophilla_intercropping systems. The fine root biomass peaked during the dry season across stand ages, implying that fine roots are positively related to seasons and soil nutrient availability. The nutrient concentration was higher under monoculture plantations to maximize the resource acquisition, which might over yield above-ground biomass in intercropping systems. Further, we provide evidence of stand age effects on soil P and K, which was highest under young plantation (12 years old) than mature plantation (25 years old). P and K peaked in the rainy season, while N peaked in the dry season. Demand for rubber trees on growth and latex production declines with the content of P and K under intercropping systems, particularly P under the mature rubber monoculture. Rubber-_F. macrophilla_intercropping systems will benefit from N-limited soil that significantly reduces the need for N fertilizers. Overall, improving ecological functions will reflect on above-ground and below-ground biomass. Studying and quantifying fine root dynamics are tedious and time-consuming but provide a better understanding of forest productivity.  
**Author Contributions:** Conceptualization, F.B.; methodology, F.B. and S.A.R.; software, F.B.; validation, F.B.; formal analysis, F.B., M.I. and D.B.; investigation, F.B. and S.A.R.; resources, S.A.R.; data curation, F.B., S.A.R. and J.S.; writing--original draft preparation, F.B. and S.A.R.; writing--review and editing, F.B., H.F.A., A.M., K.R.H. and H.A.S.; visualization, F.B., S.A.A. and M.H.B.K.; supervision, F.B. and M.S.; project administration, H.F.A. and S.A.A.; funding acquisition, S.A.R. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study was funded by the Key Program of CAS (KFZD-SW-312), the Natural Science Foundation of Yunnan Province (2016 FA047), and the Project of Xishuangbanna Science and Technology Bureau (200915, 201116). The authors would like to thank the Deanship of Scientific Research at Taif University for the Researchers Supporting Project (TURSP2020/110); Taif University, Taif, Saudi Arabia.  
**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors would like to acknowledge the support from the biogeochemistry Laboratory of the Xishuangbanna Tropical Botanical Garden for the soil elemental analysis, the authors express their gratitude to [PERSON] and [PERSON] for helping with field work and material preparation in the laboratory. We deeply thank [PERSON], [PERSON], and [PERSON], Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, Yunnan, China, for their encouragement.  
**Conflicts of Interest:** The authors declare no conflict of interest.