Species and phylogenetic lineages have evolved to differ in the way that they acquire and deploy resources, with consequences for their physiological, chemical and structural attributes, many of which can be detected using spectral reflectance form leaves. Recent technological advances for assessing optical properties of plants offer opportunities to detect functional traits of organisms and differentiate levels of biological organization across the tree of life. Here, we connect leaf-level full range spectral data (400–2400 nm) of leaves to the hierarchical organization of plant diversity within the oak genus (Quercus) using field and greenhouse experiments in which environmental factors and plant age are controlled. We show that spectral data significantly differentiate populations within a species and that spectral similarity is significantly associated with phylogenetic similarity among species. We further show that hyperspectral information allows more accurate classification of taxa than spectrally-derived traits, which by definition are of lower dimensionality. Finally, model accuracy increases at higher levels in the hierarchical organization of plant diversity, such that we are able to better distinguish clades than species or populations. This pattern supports an evolutionary explanation for the degree of optical differentiation among plants and demonstrates potential for remote detection of genetic and phylogenetic diversity.

Summary: Distinct survival strategies can result from trade‐offs in plant function under contrasting environments. Investment in drought resistance mechanisms can enhance survivorship but result in conservative growth. We tested the hypothesis that the widespread oaks (Quercus spp.) of the Americas exhibit an interspecific trade‐off between drought resistance and growth capacity. Using experimental water treatments, we isolated adaptive trait associations among species in relation to their broad climates of origin and tested for correlated evolution between plant functional responses to water availability and habitat. Across all lineages, oaks displayed plastic drought responses – typically acclimating through osmolyte accumulation in leaves and/or employing conservative growth. Oaks from xeric climates had higher osmolytes and reduced stomatal pore area index, which allows for moderated gas exchange and limits tissue loss. Patterns suggest drought resistance strategies are convergent and under strong adaptive pressure. Leaf habit, however, mediates the growth and drought resistance strategies of oaks. Deciduous species, and evergreen species from xeric climates, have increased drought tolerance through osmoregulation, which allows for continuous, conservative growth. Evergreen mesic species show limited drought resistance but could enhance growth under well‐watered conditions. Consequently, evergreen species from mesic environments are especially vulnerable to chronic drought and climate change.

We introduce a new “ecosystem‐scale” experiment at the Cedar Creek Ecosystem Science Reserve in central Minnesota, USA to test long‐term ecosystem consequences of tree diversity and composition. The experiment—the largest of its kind in North America—was designed to provide guidance on forest restoration efforts that will advance carbon sequestration goals and contribute to biodiversity conservation and sustainability. The new Forest and Biodiversity (FAB2) experiment uses native tree species in varying levels of species richness, phylogenetic diversity and functional diversity planted in 100 m² and 400 m² plots at 1 m spacing, appropriate for testing long‐term ecosystem consequences. FAB2 was designed and established in conjunction with a prior experiment (FAB1) in which the same set of 12 species was planted in 16 m² plots at 0.5 m spacing. Both are adjacent to the BioDIV prairie‐grassland diversity experiment, enabling comparative investigations of diversity and ecosystem function relationships between experimental grasslands and forests at different planting densities and plot sizes. Within the first 6 years, mortality in 400 m² monoculture plots was higher than in 100 m² plots. The highest mortality occurred in Tilia americana and Acer negundo monocultures, but mortality for both species decreased with increasing plot diversity. These results demonstrate the importance of forest diversity in reducing mortality in some species and point to potential mechanisms, including light and drought stress, that cause tree mortality in vulnerable monocultures. The experiment highlights challenges to maintaining monoculture and low‐diversity treatments in tree mixture experiments of large extent. FAB2 provides a long‐term platform to test the mechanisms and processes that contribute to forest stability and ecosystem productivity in changing environments. Its ecosystem‐scale design, and accompanying R package, are designed to discern species and lineage effects and multiple dimensions of diversity to inform restoration of ecosystem functions and services from forests. It also provides a platform for improving remote sensing approaches, including Uncrewed Aerial Vehicles (UAVs) equipped with LiDAR, multispectral and hyperspectral sensors, to complement ground‐based monitoring. We aim for the experiment to contribute to international efforts to monitor and manage forests in the face of global change.