Ecophysiological characterization of early successional biological soil crusts in heavily human-impacted areas

Ecophysiological characterizations of photoautotrophic communities are not only necessary to identify the response of carbon fixation related to different climatic factors, but also to evaluate risks connected to changing environments. In biological soil crusts (BSCs), the description of ecophysiological features is difficult, due to the high variability in taxonomic composition and variable methodologies applied. Especially for BSCs in early successional stages, the available datasets are rare or focused on individual constituents, although these crusts may represent the only photoautotrophic component in many heavily disturbed ruderal areas, such as parking lots or building areas with increasing surface area worldwide. We analyzed the response of photosynthesis and respiration to changing BSC water contents (WCs), temperature and light in two early successional BSCs. We investigated whether the response of these parameters was different between intact BSC and the isolated dominating components. BSCs dominated by the cyanobacterium <i>Nostoc commune</i> and dominated by the green alga <i>Zygogonium ericetorum</i> were examined. A major divergence between the two BSCs was their absolute carbon fixation rate on a chlorophyll basis, which was significantly higher for the cyanobacterial crust. Nevertheless, independent of species composition, both crust types and their isolated organisms had convergent features such as high light acclimatization and a minor and very late-occurring depression in carbon uptake at water suprasaturation. This particular setup of ecophysiological features may enable these communities to cope with a high variety of climatic stresses and may therefore be a reason for their success in heavily disturbed areas with ongoing human impact. However, the shape of the response was different for intact BSC compared to separated organisms, especially in absolute net photosynthesis (NP) rates. This emphasizes the importance of measuring intact BSCs under natural conditions for collecting reliable data for meaningful analysis of BSC ecosystem services

Biological soil crusts decrease infiltration but increase erosion resistance in a human-disturbed tropical dry forest

Under continuous human disturbance, regeneration is the basis for biodiversity persistence and ecosystem service provision. In tropical dry forests, edaphic ecosystem engineering by biological soil crusts (biocrusts) could impact regeneration by influencing erosion control and soil water and nutrient fluxes, which impact landscape hydrology, geomorphology, and ecosystem functioning. This study investigated the effect of cyanobacteria-dominated biocrusts on water infiltration and aggregate stability in a human-modified landscape of the Caatinga dry forest (NE Brazil), a system characterized by high levels of forest degradation and increasing aridity. By trapping dust and swelling of cyanobacterial filaments, biocrusts can seal soil surfaces and slow down infiltration, which potentially induces erosion. To quantify hydraulic properties and erosion control, we used minidisc-infiltrometry, raindrop-simulation, and wet sieving at two sites with contrasting disturbance levels: an active cashew plantation and an abandoned field experiencing forest regeneration, both characterized by sandy soils. Under disturbance, biocrusts had a stronger negative impact on infiltration (reduction by 42% vs. 37% during regeneration), although biocrusts under regenerating conditions had the lowest absolute sorptivity (0.042 ± 0.02 cm s−1/2) and unsaturated hydraulic conductivity (0.0015 ± 0.0008 cm s−1), with a doubled water repellency. Biocrusts provided high soil aggregate stability although stability increased considerably with progression of biocrust succession (raindrop simulation disturbed: 0.19 ± 0.22 J vs. regenerating: 0.54 ± 0.22 J). The formation of stable aggregates by early successional biocrusts on sandy soils suggests protection of dry forest soils even on the worst land use/soil degradation scenario with a high soil erosion risk. Our results confirm that biocrusts covering bare interspaces between vascular plants in human-modified landscapes play an important role in surface water availability and erosion control. Biocrusts have the potential to reduce land degradation, but their associated ecosystem services like erosion protection, can be impaired by disturbance. Considering an average biocrust coverage of 8.1% of the Caatinga landscapes, further research should aim to quantify the contribution of biocrusts to forest recovery to fully understand the role they play in the functioning of this poorly explored ecosystem

Biological soil crusts decrease infiltration but increase erosion resistance in a human-disturbed tropical dry forest

Under continuous human disturbance, regeneration is the basis for biodiversity persistence and ecosystem service provision. In tropical dry forests, edaphic ecosystem engineering by biological soil crusts (biocrusts) could impact regeneration by influencing erosion control and soil water and nutrient fluxes, which impact landscape hydrology, geomorphology, and ecosystem functioning. This study investigated the effect of cyanobacteria-dominated biocrusts on water infiltration and aggregate stability in a human-modified landscape of the Caatinga dry forest (NE Brazil), a system characterized by high levels of forest degradation and increasing aridity. By trapping dust and swelling of cyanobacterial filaments, biocrusts can seal soil surfaces and slow down infiltration, which potentially induces erosion. To quantify hydraulic properties and erosion control, we used minidisc-infiltrometry, raindrop-simulation, and wet sieving at two sites with contrasting disturbance levels: an active cashew plantation and an abandoned field experiencing forest regeneration, both characterized by sandy soils. Under disturbance, biocrusts had a stronger negative impact on infiltration (reduction by 42% vs. 37% during regeneration), although biocrusts under regenerating conditions had the lowest absolute sorptivity (0.042 ± 0.02 cm s−1/2) and unsaturated hydraulic conductivity (0.0015 ± 0.0008 cm s−1), with a doubled water repellency. Biocrusts provided high soil aggregate stability although stability increased considerably with progression of biocrust succession (raindrop simulation disturbed: 0.19 ± 0.22 J vs. regenerating: 0.54 ± 0.22 J). The formation of stable aggregates by early successional biocrusts on sandy soils suggests protection of dry forest soils even on the worst land use/soil degradation scenario with a high soil erosion risk. Our results confirm that biocrusts covering bare interspaces between vascular plants in human-modified landscapes play an important role in surface water availability and erosion control. Biocrusts have the potential to reduce land degradation, but their associated ecosystem services like erosion protection, can be impaired by disturbance. Considering an average biocrust coverage of 8.1% of the Caatinga landscapes, further research should aim to quantify the contribution of biocrusts to forest recovery to fully understand the role they play in the functioning of this poorly explored ecosystem

Impact of anthropogenic disturbance on ecosystem engineers and consequences for dry tropical forest regeneration in the Caatinga, Brazil

Tropical dry forests are crucial for climate adaptation, economic development, and poverty alleviation, offering vital ecosystem services. However, this understudied, and inadequately protected biome faces severe threats like deforestation and land-use changes and is often overlooked in national policies. This neglect poses risks to services like clean water provision, diverse habitats, and climate change mitigation. Changes in land use within these forests impact environmental conditions, causing reduced biodiversity and vegetation restructuring. The regeneration process relies on abiotic factors and natural soil recovery. In this dissertation, I investigated the role of two keystone organism groups—Biological soil crusts ('biocrusts') and leaf-cutting ants (LCA)—in dry forest regeneration. These ecosystem engineers can enhance topsoil quality, introduce essential nutrients and water, and influence plant germination and growth, thereby potentially affecting dry forest regeneration. My primary objectives were to determine the relevance of biocrusts in the Caatinga dry forest, their interaction with LCA, as well as both of their provision of essential ecosystem services, and their response to chronic anthropogenic disturbance. I employed various techniques to document biocrust diversity, and distribution, along with the abiotic environment alterations caused by biocrusts and LCA. Biocrusts, diverse components in the Caatinga dry forest, were present in various successional stages, including agricultural fields, regenerating areas, and old-growth forests. Dominated by cyanobacteria, their coverage depended on factors like leaf-litter burial, disturbance levels, soil stability, seasonality, and the presence/ activity of LCA nests. A balance between vascular plant cover and disturbance pressure was also crucial for biocrust distribution. Both biocrusts and LCA impacted key abiotic factors for dry forest resilience but with significantly differing ecological consequences and reactions to anthropogenic disturbances. Biocrusts, by reducing water infiltration, promoted runoff, fostering small-scale source-sink patterns, benefiting vascular vegetation. They enhanced soil fertility and provided erosion protection, with older biocrusts exhibiting more significant positive effects. Anthropogenic disturbance disrupted biocrust succession, limiting their services and leading to negative feedback loops. LCA nests increased compaction, and reduced water infiltration, potentially hindering forest regeneration. These physico-hydrological barriers persisted, especially in disturbed areas, impacting forest dynamics and resilience for years, even after colony death. Adverse effects of LCA on water availability and soil resistance escalated with anthropogenic disturbance, though LCA refuse had the potential to mitigate some negative soil property changes. Both biocrusts and LCA act as edaphic ecosystem engineers in the Caatinga dry forest, impacting vascular plants through their abiotic influence. A greenhouse experiment demonstrated the positive effects of both organisms on plant germination, development, and survival across various functional groups. This dissertation also showed for the first time that LCA can accelerate germination time. These facilitative effects are attributed to improved soil conditions, including enhanced water availability and nutrient richness. Given species-specific responses and the prevalence of LCA nests and biocrust coverage in regenerating areas, their activities likely play a pivotal role in shaping successional trajectories and regeneration dynamics in dry forests. This underscores the significant potential of both ecosystem engineers in influencing the regeneration and resilience of tropical dry forests. In summary, in the human-modified landscapes of the Caatinga, biocrusts and LCA act as ecosystem engineers, influencing vital soil properties. Biocrusts protect degraded soils and facilitate plant establishment, while the impact of LCA depends on the nest structure. These engineers play a crucial role in dry forest regeneration and sustainability. However, climate change and land degradation pose significant threats to both ecosystems and engineers, impacting their effects diametrically. This research enhances understanding of the biome's functioning, regeneration, and resilience, providing insights for sustainable management, restoration, and conservation to support biodiversity and human well-being.Tropische Trockenwälder haben eine herausragende Bedeutung für entscheidende Ökosystemdienstleistungen wie Klimaanpassung, wirtschaftliche Entwicklung und Armutsbekämpfung. Dieses wenig erforschte und unzureichend geschützte Biom ist durch Abholzung und Landnutzungsänderungen gefährdet und wird oft in nationalen Politiken übersehen. Dies führt zu Risiken hinsichtlich sauberem Wasser, Lebensraumvielfalt und Klimaschutz. Veränderungen in der Landnutzung beeinflussen Umweltbedingungen, führen zu geringerer Biodiversität und Umstrukturierung der Vegetation. Der Regenerationsprozess basiert auf abiotischen Faktoren und natürlicher Bodenerholung. Diese Dissertation erforscht die Rolle von zwei Schlüsselorganismengruppen - biologische Bodenkrusten ('Biokrusten') und Blattschneiderameisen (BSA) - bei der Trockenwald-Regeneration. Diese Ökosystem-ingenieure verbessern die Bodenqualität, führen Nährstoffe und Wasser ein und beeinflussen die Pflanzenkeimung und -entwicklung, womit sie die Regeneration prägen. Hauptziele waren die Bestimmung der Biokrustenrelevanz im Caatinga-Trockenwald, ihre Interaktion mit BSA, ihre Bereitstellung von Ökosystemdienstleistungen und ihre Reaktion auf anthropogene Störungen. Verschiedene Techniken dokumentierten Biokrustenvielfalt und -verbreitung sowie Veränderungen der abiotischen Umwelt durch Biokrusten und BSA. Biokrusten, vielseitige Bestandteile des Caatinga-Trockenwaldes, wurden in verschiedenen Sukzessionsstadien, einschließlich landwirtschaftlicher Flächen, regenerierender Gebiete und alter Wälder, gefunden. Dominiert von Cyanobakterien hing ihre Abdeckung von Laub-bedeckung, Störungsniveau, Bodenstabilität, Saisonalität und der Präsenz/Aktivität von BSA-Nestern ab. Ein Gleichgewicht zwischen Gefäßpflanzenbedeckung und Störungsdruck erwies sich ebenfalls als entscheidend für die Biokrustenverteilung. Biokrusten und BSA beeinflussten wesentliche abiotische Faktoren für die Trockenwald-Widerstandsfähigkeit, jedoch mit unterschiedlichen ökologischen Konsequenzen und Reaktionen auf anthropogene Störungen. Biokrusten förderten durch Verringerung der Wasserinfiltration den Oberflächenabfluss, begünstigten kleine Quell-Senken-Muster, verbesserten Bodenfruchtbarkeit und boten Erosionsschutz, wobei ältere Biokrusten positivere Effekte zeigten. Anthropogene Störungen hemmten die Biokrustensukzession, begrenzten Dienstleistungen und führten zu negativen Rückkopplungs-Schleifen. BSA-Nester erhöhten Bodenverdichtung und reduzierten die Wasserinfiltrationskapazität. Diese physiko-hydrologischen Barrieren beeinflussten die Wald-Dynamik und Widerstandsfähigkeit über Jahre, selbst nach Kolonie-Tod. Die nachteiligen Auswirkungen von BSA verstärkten sich mit anthropogenen Störungen, obwohl BSA-Abfall das Potenzial hatte, einige negative Bodenveränderungen zu mildern. Biokrusten und BSA wirken als edaphische Ökosystemingenieure im Caatinga-Trockenwald, beeinflussen Gefäßpflanzen durch abiotische Effekte und zeigten in einem Gewächshaus-experiment förderliche Auswirkungen auf Pflanzenkeimung, -entwicklung und -überleben. Erstmals wurde gezeigt, dass BSA die Keimungszeit beschleunigen. Diese förderlichen Effekte werden auf verbesserte Bodenbedingungen (erhöhte Wasser- und Nährstoff-verfügbarkeit) zurückgeführt. Angesichts artspezifischer Reaktionen und der Verbreitung von BSA und Biokrusten in regenerierenden Gebieten spielen ihre Aktivitäten eine entscheidende Rolle bei der Gestaltung von Sukzessionstrajektorien und Regenerationsdynamiken in Trockenwäldern. Dies unterstreicht das erhebliche Potenzial beider Ökosystemingenieure zur Beeinflussung der Regeneration und Widerstandsfähigkeit tropischer Trockenwälder. Biokrusten und BSA sind Ökosystemingenieure in der modifizierten Landschaft der Caatinga und beeinflussen maßgeblich Bodeneigenschaften. Biokrusten schützen degradierte Böden und fördern die Pflanzenetablierung, während der BSA-Einfluss von der Neststruktur abhängt. Diese Ingenieure spielen eine entscheidende Rolle in der Regeneration und Nachhaltigkeit der Trockenwälder. Trotzdem bedrohen Klimawandel und Landdegradation diese Ökosysteme und Ingenieure erheblich, was deren Effekte diametral beeinflusst. Die Dissertation vertieft das Verständnis der Funktionsweise, Regeneration und Widerstandsfähigkeit dieses bedrohten Bioms und bietet Erkenntnisse für nachhaltiges Management, Restaurierung und Naturschutz zur Unterstützung von Biodiversität und menschlichem Wohlbefinden

Ecophysiological characterization of early successional biological soil crusts in heavily human-impacted areas

Ecophysiological characterizations of photoautotrophic communities are not only necessary to identify the response of carbon fixation related to different climatic factors, but also to evaluate risks connected to changing environments. In biological soil crusts (BSCs), the description of ecophysiological features is difficult, due to the high variability in taxonomic composition and variable methodologies applied. Especially for BSCs in early successional stages, the available datasets are rare or focused on individual constituents, although these crusts may represent the only photoautotrophic component in many heavily disturbed ruderal areas, such as parking lots or building areas with increasing surface area worldwide. We analyzed the response of photosynthesis and respiration to changing BSC water contents (WCs), temperature and light in two early successional BSCs. We investigated whether the response of these parameters was different between intact BSC and the isolated dominating components. BSCs dominated by the cyanobacterium Nostoc commune and dominated by the green alga Zygogonium ericetorum were examined. A major divergence between the two BSCs was their absolute carbon fixation rate on a chlorophyll basis, which was significantly higher for the cyanobacterial crust. Nevertheless, independent of species composition, both crust types and their isolated organisms had convergent features such as high light acclimatization and a minor and very late-occurring depression in carbon uptake at water suprasaturation. This particular setup of ecophysiological features may enable these communities to cope with a high variety of climatic stresses and may therefore be a reason for their success in heavily disturbed areas with ongoing human impact. However, the shape of the response was different for intact BSC compared to separated organisms, especially in absolute net photosynthesis (NP) rates. This emphasizes the importance of measuring intact BSCs under natural conditions for collecting reliable data for meaningful analysis of BSC ecosystem services

Dataset for penetration resistance, aggregate stabiltiy and infiltration of biocrusts of the Caatinga, NE Brazil

This is the data set belonging to the publication of Szyja et al., 2023: Biological soil crusts decrease infiltration but increase erosion resistance in a human-disturbed tropical dry forest. A closer description of material and methods can be found within the publication.  All data were collected in the Caatinga dry forest, NE Brazil.  It contains:  Raw data of the penetration resistance (measured using an electronic micro penetrometer) of wet and dry biocrusts and biocrust-free control soils (vegetation free topsoil [0-5 cm] exposed to local disturbance pressure) of two study sites of different disturbance pressure (disturbed vs. regenerating). Aggregate stability measures as: Calculated geometric mean weight diameter (GMWD) and raw data and calculated kinetic energy necessary to break biocrusts (n = 6 each) of the two investigated sites.  Infiltration measurements as: Calculated sorptivity of water and ethanol, repellency index, and unsaturated hydraulic conductivity of water and ethanol in biocrusts (n= 25) and control soils   (n=20) of the two study sites Soil organic carbon content per sample per site (n = 50 biocrusts and only n = 25 controls due to sampling design of SOC determination).  </ol

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

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

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

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

Data_Sheet_1_Biological soil crusts decrease infiltration but increase erosion resistance in a human-disturbed tropical dry forest.docx

Under continuous human disturbance, regeneration is the basis for biodiversity persistence and ecosystem service provision. In tropical dry forests, edaphic ecosystem engineering by biological soil crusts (biocrusts) could impact regeneration by influencing erosion control and soil water and nutrient fluxes, which impact landscape hydrology, geomorphology, and ecosystem functioning. This study investigated the effect of cyanobacteria-dominated biocrusts on water infiltration and aggregate stability in a human-modified landscape of the Caatinga dry forest (NE Brazil), a system characterized by high levels of forest degradation and increasing aridity. By trapping dust and swelling of cyanobacterial filaments, biocrusts can seal soil surfaces and slow down infiltration, which potentially induces erosion. To quantify hydraulic properties and erosion control, we used minidisc-infiltrometry, raindrop-simulation, and wet sieving at two sites with contrasting disturbance levels: an active cashew plantation and an abandoned field experiencing forest regeneration, both characterized by sandy soils. Under disturbance, biocrusts had a stronger negative impact on infiltration (reduction by 42% vs. 37% during regeneration), although biocrusts under regenerating conditions had the lowest absolute sorptivity (0.042 ± 0.02 cm s−1/2) and unsaturated hydraulic conductivity (0.0015 ± 0.0008 cm s−1), with a doubled water repellency. Biocrusts provided high soil aggregate stability although stability increased considerably with progression of biocrust succession (raindrop simulation disturbed: 0.19 ± 0.22 J vs. regenerating: 0.54 ± 0.22 J). The formation of stable aggregates by early successional biocrusts on sandy soils suggests protection of dry forest soils even on the worst land use/soil degradation scenario with a high soil erosion risk. Our results confirm that biocrusts covering bare interspaces between vascular plants in human-modified landscapes play an important role in surface water availability and erosion control. Biocrusts have the potential to reduce land degradation, but their associated ecosystem services like erosion protection, can be impaired by disturbance. Considering an average biocrust coverage of 8.1% of the Caatinga landscapes, further research should aim to quantify the contribution of biocrusts to forest recovery to fully understand the role they play in the functioning of this poorly explored ecosystem.</p