Forest plant indicator values for moisture reflect atmospheric vapour pressure deficit rather than soil water content.

Soil moisture shapes ecological patterns and processes, but it is difficult to continuously measure soil moisture variability across the landscape. To overcome these limitations, soil moisture is often bioindicated using community-weighted means of the Ellenberg indicator values of vascular plant species. However, the ecology and distribution of plant species reflect soil water supply as well as atmospheric water demand. Therefore, we hypothesized that Ellenberg moisture values can also reflect atmospheric water demand expressed as a vapour pressure deficit (VPD). To test this hypothesis, we disentangled the relationships among soil water content, atmospheric vapour pressure deficit, and Ellenberg moisture values in the understory plant communities of temperate broadleaved forests in central Europe. Ellenberg moisture values reflected atmospheric VPD rather than soil water content consistently across local, landscape, and regional spatial scales, regardless of vegetation plot size, depth as well as method of soil moisture measurement. Using in situ microclimate measurements, we discovered that forest plant indicator values for moisture reflect an atmospheric VPD rather than soil water content. Many ecological patterns and processes correlated with Ellenberg moisture values and previously attributed to soil water supply are thus more likely driven by atmospheric water demand.

Nutrient-demanding species face less negative competition and plant-soil feedback effects in a nutrient-rich environment.

Plant-soil feedbacks (PSFs) and plant-plant competition influence performance and abundance of plants. To what extent the two biotic interactions are interrelated and thus affect plant performance in combination rather than in isolation remains poorly explored. It is also unclear how the abiotic context, such as resource availability, modifies individual and joint effects of PSFs and of plant-plant competition. Using a garden experiment, we assessed the strengths of PSFs, competition, and their combined effects explored under low and high nutrient levels, and related them to abundance of 46 plant species and their ecological optima with respect to soil nutrients. We found that PSFs reduced but did not eliminate differences in competitive ability of plant species. Isolated and combined effects of the biotic interactions poorly predicted local or regional abundance of species. They were rather related to species' ecological optima, as nutrient-demanding plants experienced less negative biotic effects but only in a nutrient-rich environment. Our study demonstrates that soil biota can mitigate differences in competitive ability among species. It remains to be tested whether such an equalizing effect can maintain coexistence under high nutrient availability, in which nutrient-demanding species may disproportionately benefit from less negative competition and PSF effects.

The impact of an invasive plant changes over time.

Many exotic plant invaders pose a serious threat to native communities, but little is known about the dynamics of their impacts over time. In this study, we explored the impact of an invasive plant Heracleum mantegazzianum (giant hogweed) at 24 grassland sites invaded for different periods of time (from 11 to 48 years). Native species' richness and productivity were initially reduced by hogweed invasion but tended to recover after ~30 years of hogweed residence at the sites. Hogweed cover declined over the whole period assessed. A complementary common garden experiment suggested that the dynamics observed in the field were due to a negative plant-soil feedback; hogweed survival and biomass, and its competitive ability were lower when growing in soil inocula collected from earlier-invaded grasslands. Our results provide evidence that the initial dominance of an invasive plant species and its negative impact can later be reversed by stabilising processes.