Trait-based functional ecology plays a key role for valuing biodiversity, based on the mechanistic link between species assembly and ecosystem-services. To evaluate the vulnerability of valuable biodiversity to human impacts, this study focused on assemblages of tree species with timber provisioning services and investigated the functional response of their utilitarian traits to forest exploitation. We first merged traditional knowledge of timber use, successional niche, and functional traits of tree species across forest biomes in Japan. Using leaf and wood traits of 514 tree species, we quantified macro-scale patterns of functional identity, richness, and divergence for the timber species and entire tree assemblages in Japan. At the 10-km grid cell level, we conducted a species loss simulation that assumed forest exploitation, in which later successional species were step-wisely removed from the potential tree assemblages. The simulation showed that species loss led to shifting functional identity and changes in functional richness and divergence. Such responses of functional structure were geographically associated with region-specific redundancy or complementarity in tree species assemblages. Specifically, the shift of functional identities was prominent in high latitudes with low species loss and low latitude with higher species loss. The utilitarian value of tree species in higher latitude forests was more closely associated with their ecological niche and functional uniqueness, which indicates vulnerability of trait space to species loss. The changes in trait space, which were associated with non-random species loss, were context-dependently influenced by climate conditions and species pool size. Our findings indicate that the insurance effect of biodiversity was a region-specific feature of forest biomes. We conclude that ethnobotany-informed trait ecology can elucidate the valuable biodiversity for human culture and its vulnerability to species loss; this information is essential for sustainable use of biodiversity resources in human society.
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