Biotic and abiotic drivers of tree seedling recruitment across an alpine treeline ecotone

www.nature.com/scientificreports OPEN Received: 6 October 2017 Accepted: 29 June 2018 Published: xx xx xxxx Biotic and abiotic drivers of tree seedling recruitment across an alpine treeline ecotone Esther R. Frei 1,2, Eva Bianchi 1,3, Giulietta Bernareggi1,4, Peter Bebi 1, Melissa A. Dawes Carissa D. Brown 5, Andrew J. Trant6, Steven D. Mamet 7 & Christian Rixen 1 1,2 , Treeline responses to climate change ultimately depend on successful seedling recruitment, which requires dispersal of viable seeds and establishment of individual propagules in novel environments. In this study, we evaluated the effects of several abiotic and biotic drivers of early tree seedling recruitment across an alpine treeline ecotone. In two consecutive years, we sowed seeds of low- and high-elevation provenances of Larix decidua (European larch) and Picea abies (Norway spruce) below, at, and above the current treeline into intact vegetation and into open microsites with artificially removed surface vegetation, as well as into plots protected from seed predators and herbivores. Seedling emergence and early establishment in treatment and in control plots were monitored over two years. Tree seedling emergence occurred at and several hundred metres above the current treeline when viable seeds and suitable microsites for germination were available. However, dense vegetation cover at lower elevations and winter mortality at higher elevations particularly limited early recruitment. Post-dispersal predation, species, and provenance also affected emergence and early establishment. This study demonstrates the importance of understanding multiple abiotic and biotic drivers of early seedling recruitment that should be incorporated into predictions of treeline dynamics under climate change. Plant species are responding to recent global temperature increases1 by shifting their ranges as populations track their fundamental niche2,3. There is increasing evidence for climate-induced latitudinal range shifts via increased shrub abundance in circumarctic tundra ecosystems4–6 and elevational shifts of shrubs and trees in mountainous regions7–11. Treeline position, i.e. the range limit of forest ecosystems, is widely considered temperature sensitive and is thus expected to respond to climate warming12–15. However, global treeline dynamics are often modulated by regional-scale drivers such as historical land use changes16 and biotic interactions17. Hence, treeline responses to global warming vary among locations and are often asynchronous with the rate of climate change17–21. Climate change-induced range expansion of treeline populations also depends on successful recruitment, which requires dispersal of viable seeds followed by successful establishment of individual propagules22. In treeline ecotones, viable seed availability commonly declines with elevation13,23 due to lower abundance of seed bearing trees and less frequent mast years, i.e. synchronous production of large seed crops24–26. Biotic interactions, such as pre-dispersal predation, may further constrain seed productivity at treeline27, impacting future treeline range expansion. Successful recruitment also depends on the availability of suitable microsites that provide the necessary conditions for emergence and establishment of seedlings28,29. Seed bed quality is determined by a complex interplay of abiotic and biotic factors such as microclimatic conditions, the presence of neighbouring vegetation, and herbivory30. Abiotic factors are considered key drivers of seedling recruitment in climatically harsh environments23. Early establishment is particularly limited by temperature and water availability31–33, but other abiotic factors, such as snow cover duration and desiccating winds, may also affect seedling recruitment34–36. 1 WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 11, 7260, Davos Dorf, Switzerland. 2Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111, 8903, Birmensdorf, Switzerland. 3 Institute of Terrestrial Ecosystems, Department of Environmental Systems Science, ETH Zurich, Universitätstrasse 22, 8092, Zurich, Switzerland. 4Dipartimento di Bioscienze, Universit di Parma, Parco Area delle Scienze 11/A, 43124, Parma, Italy. 5Department of Geography, Memorial University, 230 Elizabeth Avenue, St John’s, NL, A1B 3X9, Canada. 6School of Environment, Resources and Sustainability, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada. 7Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8, Canada. Correspondence and requests for materials should be addressed to E.R.F. (email: ) ScientiFic REporTs | (2018) 8:10894 | DOI:10.1038/s41598-018-28808-w 1 www.nature.com/scientificreports/ Site Seedling emergence (n = 1,727) 1st winter survival (n = 408) 2nd winter survival Seedling height (n = 236) (n = 54) 75.898*** 26.408*** 10.689** *** *** * 21.219*** Scarified 6.308 18.638 12.460 2.678 Year 0.146 0.521 — — Species 24.687*** 2.412 3.416 — Provenance 38.326*** 3.779 0.367 — Exclosure 15.490*** 0.253 — — Site × Scarified 7.623* 2.398 1.021 1.975 Site × Year 92.573*** 29.399*** — — Site × Species 1.515 10.249** 0.002 — Site × Provenance 0.724 0.001 0.002 — Site × Exclosure 0.033 0.134 — — Scarified × Year 4.175* 0.019 — — — Scarified × Species 0.730 0.264 — Scarified × Provenance 0.471 6.909** — — Scarified × Exclosure 0.061 1.262 — — Year × Species 1.584 2.333 — — Year × Provenance 1.021 4.199* — — Year × Exclosure 4.371* 4.695* — — Species × Provenance 16.473*** 0.148 — — Species × Exclosure 1.069 1.051 — — Provenance × Exclosure 7.896** 0.404 — — Table 1. Effects of experimental site, scarification, seeding year, species, provenance, and herbivore exclosure treatment (exclosure), as well as their interactions, on seedling emergence, first and second winter survival, and seedling height. Values and symbols are χ2-values and significances, respectively, from likelihood ratio tests of mixed-effects models. Significance levels: *P < 0.05; **P < 0.01; ***P < 0.001. Degrees of freedom: df = 1 for all factors except for site and its interactions in seedling emergence (df = 2). The forest site was excluded from survival and growth trait models because of very low seedling recruitment. Biotic interactions can be equally or even more important than abiotic factors in determining seed bed conditions37. Microsite cover effects can be highly complex, with neighbouring vegetation positively or negatively affecting tree seedlings depending on vegetation type, species, demographic state, and prevailing weather conditions29,38. On the one hand, neighbouring vegetation can facilitate recruitment by sheltering seedlings from adverse climate effects, seed predators, and herbivores23,28,39,40. On the other hand, a dense vegetation c (...truncated)