Local adaptation, genetic divergence, and experimental selection in a foundation grass across the US Great Plains' climate gradient.

Matthew Galliart,Brian Maricle,Paul St Amand,Loretta Johnson,Jesse Poland,Nora Bello,Mary Knapp,Sara Baer,Adam B. Smith

doi:10.1111/gcb.14534

Abstract

Many prior studies have uncovered evidence for local adaptation using reciprocal transplant experiments. However, these studies are rarely conducted for a long enough time to observe succession and competitive dynamics in a community context, limiting inferences for long-lived species. Furthermore, the genetic basis of local adaptation and genetic associations with climate has rarely been identified. Here, we report on a long-term (6-year) experiment conducted under natural conditions focused on Andropogon gerardii, the dominant grass of the North American Great Plains tallgrass ecosystem. We focus on this foundation grass that comprises 80% of tallgrass prairie biomass and is widely used in 20,000km2 of restoration. Specifically, we asked the following questions: (a) Whether ecotypes are locally adapted to regional climate in realistic ecological communities. (b) Does adaptive genetic variation underpin divergent phenotypes across the climate gradient? (c) Is there evidence of local adaptation if the plants are exposed to competition among ecotypes in mixed ecotype plots? Finally, (d) are local adaptation and genetic divergence related to climate? Reciprocal gardens were planted with 3 regional ecotypes (originating from dry, mesic, wet climate sources) of Andropogon gerardii across a precipitation gradient (500-1,200mm/year) in the US Great Plains. We demonstrate local adaptation and differentiation of ecotypes in wet and dry environments. Surprisingly, the apparent generalist mesic ecotype performed comparably under all rainfall conditions. Ecotype performance was underpinned by differences in neutral diversity and candidate genes corroborating strong differences among ecotypes. Ecotype differentiation was related to climate, primarily rainfall. Without long-term studies, wrong conclusions would have been reached based on the first two years. Further, restoring prairies with climate-matched ecotypes is critical to future ecology, conservation, and sustainability under climate change.

Highlights

The ability to predict the role and strength of local adaptation in natural communities
It is crucial to understand local adaptation and species interactions in long-lived perennial plants in long-term studies (Metz & Tielborger 2016). Here we investigate whether ecotypic variation in a dominant US Great Plains grass (Andropogon gerardii, common name big bluestem) is a result of local adaptation to climate using a reciprocal common garden platform established in 2009 across a precipitation gradient
Across all years, there were no significant cover differences in the mesic ecotype among all four planting sites (Fig. 4). 503 Regressions of cover by ecotype vs annual rainfall for combined years of 2014 and 2015, the latest measurement years presumably when the vegetation was stabilized, showed that the dry ecotype had highest cover with low rainfall, and decline in cover with increased rainfall as occurs in the wettest site of Southern Illinois (Carbondale, Illinois, p = 0.05, R2 = 0.50) (Fig. 5). The wet ecotype showed the opposite pattern with low cover in Western and Central Kansas and increase in cover with precipitation as occurs in Southern Illinois (Carbondale, Illinois, p = 0.007, R2=0.73)

Summary

Introduction

The ability to predict the role and strength of local adaptation in natural communities. We investigate whether ecotypic variation in a dominant US Great Plains grass (Andropogon gerardii, common name big bluestem) is a result of local adaptation to climate using a reciprocal common garden platform established in 2009 across a precipitation gradient. The study assessed experimental selection by measuring outcome of competing A. gerardii ecotypes which, arguably, should be the most robust test for local adaptation across the climate gradients. This is rarely done with perennial plants and in long term studies (Ravenscroft et al 2015). The study related both performance and genetic variation (Villemereuill et al 2016) to climate and provided a strong test for environment in structuring adaptive variation (Schneider & Mazer 2016)

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Conclusion