Scale and season determine the magnitude of invasion impacts on plant communities.

• Impacts of global plant invader Ox-eye Daisy on plant communities were studied. • Ox-eye Daisy reduced species richness and diversity at all spatial scales and sites. • Decreased species abundance distribution, density and intraspecific association. • Intensity of impact was greater in summer season and at the larger spatial scale. • Scale and season matter in standard measurement of magnitude of invasion impacts. Global synthesis of studies on impacts of invasions on plant communities is often inhibited by inconsistent results owing to plethora of methodological pitfalls. In this regard, the selection of suitable spatial scale, season of sampling and choice of diversity metrics are crucial considerations. Here we report the impacts of a global invader, Leucanthemum vulgare Lam., on plant communities selected along an altitudinal gradient (landscape scale) in Kashmir Himalaya. The environmentally similar but spatially isolated invaded and uninvaded plots were selected and then sampled at multiple spatial (quadrat and plot) and temporal (spring, summer and autumn seasons) scales during years 2016-2017. Invasion by L. vulgare had a significantly negative impact on structure and composition of the invaded communities. Invasion decreased species richness at all the spatial scales due to its effects on species abundance (density), species abundance distribution and intraspecific association. However, the magnitude of the effect was dependent on the spatial scale, being more severe at the larger scale. Renyi diversity profiles indicated that L. vulgare invasion decreased species diversity during all the three seasons, but the intensity of the invasion impact was dependent on the season, with summer being the highest. Non-multidimensional scaling analysis showed that L. vulgare significantly altered species composition of the invaded communities with higher compositional dissimilarity during the summer season. Our study suggests that the selection of suitable spatial scale and season of sampling matter in the standard measurement of magnitude of plant invasion impacts. We propose a way forward to improve methodological and analytic tools for undertaking studies on impacts of invasions on plant communities. [ABSTRACT FROM AUTHOR]