Invasion timing affects multiple scales, metrics, and facets of biodiversity outcomes in ecological restoration experiments

The need to develop optimal restoration protocols for degraded grasslands has led to experimental research aimed at determining how different restoration treatments influence outcomes for biodiversity. The magnitude and direction of diversity responses to restoration treatments may depend on the spatial scale at which diversity is measured (local, regional), the metric of diversity used (Hill numbers), and the facet of diversity (taxonomic, functional, phylogenetic) considered. We conducted a long-term factorial experiment in a degraded grassland in Missouri, USA, in which we experimentally applied a regionally appropriate biodiverse seed mixture, added soil nutrients to restore soil fertility, and varied the timing in which the invasive plant Lespedeza cuneata entered the community. We used a unified framework of Hill numbers to evaluate how treatments influenced diversity, considering different spatial scales, metrics, and facets (taxonomic, phylogenetic, functional). We find that the timing in which the invasive L. cuneata entered the community had large effects on diversity, while nutrient addition had more limited effects. This was driven by the high dominance of the focal invasive when allowed to invade early in the growing season, suppressing diversity. The positive effects of late invasion increased in magnitude with spatial grain and were higher for taxonomic than phylogenetic and functional facets of diversity. This was largely due to the dominance of the focal invasive, negatively affecting diversity within specific plant families or functional phenotypes across treatments. Under early invasion, nutrients had a negative effect, particularly at local scales, inflating beta diversity in this treatment and resulting in negative to no effect of late invasion on many aspects of beta diversity. Our results demonstrate the importance of looking at a multitude of different measures of diversity to understand the relative effects of ecological restoration treatments combined with invasion timing. Efforts to keep noxious plant invaders out of a system early in restoration approaches better allow desirable, native plants to establish and can have long-term benefits for multiple aspects of diversity.