Evolutionary responses of invasive grass species to variation in precipitation and soil nitrogen

Journal of Ecology 2016, 104, 979–986 doi: 10.1111/1365-2745.12582 Evolutionary responses of invasive grass species to variation in precipitation and soil nitrogen Monica A. Nguyen 1 *, Amy E. Ortega 1 , Kurt Q. Nguyen 1 , Sarah Kimball 2 , Michael L. Goulden 3 and Jennifer L. Funk 1 Schmid College of Science and Technology, Chapman University, Orange, CA, USA; 2 Center for Environmental Biology, University of California, Irvine, CA, USA; and 3 Department of Earth System Science, University of California, Irvine, CA, USA Summary 1. Global climate models suggest that many ecosystems will experience reduced precipitation over the next century and the consequences for invasive plant performance are largely unknown. Annual invasive species may be able to quickly evolve traits associated with drought escape or tolerance through rapid genetic changes. 2. We investigated the inﬂuence of 5 years of water and nitrogen manipulations on trait values in a southern California grassland system. Seeds from two annual grass species (Avena barbata and Bro- mus madritensis) were collected from experimental plots and grown in a common environment over two generations. We measured 14 physiological, morphological, phenological and reproductive traits. 3. Both species displayed phenotypic differences depending on the water treatment from which they were collected, but not depending on the nitrogen treatment. Both species displayed trait values characteristic of drought escape (e.g. earlier ﬂowering in A. barbata and B. madritensis, lower water-use efﬁciency in B. madritensis) when grown from the seeds collected from plots that experi- enced ﬁve years of reduced precipitation. Furthermore, A. barbata individuals grown from the seeds collected from drought plots had higher reproductive output and higher photosynthetic performance than individuals grown from water addition plots, with individuals grown from ambient plots dis- playing intermediate trait values. Notably, we found no phenotypic variation among treatments for six root traits. 4. Synthesis. Trait differences were observed following two generations in a common garden, sug- gesting that treatment differences were genetically based. This suggests that populations were responding to selection over the 5 years of water manipulations, a remarkably short time period. The rapid evolutionary responses observed here may help these two widespread invasive grass spe- cies thrive under reduced precipitation scenarios, which could have important implications for ﬁre dynamics, invasive species management and native plant restoration in communities invaded by annual grasses. Key-words: annual species, Avena barbata, Bromus madritensis, drought escape invasion ecology, invasive species, phenology, photosynthesis, root traits, water-use efﬁciency Introduction Global change, including increases in temperature, atmo- spheric CO 2 concentration and nitrogen (N) input from anthropogenic activities, is one of the most signiﬁcant threats to natural ecosystems. Further, global climate models have predicted the alterations in inter- and intra-annual precipitation in many regions (Melillo, Richmond & Yohe 2014). *Correspondence author: E-mail: nguyen.an.monica@gmail.com Biological invasion is another important agent of global change with signiﬁcant effects on global biodiversity (Millen- nium Ecosystem Assessment, 2005). A meta-analysis of 1041 cases of invasion found that alien invasive plants decrease the abundance and diversity of native resident plant species by 43.5% and 50.7%, respectively (Vil a et al. 2010). Signiﬁcant impacts on native resident species’ abundance and diversity are more likely to occur if the invading species is an annual grass (Py sek et al. 2012). Reﬁning our understanding of how invasive plants respond to environmental variation will © 2016 The Authors. Journal of Ecology © 2016 British Ecological Society