Environmental modulation of plant mycorrhizal traits in the global flora

Mycorrhizal symbioses are known to strongly influence plant performance, structure plant communities and shape ecosystem dynamics. Plant mycorrhizal traits, such as those characterizing mycorrhizal type (arbuscular (AM), ecto-, ericoid, or orchid mycorrhiza) and status (obligately (OM), facultatively (FM), or non-mycorrhizal) offer valuable insight into plant belowground functionality. Here, we compile available plant mycorrhizal trait information and global occurrence data (~100 million records) for 11,770 vascular plant species. Using a plant phylogenetic mega-tree and high-resolution climatic and edaphic data layers, we assess phylogenetic and environmental correlates of plant mycorrhizal traits. We find that plant mycorrhizal type is more phylogenetically conserved than plant mycorrhizal status, while environmental variables (both climatic and edaphic; notably soil texture) explain more variation in mycorrhizal status, especially FM. The previously underestimated role of environmental conditions has far-reaching implications for our understanding of ecosystem functioning under changing climatic and soil conditions. 

Methods
Data collection In order to identify phylogenetic and environmental correlates of plant mycorrhizal traits, we compiled four data sources: 1) plant species mycorrhizal trait data; 2) plant species occurrence data; 3) global climatic and soil environmental data and 4) plant phylogenetic information. The plant occurrence and environmental data were used to estimate plant species' environmental associations. The environmental association data and phylogenetic information were then used to model variations in plant mycorrhizal trait expression (Figure 1).

First, plant mycorrhizal trait data were obtained from the most up-to-date literature available, including data published by Harley & Harley (1987, 1990), Wang & Qiu (2006), Hempel et al. (2013), Bueno et al. (2017), Gerz et al. (2018) and Soudzilovskaia et al. (2020). We distinguished four plant mycorrhizal types (arbuscular (AM), ecto- (ECM), orchid (ORM), and ericoid (ERM) mycorrhiza, as defined by Smith & Read (2008)) and three plant mycorrhizal statuses (obligately (OM), facultatively (FM) and non-mycorrhizal (NM)). We compiled plant mycorrhizal trait data for 14,722 taxa at the species level (Appendix S2). Second, plant species occurrence records were retrieved from the Global Biodiversity Information Facility (GBIF; www.gbif.org) for 13,479 species that were present in both the standardised GBIF species list and the mycorrhizal trait species list. Third, plant occurrence data of 13,479 species were intersected with a 30 arc-seconds (approximately 1 km) global grid, with the presence or absence of each species in each cell recorded. A sensitivity test indicated that sampling 20 grid-cell records produced environmental parameter estimates that were representative of wider distribution areas (Appendix S1); retaining species recorded in ≥ 20 cells left 62,540,387 grid-cell level records for 11,770 species (Appendix S3). Environmental associations were approximated by intersecting the distribution data for each species with a raster stack of 54 environmental data layers (Table S1). Fourth, a phylogenetic tree containing the 11,770 species in our dataset was compiled, and phylogenetic signal in plant mycorrhizal traits was examined using the δ statistic (Borges et al., 2019). See Appendix S1 for details of datasets and data filtering. 710 dual mycorrhizal plant species (AM + ECM) were distinguished (Appendix S2), of which 665 were matched with geographic location information in GBIF. Except where stated otherwise, these species were grouped with ECM plant species in further analyses, reflecting ongoing controversy concerning the definition of dual mycorrhizal plant species (Teste et al., 2020; Brundrett, 2021a) and the fact that the niches of dual mycorrhizal plants most closely resemble those of ECM plants (Gerz et al., 2018).

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