With the pressing challenges of global change and the imperative for sustainable ecosystems, it becomes essential to ensure a balanced and efficient utilization of materials and energy within each ecosystem. Litter production and decomposition represent pivotal aspects of ecosystem dynamics, playing a crucial ecological role (Liu and Sun, 2013). The biophysical processes involved in litterfall contribute significantly to maintaining soil nutrient levels, enhancing primary productivity, regulating nutrient cycling and energy flow, and even elucidating litter response to global change (Waring and Schlesinger et al., 1985; Liu et al., 2024). Particularly in forest ecosystems, the litterfall process is instrumental in biogeochemical cycling of nutrients (Krishna and Mohan, 2017). Over half of the net primary productivity in forest ecosystems undergoes decomposition as litterfall, replenishing soil nutrients (Schlesinger, 1997). The litterfall process is influenced by various environmental factors (Piscart et al., 2011; Wang et al., 2015). Therefore, studying litterfall process and the influencing factors holds significant practical importance for forest ecosystem management amid global change.

As the world economy globalizes and the material needs of the human population worldwide continue to escalate, the scale and intensity of plantations have been expanding worldwide. However, increasing management intensity in plantations poses challenges in simultaneously practicing sustainability measures and ensuring quality assurance (Liu JQ et al., 2018; Singh et al., 2021). Hence, there arises a need to explore the diverse and crucial roles of plantation systems in material circulation, product supply, ecological environment enhancement, and response to global change. The sustainability of growing plantation systems is fundamental in achieving these goals. The rubber tree [Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg.] stands out as an economically significant crop in the tropical systems worldwide. Since its introduction in 1904, it has occupied 1,167,300 ha in southern China (Tang et al., 2016). Unfortunately, rubber monocultures have led to severe soil problems, including nutrient leaching, soil erosion, and compaction (Mann, 2009; Liu, 2014; Liu et al., 2015; Liu SR et al., 2018). Notably, soil total nitrogen (TN), available phosphorus, and total phosphorous (TP) degraded more rapidly in rubber monocultures than in rainforests, posing a threat to green development and biodiversity in tropical systems. This trend threatens green development and biodiversity in tropical systems (Sun et al., 2016; Jiang et al., 2017). Agroforestry ecosystems, or intercropping systems, represent sustainable land use patterns wherein agricultural and production systems harmonize to achieve efficient and sustainable utilization of materials and energy, thereby enhancing total productivity and economic benefits (Arisara et al., 2018). By leveraging ecological complementarity between different species, agroforestry systems can conserve water and soil and increase carbon storage (Ehrenbergerová et al., 2016; Weerasekara et al., 2016). Hence, some studies advocate for the establishment of rubber-based agroforestry systems using Hevea and other crops or tree species as an effective method to overcome the aforementioned obstacles (Wu et al., 2016; Qi et al., 2021).

Several studies have explored litterfall dynamics within rubber-based agroforestry ecosystems (Liu et al., 2017; Zhu et al., 2021). However, the challenges presented by rubber plantation ecosystems concerning soil quality degradation and their impact on global climate change remain unclear. Moreover, there is a notable gap in research regarding the influence of rubber-based agroforestry on litterfall composition and nutrient return patterns. We hypothesize that rubber intercropping with native trees can significantly change litterfall and litter main nutrient return. This study speculates that rubber-based agroforestry may alter litterfall composition, leading to increased litterfall and nutrient content in rubber plantations. Through the analysis of litterfall dynamics and its primary nutrient return mechanisms in rubber agroforestry systems, this research aims to provide insights crucial for sustainable development of rubber plantations.

The litterfall of rubber plantations has been a topic concern and study since the 1990s (Yang ZX et al., 1997; Chaudhuri et al., 2003). However, few studies have examined litterfall and its main nutrient return dynamics in rubber plantations from the perspective of the rubber-based agroforestry system. In this study, we observed that the litterfall dynamics of the Hevea monoculture system, Hevea–Michelia intercropping system, and Hevea–Mytilaria intercropping system showed seasonal fluctuations, with a peak period of concentrated litterfall occurring from February to March. In the rubber planting areas of Yunnan Province, China, the annual variation in litterfall from rubber trees in the high-altitude terrain areas displayed a unimodal characteristic, peaking around January. Conversely, in middle- and low-altitude terrain areas, the annual volume of litterfall in rubber trees showed a bimodal characteristic, with litter concentrated from January to early February and again in September during the period of concentrated rubber seed production (Ren et al., 1999; Jia, 2006; Zhou et al., 2017). Additionally, in middle-aged and old rubber plantations in the Xishuangbanna area of Yunnan Province, the annual variation in the litterfall volume exhibited bimodal characteristics (Zhou et al., 2017). The presence of two peaks in the annual variation in the litter volume in the middle- and low-altitude areas of Yunnan Province may be attributed to the abundant and efficient light and heat resources in the unique elevation and terrain, the prolific flowering and fruiting of rubber trees, and the abundance of withered fruits during ripening of rubber fruits.

In this study, leaf litter comprised the largest proportion of litterfall, with leaf litter and litter branches primarily determining the overall litterfall pattern across the three systems. Specifically, the litterfall in the Hevea monoculture system, Hevea–Michelia intercropping system, and Hevea–Mytilaria intercropping system accounted for 77.6%, 79.80%, and 88.25% of the total litterfall, respectively. Furthermore, the branch litterfall in these systems represented 13.72%, 11.27%, and 6.33% of the total litterfall, respectively. Some studies conducted in rubber plantations in Guangdong Province, China, also reported the litter composition. These studies revealed that rubber leaves, branches, and flowers and fruits constituted 91.2%, 7.3%, and 1.5% of the litter, respectively, with the total litter pattern being predominantly characterized by leaf and branch litterfall (Yang ZJ et al., 1997). Similarly, research on middle-aged rubber plantations in the Xishuangbanna area of Yunnan Province in China indicated that the proportion of leaves, branches, and flowers and fruits in the litter was 73.8%, 22.06%, and 5.5% of the total litter, respectively, with litter leaves and branches also dominating the total litterfall pattern (Zhou et al., 2017). These findings align with conclusions drawn from other studies on litterfall in tropical forests (Wan et al., 2015). Ren et al. (1999) conducted a study on the litter composition of rubber multilayer plantations comprising old rubber trees, Baccaurea ramiflora and Rauvolfia vomitoria, which had been growing for over 30 years. The total litterfall pattern in this context was dominated by litter leaves and littered flowers and fruits, which accounted for 52.48% and 33.09%, respectively. This outcome may be attributed to the actual plant composition and the age of tree multilayer plantations.

In this study, the total annual litterfall of the Hevea–Michelia intercropping system (27,309 kg ha⁻¹) and Hevea–Mytilaria intercropping system (34,477 kg ha⁻¹) was significantly higher than that of the Hevea monoculture system (22,364 kg ha⁻¹). Specifically, the annual litter content of the Hevea–Michelia intercropping system and Hevea–Mytilaria intercropping system was 1.22 times and 1.54 times that of the Hevea monoculture system, respectively. Furthermore, the annual litter content of the rubber mixed cropping model exceeded that of the southern subtropical broad-leaved forest in China (ranging from 7,000 kg ha^-1 to 11000 kg ha⁻¹) and the mountainous tropical rainforest of Hainan (ranging from 7,700 kg ha^-1 to 9,700 kg ha⁻¹), as well as that of the tropical rainforest in Malaysia in Southeast Asia (ranging from 7,500 kg ha⁻¹ to 10,200 kg ha⁻¹) (Lim, 1978; Lu and Liu, 1988; Tu et al., 1993; Yang ZX et al., 1997). Comparatively, the annual litter content of rubber monocropping (7,510 kg ha⁻¹) was higher than that of rubber cocoa hybrid (6,810 kg ha⁻¹) and rubber large-leaf jack pulling, although no significant difference was observed between the above monocropping and mixed species (Lu et al., 2021). This comparative analysis of annual litterfall highlights that the annual litterfall volume in the rubber Michelia/Mytilaria intercropping system not only surpasses that of the rubber Amomum villosum intercropping pattern but also exceeds the annual litterfall volume of the southern subtropical broad-leaved forest in China, the mountainous tropical rainforest in Hainan, and the tropical rainforest of Malaysia in Southeast Asia (Lim, 1978; Lu and Liu, 1988; Tu et al., 1993; Yang ZJ et al., 1997).

Rubber intercropping with native trees significantly changed the return of main litter nutrients. Specifically, compared to the Hevea monoculture system, the TN content of litterfall increased by 185 kg ha⁻¹ and 120 kg ha⁻¹ in the Hevea–Mytilaria and Hevea–Michelia intercropping systems, respectively, representing a 103.70% and 67.88% increase for the whole year. Similarly, the TP content in litterfall in the Hevea–Mytilaria intercropping systems and Hevea–Michelia intercropping system increased by 35 kg ha⁻¹ and 30 kg ha⁻¹, respectively, indicating a 159.15% and 122.79% increase for the whole year. Additionally, the TK content litterfall in the Hevea–Mytilaria intercropping system and Hevea–Michelia intercropping system litterfall increased by 170 kg ha⁻¹ and 139 kg ha⁻¹, respectively, representing a 139.46% and 96.27% increase for the whole year. In terms of the nutrient content of litter, TN was the highest, followed by TK, and the lowest was TP. Regarding the nutrient content of litter, TN exhibited the highest levels, followed by TK, with TP being the lowest. Concerning the annual growth rate of main litter nutrient return, the TP content of litter showed the most significant increase in the intercropping system, followed by TK and TN, respectively. Additionally, the nutrient content of litter in the three systems displayed a seasonal trend, particularly in February and March during spring. Some scholars have also observed dynamic monthly trends in the N, P, and K contents of leaf litter in young and old rubber plantations in Hainan, noting a decrease in these elements during autumn and winter, with enrichment occurring in winter and spring (Miao et al., 2012; Xue et al., 2022). The above-mentioned research shows that the intercropping production practice of rubber plantations has significantly increased the amount of litter on the rubber plantation surface. We believe that these abundant litters are expected to significantly improve the soil and water conservation capacity of the surface soil, thereby optimizing the surface ecological environment. More importantly, these returned rubber plantations have enriched litter nutrients, which can offset the input of artificial fertilizer to the rubber plantation to a certain extent, which is equivalent to reducing labor input and fertilizer input.

Some scholars tend to focus on the effects of rubber tree age on litter and nutrient return, while paying little attention to the influence of meteorological factors on litter and nutrient return in rubber plantations. The litterfall yield in rubber plantations in Hainan showed a “double-peak” dynamic change (Miao et al., 2012; Xue et al., 2022). In this study, we selected the monthly mean temperature, monthly maximum temperature, monthly minimum temperature, monthly average wind speed, monthly rainfall, and other meteorological and environmental factors to analyze their association with the litter and nutrient content of rubber plantations. We found that litterfall was significantly correlated with monthly average temperature, monthly minimum temperature, and monthly average wind speed, contributing 80.5%, 75.5%, 69.8%, and 69.6% to the total litterfall and its leaf, branch, and flower and fruit components, respectively. Moreover, the contribution rates of monthly average temperature, monthly minimum temperature, and monthly average wind speed factors to TN and TK content of the annual litterfall were 73.9% and 66.6%, respectively, indicating the predominant role of temperature and wind speed. Interestingly, the changes in litter composition and nutrient content in the three patterns, including the Hevea monoculture system, Hevea–Michelia intercropping system, and Hevea–Mytilaria intercropping system, were not significantly affected by monthly maximum temperature and precipitation. Previous studies have also indicated that average monthly temperature is the primary climatic factor influencing the leaf litter pattern in rubber plantations (Cao et al., 2011). Similar conclusions have been drawn in other forest types, where temperature and extreme wind speed profoundly affected litter yield and different components in monsoon evergreen broad-leaved forests and mixed coniferous and broad-leaved forests (Liu et al., 2024).

Based on the findings regarding the effect of intercropping rubber with native tree species on litterfall, our conclusions are summarized as follows:

(1) Rubber intercropping with native tree species significantly enhances litter production in rubber plantations. The increase is expected to effectively cover the rubber plantation surface, supplement the deficit in soil organic matter, and mitigate or reverse the soil degradation trend resulting from long-term rubber plantation operations.