Responses of Dune Plant Communities to Continental Uplift from a Major Earthquake: Sudden Releases from Coastal Squeeze

<div><p>Vegetated dunes are recognized as important natural barriers that shelter inland ecosystems and coastlines suffering daily erosive impacts of the sea and extreme events, such as tsunamis. However, societal responses to erosion and shoreline retreat often result in man-made coastal defence structures that cover part of the intertidal and upper shore zones causing coastal squeeze and habitat loss, especially for upper shore biota, such as dune plants. Coseismic uplift of up to 2.0 m on the Peninsula de Arauco (South central Chile, ca. 37.5º S) caused by the 2010 Maule earthquake drastically modified the coastal landscape, including major increases in the width of uplifted beaches and the immediate conversion of mid to low sandy intertidal habitat to supralittoral sandy habitat above the reach of average tides and waves. To investigate the early stage responses in species richness, cover and across-shore distribution of the hitherto absent dune plants, we surveyed two formerly intertidal armoured sites and a nearby intertidal unarmoured site on a sandy beach located on the uplifted coast of Llico (Peninsula de Arauco) over two years. Almost 2 years after the 2010 earthquake, dune plants began to recruit, then rapidly grew and produced dune hummocks in the new upper beach habitats created by uplift at the three sites. Initial vegetation responses were very similar among sites. However, over the course of the study, the emerging vegetated dunes of the armoured sites suffered a slowdown in the development of the spatial distribution process, and remained impoverished in species richness and cover compared to the unarmoured site. Our results suggest that when released from the effects of coastal squeeze, vegetated dunes can recover without restoration actions. However, subsequent human activities and management of newly created beach and dune habitats can significantly alter the trajectory of vegetated dune development. Management that integrates the effects of natural and human induced disturbances, and promotes the development of dune vegetation as natural barriers can provide societal and conservation benefits in coastal ecosystems.</p></div

Location of the study sites at the beach of Llico, south central Chile.

<p>Before (July 2009) and after (December 2011) Maule earthquake (27^th February 2010) pictures of Llico are presented to show approximate locations (red dots) of sandy sites in front of the seawall (Sw) and the rocky revetment (Rv), as well as that of the unarmoured beach (B). Yellow arrows indicate location of a jetty in relation to low tide level. Note differences in intertidal width before and after the earthquake.</p

Similarity in the dune plant assemblages among the beach sites over time.

<p>Similarity of plant assemblages (note scale: 40–100%) between a) Revetment—Sewall, b) Revetment—Beach, c) Sewall—Beach were calculated from the Bray-Curtis similarity matrix (ANOSIM, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0124334#pone.0124334.s003" target="_blank">S2 Table</a>). Sampling times as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0124334#pone.0124334.g003" target="_blank">Fig 3</a>. *significant differences (<i>p</i> < 0.05) in the similarity of plant assemblages between sites (Rv <i>vs</i> Sw, Rv <i>vs</i> B, and Sw <i>vs</i> B) at each time.</p

Responses of the plant species and vegetated area to the Maule earthquake.

<p>a) Mean (1 + SE, n = 4 transects) number of species at each site, b) total number of species including all sites, c) mean (1 ± SE, n = 4 transects) width of the vegetated area at each site, d) absolute sand cover (standardized mean ± SE, n = 4 transects) of the three study sites over time. The study sites at the beach of Llico are: Sw (in front of a seawall), Rv (in front of a revetment) and B (unarmoured beach). The sampling times were: February (F), April (A), August (Au) and October (O) 2012, January (J), June (Ju), and November (N) 2013, and January (J) and November (N) 2014.</p

Across-shore distribution over time of the dune plant species (plant cover in %).

<p>Sandy beach site located in front of the seawall at Llico (scale: 0–50%). Upper-shore levels started at 0 m. The sampling times were: February, April, August, October 2012 January, June, November 2013, and January 2014. Repair work on the concrete wall affected the vegetated zone at this site during November 2014; thus, no sampling was carried out at that month.</p

Absolute cover of <i>Atriplex</i> spp., <i>Schoenoplectus americanus</i>, and <i>Hordeum</i> spp.

<p>The plots show the standardized mean values (1 ± SE, n = 4 transects) of the three study sites over time (sampling times and study sites as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0124334#pone.0124334.g003" target="_blank">Fig 3</a>).</p

Before/after Maule earthquake (27th February 2010).

<p>Pictures of the armoured sites located in front of a seawall and a rocky revetment, and those of the unarmoured site taken at the beach of Llico at different stages of the study. Note the increasing human activities at the seawall site and the developed vegetation at the unarmoured site after the earthquake. Repair work on the concrete wall affected the vegetated zone at that site during November 2014.</p

Across-shore distribution (m) over time of the dune plant species (plant cover in %).

<p>Located at the unarmoured sandy beach site at Llico (scale: 0–50%). Upper-shore levels started at 0 m. The sampling times were: February, April, August, October 2012 January, June, November 2013, and January, November 2014.</p

Results of GLMMs (model fit by maximum likelihood, Laplace approximation) containing pair-wise contrasts for the main response variables of the vegetated area at each study site.

<p>Models included site (B: unarmoured beach, Rv: revetment and Sw: seawall) as a fixed factor, time (9 dates) as a covariate, and random effects were associated with transects as a grouping factor. The best fitted full model is indicated per response variable. SE: standard error; var: variance; SD: standard deviation; AIC: Akaike’s information criterion. z values for models with Poisson error structures, t values for models with Gaussian error structures. Significant effects appear in bold.</p><p>Results of GLMMs (model fit by maximum likelihood, Laplace approximation) containing pair-wise contrasts for the main response variables of the vegetated area at each study site.</p

Results of GLMMs (model fit by maximum likelihood, Laplace approximation) containing pair-wise contrasts for the main plant species (standardized absolute cover, see Methods) of the vegetated area at each study site.

<p>Models included site (B: unarmoured beach, Rv: revetment and Sw: seawall) as a fixed factor, time (9 dates) as a covariate, and random effects were associated with transects as a grouping factor. The best fitted full model is indicated per response variable. SE: standard error; var: variance; SD: standard deviation; AIC: Akaike’s information criterion. Significant effects appear in bold.</p><p>Results of GLMMs (model fit by maximum likelihood, Laplace approximation) containing pair-wise contrasts for the main plant species (standardized absolute cover, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0124334#sec002" target="_blank">Methods</a>) of the vegetated area at each study site.</p