Advancing our understanding of plant diversity-biological invasion relationships using imaging spectroscopy

Invasive plants can alter ecosystem composition, structure and function, which in turn may have significant impacts on plant diversity. Although the impacts of biological invasions on plant diversity have been studied in previous literature, results have been inconsistent and occasionally counterintuitive. The crux of the matter is that most of these studies have been fine- or local-scale experiments. But ecological inferences made at local scales are not usually generalizable to broad spatial scales, highlighting the critical need for large-scale studies. Remote sensing is one of the few viable means to achieve this goal due to its unique capability to map plant diversity and biological invasions at broad spatial scales. Particularly, imaging spectroscopy or hyperspectral imaging – which measures the reflected light from the Earth surface in many narrow, contiguous spectral bands ranging from visible to shortwave infrared – can capture several key ecosystem characteristics related to biodiversity and biological invasions with high level of accuracy. Here, we used imaging spectroscopy and in situ observations to determine the association between plant diversity and biological invasions in a naturally-assembled grassland subject to common management practices, such as prescribed fire. We focused on Lespedeza cuneata (hereafter L. cuneata), an invasive legume in grasslands of the U.S. Southern Great Plains. We used airborne hyperspectral data along with in situ observations to estimate functional traits and calculate functional diversity using Rao's Q index. We then assessed the associations between functional diversity and biological invasion with generalized additive models across different spatial scales – here, referring to dimensions of a sampling plot, not pixel size – ranging from 30 m × 30 m to 250 m × 250 m plots distributed across a 67 km2 study area. Three main findings emerged from our analyses. First, results obtained from in situ observations showed that L. cuneata invasion, in general, did not affect taxonomic diversity when expressed using the first three Hill numbers, including species richness, exponential Shannon entropy index, and inverse Simpson concentration index. Second, results from our remote sensing analyses suggested that, unlike taxonomic diversity, functional diversity was significantly affected by L. cuneata invasion, but the association between functional diversity and biological invasion was not always linear. The association between functional diversity and biological invasion was linear and positive across low rates of invasion (L. cuneata percent cover of 0–25%) but plateaued for moderate rates of invasion (L. cuneata percent cover of 25–35%). Third, the association between functional diversity and biological invasion was scale-dependent, and it was also influenced by time-since-fire. Overall, our experiment reveals that while the introduction of an invasive plant may minimally impact taxonomic diversity, its impact on functional characteristics and functional diversity can be significant, and therefore should not be neglected. Quantifying functional diversity and its association with biological invasion across large spatial extents based solely on field surveys is a daunting task; this study served as a showcase of how imaging spectroscopy can transform our understanding of biodiversity-biological invasion linkages in ways not previously possible and at spatial scales not previously achievable.