Diversifying tropical agroforestry: High-value understory crops in rubber and coconut systems

CONTEXT Tropical agriculture faces increasing challenges in sustaining production and rural livelihoods while addressing climate change, biodiversity loss, soil degradation, and resource limitations. Multi-strata agroforestry systems integrating perennial overstory crops with high-value understory species offer a promising pathway for climate-smart intensification and sustainable tropical agriculture. OBJECTIVE This review synthesizes evidence from multi-strata agroforestry systems combining major overstory crops, including Hevea brasiliensis , Cocos nucifera , and Areca catechu , with high-value understory crops such as spices, beverage, and medicinal plants. We develop a trait-based framework linking overstory characteristics, ecological interactions, management strategies, and ecosystem multifunctionality. METHODS A comprehensive literature synthesis was conducted using Web of Science Core Collection, Scopus, and Google Scholar, focusing on productivity, ecosystem services, resource-use efficiency, management practices, and socioeconomic outcomes. The evidence was organized around overstory–understory interactions, climate resilience, ecosystem services, productivity, and adoption challenges. RESULTS AND CONCLUSIONS Multi-strata agroforestry systems enhance carbon (C) storage, biodiversity, soil functions, and land-use efficiency compared with monocultures, with land equivalent ratios (LER) frequently exceeding 1.0. Reported evidence indicates that agroforestry systems can increase C storage by 20–50%, improve soil organic carbon (SOC) stocks by 10–20% compared with monocultures, and reduce midday air and soil temperatures by approximately 3–5 °C and 2–4 °C, respectively. High-value understory crops further enhance productivity and income diversification, with LER values commonly ranging from 1.2 to 1.8 and exceeding 2.0 in some shade-adapted crops. However, outcomes remain context-dependent due to competition for light, water, and nutrients, as well as potential pest and disease risks. Optimized performance depends on complementary species selection, canopy regulation, and adaptive soil fertility management. Key knowledge gaps remain in long-term productivity, C sequestration dynamics, resource competition thresholds, and socioeconomic barriers. SIGNIFICANCE Diversified overstory–understory agroforestry systems represent a nature-based solution for developing resilient tropical landscapes by integrating agricultural productivity, biodiversity conservation, ecosystem services, and climate change mitigation.