Plants living on the edge

207 Pages·2016·27.13 MB·English

Plants living on the edge
Colonization processes of aquatic and riparian vegetation
along restored lowland streams
Rob G.A. Fraaije
Thesis committee: Dr. A. Baattrup-Pedersen
Prof.dr. E. van Donk
Prof.dr. H. de Kroon
Prof.dr. J. Silvertown
Prof.dr.ir. P.F.M. Verdonschot

Photo courtesy of: R.G.A. Fraaije and M.B. Soons
Layout and printing: Print Service Ede, Ede, the Netherlands
ISBN: 978-94-91602-67-2
This thesis should be cited as:
Fraaije R.G.A. (2016) Plants living on the edge: colonization processes of aquatic and
riparian vegetation along restored lowland streams. PhD thesis. Utrecht University,
Utrecht, the Netherlands
Plants living on the edge: colonization processes of aquatic and
riparian vegetation along restored lowland streams
Planten die op de rand leven: kolonisatie processen van beek- en oevervegetatie
langs herstelde laaglandbeken
(met een samenvatting in het Nederlands)
Proefschrift
ter verkrijging van de graad van doctor aan de Universiteit Utrecht
op gezag van de rector magnificus, prof.dr. G.J. van der Zwaan,
ingevolge het besluit van het college voor promoties in het openbaar te verdedigen
op woensdag 31 augustus 2016 des ochtends te 10.30 uur
door
Robertus Gerardus Aloijsius Fraaije
geboren op 23 augustus 1984 te Ammerzoden
Promotor: Prof.dr. J.T.A. Verhoeven
Copromotor: Dr. M.B. Soons
This study was supported by Agentschap NL (Rijksdienst voor Ondernemend
Nederland), the Foundation for Applied Water Research (STOWA) and grants from
the Netherlands Organization for Scientific Research (NWO) to M.B. Soons: NWO-ALW
Meervoud and Vidi grants.
Contents
Chapter 1 7
General Introduction
Chapter 2 21
Elevational patterns of hydrochorous seed deposition along stream
riparian gradients
Chapter 3 51
Early plant recruitment stages set the template for the development of
vegetation patterns along a hydrological gradient
Chapter 4 85
Dispersal versus environmental filtering in a dynamic system: drivers of
vegetation patterns and diversity along stream riparian gradients.
Chapter 5 119
Functional responses of aquatic and riparian vegetation to hydrogeomorphic
restoration of lowland streams and their valleys
Chapter 6 153
Synthesis
References 167
Summary 185
Nederlandse samenvatting 191
Dankword 19
Author’s affiliations 203
Publications 205
About the author 207
Chapter 1
General Introduction
Chapter 1
PLANT SPECIES DISTRIBUTIONS AND BIODIVERSITY
Plant diversity is increasingly threatened across the globe by habitat loss and
deterioration (Tilman et al. 2001; Dirzo & Raven 2003; Butchart et al. 2010). Besides
threats to the conservation of species (Myers et al. 2000), these biodiversity declines
may impair ecosystem functioning and stability (Tilman & Downing 1994; McCann
2000; Tilman et al. 2012). In order to successfully protect sensitive species and
biodiversity in general, a central goal in ecology is to understand the mechanisms that
determine species distributions and biodiversity.
Determinants of species distributions and biodiversity
Niche differentiation is a major concept in this field, explaining species distributions
by interspecific differences in the ability to acquire limiting resources (Tilman 1982)
or withstand adverse conditions (Kenkel et al. 1991; Lenssen & de Kroon 2005). This
combination of abiotic constraints and biotic interactions forms an ‘environmental
filter’ (sensu van der Valk 1981; Keddy 1992; ter Steege & Zagt 2002) for species
establishment and persistence, which segregates species along environmental or
temporal niches (Silvertown 2004). With every species occupying its own niche, stable
coexistence in an ecosystem is promoted which generally increases biodiversity.
A concept contrasting to niche differentiation by environmental filtering, is
the explanation of species distribution by differences in arrival, or ‘dispersal filtering’.
According to this view, size of the local and regional species pools, chance colonization
events and differences in species dispersal abilities are critical factors structuring
plant communities, with species being ‘neutral’ to differences in habitat conditions
(MacArthur & Wilson 1967; Zobel 1997; Hubbell 2001; Myers & Harms 2009).
Mechanisms of environmental and dispersal filtering
Although there is general consensus that both environmental filtering and dispersal
filtering play a role in natural community assemblages (Gravel et al. 2006; Adler et
al. 2007; Myers & Harms 2009; Götzenberger et al. 2012; Fig. 1.1), the assessment of
their relative importance across time, space, or ecosystem types remains challenging
(Chase & Myers 2011; Shipley et al. 2012). Moreover, although niche differentiation
along environmental axes has been shown in many studies (Silvertown 2004) it is
often unclear at which stage in a plant’s life cycle such filters operate: during early
stages, thereby preventing occupation of the full potential niche of adult plants (Grubb
1977; Grime 2001), or at later stages, e.g. through competitive interactions among
adults (Wedin & Tilman 1993). Additionally, several early recruitment processes can
be distinguished, namely germination, seedling survival and seedling growth. While
early recruitment stages are increasingly recognized as determinants of adult species
distributions (Evans & Etherington 1990; Eriksson & Ehrlén 1992; Kotorová & Lepš
8
General Introduction
1999; Stampfli & Zeiter 2008), studies to date have focused either on one particular
recruitment stage or on the combined result of all stages (total seedling establishment).
Hence, quantitative information on the relative importance of the different stages to
recruitment and adult species distributions is generally lacking. Such information 1
would add significantly to our understanding of community assembly and biodiversity.
Also for dispersal filtering several mechanisms may play a role. Dispersal
limitation may occur due to a lack of nearby source populations (Bakker & Berendse
1999; Brederveld te .la 2011; Baattrup-Pedersen te .la 2013; Lorenz & Feld 2013),
by limited dispersal opportunities (blocked dispersal pathways; Ozinga te .la 2009;
Nilsson te .la 2010), but also by a mismatch in spatial patterns of deposited seeds
hampering seeds in reaching certain microhabitats (Nathan & Muller-Landau 2000;
Jordano .la te2007; Sarneel & Soons 2012; Sarneel .la te2014a). A better understanding
of plant dispersal pathways is required to be able to predict species (re-)colonization
potential and their effects on plant diversity.
Figure 1.1. Overview of the processes that determine plant community assembly (modified from
Götzenberger et al. 2012). The global species pool defines a regional species pool through the speciation,
extinction and migration of species (phylogeographic assembly). At a given local site the species pool
constitutes species from the regional species pool that are able to disperse there (dispersal assembly). At the
local site, habitat filtering and biotic interactions define the actual assemblage of plant species (ecological
assembly).
9
Chapter 1
Relative importance of environmental and dispersal filtering in dynamic habitats
Recent studies suggest that environment-driven community assembly may become
more important under stressful environmental conditions (Chase 2007; Jiang & Patel
2008; Lepori & Malmqvist 2009), whereas dispersal filtering may become more
prevalent under benign conditions (Myers & Harms 2009; Germain et al. 2013).
Species interactions may additionally enforce both types of filtering by competitive
displacement of subordinates towards peripheral ends of environmental gradients
(Wisheu & Keddy 1992), narrowing species’ realized niche ranges (Hutchinson 1957;
Pickett & Bazzaz 1978; Silvertown et al. 1999), or by preventing establishment of
later arriving species (‘priority effects’), especially in more productive environments
(Chase 2010; Kardol et al. 2013).
In dynamic habitats that are prone to natural disturbance regimes, it is
particularly unclear whether environmental filtering or dispersal filtering dominates,
or how the two interact in community assembly (Lepori & Malmqvist 2009; Myers &
Harms 2009; Brederveld et al. 2011). Disturbance regimes, i.e. “any relatively discrete
event that disrupts the structure of an ecosystem, community, or population, and
changes resource availability or the physical environment” (White & Pickett 1985),
are an intrinsic feature of many highly diverse and ecologically important habitats,
that are increasingly disrupted by human interventions across the globe (Mack &
D’Antonio 1998; Shea et al. 2004; Turner 2010). Information on the relative importance
of dispersal filtering versus environmental filtering is of highest importance for the
design of effective conservation and restoration strategies of these habitats. When
environmental filtering dominates, measures should first seek to improve or maintain
proper local environmental conditions (e.g. Roelofs et al. 2002; Jähnig et al. 2009;
Lamers et al. 2015) but when dispersal filtering dominates, it is of primary importance
to protect nearby source populations and secure dispersal pathways (Soons et al.
2005; Verhoeven et al. 2008; Brederveld et al. 2011)
STREAMS AND THEIR RIPARIAN ZONES
Riparian zones along streams provide a typical example of a dynamic and highly diverse
habitat (Naiman & Décamps 1997), in which the relative roles of dispersal filtering and
environmental filtering, as well as a full mechanistic understanding of both filtering
processes, are yet unresolved. Riparian zones form the interface between the aquatic
and terrestrial environments, and comprise the area between the low and (extremely)
high water marks (Naiman & Décamps 1997). Spatial and temporal variation in
flooding (limitation by abiotic stress) and drought (resource limitation) stimulate
community re-assembly and impose strong environmental filters. These dynamics
coincide with a strong hydrological gradient from the channel to the upland, together
generating gradients in the vegetation that are commonly explained by species niche
10