Rubber-timber Tree Agroforests Produce Greater Litter and Enhance Litter Decomposition more than Rubber Monocultures in Southern Thailand

Background and Objectives: Rubber monoculture has replaced large areas of tropical rain forest across Southeast Asia and China, and its continued expansion is now recognized to be substantially greater than previous estimates, including within Key Biodiversity Areas. This intensive land use impairs nutrient cycling by disrupting litter production and decomposition, and is further associated with declining soil microbial diversity, depleted soil organic matter, and reduced water retention capacity, all of which threaten the long-term productivity of tropical agricultural landscapes. Agroforestry, which integrates timber or other tree species into rubber plantations, has been proposed as a more sustainable land-use alternative, and prior work has linked diversified rubber systems to higher soil organic matter, greater nutrient availability, improved soil physical quality, and higher carbon sequestration potential relative to monocultures. However, how rubber-timber intercropping specifically influences litter dynamics and nutrient cycling, particularly across geographically and climatically distinct regions, remains poorly understood. This study aimed to compare litter production and rubber leaf litter decomposition rates between rubber agroforests and monocultures in southern Thailand, and to identify the environmental and geographical factors underlying observed differences. Methodology: Litter production and rubber leaf litter decomposition were compared over one year across three sites of 20-year-old para rubber agroforest and monoculture farms in southern Thailand. Two sites were located on the eastern coast of peninsular Thailand, in the Chana district and Namnoi subdistricts of Songkhla province, and other was located on the western coast in Na Toh Ming subdistrict of Trang province, allowing comparison across geographically distinct monsoon regimes; annual rainfall during the study period was about 2,180 mm, with rainy-season timing differed between coasts. All sites were similar on para rubber density (375–468 individuals ha-1); intercropped timber species (Dipterocarpus alatus, Aquilaria crassna, and Hopea odorata) were 7–13 years old with a total basal area of 4–12 m² ha-1. Litterfall was collected monthly using 1 m² litter traps (15 per farm, 90 total), and rubber leaf litter decomposition was assessed using the litterbag technique across both the rainy and dry seasons. Environmental variables, including temperature, relative humidity, canopy cover, and soil moisture, were also measured at each site to help explain observed differences in litter dynamics between farm types. Main Results: Litter production was consistently higher in rubber agroforests than in rubber monocultures throughout the year. Total annual litter production was significantly different (p < 0.05) between rubber practices; 4.18 ± 0.35 Mg ha-1 yr-1 in agroforests, and 2.56 ± 0.34 Mg ha-1 yr-1 in monocultures. Leaf litter contributed approximately 80% of total litter production in both systems, and intercropped timber trees contributed roughly 50% more leaf litter to agroforests. In both systems, litter production was greater during the dry season than the rainy season (0.34 ± 0.31 vs. 0.22 ± 0.21 Mg ha-1 mo-1; p < 0.001), with farm type and season each having significant, independent effects and no significant interaction between them. Geographical variation in the timing of peak litter production was also observed: peak leaf litter production occurred in March at the eastern sites but as early as January–February at the western site, reflecting differences in the timing of the southwest and northeast monsoons as modulated by the Tenasserim mountain range—apparently the first documented evidence of such geographical variation in rubber leaf litterfall timing between the two coasts of southern Thailand. Rubber leaf litter also decomposed significantly (p < 0.001) faster in agroforests than in monocultures, with mean annual decomposition rates of 6.57 ± 0.45 g yr-1 versus 4.56 ± 0.74 g yr-1. The half-life of rubber leaf litter (t50%) was 0.11 ± 0.01 year in agroforests versus 0.15 ± 0.02 year in monocultures, while the time required for 95% mass loss (t95%) was 0.46 ± 0.03 versus 0.67 ± 0.11 year, indicating substantially faster nutrient return in agroforestry systems. Decomposition rates were markedly lower during the dry season than the rainy season in both systems; in agroforests, rainy-season decomposition (3.98 ± 0.64 g mo-1) was approximately four times higher than dry-season rates (0.99 ± 0.18 g mo-1). The difference in decomposition rates between farm types was significant during the rainy season (p < 0.01) but not the dry season (p > 0.05), suggesting that the favorable microclimate of agroforests is particularly important for decomposer activity during the wet period. Agroforests also had significantly higher in all environments than monoculture, for instant, relative humidity (86.61 ± 7.37% vs. 75.66 ± 8.97%), canopy cover (89.67 ± 7.96% vs. 59.97 ± 5.59%), and soil moisture content (3.90 ± 0.64% vs. 2.00 ± 0.86%). Conclusion: Rubber-timber agroforestry enhances both litter quantity and decomposition dynamics relative to rubber monoculture, providing greater and more consistent annual nutrient returns to the soil. The higher decomposition rates observed in agroforests are likely driven by more favorable microclimatic conditions, dense crown canopy, high relative humidity and soil moisture content—together with enhanced biological activity of soil microbes and macrofauna promoted by more diverse litter inputs. The greater litter cover in agroforests may also help reduce the impact of heavy rainfall on the soil surface, thereby mitigating erosion. Given the ongoing and often underestimated expansion of rubber plantations across Southeast Asia, these findings support the promotion of rubber-timber agroforestry as a more ecologically sustainable land-use practice than conventional rubber monoculture.