Biodiversity in mountain soils above the treeline

Abstract

Biological diversity in mountain ecosystems has been increasingly studied over the last decade. This is also the case for mountain soils, but no study to date has provided an overall synthesis of the current state of knowledge. Here we fill this gap with a first global analysis of published research on cryptogams, microorganisms, and fauna in mountain soils above the treeline, and a structured synthesis of current knowledge. Based on a corpus of almost 1400 publications and the expertise of 37 mountain soil scientists worldwide, we summarise what is known about the diversity and distribution patterns of each of these organismal groups, specifically along elevation, and provide an overview of available knowledge on the drivers explaining these patterns and their changes. In particular, we document an elevation-dependent decrease in faunal diversity above the treeline, while for cryptogams there is an initial increase above the treeline, followed by a decrease towards the nival belt. Thus, our data confirm the key role that elevation plays in shaping the biodiversity and distribution of these organisms in mountain soils. The response of prokaryote diversity to elevation, in turn, was more diverse, whereas fungal diversity appeared to be substantially influenced by plants. As far as available, we describe key characteristics, adaptations, and functions of mountain soil species, and despite a lack of ecological information about the uncultivated majority of prokaryotes, fungi, and protists, we illustrate the remarkable and unique diversity of life forms and life histories encountered in alpine mountain soils. By applying rule- as well as pattern-based literature-mining approaches and semi-quantitative analyses, we identified hotspots of mountain soil research in the European Alps and Central Asia and revealed significant gaps in taxonomic coverage, particularly among biocrusts, soil protists, and soil fauna. We further report thematic priorities for research on mountain soil biodiversity above the treeline and identify unanswered research questions. Building upon the outcomes of this synthesis, we conclude with a set of research opportunities for mountain soil biodiversity research worldwide. Soils in mountain ecosystems above the treeline fulfil critical functions and make essential contributions to life on land. Accordingly, seizing these opportunities and closing knowledge gaps appears crucial to enable science-based decision making in mountain regions and formulating laws and guidelines in support of mountain soil biodiversity conservation targets.

Keywords: alpine soils; bacteria; biogeography; cryptogams; fungi; invertebrates; lichens; microbial diversity; protists; systematic mapping.

Fig. 1

Global map of the number of scientific publications on biodiversity in temperate and continental mountain soils above the treeline (cryptogams, soil microbiota, and soil fauna) by alpine mountain region. The dark core of the pies represents the number of publications in the respective area compared to the number for the region with the most publications (i.e. Central Asia). Total N = 1380 publications, which include all studies specifically or generically describing alpine soil biodiversity. The alpine mountain regions here are numbered as in Table S1: (1) North American Cordillera, (2) Appalachians & Northeast Ranges, (3) Andes & South America, (4) Northern Europe, (5) Central & Southern Europe, (6) North Asia, (7) Caucasus, (8) Central Asia, (9) East Asia, (10) Southern African Ranges, and (11) Australia & New Zealand. See Fig. S1 in Appendix S1 for the same data presented as density of publications per 1000 km² of alpine area and Fig. S2 in Appendix S1 for a visual comparison of the areas of the global alpine mountain regions discussed in this review. Icons are taken from Biorender.com. Photographs from left to right: for cryptogams: arctic–alpine lichens Thamnolia vermicularis (sw.) schaer. and Peltigera aphthosa (l.) willd. (credits: Bettina Weber); for soil microbiota: DNA (green)‐stained soil bacteria and soil particles (red) (credit: Nadine Praeg & Paul Illmer), Russula sp. (credit: Andrea J. Britton); for soil fauna: nematodes and a male velvet spider Eresus sandaliatus (martini & goeze, 1778) (credits: CSIRO Entomology and Michael Steinwandter, respectively).

Fig. 2

(A) Number of scientific publications with a primary focus on one of the three alpine soil organismal groups discussed in this review: biocrusts and other cryptogams; soil microbiota (viruses, archaea, bacteria, fungi, and protists); and soil fauna (micro‐, meso‐, and macrofauna). The counts include publications applying generic terms with no explicit mentions of specific species or organismal groups (e.g. ‘microbes’, ‘arthropods’: indicated with grey icons). (B) Number of publications with a primary or secondary focus on these organisms but excluding publications using only generic organismal terms. The darker base of the bars indicates the number of publications where that group is the primary focus, the lighter part where it is the secondary focus. Icons are taken from Biorender.com.

Fig. 3

Line graphs showing the cumulative number of scientific publications through time dealing with soil biodiversity above the treeline in temperate and continental mountain regions. The numbers represent the publications with a primary focus on the specific alpine soil organismal groups shown.

Fig. 4

Overview of mountain characteristics and abiotic factors influencing the alpine landscape, mountain soils, and alpine soil organisms. Icons are taken from Biorender.com. Photograph credits: Paul Illmer, Nadine Praeg, Michele D'Amico, Emanuele Pintaldi.

Fig. 5

Global map of the number of scientific publications on cryptogam biodiversity in temperate and continental mountain soils above the treeline by alpine mountain region. Number of publications is given per crust type and relative to the maximum number of publications found (Central & Southern Europe, N = 51): the dark‐coloured part of the bar represents those publications in which this group was deemed the primary focus, the light‐coloured part represents those publications where this group was the secondary focus. See Appendix S1 for a detailed description of the methods and Table S5 for publication numbers per alpine mountain region and soil organismal group. Icons are taken from Biorender.com. Photographs from left to right: for cryptogams: arctic–alpine lichen Thamnolia vermicularis (sw.) schaer., alpine zone of the Großglockner, Austria (credit: Stefan Herdy); arctic–alpine lichen Peltigera aphthosa (l.) willd. and arctic–alpine moss Polytrichum sp. in vicinity of Kangerlussuaq, Greenland (credit: both Bettina Weber); for biocrusts: cyanobacteria‐dominated soil crust (dark surface colouration) intermingled with bryophytes; cyanobacteria‐dominated biocrust mixed with chlorolichens, dominated by Fulgensia sp.; cyanobacteria‐dominated biocrust mixed with mosses, dominated by Tortella sp. (all three from the alpine zone of the Großglockner, Austria; credits: Stefan Herdy).

Fig. 6

Global map of the number of publications on microbiota biodiversity in temperate and continental mountain soils above the treeline by alpine mountain region. Number of publications is given per microbial group and relative to the maximum number of publications found (Central Asia, N = 391): the dark‐coloured part of the bar represents those publications in which this group was deemed the primary focus, the light‐coloured part represents those publications where this group was the secondary focus. See Appendix S1 for a detailed description of the methods and Table S6 for publication numbers per region and soil organism group. Icons from Biorender.com. Photographs from left to right: for archaea: Methanosarcina sp. (credit: Paul Illmer); for bacteria: Methylosinus sporium (credit: Nadine Praeg), DNA (green)‐stained soil bacteria attached to soil particle (red) (credit: Nadine Praeg & Paul Illmer); for fungi: Trichoderma asperellum samuels, lieckf. & nirenberg intercoiled with Botrytis sp. (credit: Siebe Pierson), Leccinum vulpinum watling (credit: Andrea J. Britton); for protists: Acanthamoeba sp. (credit: Kenneth Dumack).

Fig. 7

Global map of the number of publications on fauna biodiversity in temperate and continental mountain soils above the treeline by alpine mountain region. Number of publications is given per faunal group and relative to the maximum number of publications found (Central & Southern Europe, N = 72): the dark‐coloured part of the bar represents those publications in which this group was deemed the primary focus, the light‐coloured part represents those publications where this group was the secondary focus. See Appendix S1 for a detailed description of the methods and Table S7 for publication numbers per region and soil organismal group. Icons are taken from Biorender.com. Photographs from left to right: for microfauna: microscope images of nematodes (credit: CSIRO Entomology), and the tardigrade Macrobiotus sp. (credit: Michala Tůmová); for mesofauna: the springtail Entomobrya nivalis (c. linnaeus, 1758) and the mite Platynothrus pelitfer (koch, 1989) (credits: Frank Ashwood); for macrofauna: the ‘green’ earthworm Aporrectodea smaragdina (rosa, 1892), which inhabits calcareous mountain soils in the European Alps and Dinaric Alps, and the male velvet spider Eresus sandaliatus (martini & goeze, 1778) found in alpine dry pastures in the Central European Alps (credits: Michael Steinwandter).

Fig. 8

Taxonomic and geographic distribution of studies in temperate and continental mountain soil biodiversity research above the treeline. The number of studies include those where the organismal group was either the primary or secondary focus of the study.

Fig. 9

Frequencies of main research focus on the three main soil organismal groups, including all specific and generic mentions of cryptogams, microbiota, and fauna (A) and on specific taxa within these groups (B) as reported in the corpus of literature on biodiversity in mountain soils above the treeline. Research focus is a category in the custom thesaurus. For each paper, the main research focus is the one with the highest count of corresponding key words extracted from the title and abstract (see Appendix S1: Material and Methods). The numbers of studies include those where the organismal group was either the primary or secondary focus of the study.