Abstract
<jats:p>Conventional forest tree breeding has largely focused on species that perform well in monoculture systems, where genetic evaluation, seed production, and management can be optimized under uniform conditions. While this approach has delivered substantial genetic gains, it does not adequately address the growing demand for forests that function as diverse, climate resilient ecosystems. Most tropical tree species occur native in species diverse ecosystems, where species have co-evolved under various degrees symbiotic interactions from mutualism to commensalism. It is reflected in plantings, where many of the species therefore grow best/only in mixed species stands, where ecological interactions shape their performance, adaptability, and long-term survival. Yet breeding strategies specifically designed for such mixed systems remain largely unexplored. While the concept of mixed-species evolutionary breeding has been explored in crop systems, comparable approaches remain largely unexplored in tree breeding. This is paradoxical, given that the genetic basis of interspecific interactions is likely to be particularly important for tree species that exhibit reduced fitness in monocultures. Rather than exploiting the genetics underlying species interactions, conventional tree breeding has largely focused on selecting individual genotypes that perform best under single-species conditions. In response to this discrepancy, we propose a shift toward designing and selecting seed sources based on their performance, ecological function, and economic value within relevant species mixtures rather than in single-species systems. We advocate breeding programs that maintain high levels of genetic diversity in planting material to enhance the resilience and adaptive capacity of restored forests, while simultaneously providing a cost-effective framework capable of accommodating multiple tree species. Such an approach reflects the growing recognition that future forestry and restoration efforts will increasingly rely on diverse, mixed-species plantings to deliver resilient and multifunctional forest ecosystems. Based on these considerations, we propose the novel concept of Multi-species Breeding Seed Orchards (MBSOs) as a transformative approach to tropical tree improvement. Unlike traditional single-species orchards, MBSOs are designed to mimic natural forest dynamics while enabling systematic genetic improvement, and to serve as a reliable source of genetically improved seeds for diverse applications including plantation forestry, landscape restoration, agroforestry, and broader tree planting and climate-smart forestry initiatives. By integrating ecological principles with quantitative and genomic breeding methods, this concept seeks to reconcile productivity, biodiversity, and resilience within a single breeding framework. We discuss the scientific and operational challenges including species compatibility, reproductive synchrony, spatial design and competition management, maintenance of genetic integrity within and across species, and the complexity of estimating genetic parameters such as heritability and breeding values in mixed species contexts. We critically examine these biological, genetic, and design constraints and discuss emerging methodological and analytical solutions to overcome them. Finally, we present the conceptual design and layout of a planned MBSO to be established at Karura forest in Nairobi, Kenya, envisioned as the first breeding seed orchard of its kind. This novel initiative will serve as both a research platform and a demonstration site, advancing the science of breeding trees for mixed species systems and redefining the future of tropical tree improvement under the pressures of climate change and the urgent need for tree plantings as part of a sustainable green transition toward biobased and resilient economies.</jats:p>