Variation in d 13 C among species and sexes in the family Restionaceae along a fine-scale hydrological gradient YOSEPH N. ARAYA, 1 * JONATHAN SILVERTOWN, 1 DAVID J. GOWING, 1 KEVIN MCCONWAY, 2 PETER LINDER 3 AND GUY MIDGLEY 4 1 Department of Life Sciences, Open University,Walton Hall, Milton Keynes MK7 6AA (Email: y.n.araya@open.ac.uk), 2 Department of Mathematics and Statistics, Open University,Walton Hall, Milton Keynes, UK; and 3 Institute of Systematic Botany, University of Zurich, Zurich, Switzerland; and 4 Kirstenbosch Research Centre, South African National Biodiversity Institute, Cape Town, South Africa Abstract Consistent, repeatable 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. In C3 species tissue carbon isotope discrimination (d 13 C) is known to vary sensitively in response to stomatal behaviour, reflecting stomatal limitation of photosynthesis during the period of active growth. However, this has not been studied at fine-spatial scale in natural communities.We tested how d 13 C varied between species and sexes of individuals in the family Restionaceae growing along a monitored hydrological gradient.Twenty Restionaceae species were investigated using species-level phylogeny at two sites in the Cape Floristic Region, a biodiversity hotspot. A spatial overlap analysis showed the Restionaceae species segregated significantly (P < 0.001) at both sites. Moreover, there were significant differences in d 13 C 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 d 13 C values did not show any significant correlation with the hydrological gradient. We suggest that some other variable (e.g. plant phenology) could be responsible for masking a simple response to water availability. Key words: cape floristic region, carbon isotope discrimination, hydrological gradient, plant gender, Restionaceae, soil water regime trade-off, water-use efficiency. INTRODUCTION Plants are in general very sensitive to small differences in soil moisture, even within a range of moisture ten- sions where water is still freely available to them (Davies & Gowing 1999). Classical experiments by Ellenberg (1953) with meadow grassland plants showed that interspecific competition leads to segre- gation of species’ distributions along soil moisture gra- dients and Silvertown et al. (1999) further showed that this structures meadow plant communities in the field. Niche separation on fine-scale hydrological gradi- ents may well be common in plant communities, although the physiological basis of this segregation is still unclear. In general, separation of species into dis- tinct niches along any resource or environmental axes is caused by trade-offs that force species to specialize (MacArthur 1972). Possible causes of hydrological niche specialization that have been suggested include trade-offs between tolerance of aeration stress (caused by waterlogging) and drought stress (Davies & Gowing 1999; Silvertown et al. 1999) and tolerance of low soil nutrient availability (in wet conditions) and drought stress (e.g. Neill 1990; Castelli et al. 2000; Araya 2005). Whatever the precise trade-offs may be that underlie the specialization of species into hydrological niches, these must involve the water economy of plants and hence must be related to stomatal behaviour. Stomatal function imposes a fundamental trade-off between water conservation and carbon assimilation (and hence growth) because stomata must open to allow CO 2 uptake, but must close to limit water loss. Thus water stress may cause stomatal limitation of photo- synthesis during periods of active growth (e.g. Henson et al. 1989). The ratio of CO2 assimilated to stomatal conductance determines the intrinsic water-use effi- ciency (WUE) of a plant. Could this fundamental trade-off arising from the two conflicting functions of stomata be the ultimate explanation for niche separa- tion along hydrological gradients? If it is, then intrinsic WUE ought to vary between species in a systematic manner along soil moisture gradients. Precisely how intrinsic WUE is expected to vary along soil moisture gradients depends on how other *Corresponding author. Accepted for publication October 2009. Austral Ecology (2010) ••, ••–•• © 2010 The Authors doi:10.1111/j.1442-9993.2009.02089.x Journal compilation © 2010 Ecological Society of Australia