Patterns and processes of initial terrestrial-ecosystem development

J. Plant Nutr. Soil Sci. 2011, 174, 229–239

DOI: 10.1002/jpln.201000158

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Review Article

Patterns and processes of initial terrestrial-ecosystem development Wolfgang Schaaf1*, Oliver Bens2, Anton Fischer3, Horst H. Gerke4, Werner Gerwin5, Uwe Grünewald6, Hartmut M. Holländer 6, Ingrid Kögel-Knabner7, Michael Mutz8, Michael Schloter9, Rainer Schulin10, Maik Veste5, Susanne Winter3, and Reinhard F. Hüttl1 1

Soil Protection and Recultivation, Brandenburg University of Technology, P. O. Box 101344, 03013 Cottbus, Germany German Research Center for Geosciences GFZ, Telegrafenberg, 14473 Potsdam, Germany 3 Geobotany, Department of Ecology and Ecosystem Management, Technische Universität München, Hans-Carl-von-Carlowitz-Platz 2, 85354 Freising, Germany 4 Institute of Soil Landscape Research, Leibniz Center for Agricultural Landscape Research (ZALF), Eberswalder Straße 84, 15374 Müncheberg, Germany 5 Research Center Landscape Development and Mining Landscapes (FZLB), Brandenburg University of Technology, P. O. Box 101344, 03013 Cottbus, Germany 6 Hydrology and Water Resources Management, Brandenburg University of Technology, P. O. Box 101344, 03013 Cottbus, Germany 7 Lehrstuhl für Bodenkunde, Technische Universität München, Department of Ecology and Ecosystem Management, 85350 Freising, Germany 8 Freshwater Conservation, Brandenburg University of Technology, Seestraße 45, 15526 Bad Saarow, Germany 9 Department of Terrestrial Ecogenetics, Institute of Soil Ecology, Helmholtz-Zentrum München, German Research Center for Environmental Health, Ingolstädter Landstraße 1, 85746 Neuherberg, Germany 10 Soil Protection Group, Institute of Terrestrial Ecosystems, ETH Zürich, Universitätstraße 16, 8092 Zürich, Switzerland 2

Abstract Ecosystems are characterized as complex systems with abiotic and biotic processes interacting between the various components that have evolved over long-term periods. Most ecosystem studies so far have been carried out in mature systems. Only limited knowledge exists on the very initial phase of ecosystem development. Concepts on the development of ecosystems are often based on assumptions and extrapolations with respect to structure–process interactions in the initial stage. To characterize the effect of this initial phase on structure and functioning of ecosystems in later stages, it is necessary to disentangle the close interaction of spatial and temporal patterns of ecosystem structural assemblages with processes of ecosystem development. The study of initial, less complex systems could help to better identify and characterize coupled patterns and processes. This paper gives an overview of concepts for the initial development of different ecosystem compartments and identifies open questions and research gaps. The artificial catchment site “Chicken Creek” is introduced as a new research approach to investigate these patterns and processes of initial ecosystem development under defined boundary conditions. This approach allows to integrate the relevant processes with related pattern and structure development over temporal and spatial scales and to derive thresholds and stages in state and functioning of ecosystems at the catchment level. Key words: soils / pedogenesis / vegetation succession / microbial succession / artificial catchment

Accepted August 27, 2010

1 Introduction Ecosystems are highly complex systems composed of many different abiotic and biotic compartments that are closely linked by interacting processes and codevelop over long-term periods. The transport and cycling of water and elements integrates all compartments over scales and relates processes and patterns to the overall functioning of an ecosystem. Most integrating studies have been carried out in “climax” ecosystems (e.g., Pennisi, 2010), but only limited knowledge exists about the initial phase of ecosystem development although it is hypothesized that the conditions at “point zero” and the processes of the initial phase determine and control further development (e.g., Lichter, 1998a) and

that “small differences early in the process can ramify over time” (Walker and del Moral, 2003). It is rarely possible to study the “point zero” of ecosystem development and primary ecosystem genesis under natural conditions. Examples are landscapes that have been completely destroyed, transformed or even newly created by volcanic activity such as Mount St. Helens (Bishop, 2002; Dahlgren et al., 1999; del Moral and Wood, 1993) and Hawaii (Müller-Dombois and Fosberg, 1998) in the USA or Surtsey Island on Iceland (Friðriksson, 2005). Other examples of initial ecosystems are found in glacier retreat areas in arctic or alpine environments

* Correspondence: Dr. W. Schaaf; e-mail: [email protected]

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(Cooper, 1923; Matthews, 1992), tectonic uplift zones along coastlines, coastal and inland sand dunes, areas like the newly exposed seashores of the Dead Sea and the Aral Sea (Aloni et al., 1997; Breckle, 2002; Dimeyeva, 2007) or marsh (Gröger et al., 2009). In Central Europe today, natural initial ecosystem development can rarely be observed and it is mostly restricted to small-scale areas like inland dunes or stretches of wild rivers. Due to this small-scale occurrence, the initial development is strongly influenced by surrounding landscapes. Anthropogenic disturbances can result in large-scale impacts on landscapes creating new land surfaces and spatial patterns (Walker and Willig, 1999). An example for these types of large-scale disturbances initiating new development is open-pit mining of metals or lignite (Bradshaw, 1983; Hüttl and Weber, 2001; Schaaf, 2001; Zikeli et al., 2002). A methodological approach to study the development of ecosystems are chronosequence studies. By substituting time with space it is possible to extend the time scale for investigations (Pickett, 1989). The value of chronosequences is often restricted by the difficulty in finding sites that are really comparable with respect to all environmental factors except for time (Hugget, 1998). Given sufficient knowledge on processes, it is possible to reveal chronofunctions, e.g., for pedogenesis (Johnson et al., 1990). However, for highly complex systems the degree of process understanding is often incomplete (Schaetzl et al., 1994). Further uncertainties exist due to the insufficient knowledge of the “point zero” of development. Chronosequences have been used to study weathering rates and soil formation (Hodson et al., 1998; Lichter, 1998b; Turk et al., 2008; White et al., 1996), pedogenesis (Harden et al., 1991; Jenny, 1941; Kennedy et al., 1998; Scalenghe and Ferraris, 2009; Stevens and Walker, 1970), vegetation succession (Fischer, 1982; Kremsater and Bunnell, 1998; Millner and Gloyne-Phillips, 2005), and forest growth (Blackwell and Trofymow, 1998) over long time periods. Whereas individual components of ecosystems have been studied in detail during ecosystem development, less attention has been directed to the complex interaction of ecosystem components during codevelopment and the interacting effects of spatial patterns and processes on the development of ecosystem functions and stages (Chadwick and Chorover, 2001; Groffman et al., 2006; Kelly et al., 1998; Lambers et al., 2009; Torrent and Nettleton, 1978; van Breemen et al.,

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2000). Current research indicates the importance of hot spots and patches as starting points of initial development (Ludwig et al., 2000; McClain et al., 2003). The objective of this paper is to give an overview of the current knowledge, existing concepts, open questions, and research gaps about the initial development of ecosystems and their components (e.g., soil, vegetation, microbial communities, biological crusts). Special emphasis is put on the importance of temporal and spatial patterns and their interaction, interdependency and codevelopment with relevant processes of pedogenesis, vegetation succession, soil structural development, and microbial succession during the initial phase of ecosystem development.

2 Initial soil formation The basic conceptual frameworks of pedogenesis are mainly based on changes in soil properties over time (Bockheim, 1980; Dokuchaev, 1893; Jenny, 1941; Yaalon, 1975). Rate and direction of development are controlled by both exogenic and endogenic factors (e.g., Heimsath et al., 1997). During this course, the soil system “accumulates” properties and reaches new stages by transgressing thresholds (Bockheim et al., 2005; Chadwick and Chorover, 2001; Schaetzl et al., 1994). Chronofunctions can be derived by inferring temporal patterns of soil properties across chronosequences. Examples are processes of mineral weathering, formation of soil horizons, leaching, decalcification, secondary mineral formation, the occurrence of buffering systems, or the stability of solid phases. In many cases (cf. references above), these delineations involve uncertainties due to extrapolations from and estimations of unknown conditions at “point zero” of development with insufficient information on initial offsets and are often enhanced by large time gaps between “point zero” and the first chronosequence stage (Fig. 1). The concept of interacting processes and their significance for soil formation are mostly based on chronosequence studies or on delineations from already well-developed soils (Lichter, 1998b; Melkerud et al., 2003). Several studies have shown the importance of the interaction of the soil mineral phase with the biological components of the ecosystem (Banfield et al., 1999; Jongmans et al., 1997; Kelly et al.,

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Figure 1: Problems and uncertainties in the conceptual approach of soil formation as derived from chronosequence studies.

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1998; Smits et al., 2005; van Breemen et al., 2000). For example, roots and the composition of soil solution as affected by biochemical processes are influencing mineralweathering rates (Lindroos et al., 2003; Raulund-Rasmussen et al., 1998). Soil solution and solid phases are therefore tightly coupled with the solid phase acting both as a source for nutrients and as a sink as could be shown for example for the stabilization of soil organic matter (Eusterhues et al., 2005; Guggenberger and Kaiser, 2003; Hagedorn et al., 2003; Kalbitz et al., 2000; Kögel-Knabner et al., 2008). For the initial phase of ecosystem genesis, the temporal development and the spatial patterns of these interactions between solute and solid phases are largely unknown (Fig. 2). For mineral soils, the solid phase is made of sediment particles of certain textures that are initially organized in space according to the deposition process. Initial soil development may vary in a wide range depending on the textural and mineral composition and the spatial arrangement of the parent material. Unconsolidated coarsely textured sediments undergo shrinkage upon wetting leading to increased bulk densities and crack formation. In finely textured sediments, aggregate formation and strength depend on biological activity as well as on the intensity, number, and timing of swelling and drying events or freezing-and-thawing cycles. Aggregate formation enhances spatial heterogeneity of physical and chemical properties by splitting the original sediment into a denser intraaggregate matrix and a more continuous interaggregate pore network (Horn and Smucker, 2005). Bioturbation, on the other hand, may counteract these separations (Wilkinson et al., 2009). Soil-structure formation (i.e., cracks, aggregates, and biopores) may allow rainwater to infiltrate and flow preferentially along these pathways and to bypass the rest of the porous soil matrix. Such preferential-flow paths can form porous structures in the soil that are important hot spots for initial development. Transport of dissolved organic matter (DOM) and nutrients to the subsoil is often channeled along preferential-

flow paths (Hagedorn et al., 2000; Hagedorn and Bundt, 2002) resulting in an increased biological activity and root growth along these patterns (Bundt et al., 2001). As found in the above studies for established forest soils, preferentialflow paths may similarly serve as zones of enhanced mineral weathering and element transformation in fresh sediments during ecosystem development. Developing macropores represent microsites in the soil that are more biologically active, and often more chemically reactive than the bulk soil (Jarvis, 2007), in particular if coatings along pore walls develop in structured soil (Mallawatantri et al., 1996). Little is known on the dynamics of soil structure in relation to initiation of macropore flow (Ahuja et al., 2006). The surface topography formed via soil, plant, and climate interactions, creates local niches for vegetation establishment (GutierrezJurado et al., 2006), which may lead to differences in soil-development rates. These structure-and-process interactions provide a feedback between vegetation, soil water fluxes, and geomorphic processes in soils and watersheds (Lin et al., 2005). Flow patterns seem to be characteristic for each development stage. In this sense, soil formation follows these initial patterns depending on initial soil structures. The accumulation and formation of stable soil organic matter (SOM) is an essential process in pedogenesis and ecosystem development. The type and properties of accumulated SOM control element fluxes at different scales from microsite to catchment and may also influence vegetation succession. The initial phase of soil development is characterized by a strong increase of SOM content (Šourková et al., 2005). Longer chronosequence studies show a decrease and then leveling off of the accumulation rate at a soil-specific steady state (Harden et al., 1991; Lichter, 1998a; Turk and Graham, 2009). The time until steady state is reached in soils developed on coastal dunes of Lake Michigan is 440 y for the total amount of organic C (Lichter, 1998a) and for forest-floor horizon thickness 285 y. In contrast, Harden et al. (1991) report that steady state for organic C is not reached within more vegetation (patterns)

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Figure 2: Important processes (arrows) and compartments (rectangles) of initial soil development. Black boxes and brackets indicate insufficient existing knowledge on sections of the diagram (see also Fig. 3).

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than 8000 y in soils developed after deglaciation of the Laurentian ice sheet. These discrepancies may be due to different climatic conditions as well as the soils being in different stages of development controlled by pedogenetic thresholds (Chadwick and Chorover, 2001).

most studies lack defined characteristics of the starting conditions at “point zero”. Data are necessary on volumetric changes, spatial distribution of patterns, related processes, and codeveloping ecosystem functions (Fig. 2).

Only few studies report changes in the composition of SOM during pedogenesis (e.g., Skjemstad et al., 1992). In a pot experiment, under grass over 34 y increasing amounts of organic matter (OM) accumulated during the experiment were found in the medium- to fine-silt and clay fraction (Leinweber and Reuter, 1992). The OM in the clay fraction displayed a higher thermal stability upon pyrolysis than the OM associated with the silt fractions. These more stable organomineral associations developed only in the last two decades of the experiment. 13C-NMR-spectroscopic analysis of the clay fraction revealed a dominance of alkyl-C compounds, ascribed to a microbial origin.

3 Development of the root zone

The quantity and composition of soil organic substances accumulated during initial soil formation have a decisive influence on structure and physical parameters of soils. Hydrophobicity may affect hydraulic properties, hydrological processes, water content, structure, and humus content of soils. Water repellency results in a pronounced hysteresis of the water-retention function (Bauters et al., 2000), reduced infiltration capacity (Lamparter et al., 2006; Wahl et al., 2003) and consequently increased surface runoff and erosion (Lemmnitz et al., 2008) as well as preferential flow (Ritsema et al., 1998). Hydrophobic interfaces were found to increase aggregate stability (Goebel et al., 2005) and to affect the stabilization and accumulation of SOM (Spaccini et al., 2002). Most studies on interactions of mineral phases and SOM during ecosystem development are not comprehensive in terms of the parameters collected (cf. Totsche et al., 2010). Future investigations should examine the evolution of both the amount and composition of OM and the interaction with mineral surfaces during soil development to allow for the derivation of chronofunctions. As pointed out by Lichter (1998b),

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Roots are key factors in soil formation and development. The processes by which roots interfere with pedogenesis are manyfold and characterized by complex mutual interactions (e.g., Lambers et al., 2009). Roots both are influenced by existing soil structures and create soil heterogeneities (Fig. 3). Separating root-derived from pre-existing soil heterogeneities is usually very difficult. Roots are a primary source of OM in soil, through the release of organic exudates, including chelators, enzymes, and mucilages, and the turnover of their biomass (Hütsch et al., 2002). Rhizodeposition and microbial re-synthesis are major pathways for the entry of chemically stabilized OM into soil (Bottner et al., 1999). The so-called rhizosphere effect, i.e., the establishment of different conditions in the immediate surroundings of roots as compared to the bulk soil, is created by the exudation of organic compounds, the release of protons and CO2, the consumption of oxygen, the uptake of nutrients and water (Hinsinger et al., 2005). In nutrient-poor soils, which are typical for the early phase of ecosystem development, a dominating influence on root growth may come from spatial and temporal variations in the availability of essential nutrients forcing roots to grow into the vicinity of such nutrients to capture sufficient amounts (Robinson, 1994). The patterns which plants show in the allocation of assimilates to root growth can be interpreted as strategies (Canham et al., 1996). Knowing these strategies could provide an important key to understand soil structural development (Fig. 3). Patchiness in soil nutrients was found to increase root growth in various studies (e.g., Jackson and Caldwell, 1996), vegetation (patterns)

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assimilate structure, solution composition allocation roots/ exudates, heterogeneity water + nutrient availability rhizosphere soil (cf. Fig. 1) root-derived vs. preexisting heterogeneity

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Figure 3: Important processes (arrows) and compartments (rectangles) of initial biotic ecosystem development. Black boxes with arrows indicate insufficient existing knowledge of effects on processes and compartments (see also Fig. 2).

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J. Plant Nutr. Soil Sci. 2011, 174, 229–239 although others did not find such an effect (e.g., Kembel and Cahill, 2005). The positive effect of patchiness on plant growth was related to precision foraging, the capability of root systems to increase growth and activity locally at spots of increased nutrient concentrations (Hodge, 2004). Plant responses to soil nutrient patchiness are affected by many factors, including plant species, genotype, nutritional status, and developmental stage, size, and duration of patches, intensity of patchiness (magnitude of contrasts in nutrient concentrations between nutrient-rich patches and surrounding soil), as well as availability and distribution of other nutrients and water (Yano and Kume, 2005). Thus, the competition for heterogeneously distributed nutrients and an eventual stabilization of patchiness by the vegetation in the initial phase of succession may become crucial for the development of root-induced soil structures during subsequent stages (Aerts, 1999). Studying the initial stages of ecosystem development offers an opportunity to disentangle the interactions between soil and roots leading to the formation of soil structures and patterns (Fig. 3).

4 Succession of microbial communities Microbes are responsible for an effective element turnover in soil and supply plants with nutrients (Sharma et al., 2005). Best understood are symbiotic interactions between plant and microbes, e.g., the interplay between mycorrhizal fungi and plants or the association between rhizobia and legumes. Most important in these cases is the “communication” between microbes and their host plants by specific signal molecules (Pierson and Pierson, 2007). There is a lack of knowledge about the mode of interaction between plants and microbes in developing ecosystems (Fig. 3). Despite some nutrient-rich patches, which are quickly colonized by plants (see above), mainly ammonia and nitrate are in general limited in the initial phase of ecosystem development in soil and inhibit plant growth. Therefore plants must rely on an efficient mobilization and transport of bio-available forms of N by microbes. However, as plant productivity in these young ecosystems is often reduced or assimilates are used to produce secondary metabolites to protect itself for grazing or competition, which results in low amounts of C input into the soil, microbial activities, also in the rhizosphere, are consequently ignoble. This in turn results in low mineralization rates of N and missing N for plant growth. How and at what stage of ecosystem development this vicious circle is burst is completely unclear. Maybe “fungal highways” (Warmink and van Elsas, 2009) play a key role in transporting nutrients and organisms from nutrient-rich hotspots in soil (e.g., biological crusts) to other soil compartments. Most studies on the development of microbial-community structure and function during the initial phase of ecosystem formation have been carried out on glacier forefields indicating an increase in biomass over time and changes in microbial-community structure during ecosystem development (Ohtonen et al., 2005; Sigler et al., 2002; Tscherko et al., 2004). Bardget and Walker (2004) showed that during ecosystem development, plants are the drivers for the evolution of microbial-community structure in soil.  2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Initial terrestrial-ecosystem development 233 Changes in the complexity of the microbial-community structure during ecosystem development also induce changes in microbial functionality (Tscherko et al., 2003). Rhizodeposition and exudation of the same plant changed along a gradient of a glacier forefield (Edwards et al., 2006), which could influence microbial activity, functionality, and community structure in addition to increased nutrient availability along the gradient. These results clearly indicate a close relation between ecosystem development, plant-community composition, and microbial-community structure and function (Fig. 3). However, direct links between plant and microbial communities are still missing. By following the fluxes of stable-isotope labels from exudates respectively plant litter into the microbes, it is possible to directly link plant performance and microbial functionality (Griffiths et al., 2004). This kind of investigation may help to identify microbial food webs during the initial phase of ecosystem development (Rangel-Castro et al., 2005). Initial ecosystems would be also an ideal playground for testing theories of general ecological relevance. For example, whereas in macroecology the role of functional redundancy for the stability or resilience of ecosystems has been studied in great detail (Loreau, 2004), due to the complexity of microbial communities in soil, there is no data available to supporting or declining this theory for microbial ecology. If this theory is valid also for microbiota living in soils, then initial ecosystems, with reduced diversity pattern of microbes, should be disturbed by biotic and abiotic stressors more easily than climax ecosystems, with well-developed microbial populations. However, to perform this type of studies, (molecular) approaches targeting the overall functional gene respectively transcript pools in soils are needed to connect this data with corresponding turnover rates and fluxes of nutrients (Sharma et al., 2006). New high throughput sequencing strategies may help us to reach this goal (Urich et al., 2008).

5 Formation of biological soil crusts—an example of biotic and abiotic interaction in initial ecosystems First colonizers of new land surfaces are cyanobacteria, green algae, mosses, liverworts, lichens, fungi, and bacteria. These organisms bound together the soil particles with organic material forming biological soil crusts (BSC), which cover the first millimeter of the top soil (Belnap and Lange, 2001). The timeframe for the establishment of these crust communities and their succession ranges from a few years on arid sand dunes (Veste et al., 2001) up to more than a century for soil lichens communities (Eldrige and Greene, 1994). Biological soil crusts are highly stress-tolerant under extreme environmental conditions and therefore widespread in many ecosystems from deserts to polar regions. Biological soil crusts stabilize the soil against water and wind erosion. The development of soil surface crusts influences vegetation pattern formation through adaption to the physico-chemical conditions. Biological N fixation by free-living cyanobacteria and by cyanobacteria in lichens is an important N input pathway in nutrient-limited ecosystems (Russow et al., 2005; Veluci et al., 2006). Mucilaginous material exuded by cyanobacteria and mosses binds to mineral particles resulting in a www.plant-soil.com

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stabilization of the topsoil, reduction of soil erosion, and accumulation of OM in the first millimeters of the topsoil (Veste et al., 2001). Biological soil crusts play a key role in hydrological processes, e.g., by decreasing soil infiltration rates (Fischer et al., 2010; Yair et al., 2008). Surface stabilization and N fixation are positive effects of BSC for the establishment of vascular plants during ecosystem development (see Fig. 3; Beyschlag et al., 2008). The importance of microscale structures within the BSC for ecosystem processes and functions is, however, not fully understood (Bowker et al., 2010).

6 Vegetation succession Two contrasting succession concepts for vegetation development on bare surfaces were developed by Egler (1954): (1) The “Relay Floristics” concept, where first invaders improve soil conditions by OM accumulation and root penetration into the substrate bringing OM to deeper layers; growing conditions for plants improve and new groups of species with higher demands, e.g., on water and nutrient availability, invade step by step. (2) The “Initial Floristic Composition” (IFC) concept where all species that will establish in the future already occur at the beginning of succession; while soil conditions are improving, successive groups of species out of the represented species pool will become dominant. Wilson et al. (1992) modified the IFC and distinguished between complete initial floristics, i.e., all species that are likely to be involved in the succession are present at the beginning, and preemptive initial floristics in the sense that from the regional species pool certain species arrive early, and theses species will influence the course of succession for a long time. These concepts were applied to both natural habitats and to vegetation dynamics in intensively managed landscapes (e.g., Felinks et al., 1999; Tischew, 2004). General outcomes from vegetation succession studies are: (1) Conditions at “point zero” are important determinants for further vegetation development. According to the IFC concept, the first established species have a chance to regulate the species composition and vegetation structure for a longer time period because they occupy all suitable growing sites at the very beginning. (2) Disturbances open up closed vegetation cover and facilitate additional species to expand the vegetation composition. In resource-limited ecosystems, spatial heterogeneity is an important factor for the development of vegetation. Nitrogenfixating plants increase soil nitrogen content and create “fertile islands”, which facilitates plants in the near neighborhood (e.g., Jacob et al., 2005). These findings support conceptual models for the initial colonization, which emphasize nucleation, growth, and aggregations of discrete patches of pioneer vegetation (Franks, 2003). Cutler et al. (2008) tested three different models of spatial dynamics of primary succession on a chronosequence of volcanic substrate on Iceland: (1) a classic model, in which internal processes continue to dominate and later-stage species expend their population through patch growth; (2) a patch-dynamics model, where life history, biotic interactions, and disturbance plays an important role in creating gaps which revert to the pioneer stage; and (3) a geo-ecological model, which emphasizes the interactions of  2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

J. Plant Nutr. Soil Sci. 2011, 174, 229–239 physical and biological processes resulting in a mosaic of patches at different stages of development. In the chronosequence study on volcanic substrate, pioneer nucleation in safe sites at microscales occurred in the early stages of ecosystem development and could be described by the patchdynamics model. Spatial patterns in later stages of succession support the geo-ecological model, where spatial differentiation of higher vegetation relates to mesoscale substrate topography. The integration of ecosystem processes at different scales and their importance for the vegetation development at the ecosystem level needs further field studies and is still a challenge for ecological research.

7 Initial formation of surface waters Based on investigations of stream succession after disturbance in deserts, and primary succession in volcanic, glacier retreat, and post-mining areas (Mutz et al., 2002; Milner and GloynePhillips, 2005), initial stream development is expected to follow a sequence of three stages: (1) initial open-land stage without riparian vegetation and in-stream vascular plants characterized by the presence of benthic algae; (2) open-land stage with vascular plants along and in the channels; (3) riparian stage, characterized by the presence of woody riparian vegetation and instream dead wood. Each of these plant communities acts as a source for organic-C transformation within the stream. With transition from one to the following stage, OM of new quality is delivered to the stream increasing the amount of total OM. Vegetation has the potential to function as a critical physical structure affecting the vertical connectivity between the channel and deeper bed sediments and on hyporheic flowpaths, thereby controlling metabolic potential and C transformation (Fellows et al., 2001; Malard et al., 2002). Benthic algal mats can clog the stream bed and inhibit vertical exchange (Battin and Sengschmitt, 1999). In-stream vascular plants influence flow and sedimentation in the channel, altering patterns in directions and magnitude of vertical exchange (Salehin et al., 2003). In-stream wood increases vertical water exchange and the depth of bed sediments that participate in stream metabolism (Mutz et al., 2007). In semiarid landscapes and drying streams, the availability of water increases microbial processes, decomposition of OM, and colonization of vascular plants (Belnap et al., 2005; Langhans and Tockner, 2006). Hence, streams and riparian areas are likely to be “hot spots” for C transformation (McClain et al., 2003). If disturbance by flash floods and bed scour is moderate, vascular plant colonization initiates the streams transition to stage 2. Transition to stage 3 was observed in a chronosequence following glacial recession in Alaska (Milner and Gloyne-Philips, 2005). More than 90% of the banks were covered with vegetation within four decades, although significant accumulation of large wood was not found until ≈ 130 y of stream development. The shifts in energy sources, environmental factors and vertical connectivity with the transition among the three suggested initial development stages, are likely to set off shifts in the streams microbial structure, transformation, and accumulation of C. However, no data on these processes exist to our knowledge. www.plant-soil.com

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8 An artificial-catchment approach to study patterns and processes of initial ecosystem development The above described interacting dynamics of structures and processes in the early development stages are manifested by affecting the water and element balance of the system. For a hydrological watershed or defined catchment area, the output signal reflects the integrated effects of all processes and interrelations (Likens, 1985). The signal variations may be used to identify the complexity of system structural changes (Feng et al., 2004; Kirchner, 2003). Nevertheless, quantitative methods how to describe smaller-scale processes and complex interrelation with structure changes converted into the integrated signal from catchments are not available. Since the initial and boundary conditions of natural catchments are mostly not well-defined, an artificially created catchment was constructed to combine process-oriented research on initial development of ecosystems with interactions and codevelopment of spatial patterns and structures. The approach focuses on the processes during the initial phase which are affected by existing and newly emerging patterns and their interdependencies to test if the initial phase of ecosystem development controls and determines the later state of ecosystems. Our objective is to enhance integral models of structure genesis in ecosystems and of process dynamics as well as their interactions during the initial development phase. The aim is to integrate these feedback mechanisms in the analysis of water and element budgets at the catchment scale and to implement them into models. To allow the clear definition of as-homogeneous-as-possible starting conditions at “point zero” and to be able to integrate spatially distributed processes and patterns to larger units, an artificial catchment was constructed in the mining area of Lusatia/Germany as the main research site (Gerwin et al., 2009). With an area of ≈ 6.5 ha, this catchment “Chicken Creek” is to our knowledge the largest artificial catchment worldwide. It was constructed as a 2–4 m layer of postglacial sandy to loamy sediments overlying a 1–2 m layer of Tertiary clay that forms a shallow pan and seals the whole catchment at the base (Fig. 4). No further measures of restoration like planting, amelioration, or fertilization were carried out to allow natural succession and undisturbed development. Due to the

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Figure 4: Aerial view and main features of the artificial catchment “Chicken Creek” near Cottbus, NE Germany in May 2007, 18 months after finishing of the construction work.

artificial construction, boundary conditions of this site are clearly defined including well-documented inner structures as compared to natural catchments. It is assumed that the interaction of patterns and processes during initial development will proceed from simpler to more complex states of the systems and that different stages along this phase can be identified at the catchment level (Fig. 5). Changes within the catchment and development of biotic and abiotic structures and patterns are intensively monitored since 2005, when construction finished (Schaaf et al., 2010).

9 Concluding remarks and perspectives To study patterns and processes of initial ecosystem development at an artificial catchment is a novel and promising approach to disentangle the complex interactions and feedback mechanisms typically found in mature ecosystems and to understand the relevance and importance of initial conditions on further development and future state of an ecosystem. This artificial catchment can, however, not be regarded as representative nor be reproducible at the catchment level. This approach allows to develop tools for the integrative analysis of effects of relevant processes together with related

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Figure 5: Conceptual model of dominating patterns and processes during initial ecosystem development.

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pattern and structure development over temporal and spatial scales and to derive thresholds and stages in state and functioning of ecosystems at the catchment level. Beside these fundamental objectives and questions, the approach aims at transforming its result to more applied problems like improving hydrologic catchment models as well as restoration measures and management options for disturbed or degraded landscapes. This brief review of research gaps suggests that the integrated analysis of patterns and processes of initial terrestrialecosystem development can be improved by the combination of existing concepts from the individual fields of research with water and element budgets in a catchment experiment that comprises all main system components. It is obvious that this unique site does not allow for replications or treatments, it is used for monitoring, observations, and method development, while additional studies are carried out at a nearby experimental site with the same sediments but without clay layer, and under controlled glasshouse and laboratory conditions. To include the impact of environmental site conditions on initial development, comparable studies are carried out using a chronosequence approach, e.g., along the forefield of the Damma glacier in Switzerland. The project is also thought to encourage similar studies in different regions, climates, and regoliths by other research groups.

Acknowledgments The Transregional Collaborative Research Centre (SFB/ TRR) 38 “Structures and Processes of the Initial Ecosystem Development Phase in an Artificial Water Catchment” (www.tu-cottbus.de/sfb_trr) funded by the Deutsche Forschungsgemeinschaft (DFG) and by the Brandenburg Ministry of Science and Research was established in July 2007 as an initiative of three universities (BTU Cottbus, TU München, and ETH Zürich). The artificial catchment “Chicken Creek” was constructed with the technical and financial support provided by Vattenfall Europe Mining. We thank three reviewers for valuable comments and suggestions.

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