Plant functional trait variation in relation to riparian geomorphology: The importance of disturbance GARRETH KYLE* AND MICHELLE R. LEISHMAN Department of Biological Sciences, Macquarie University NSW 2109, Australia (Email: gkyle@bio.mq.edu.au) Abstract This study examined the patterns of plant functional trait variation in relation to geomorphology, disturbance and a suite of other environmental factors in the riparian margin of the Upper Hunter River, New South Wales, Australia.Vegetation was surveyed on three geomorphic surfaces (point bar, bench and bank) along a 5.5-km stretch of the Upper Hunter River. Functional traits relating to plant growth and reproduction were collected for the identified species.  and principal components analysis were used to compare the trait assemblages of species associated with each geomorphic unit. Pearson’s correlation coefficients were used to investigate trait variation with respect to environmental variables. There were clear differences in the plant functional trait assemblages associated with the three geomorphic units. Generally the point bar was associated with species that were herbaceous, with small seed mass, a short stature and a high specific leaf area (SLA). Conversely, the bench was associated with grasses that had unassisted seed dispersal and intermediate seed mass and SLA, while species on the bank had tall stature, large seed mass, a high SLA and a perennial life cycle. Variation along the primary gradient of plant functional trait composition was most strongly related to disturbance frequency and to a lesser extent soil nutrients and the proportion of clay and silt, while variation along the secondary gradient was associated with variation in substrate texture as well as soil nutrients. Key words: flooding, seed mass, soil nutrients, soil texture, specific leaf area. INTRODUCTION Understanding the role of environmental factors in controlling vegetation structure and function has been a central theme of ecological research for decades. Riparian zones are considered particularly dynamic habitats with high spatial and temporal variability and consequent high species diversity (Naiman et al., 1993).Within the riparian margin a number of factors influence plant community patterns including distur- bance (Lyon & Sagers 1998, 2003), inundation period (Sorrell et al. 2000; Parolin 2002; Sluis & Tandarich 2004), groundwater levels (Lite et al. 2005), geomor- phology (Van Coller et al. 1997, 2000), substrate com- position (Maingi & Marsh 2006), organic matter and aspects of fertility such as pH, nitrogen and phospho- rus (Lyon & Sagers 1998, 2003; Budke et al. 2007). Patterns of substrate texture and nutrient composition and deposition have also been shown to vary with both geomorphology and disturbance (Pinay et al. 1992; Steiger & Gurnell 2002), illustrating the intercon- nected nature of these factors. Many of the studies that have examined community composition within the riparian margin have employed a species-based approach (e.g. Brierley & Cunial 1998; Lyon & Sagers 1998; Corbacho et al. 2003), dealing indirectly with functional groups through a priori clas- sifications of species assemblages, and hence may be of little predictive value in systems where the same species are not present (Roberts 2002). Further to this while taxonomic information is easy and relatively inexpensive to collect (Keddy & Schlieder 1999), it is not necessarily indicative of important underlying eco- system processes (Brooks et al. 2002). An alternative and more insightful approach is to focus on the func- tional traits of the species. This approach has previ- ously been used for several purposes including comparisons of ecological strategies between diverse vegetation communities (Diaz et al. 2004), the assess- ment of the response of plant communities to distur- bance (McIntyre & Lavorel 2001; Boer & Smith 2003; Lloret & Vila 2003; Adler et al. 2004; Vesk &Westoby 2004; Vesk et al. 2004) and to predict the impact of climate change (Diaz & Cabido 1997). Typically the plant functional trait comparisons between disturbance types have focused on grazing and fire with little consideration for flooding. However, there are some important distinctions between the characteristics of each disturbance type. Grazing is a predictably selective form of disturbance with animal behaviour through browse choice playing a significant role in determining which species are impacted. In contrast, both flooding and fire are much *Corresponding author. Accepted for publication November 2008. Austral Ecology (2009) 34, 793–804 © 2009 The Authors doi:10.1111/j.1442-9993.2009.01988.x Journal compilation © 2009 Ecological Society of Australia