The artificial Hühnerwasser catchment has experienced a significant and monitored evolution since 2005, changing from a post-mining landscape to an almost fully vegetated ecosystem. The early stages showed a fast rate of ecohydrological evolution with changing dominating processes and feedbacks. The evolution of rill vegetation encroachment is one of such complex co-evolving processes.
We hypothesise that rill vegetation encroachment is driven by the evolution of the hydrologic/hydraulic regime of the rill network, which in turn affects the regime, potentially creating
a stabilising positive feedback. We further hypothesise that rill vegetation occurs later than hillslope vegetation, and follows a particular establishment and encroachment timeline in
response to the changing hydrological/hydraulic regimes. That is, the early runoff-dominated regime results in higher flows, velocities, transport and erosion capacity, thus favouring seed
flushing and seedling uprooting. On the other hand, as the system transitions from a runoffdominated into an infiltration- and ET-dominated system, flow, velocity, transport and erosion
capacity in the rill network are reduced, making seed establishment in the rills more likely. We explore these hypothesis with two complementary approaches: an analysis of the spatiotemporal distribution of vegetation and a process-based numerical modelling study. Firstly, we assess aerial photography of rill vegetation encroachment between 2007 and 2012 in terms of several vegetation types to derive temporal indicators of encroachment. The
analysis reveals that in the initial stages, a rill network developed in the hillslope. Shortly after vegetation first established on hillslopes, the rill network became progressively vegetated. Different pioneering species established heterogeneously, at different times and encroached into the rills at different rates. However, despite the volume of data, it is difficult to assess
which are the governing and limiting processes which respectively drive and constrain how and at which rate vegetation encroaches into the rills. In consequence, a pilot modelling study to identify the relative relevance of rill network geometry, bare soil infiltration, hillslope vegetation heterogeneity and intra-storm variability on the hydraulic response of the rill network and its possible impact on encroachment. The overall results suggest that vegetation
encroachment may be controlled by the rill network hydraulic regime, but such regime is the result of a complex superposition of responses of all the aforementioned factors, of which rill geometry appears to be a dominant one. Furthermore, the simulations showed
that vegetation spatial heterogeneity has an impact on the hydraulic regime coupled to the presence of temporal rainfall variability. Altogether, these results show that the governing coevolving
ecohydrological processes are interacting and are strongly affected by spatial and temporal heterogeneities.
The artificial catchment “Hühnerwasser” was built in a post-mining landscape, as a field experiment to observe and monitor early-development ecosystems at hillslope scale. Early on, rain-induced rill and channels formation was observed, followed by vegetation growth in between rills, and later on inside the rills.
In this work, we aim to describe the temporal evolution of the spatial distribution of rill vegetation. In general terms, we hypothesize four different encroachment patterns might occur: (1) The vegetation spreads from the top of the rills downstream. The underlying hypothesis would be a higher establishment probability due to a lower velocity of surface runoff and therefore less transport probability compared to downstream rill segments. (2)
The vegetation starts growing downstream and moves upwards. In this case we can hypothesize that the spatial distribution of the vegetation is dominated by water availability, which is higher or more stable downstream. (3) The vegetation encroaches from the sides into the rills, meaning that inter-rill vegetation governs vegetation encroachment inside the rills. (4) If no patterns are detected it might mean that the encroachment process depends
strongly on very local conditions, or perhaps that the underlying assumption of an initially homogeneous seed distribution is false.
To investigate the rills formation and rill vegetation encroachment processes, rills and vegetation patches inside these rills were identified and digitized from aerial photographs of the catchment from 2007 to 2012. Different vegetation types were identified based on the colour and texture of the patches. The geometrical properties of the rill segments and vegetation patches were used to define how the different vegetation types have distributed in space and how this distribution has changed over time. Rill mapping reveals a large increase of rill area from 2007 to 2008 and only a small rise from 2008 to 2009. Starting in 2010 dense vegetation prevents a precise mapping of the rills in the aerial photographs, so we assume that there is no change in rill area from 2009 to 2012. In 2007 vegetation covered only 1.4% of the rill area. There is only a small increase of this area in 2008 compared to the later years. In 2012 more than 50% of the rill area is covered by plants. Ten different vegetation types have been identified in the aerial photographs, starting with two types in 2007. By 2012 nine types are spotted in
the rills of the catchment. Some of these vegetation types show an encroachment from up- to downstream (e.g. Tussilago farfara), some from down- to upstream like Phragmites australis, whose creeping rhizomes can also promote such spreading. Others reveal no patterns. To better assess and identify the underlying processes leading
to these encroachment processes shown by the different vegetation types further data analysis –e.g. comparison with ground based vegetation mapping– and process-based hydrological modelling is necessary to fully explain these observations and assessing additional competition effects, which may be at play.
While there are hints that biological soil crusts (BSCs) can constitute physical barriers for the emergence of vascular plants, a conceptual approach for the quantitative evaluation of these effects is still missing. Here we present an experimental design to test the emergence of seedlings in situ with (i) capping natural intact, (ii) destroyed and (iii) removed BSC. The selected field site is directly adjacent to the constructed Hühnerwasser catchment (Lusatia,
Germany). This site exists since the end of 2008 and consists of loamy sand. Serving as proxy for seedling thrust, we inserted pre-germinated seeds of three confamiliar plant species with different seed masses (members of the Fabaceae family: Lotus corniculatus
L., Ornithopus sativus Brot., and Glycine max
(L.) Merr.). In each treatment as well as in the control group planting depths were 10 mm. We took care that experimental plots had identical crust thickness, slightly less than 4 mm, serving as proxy for mechanical resistance.
A plot became established as follows: Firstly, the pristine crusted surface was vertically cut. To the windward side the BSC remained intact (i: “with BSC” stripe). To the downwind side soil material was temporarily excavated for laterally inserting the seeds beneath the surface of the first stripe. Then at the thereby disturbed second stripe pulverised BSC material became filled as a top layer (ii: “BSC mix” stripe). From the next stripe the BSC was removed (iii : “no BSC” stripe). Thus each plot had each experimental group in spatial contiguity (within 50 cm * 50 cm). The overall 50 plots were distributed across an area of 40 m * 12 m. When individuals of a species either emerged at all stripes, “XXX”, or at no stripe of a plot, “– – –“, there was no reason to suppose any effect of a crust. The “–XX” emergence pattern (depicting the appearance of seedlings in both stripes possessing manipulated surfaces) points towards hindrance more clearly than “–X–” or “– - X”. Altogether eight possible combinations exist. Combinatorial analysis turned out that seedling emergence had been notably impeded for light-weighted seeds but little for heavy seeds. Repeated recordings enable to account for adaptable emergence of seedlings according to varying crust conditions – in spatial as well as temporal terms. The proposed experimental procedure hence is highly recommended as a viable instrument to further investigate filter and facilitation processes between BSCs and vascular plants.
The aim of this study was to detect a potential temporal shift from sink to source dynamics of pioneer plants during primary succession. The study was done in an initial ecosystem, namely an artificially constructed catchment on Pleistocene sands in eastern Germany. The import and export of seeds (including soil seed bank and seed rain), the development of vegetation cover and the population size of all pioneer species were recorded in 2005–2009. The interplay between the spatial distribution of seed rain and the prevailing wind direction provide evidence that seeds of Conyza canadensis (Asteraceae) first and to a considerable amount left the developing ecosystem. A change from a sink to source dynamics was detected already in the first year of both the existence of the site and occurrence of C. canadensis individuals within the site. Population patterns at the landscape unit scale demonstrated extensive auto-correlations in the coverage of this invasive plant at first. Our results underline the pioneer traits of C. canadensis as a prolific seeder and excellent wind disperser, which enables this species to become rapidly dominant over relatively large areas.
Successional phases describe changes in ecological communities that proceed in steps rather than continuously. Despite their importance for the understanding of ecosystem development, there still exists no reliable definition of phases and no quantitative measure of phase transitions. In order to obtain these data, we investigated primary succession in an artificial catchment (6 ha) in eastern Germany over a period of 6 years. The data set consists of records of plant species and their cover values, and initial substrate properties, both from plots in a regular grid (20 m × 20 m) suitable for spatial data analysis. Community assembly was studied by analyses of species co-occurrence and nestedness. Additionally, we correlated lognormal and log series distributions of species abundance to each community. We here introduce a new general method for detection of successional phases based on the degree of transient spatial homogeneity in the study system. Spatially coherent vegetation patterns revealed non-overlapping partitions within this sequence of primary succession and were characterized as two distinct ecological phases. Patterns of species co-occurrence were increasingly less random, and hence the importance of demographic stochasticity and neutral community assembly decreased during the study period. Our findings highlight the spatial dimension of successional phases and quantify the degree of change between these steps. They are an element for advancing a more reliable terminology of ecological successions.
Landscapes and ecosystems are complex systems with many feedback mechanisms acting between the various abiotic and biotic components. The knowledge about these interacting processes is mainly derived from mature ecosystems. The initial development of ecosystem complexity may involve state transitions following catastrophic shifts, disturbances, or transgression of thresholds. We propose a conceptual framework of feedback processes in early states of ecosystem development affected by spatiotemporal environmental drivers. To test this concept, we used 10-year time series of hydrological, biological, geomorphological, and soil data from the constructed catchment Chicken Creek.” The 6ha site was left to unrestricted development since 2005 and was intensively monitored. The data showed a very rapid development of the site with an increasing complexity and heterogeneity. In the first years, stochastic signals like the initial substrate conditions and external drivers like extreme weather events were the most important factors resulting in abiotic/abiotic feedback mechanisms shaping the morphology of the site and creating site diversity. Initial abiotic feedback mechanisms between water and substrate were soon followed by abiotic/biotic feedbacks between biological soil crusts, invading vegetation, geomorphology, and hydrology resulting in state transitions of catchment functioning.
Conclusions
(2011)