Resilience of beach grasses along a biogeomorphic successive gradient: resource availability vs. clonal integration

Valérie C Reijers,Anne J A De Rond,Sean C S Hoetjes,Tjisse Van Der Heide,Leon P M Lamers,Carlijn Lammers

doi:10.1007/s00442-019-04568-w

Abstract

Coastal ecosystems are often formed through two-way interactions between plants and their physical landscape. By expanding clonally, landscape-forming plants can colonize bare unmodified environments and stimulate vegetation–landform feedback interactions. Yet, to what degree these plants rely on clonal integration for overcoming physical stress during biogeomorphological succession remains unknown. Here, we investigated the importance of clonal integration and resource availability on the resilience of two European beach grasses (i.e. Elytrigia juncea and Ammophila arenaria) over a natural biogeomorphic dune gradient from beach (unmodified system) to foredune (biologically modified system). We found plant resilience, as measured by its ability to recover and expand following disturbance (i.e. plant clipping), to be independent on the presence of rhizomal connections between plant parts. Instead, resource availability over the gradient largely determined plant resilience. The pioneer species, Elytrigia, demonstrated a high resilience to physical stress, independent of its position on the biogeomorphic gradient (beach or embryonic dune). In contrast, the later successional species (Ammophila) proved to be highly resilient on the lower end of its distribution (embryonic dune), but it did not fully recover on the foredunes, most likely as a result of nutrient deprivation. We argue that in homogenously resource-poor environments as our beach system, overall resource availability, instead of translocation through a clonal network, determines the resilience of plant species. Hence, the formation of high coastal dunes may increase the resistance of beach grasses to the physical stresses of coastal flooding, but the reduced marine nutrient input may negatively affect the resilience of plants.

Highlights

Vegetated coastal ecosystems including coastal dunes, salt marshes and seagrass meadows underpin vital services in coastal zones (Costanza et al 1997; Barbier et al 2008, 2011)
Based on our overarching hypothesis, we suggest that for coastal dune systems in general: (1) dune-forming species rely on clonal integration in the early successional phase of beach colonization, but that this effect wears off in later successional phases and (2) exposure to physical stress and resource availability synergistically determine the resilience of beach grasses to severe physical stress
We addressed the following questions: (1) How do both beach grasses respond to severe physical stress? (2) How does the position on the successive gradient affect the resilience of both beach grasses? (3) Does clonal integration help both beach grasses recover after severe physical stress? (4) Does the importance of clonal integration in overcoming physical stress decrease over a successive gradient when there are fewer resources available?

Summary

Introduction

Vegetated coastal ecosystems including coastal dunes, salt marshes and seagrass meadows underpin vital services in coastal zones (e.g. flood protection, water and carbon storage, biodiversity enhancement) (Costanza et al 1997; Barbier et al 2008, 2011). Because these feedbacks require a minimum plant shoot density and patch size to operate adequately, feedback-dependent ecosystems can suddenly collapse below this threshold, and (re-)establishment is impeded (Christianen et al 2014; Silliman et al 2015; Angelini et al 2016) To rapidly overcome these density- and patch sizedependent establishment thresholds, many landscape-forming plants rely on clonal expansion as their main mode of dispersal (Bouma et al 2005; Kendrick et al 2005; Hacker et al 2012, Reijers et al 2019a). The stressful conditions prevailing in exposed bare, unmodified coastal systems (e.g. intertidal mudflats, beach plains) hamper successful establishment of plant species Expansion into these stressful environments can be facilitated by physiological integration of pioneer shoots with a sufficiently large original patch through rhizomal connections (Amsberry et al 2000; Silinski et al 2016). To what degree landscape-forming plants rely on clonal integration throughout the various phases of biogeomorphological succession, from bare unmodified environments to biologically engineered habitat, remains poorly understood (Kendrick et al 2005; Corenblit et al 2011, 2015a, b)

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