title: Experiment, monitoring, and gradient methods used to infer climate change effects on plant communities yield consistent patterns.
abstract: Inference about future climate change impacts typically relies on one of three approaches: manipulative experiments, historical comparisons (broadly defined to include monitoring the response to ambient climate fluctuations using repeat sampling of plots, dendroecology, and paleoecology techniques), and space-for-time substitutions derived from sampling along environmental gradients. Potential limitations of all three approaches are recognized. Here we address the congruence among these three main approaches by comparing the degree to which tundra plant community composition changes (i) in response to in situ experimental warming, (ii) with interannual variability in summer temperature within sites, and (iii) over spatial gradients in summer temperature. We analyzed changes in plant community composition from repeat sampling (85 plant communities in 28 regions) and experimental warming studies (28 experiments in 14 regions) throughout arctic and alpine North America and Europe. Increases in the relative abundance of species with a warmer thermal niche were observed in response to warmer summer temperatures using all three methods; however, effect sizes were greater over broad-scale spatial gradients relative to either temporal variability in summer temperature within a site or summer temperature increases induced by experimental warming. The effect sizes for change over time within a site and with experimental warming were nearly identical. These results support the view that inferences based on space-for-time substitution overestimate the magnitude of responses to contemporary climate warming, because spatial gradients reflect long-term processes. In contrast, in situ experimental warming and monitoring approaches yield consistent estimates of the magnitude of response of plant communities to climate warming.
authors.affiliation: National Ecological Observatory Network, Boulder, CO 80301; Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309; selmendorf@neoninc.org.
authors.forename: Sarah C
authors.initials: SC
authors.lastname: Elmendorf
authors.affiliation: Department of Geography, University of British Columbia, Vancouver, BC, Canada V6T 1Z2;
authors.forename: Gregory H R
authors.initials: GH
authors.lastname: Henry
authors.affiliation: Biology Department, Grand Valley State University, Allendale, MI 49401;
authors.forename: Robert D
authors.initials: RD
authors.lastname: Hollister
authors.affiliation: Botanical Department, Faroese Museum of Natural History, FO-110 Tórshavn, Faroe Islands;
authors.forename: Anna Maria
authors.initials: AM
authors.lastname: Fosaa
authors.affiliation: International Institute of Tropical Forestry, US Forest Service, Rio Piedras, PR 00926;
authors.forename: William A
authors.initials: WA
authors.lastname: Gould
authors.affiliation: Department of Biology, Memorial University, St. John's, NL, Canada A1B 3X9;
authors.forename: Luise
authors.initials: L
authors.lastname: Hermanutz
authors.affiliation: Terrestrial Ecology Department, Norwegian Institute for Nature Research, NO-7034 Trondheim, Norway;
authors.forename: Annika
authors.initials: A
authors.lastname: Hofgaard
authors.forename: Ingibjörg S
authors.initials: IS
authors.lastname: Jónsdóttir
authors.affiliation: Faculty of Life and Environmental Sciences, University of Iceland, 101 Reykjavík, Iceland; Department of Biology, University Centre in Svalbard, 9171 Longyearbyen, Norway;
authors.forename: Ingibjörg I
authors.initials: II
authors.lastname: Jónsdóttir
authors.affiliation: Arctic National Wildlife Refuge, US Fish and Wildlife Service, Fairbanks, AK 99701;
authors.forename: Janet C
authors.initials: JC
authors.lastname: Jorgenson
authors.affiliation: Département des Sciences de l'environnement et Centre d'études nordiques, Université du Québec à Trois-Rivières, Trois-Rivières, QC, Canada G9A 5H7;
authors.forename: Esther
authors.initials: E
authors.lastname: Lévesque
authors.affiliation: Icelandic Institute of Natural History, 212 Gardabaer, Iceland;
authors.forename: Borgþór
authors.initials: B
authors.lastname: Magnusson
authors.affiliation: Department of Biological and Environmental Sciences, University of Gothenburg, SE 405 30 Gothenburg, Sweden;
authors.forename: Ulf
authors.initials: U
authors.lastname: Molau
authors.affiliation: School of GeoSciences, University of Edinburgh, Edinburgh EH9 3FF, United Kingdom;
authors.forename: Isla H
authors.initials: IH
authors.lastname: Myers-Smith
authors.affiliation: Department of Biological Sciences, Florida International University, Miami, FL 33199;
authors.forename: Steven F
authors.initials: SF
authors.lastname: Oberbauer
authors.affiliation: Community Ecology Group, Swiss Federal Institute for Snow and Avalanche Research, CH-7260 Davos, Switzerland;
authors.forename: Christian
authors.initials: C
authors.lastname: Rixen
authors.affiliation: Department of Biology, University of Texas at El Paso, El Paso, TX 79912; and.
authors.forename: Craig E
authors.initials: CE
authors.lastname: Tweedie
authors.forename: Marilyn D
authors.initials: MD
authors.lastname: Walker
authors.affiliation: HOMER Energy, Boulder, CO 80301.
authors.forename: Marilyn
authors.initials: M
authors.lastname: Walker
mesh_terms: D044822:Biodiversity; D057231:Climate Change; D064308:Ecological Parameter Monitoring / Q000379:methods*; D017753:Ecosystem; D057232:Global Warming; D018521:Plant Physiological Phenomena; D010944:Plants
keywords: climate change; space-for-time substitution; thermophilization; tundra; warming experiment