Worldwide, invasive alien plant species threaten the biodiversity and the functioning of ecosystems. Most invasion research so far has focused on the properties underlying species invasiveness and community invisibility. Yet invasion impact on ecosystem functioning, and the underlying causal pathways remain largely unknown. Here we dealt with this knowledge gap by extending the traditional functional trait framework to spectral data, by using traits estimated from reflectance, as a surrogate for conventionally measured traits. Our study thus proposes an interdisciplinary approach to “harness the ecological data revolution”. We focused on two functionally distinct species that are invasive in Belgium: the acquisitive, annual forb Impatiens glandulifera Royle, and the conservative, rhizomatous perennial forb Solidago gigantea Ait. Over a continuous gradient of invader cover, we quantified six ecosystem functions involved in the cycling of carbon and nutrients. We also measured a set of plant functional traits both conventionally, following standard protocols, and optically, from hyperspectral reflectance obtained through proximal field spectroscopy. We asked 1) which ecosystem functions are affected by the invaders; 2) via which trait-based mechanism – mass ratio or niche complementarity – they are mediating these changes; and 3) if the optically measured traits deliver the same ecological insights as conventionally measured traits. To answer these questions, we applied linear mixed models and structural equation models.
Results/Conclusions
Analyses based on either conventionally or optically measured traits revealed similar results: the invaders altered aboveground dry biomass (decrease and increase under I. glandulifera and S. gigantea respectively), litter stabilization (decrease under both invaders) and soil available phosphorus (increase under both invaders) through mass ratio effects, rather than through decreasing the functional diversity of the community. Whereas S. gigantea did so by shifting the community towards more conservative traits, I. glandulifera achieved this by making the community taller and richer in leaf nutrients. We found no significant invasion effects on soil nitrogen and organic carbon, and litter decomposition rate. The use of remote sensing through optically measured traits, is not only useful to advance our understanding of the mechanisms and consequences of plant invasion, but may also be valuable to the broader field linking plant community composition to ecosystem functioning. Its potential for studying larger spatial scales over time may contribute to even more comprehensive insights.