LETTER Phylogenetic conservatism in plant-soil feedback and its implications for plant abundance Brian L. Anacker, 1 * John N. Klironomos, 2 Hafiz Maherali, 3 Kurt O. Reinhart 4 and Sharon Y. Strauss 1 Abstract We examined whether plant-soil feedback and plant-field abundance were phylogenetically con- served. For 57 co-occurring native and exotic plant species from an old field in Canada, we col- lected a data set on the effects of three soil biota treatments on plant growth: net whole-soil feedback (combined effects of mutualists and antagonists), feedback with arbuscular mycorrhizal fungi (AMF) collected from soils of conspecific plants, and feedback with Glomus etunicatum,a dominant mycorrhizal fungus. We found phylogenetic signal in both net whole-soil feedback and feedback with AMF of conspecifics; conservatism was especially strong among native plants but absent among exotics. The abundance of plants in the field was also conserved, a pattern under- lain by shared plant responses to soil biota. We conclude that soil biota influence the abundance of close plant relatives in nature. Keywords AMF, Arbuscular mycorrhizal fungi, exotic, Glomus etunicatum, introduced, old field, phylo- genetic signal, phylogeny, plant community assembly, plant-soil feedbacks. Ecology Letters (2014) 17: 1613–1621 INTRODUCTION Plant interactions with macro-mutualists and macro-antago- nists often exhibit phylogenetic signal, defined as the tendency for relatives to resemble each other more closely in their char- acteristics than expected by chance (Harvey & Pagel 1991). Moreover, evolutionary history is a useful predictor of the distribution and abundance of species (Donoghue 2008), spe- cies associations (Cavender-Bares et al. 2009) and the out- comes of species interactions (Becerra 1997; Agrawal & Fishbein 2006; Weiblen et al. 2006). In contrast, relatively few studies have examined whether reciprocal effects between plant growth and soil community development (plant-soil feedback) are shared above the level of species (i.e. plant clades) (reviewed below), despite the important linkage between soil biota and plant growth and plant abundance in natural communities (van der Putten et al. 1993; Bever et al. 1997; Klironomos 2002; Wardle et al. 2004; Bezemer et al. 2006; Kardol et al. 2006; Reinhart et al. 2012). Conservatism in species responses to soil biota has direct implications for species invasibility and coexistence. For example, if close relatives share positive soil feedbacks, then occupancy by one species will favour co-establishment of close relatives and coexistence. In contrast, if relatives share nega- tive soil feedbacks, then an invader might experience biotic resistance by a community with an already-established close relative. Understanding the degree of phylogenetic conserva- tism in plant responses to soil biota can therefore shed light on the cause of conflicting reports of whether species are better (e.g. Duncan & Williams 2002) or poorer (e.g. Strauss et al. 2006) invaders of communities with close relatives. If plant clades share positive soil feedbacks and typically thrive in each others’ soils while others share negative soil feedbacks and therefore cannot stably coexist, then assessing the degree of phylogenetic signal in plant-soil feedbacks can assist in developing strategies for ecological restoration. Several studies have shown that close plant relatives have similar plant-soil feedbacks, but these are all based on a small number of soil microbe or plant species. Reinhart et al. (2012) found significant phylogenetic signal in the response of 95 plant species of tallgrass prairies to two arbuscular mycorrhi- zal fungi (AMF) species in the genus Glomus. For seven sub- tropical tree species, close relatives were shown to have more similar negative growth responses to soil fungal pathogens (Liu et al. 2012). Soils trained by two native Potentilla species caused strong negative feedbacks on the growth of a distantly related, non-native Potentilla species (Callaway et al. 2013). Experiments with six grass species showed that close relatives had similar plant-soil feedbacks (Brandt et al. 2009). Plants in the Asteraceae family responded similarly to the application of a systemic fungicide (Bennett & Cahill 2013). Introduced species in New Zealand experienced negative soil feedback in conspecific and congeneric soils, but not in distant relative soils (Diez et al. 2010). These studies suggest that evolution- ary processes, like strong developmental constraints or the absence of genetic variation, could interact with ecological processes to determine the nature of plant-soil feedbacks. Phylogenetic signal in plant-soil feedback would, however, need to be quite strong in order to be detected in the face of context-dependent variation in the interaction. Communities 1 Department of Evolution and Ecology, University of California, Davis, CA, 95616, USA 2 Department of Biology, University of British Columbia, Okanagan campus, Kelowna, British Columbia, Canada 3 Department of Integrative Biology, University of Guelph, Ontario, Canada 4 Fort Keogh Livestock and Range Research Laboratory, USDA-Agricultural Research Service, Miles City, MT, 59301, USA *Correspondence: E-mail: blanacker@ucdavis.edu © 2014 John Wiley & Sons Ltd/CNRS Ecology Letters, (2014) 17: 1613–1621 doi: 10.1111/ele.12378