Effects of Dominance and Diversity on Productivity along Ellenberg’s Experimental Water Table Gradients Andy Hector 1,2 *, Stefanie von Felten 1 , Yann Hautier 1¤ , Maja Weilenmann 1 , Helge Bruelheide 3 1 Institute of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland, 2 Microsoft Research, Cambridge, United Kingdom, 3 Institute of Biology/Geobotany and Botanical Garden, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany Abstract Heinz Ellenberg’s historically important work on changes in the abundances of a community of grass species growing along experimental gradients of water table depth has played an important role in helping to identify the hydrological niches of plant species in wet meadows. We present a previously unpublished complete version of Ellenberg’s dataset from the 1950s together with the results of a series of modern statistical analyses testing for hypothesized overyielding of aboveground net primary production as a consequence of resource-based niche differentiation. Interactions of species with water table depth and soil type in the results of our analyses are qualitatively consistent with earlier interpretations of evidence for differences in the fundamental and realized niches of species. Arrhenatherum elatius tended to dominate communities and this effect was generally positively related to increasing water table depth. There was little overyielding of aboveground net primary production during the two repeats of the experiment conducted in successive single growing seasons. Examination of how the effects of biodiversity on ecosystem processes vary across environmental gradients is an underutilized approach – particularly where the gradient is thought to be an axis of niche differentiation as is the case with water availability. Furthermore, advances in ecology and statistics during the 60 years since Ellenberg’s classic experiment was performed suggest that it may be worth repeating over a longer duration and with modern experimental design and methodologies. Citation: Hector A, von Felten S, Hautier Y, Weilenmann M, Bruelheide H (2012) Effects of Dominance and Diversity on Productivity along Ellenberg’s Experimental Water Table Gradients. PLoS ONE 7(9): e43358. doi:10.1371/journal.pone.0043358 Editor: Adam Siepielski, University of San Diego, United States of America Received November 23, 2011; Accepted July 20, 2012; Published September 12, 2012 Copyright: ß 2012 Hector 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: The authors have no support or funding to report. Competing Interests: Andy Hector was employed visiting researcher in computational ecology and environmental science at MSR (Microsoft Research) Cambridge. This does not alter the authors’ adherence to all the PLOS ONE policies on sharing data and materials. * E-mail: andrew.hector@uzh.ch ¤ Current address: Department of Ecology, Evolution and Behavior, University of Minnesota, Saint Paul, Minnesota, United States of America Introduction There is a long tradition in ecology of investigating interactions by growing species alone and in competition with others. In this article we present a previously unpublished complete dataset from a classic example of this type of experiment: Heinz Ellenberg’s ‘‘Hohenheim groundwater table experiment’’ on the effects of water table depth on communities of grassland plant species grown in monoculture and mixture [1,2]. In plant ecology this monoculture vs. mixture approach has been used to investigate competition between species and the consequences of species interactions for ecosystem primary productivity [3–8]. Our paper therefore also presents the results of a contemporary analysis of overyielding and the effects of diversity on productivity using Ellenberg’s data and additive partitioning methods [6,9]. Heinz Ellenberg is well known for having introduced the concept of indicator values, based on the occurrence of species along gradients in nutrient and water supply, pH and climate and other environmental variables [10]. What is less well known is that his concept of fundamental and realized niches predated the frequently cited paper of Hutchinson [11] and was derived from grass communities grown along experimental gradients of depth to the water table [1,2]. Ellenberg created the experimental gradients using a concrete tank with sides that gradually increased in height from one end to the other (Fig. 1). The tank was filled with soil that also varied in depth along its length by following the height of the walls. Water flowed through the tank from an inlet at the deep end to a spill way and outlet at the shallow end to produce a gradient of depth to the water table. As a supplement to our paper we present a newly discovered complete version of Ellenberg’s data from his Hohenheim experiment [1,2]. Although Ellenberg’s Hohenheim experiments date from the early 1950s they more recently played an important role in defining the hydrological niches of plants in wet meadows in S.W. England. Silvertown and colleagues [12] did this using data on species occurrence in the Somerset Levels in relation to the depth of the water table (estimated using bore hole measurements). Randomisation tests of the relative abundance of species from Ellenberg’s published Hohenheim data showed that the funda- mental niches of species, as measured in monoculture, overlapped far more than expected by chance and more than the realized niches of plants grown together in mixed communities. The first result suggests that, when grown alone, species tend to favour the same conditions (have similar fundamental niches), while the second result suggests that competition drives species to have different realized niches [12]. The aim of Ellenberg’s experiments was to create a gradient in soil moisture - a resource-based potential niche axis. Coexistence of species through resource partitioning is generally expected to result in overyielding of community productivity. We recognized that Ellenberg’s species abundance data in monoculture and PLOS ONE | www.plosone.org 1 September 2012 | Volume 7 | Issue 9 | e43358