Biotic and abiotic drivers of intraspecific trait variation within plant populations of three herbaceous plant species along a latitudinal gradient.

Background: The importance of intraspecific trait variation (ITV) is increasingly acknowledged among plant ecologists. However, our understanding of what drives ITV between individual plants (ITV _BI ) at the population level is still limited. Contrasting theoretical hypotheses state that ITV _BI can be either suppressed (stress-reduced plasticity hypothesis) or enhanced (stress-induced variability hypothesis) under high abiotic stress. Similarly, other hypotheses predict either suppressed (niche packing hypothesis) or enhanced ITV _BI (individual variation hypothesis) under high niche packing in species rich communities. In this study we assess the relative effects of both abiotic and biotic niche effects on ITV _BI of four functional traits (leaf area, specific leaf area, plant height and seed mass), for three herbaceous plant species across a 2300 km long gradient in Europe. The study species were the slow colonizing Anemone nemorosa, a species with intermediate colonization rates, Milium effusum, and the fast colonizing, non-native Impatiens glandulifera.
Results: Climatic stress consistently increased ITV _BI across species and traits. Soil nutrient stress, on the other hand, reduced ITV _BI for A. nemorosa and I. glandulifera, but had a reversed effect for M. effusum. We furthermore observed a reversed effect of high niche packing on ITV _BI for the fast colonizing non-native I. glandulifera (increased ITV _BI ), as compared to the slow colonizing native A. nemorosa and M. effusum (reduced ITV _BI ). Additionally, ITV _BI in the fast colonizing species tended to be highest for the vegetative traits plant height and leaf area, but lowest for the measured generative trait seed mass.
Conclusions: This study shows that stress can both reduce and increase ITV _BI , seemingly supporting both the stress-reduced plasticity and stress-induced variability hypotheses. Similarly, niche packing effects on ITV _BI supported both the niche packing hypothesis and the individual variation hypothesis. These results clearly illustrates the importance of simultaneously evaluating both abiotic and biotic factors on ITV _BI . This study adds to the growing realization that within-population trait variation should not be ignored and can provide valuable ecological insights.