Functional traits are more variable at the intra- than inter-population level: a study of four calcareous dry-grassland plant species

The majority of studies investigating plant functional traits have used species average trait values, and assumed that average values were sufficiently representative of each species considered. Although this approach has proven valuable in community ecology studies, plant functional traits can significantly vary at different scales, i.e. between but also within populations. The study of species functional trait variability can facilitate increasingly accurate studies in community ecology. Nevertheless, the current extent of within-site plant trait variability has been poorly addressed in the literature. Calcareous grasslands are ecosystems well-suited to study plant trait variation at small spatial scales. Many species are present on heterogeneous calcareous sites, with significant differences in hydric status due to variations in soil depth, soil moisture, aspect, and slope. This study assesses the extent of intra-population functional trait variability and tests the hypothesis that this variability can be explained by within-site environmental heterogeneity. Three functional traits (SLA-specific leaf area, LDMC-leaf dry matter content, and plant vegetative height) were assessed in three populations of four calcareous grassland species totalling 950 individuals. The heterogeneity in soil depth and potential direct incident radiation was also quantified and related to plant functional trait variability. The intra-population functional trait variability was compared to the inter-population variability of collected data and global inter-population variability data obtained from the worldwide TRY functional traits database. The results showed that SLA, LDMC, and plant height are characterized by considerable intra-population variation (SLA: 72–95%, LDMC: 78–100% and vegetative height: 70–94% of trait variability). The results also indicate higher plant height and larger SLA for individuals located in plots with deeper soils or lower potential direct incident radiation, on gentle slopes or north-facing slopes. Our findings additionally support the concept that higher plant height, higher SLA, and lower LDMC are related to higher availability of soil water. Individuals on shallow soils or in more exposed areas are better equipped to cope with environmental stress. Our results indicate plasticity or local adaptation in individuals to environmental heterogeneity. This study suggests that detailed analyses involving plant functional traits require measurements in situ from a large number of individuals, as the degree of individual response strongly depends on an individual’s location and its micro-environmental conditions. Neglecting intra-population trait variability may be critical, as intraspecific variation can be very high at the population scale, and is likely to be driven by local environmental heterogeneity. Manuscript received 26 November 2015, accepted 29 April 2016 Co-ordinating Editor: Thomas Becker 322