Understanding Plant Community Responses to Combinations of Biotic and Abiotic Factors in Different Phases of the Plant Growth Cycle Kevin A. Wood 1,2 *, Richard A. Stillman 2 , Ralph T. Clarke 2 , Francis Daunt 1 , Matthew T. O’Hare 1 1 Centre for Ecology and Hydrology, Bush Estate, Penicuik, Edinburgh, Midlothian, United Kingdom, 2 School of Applied Sciences, Bournemouth University, Talbot Campus, Poole, Dorset, United Kingdom Abstract Understanding plant community responses to combinations of biotic and abiotic factors is critical for predicting ecosystem response to environmental change. However, studies of plant community regulation have seldom considered how responses to such factors vary with the different phases of the plant growth cycle. To address this deficit we studied an aquatic plant community in an ecosystem subject to gradients in mute swan (Cygnus olor) herbivory, riparian shading, water temperature and distance downstream of the river source. We quantified abundance, species richness, evenness, flowering and dominance in relation to biotic and abiotic factors during the growth-, peak-, and recession-phases of the plant growth cycle. We show that the relative importance of biotic and abiotic factors varied between plant community properties and between different phases of the plant growth cycle. Herbivory became more important during the later phases of peak abundance and recession due to an influx of swans from adjacent pasture fields. Shading by riparian vegetation also had a greater depressing effect on biomass in later seasons, probably due to increased leaf abundance reducing light intensity reaching the aquatic plants. The effect of temperature on community diversity varied between upstream and downstream sites by altering the relative competitiveness of species at these sites. These results highlight the importance of seasonal patterns in the regulation of plant community structure and function by multiple factors. Citation: Wood KA, Stillman RA, Clarke RT, Daunt F, O’Hare MT (2012) Understanding Plant Community Responses to Combinations of Biotic and Abiotic Factors in Different Phases of the Plant Growth Cycle. PLoS ONE 7(11): e49824. doi:10.1371/journal.pone.0049824 Editor: Martin Heil, Centro de Investigacio ´ n y de Estudios Avanzados, Mexico Received June 27, 2012; Accepted October 17, 2012; Published November 14, 2012 Copyright: ß 2012 Wood et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was funded by a Centre for Ecology & Hydrology Algorithm Studentship (number NEC3579) from the Natural Environment Research Council (www.nerc.ac.uk). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: kevinwoodecology@hotmail.co.uk Introduction Vascular plants are critical to the structure, functions and service provision in a wide range of ecosystems [1]. The roles of plants within ecosystems can vary with changes in plant community structure and function, for example changes in abundance or species composition [2,3,4,5]. Thus in order to understand how the roles of plants within ecosystems will vary over time it is necessary to quantify how plant community structure and function respond to the range of biotic and abiotic factors found in nature. Among the key factors that may regulate plant community structure and function are herbivory [6,7,8,9], temperature [10,11], light availability [12,13,14] and concentrations of growth-limiting nutrients [15,16]. However, few studies address how such additive and interactive biotic and abiotic factors regulate plant community structure and function over time. In temperate regions, plants typically exhibit seasonal cycles of growth and recession mediated by strong changes in growth rates [17]. Such seasonal differences in growth rate can mediate the response to biotic and abiotic factors [18]. The factors which affect plant community properties may also exhibit temporal gradients; for example, seasonal variance in herbivore densities can mediate the effect of grazing on the plant community [18,19]. Therefore the factors which regulate plant community structure and function may vary between different phases of the plant growth cycle, due to variance in plant growth rate, changes in the magnitude of the biotic and abiotic factors, and the strength of the responses of plants to these factors. To date few studies have examined the regulation of plant community structure and function in shallow, lowland rivers, despite the high abundances and keystone roles of plants within these ecosystems [16,20]. In such ecosystems three main phases of the plant growth cycle can be observed: plant growth is strong in spring (April-June), peak abundances are reached in July and declines occur thereafter [12,21,22]. The relative importance of community drivers may also vary between these different phases of the plant growth cycle. Mute swans (Cygnus olor Gmelin 1789) use this resource seasonally, switching from riparian pasture in winter and spring to the river during summer and autumn [23]. Seasonal growth and fall of leaves on riparian trees varies light availability for aquatic plants [24]. Water temperature also shows a distinct seasonal pattern, peaking around July [25]. Seasonal changes in water depth and discharge, which increase downstream, mean that the distance downstream of the river source must also be considered in the context of temporal variation in the regulation of plant community structure and function [26]. Swan herbivory, riparian shading, and the factors correlated with distance downstream are known to decrease the growth rates of aquatic plants within shallow temperate rivers [11,16,20]. Changes in PLOS ONE | www.plosone.org 1 November 2012 | Volume 7 | Issue 11 | e49824