Self‐organised patchiness and scale‐dependent bio‐geomorphic feedbacks in aquatic river vegetation

Abstract

Spatial self‐organisation of ecosystems is the process by which large‐scale ordered spatial patterns emerge from disordered initial conditions through local feedbacks between organisms and their environment. Such process is considered important for ecosystem functioning, providing increased productivity, resistance and resilience against environmental change. Although spatial self‐organisation has been found for an increasing number of ecosystems, it has never been shown so far for aquatic river vegetation. Here we explore the existence of spatial self‐organisation of freshwater macrophyte patches in a typical lowland river (Belgium), showing that the underlying mechanisms for pattern formation are scale‐dependent feedbacks between plant growth, water flow and local river bed erosion and sedimentation. The mapping of vegetation patches showed that the frequency distribution of patch sizes is governed by a power‐law function, suggesting that the patches are self‐organised. Scale‐dependent feedbacks, likely to lead to this self‐organised pattern, were demonstrated with a mimic experiment. Both positive and negative feedbacks on plants were confirmed by a transplantation experiment. Placing vegetation patch mimics in the river showed experimentally that on a short range (within and behind the mimics) flow reduction and increased sedimentation occurred, while on a larger range (next to patches) the flow was accelerated and decreased sedimentation took place. By transplanting macrophytes within, next to and further away from existing patches, it was proven that the conditions within the patches favoured the survival and growth of transplants (i.e. short‐range positive feedback), while the conditions just next to patches led to decreased survival and growth (i.e. long‐range negative feedback).

Physical modelling of water, fauna and flora: knowledge gaps, avenues for future research and infrastructural needs

“…In recent years, much work has been done on feedbacks in "biogeomorphology" and/or "ecogeomorphology" (Fisher et al 2007, Darby 2010, Hession et al 2010, Reinhardt et al 2010, Rice et al 2010b, Wheaton et al 2011, Schoelynck et al 2012 …”

“…Macrophytes, for instance, often form in clustered groups of individuals of one or more species on the beds of rivers. The deposition of sediment and organic matter is promoted within these "patches" (Schoelynck et al 2012), making them bio-geochemical hotspots (Schoelynck 2011). There is also growing evidence that Silicon concentration in aquatic species litter is a major factor driving decomposition rates (Schaller et al 2012a(Schaller et al , 2012b, which can impact upon ecosystem productivity and nutrient availability and thus affect vascular plant distribution (Bragazza and Gerdol 2002).…”

Patches in a side-by-side configuration: A description of the flow and deposition fields

“…Rietkerk and Van de Koppel (2008) explained the process called spatial self-organization, in which large-scale, ordered spatial patterns occur because of feedbacks between small-scale landscape elements. Such landscape evolution has been demonstrated for a wide variety of ecosystems, such as mussel beds (van de Koppel et al 2005), vegetation on tidal flats (van Wesenbeeck et al 2008), and vegetation in lowland rivers (Schoelynck et al 2012). In each case, the introduction of an organism produces positive feedbacks (stress reduction, accumulation of nutrients) and negative feedbacks (stress enhancement, depletion of nutrients), which influence the pattern of growth.…”

“…reviewed in Nepf (2012). However, vegetation is often found in patches of finite length and width (Naden et al 2006, Temmerman et al 2007, Schoelynck et al 2012, rather than in continuous segments. As such, recent attention has been focused on the study of finite patches of vegetation, both in the lab and in the field (Cotton et al 2006, Bouma et al 2009, Zong and Nepf 2011.…”

“…Sites of deposition, in contrast, are where seeds and organic matter will tend to accumulate, leading to favorable conditions for plant growth (Gurnell et al 2005). Deposition of fine sediments in flow influenced by vegetation has been related to the characteristics of the mean and turbulent velocity field through laboratory studies (Chen et al 2012) and field studies (Sand-Jensen 1998, Cotton et al 2006, Schoelynck et al 2012. Chen et al (2012) modelled patches of emergent vegetation in a laboratory flume.…”