Above-belowground effects of foundational tree range-expansions in treeless ecosystems

Gabriel Munoz Sr., Faculty of Arts and Science, Concordia University, Montreal, QC, Canada, Isaac Eckert and Tonia DeBellis, Biology, Concordia University, Montreal, QC, Canada, Martin Nuñez, Grupo de Ecología de Invasiones, Universidad Nacional del Comahue, Bariloche, Argentina, Jean-Philippe Lessard, Department of Biology, Concordia University, Montreal, QC, Canada

Background/Question/Methods
Whole ecosystem transformations may follow after the expansion of species into novel geographical ranges. The impacts of species range expansions can ripple across trophic levels and modify the structure of the receiving ecosystems. Particularly, when populations of foundation species expand into species-poor, isolated ecosystems such as in islands or in high-altitude alpine tundra. Here, we experimentally evaluate the effects in the Patagonian alpine tundra ecosystem following the expansion of a non-native foundation species, the lodgepole pines (Pinus contorta). We studied a chronosequence of lodgepole pines expanding onto the treeless alpine tundra of at plateau, close to the summit of the Piltriquitrón, a massif that overlooks the town of El Bolsón, in Rio Negro, Argentina (41°59′00″S). At this high-altitude plateau, thousands of individuals of lodgepole pines have successfully established and have been growing in numbers for the past 10-15 years ago. We placed a total of forty-five (45) plots of two (2) sq-meter randomly placed but set to cover the full spatial extent of the lodgepole pine altitudinal expansion front. We ask 1) what are the per-capita of novel lodgepole pine effects in species diversity and environmental gradients above and belowground? 2) How does a novel foundation species influence the assembly of native interspecific interactions across trophic levels?
Results/Conclusions
By combining field observations with causal statistical inference, we successfully identified causal effect cascades which allow for a more comprehensive understanding of the multilevel-multitrophic above-belowground effects of foundation species expanding to ecosystems where they are ecologically novel. We found that the effects of expanding lodgepole pines decrease soil temperature, increases soil water content, and have variable effects on soil nutrient availability along the expansion chronosequence. We observed relatively small changes in the taxonomic and functional composition of plants and arthropods aboveground. However, we found significant differences in the way how plants organize their interactions with root-associated fungi, leading to the assembly of distinct network structures for the exchange of resources at the above-belowground interphase. These changes in plant-fungi interaction diversity were associated with deterministic but contrasting influences of nitrogen and phosphorus gradients in the soil. Finally, we show that interaction rewiring during the early stages of expansion leads to irreversible changes in the structure of native plant-root associated fungi networks at the later stages of the expansion. Our study shows how even small-scale per-capita contributions of expanding lodgepole pines can lead to whole changes in the structure and stability of ecosystems.