Segregation of plant species along hydrological gradients is an established phenomenon that must in some way reflect a trade-off between plants’ abilities to tolerate the opposing constraints of drought and waterlogging (Silvertown et al. 1999 Nature 400: 61-63). In C₃ species such as Restios, tissue carbon isotope discrimination (δ¹³C) is known to vary in response to stomatal behaviour, reflecting stomatal limitation of photosynthesis during the period of active growth. In this connection, we tested how δ¹³C varied between species and sexes of individuals belonging to the family Restionaceae growing along finely-monitored hydrological gradient in the field. Thirty species of Restionaceae were recorded on hydrological gradients at two sites in the Cape Floristic Region, South Africa. Stem samples were taken from flowering individuals and analysed using an elemental analyzer and isotope ratio mass-spectrometer. At each site, a hydrologic model was built using inputs of bi-weekly water table depth, micro-topographic elevation, soil characteristics and meteorological data. Model output of water-table depth for each quadrat was then used to calculate hydrological niche metrics for each species. Niche segregation between species was tested using Stone & Roberts’ C-score. The relationship between the water table depth and δ¹³C was investigated by the method of Phylogenetically Independent Contrasts using a species-level phylogeny on CACTUS 1.13^®.  

Results/Conclusions

10 restio species, out of total 18 were found to be present in >10% of the quadrats at Riverlands, while for Limietsberg it was 7 out of 12 species. Restionaceae species segregated significantly (p<0.001) along the hydrological gradient at both sites. δ¹³C values observed ranged from -26.90‰ (Cannomois acuminata) to -28.77‰ (Willdenowia arescens) in Riverlands and from -25.18‰ (Anthochortus crinalis) to -27.23‰ (Elegia coleura) in Limietsberg.  δ¹³C values for females were generally more negative than for their male counterpart species. Analysis of variance showed there were significant differences in δ¹³C values among the Restionaceae species (p<0.001) and between male and female individuals of each species (p<0.01). However, after accounting for phylogeny, species δ¹³C values did not show any significant correlation with the hydrological gradient.