Physiological mechanisms of facilitation and complementarity that contribute to overyielding in experimental forests

Changes in climate and land use are posing challenges for organisms at the edges of their geographic ranges. Plant diversity and community composition can alter microenvironments, potentially mitigating novel stressors associated with climate change. However, the mechanisms by which plants support the survival and growth of neighbors are still being uncovered. To conserve biodiversity and ecosystem functionality, it is essential to find ways to support vulnerable species in this rapidly changing world. In this study, we show that tree diversity and community composition can benefit the growth and survival of slower-growing tree species through mechanisms of facilitation and complementarity. We selected four slower-growing species with varying levels of shade tolerance to study in a tree diversity experiment: Quercus alba, Quercus rubra, Acer rubrum and Tilia americana. Throughout the growing season, surveys of phenology, herbivory, and disease along with physiological measurements were taken on these species in three diversity treatments: monocultures, angiosperm-gymnosperm bi-cultures, and diverse 12-species mixtures. The angiosperm-gymnosperm bi-cultures and 12-species mixtures facilitated the survival of slower-growing, shade-tolerant species by generating shade, thereby reducing light-induced stress. Complementarity of soil water usage in the diverse 12-species forests likely contributed to the maintenance of soil water availability that could support higher rates of photosynthesis and growth in the shade-tolerant species. We further found that diverse mixtures of trees created unique conditions that influenced species-specific dynamics of disease and herbivory, which can influence growth and survival. We observed patterns that support the concept of a tradeoff between resource investments in growth and survival in the four slower-growing tree species, but especially in the shade-tolerant T. americana. In facing the challenges of climate change and habitat degradation, we present a mechanistic basis for how tree diversity can increase the growth and survival of slower-growing species by facilitating the reduction in light-stress and contributing to complementarity of local resource usage.