Neglected but Potent Dry Forest Players: Ecological Role and Ecosystem Service Provision of Biological Soil Crusts in the Human-Modified Caatinga

Michelle Szyja,Flávia D A Oliveira,Artur Gonçalves De Souza Menezes,Burkhard Büdel,Inara Leal,Rainer Wirth,Marcelo Tabarelli

doi:10.3389/fevo.2019.00482

Michelle Szyja, Flávia D A Oliveira + Show 5 more

Open Access

https://doi.org/10.3389/fevo.2019.00482

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Abstract

Biological soil crusts (biocrusts) have been recognized as key ecological players in arid and semiarid regions at both local and global scales. They are important biodiversity components, provide critical ecosystem services, and strongly influence soil-plant relationships, and successional trajectories via facilitative, competitive, and edaphic engineering effects. Despite these important ecological roles, very little is known about biocrusts in seasonally dry tropical forests. Here we present a first baseline study on biocrust cover and ecosystem service provision in a human-modified landscape of the Brazilian Caatinga, South America's largest tropical dry forest. More specifically, we explored (1) across a network of 34 0.1 ha permanent plots the impact of disturbance, soil, precipitation, and vegetation-related parameters on biocrust cover in different stages of forest regeneration, and (2) the effect of disturbance on species composition, growth and soil organic carbon sequestration comparing early and late successional communities in two case study sites at opposite ends of the disturbance gradient. Our findings revealed that biocrusts are a conspicuous component of the Caatinga ecosystem with at least 50 different taxa of cyanobacteria, algae, lichens and bryophytes (cyanobacteria and bryophytes dominating) covering nearly 10% of the total land surface and doubling soil organic carbon content relative to bare topsoil. High litter cover, high disturbance by goats, and low soil compaction were the leading drivers for reduced biocrust cover, while precipitation was not associated Second-growth forests supported anequally spaced biocrust cover, while in old-growth-forests biocrust cover was patchy. Disturbance reduced biocrust growth by two thirds and carbon sequestration by half. In synthesis, biocrusts increase soil organic carbon (SOC) in dry forests and as they double the SOC content in disturbed areas, may be capable of counterbalancing disturbance-induced soil degradation in this ecosystem. As they fix and fertilize depauperated soils, they may play a substantial role in vegetation regeneration in the human-modified Caatinga, and may have an extended ecological role due to the ever-increasing human encroachment on natural landscapes. Even though biocrusts benefit from human presence in dry forests, high levels of anthropogenic disturbance could threaten biocrust-provided ecosystem services, and call for further, in-depth studies to elucidate the underlying mechanisms.

Highlights

IntroductionBiological soil crusts (hereafter referred to as “biocrusts”) are communities consisting of photosynthetic (i.e., cyanobacteria, eukaryotic algae, lichens, and bryophytes) and non-photosynthetic organisms, such as heterotrophic bacteria and microfungi (Chamizo et al, 2013), covering around 12% of the Earth’s terrestrial surface (RodriguezCaballero et al, 2018)
Biological soil crusts are communities consisting of photosynthetic and non-photosynthetic organisms, such as heterotrophic bacteria and microfungi (Chamizo et al, 2013), covering around 12% of the Earth’s terrestrial surface (RodriguezCaballero et al, 2018)
Biocrusts have been identified as a key ecological component across many arid and semi-arid regions regarding biodiversity, ecosystem functions.They promote both primary and secondary succession and drive many ecological processes associated to early communities, such as biogenic weathering, soil development, nutrient uptake, and water balance (Belnap, 2001a; Chamizo et al, 2016)

Summary

Introduction

Biological soil crusts (hereafter referred to as “biocrusts”) are communities consisting of photosynthetic (i.e., cyanobacteria, eukaryotic algae, lichens, and bryophytes) and non-photosynthetic organisms, such as heterotrophic bacteria and microfungi (Chamizo et al, 2013), covering around 12% of the Earth’s terrestrial surface (RodriguezCaballero et al, 2018). While biocrusts cover a wide range of latitudes, from tropical to temperate and polar ecosystems (Belnap et al, 2001), they are abundant across arid and semiarid regions, where vascular plants are not able to outcompete them (Belnap and Lange, 2013) Across suitable habitats, they can cover up to 100% of the terrestrial surface throughout the year (Kleiner and Harper, 1972). Biocrusts follow a successional process initiated by cyanobacteria and algae, which favor the subsequent establishment of lichens and bryophytes (Belnap, 1995, 2001a) In this perspective biocrusts can facilitate, inhibit or have neutral effects on succession in vascular plant communities (Zhang et al, 2016), acting as ecosystem engineers. In addition to soil conditions and nutrient budgets, these organisms can directly affect seed germination of vascular plants, either positively or negatively, indicating that biocrust-mediated processes have relevance beyond simple soil development (Deines et al, 2007; Song et al, 2017)

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