Climate Change Alters Seedling Emergence and Establishment in an Old-Field Ecosystem Aime ´ e T. Classen 1,2 *, Richard J. Norby 2 , Courtney E. Campany 1 , Katherine E. Sides 2 , Jake F. Weltzin 1,3 1 Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee, United States of America, 2 Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States of America, 3 USA National Phenology Network, Tucson, Arizona, United States of America Abstract Background: Ecological succession drives large-scale changes in ecosystem composition over time, but the mechanisms whereby climatic change might alter succession remain unresolved. Here, we asked if the effects of atmospheric and climatic change would alter tree seedling emergence and establishment in an old-field ecosystem, recognizing that small shifts in rates of seedling emergence and establishment of different species may have long-term repercussions on the transition of fields to forests in the future. Methodology/Principal Findings: We introduced seeds from three early successional tree species into constructed old-field plant communities that had been subjected for 4 years to altered temperature, precipitation, and atmospheric CO 2 regimes in an experimental facility. Our experiment revealed that different combinations of atmospheric CO 2 concentration, air temperature, and soil moisture altered seedling emergence and establishment. Treatments directly and indirectly affected soil moisture, which was the best predictor of seedling establishment, though treatment effects differed among species. Conclusions: The observed impacts, coupled with variations in the timing of seed arrival, are demonstrated as predictors of seedling emergence and establishment in ecosystems under global change. Citation: Classen AT, Norby RJ, Campany CE, Sides KE, Weltzin JF (2010) Climate Change Alters Seedling Emergence and Establishment in an Old-Field Ecosystem. PLoS ONE 5(10): e13476. doi:10.1371/journal.pone.0013476 Editor: Jane Catherine Stout, Trinity College Dublin, Ireland Received May 18, 2010; Accepted September 20, 2010; Published October 18, 2010 Copyright: ß 2010 Classen 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: Research was sponsored by the US Department of Energy, Office of Science, Biological and Environmental Research Program, Grant No. DE-FG02- 02ER63366. 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: aclassen@utk.edu Introduction Predicting the response of ecosystems — especially ecosystems in transition — to projected climatic change is a compelling but formidable challenge. Some models predict and research support that global change driven changes in plant community composi- tion may be more important to regional terrestrial productivity and its feedback to the climate than will physiological responses of individual plant taxa [1,2,3]. However, few studies have experimentally examined the effects of multiple climatic drivers on ecosystems in transition [4,5,6]. Hence, it is important to measure the responses of transitional ecosystems under realistic simulations of climatic change and seek new approaches for generalizing from experimental observations. Succession from abandoned agricultural and grassland ecosys- tems to forested ecosystems represents a key transition in ecosystem structure and function, and these transitions have the potential to generate large carbon sinks [7,8]. Climatic change, such as warming and altered precipitation regimes, is causing shifts in species distributions [9,10] and phenologies [11]. These changes can alter forest composition; for example, warming could increase the growth rate of established individuals or select for warm- adapted species. Although shifts in plant species distributions have been observed [12,13,14], we know little about the mechanisms behind these transitions [15,16,17], which limits our ability to predict future responses. We investigated whether multiple climate change factors would alter the successional trajectory associated with woody plant establishment in an old-field ecosystem. We introduced seeds from three early successional tree species into constructed old-field plant communities that had been subjected for 4 years to altered tem- perature, precipitation, and atmospheric CO 2 regimes in an experi- mental facility. The experiment was a randomized, complete block, split plot design where atmospheric CO 2 concentrations (ambient or +300 ppm) and air temperature (ambient or +3uC) were applied at the plot level and precipitation (dry or wet) was applied at the split plot within each treatment. Atmospheric [CO 2 ], air temperature, and precipitation were all main effects. Allowing the experiment to run for 4 years prior to the introduction of the tree seeds en- abled the plant and soil communities to respond to the treatments [18,19,20,21]. The tree seeds we used are common early suc- cessional species found invading old fields adjacent to our research site in the southeastern US: Pinus taeda (loblolly pine), Liquidambar styraciflua (sweetgum), and Acer saccharinum (silver maple). Population demographics of woody plants establishing within intact successional plant communities are likely to be constrained by factors that alter germination and seedling establishment, and ultimately, recruitment of individuals into the population [22,23,24]. Light and water are the two most important abiotic determinants of seedling establishment and success [25]. The three environmental factors being manipulated in this experiment, atmospheric [CO 2 ], air temperature, and precipitation, alter light availability and soil PLoS ONE | www.plosone.org 1 October 2010 | Volume 5 | Issue 10 | e13476