<i>Avicennia germinans</i> (black mangrove) and <i>Spartina alterniflora</i> (smooth cordgrass) density and height values.

<p>(A,C) <i>Avicennia</i> and (B,D) <i>Spartina</i> (smooth cordgrass) total trunk/stem densities per square meter (# m^-2) and maximum heights (cm) from control (gray) and fertilized (black) treatment plots for each sampling year (2010–2013). Data are mean values ± standard error; n = 11. Upper case letters indicate temporal trends within control plots; lower case letters indicate temporal trends within fertilized plots. Different letters indicate significance at perm p < 0.05 within control or fertilized treatments; * indicates significance at perm p < 0.05 between treatments per year. (See Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193617#pone.0193617.t001" target="_blank">1</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193617#pone.0193617.t003" target="_blank">3</a> for statistical analyses).</p

A conceptual diagram of a proposed positive feedback loop for mangrove encroachment in enriched conditions.

<p>Global changes (large bold arrow) are driving mangrove stand growth and subsequent encroachment into marshes, and this expansion may be facilitated by high nutrient conditions. Fertilization (small bold arrow) increases mangrove height, expediting mangrove canopies exceedance of species-specific growth-limiting height thresholds, such as reduced negative neighbor effects¹ and increased resiliency to freeze damage². This in turn increases mangrove stand growth and expansion. (¹ Guo et al. 2013; ² Osland et al. 2015).</p

Study site location.

<p>(A) Plots were located in Port Aransas, TX, USA (B) in co-occurring <i>Avicennia germinans</i> (black mangrove) and <i>Spartina alterniflora</i> (smooth cordgrass) stands.</p

<i>Avicennia germinans</i> (black mangrove) height class densities.

<p>Black mangrove density per square meter (# m^-2) values in control (gray) and fertilized (black) treatment plots categorized as (A) sub-shrub = <i>Avicennia</i> < 0.5 m, (B) shrub = <i>Avicennia</i> 0.5–1.0 m, and (C) tall shrub = <i>Avicennia</i> > 1.0 m within each sampling year (2010–2013). Data are mean values ± standard error; n = 11. Upper case letters indicate temporal trends within control plots; lower case letters indicate temporal trends within fertilized plots. Different letters indicate significance at perm p < 0.05 within control or fertilized treatments; * indicates significance at perm p < 0.05 between treatments per year. (See Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193617#pone.0193617.t001" target="_blank">1</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193617#pone.0193617.t003" target="_blank">3</a> for statistical analyses).</p

PermANOVA results determining treatment and sampling year differences for density and maximum height values.

<p>PermANOVA results determining treatment and sampling year differences for density and maximum height values.</p

PermANOVA pairwise results comparing density and maximum height values between sampling years.

<p>PermANOVA pairwise results comparing density and maximum height values between sampling years.</p

A conceptual ball-in-cup diagram hypothesizing the direction nutrient enrichment may drive the mangrove-marsh ecotone.

<p>Global changes (e.g., reductions in freezing events, sea level rise, and higher atmospheric CO₂) are often evoked as the main driver in increased mangrove coverage. Nutrient enrichment, on a local scale, may augment marsh growth and reduce mangrove growth, subsequently contributing to slower mangrove encroachment.</p

Summary of two-way repeated-measures ANOVA results for arthropod densities in response to plant removal disturbance treatments in mound (small patch) and terrace (large patch) habitats over time.

a<p>p-values for <i>Prokelesia</i> and <i>Ischnodemus</i> follow the Greenhouse-Geisser correction.</p

Disturbance and patch size effects on plant characteristics.

<p>Effects of disturbance treatments in large (terraces) and small (mounds) patches on plant characteristics: a) total plant cover, b) total live stem density, c) <i>Spartina alterniflora</i> stem height, d) relative <i>Spartina alterniflora</i> chlorophyll <i>a</i> content. Error bars represent standard error. Significant treatment effects from two-way ANOVA are noted; D = Disturbance, H = Habitat patch size.</p

Disturbance and patch size effects on arthropod characteristics.

<p>Effects of disturbance treatments in large (terraces) and small (mounds) patches over time on arthropod characteristics: a) <i>Ischnodemus</i> density, b) <i>Prokelesia</i> density, c) spider density. Error bars represent standard error. Significant treatment effects from two-way repeated measures ANOVA are noted; T = time, D = Disturbance, H = Habitat patch size, NS = no significant treatment effects.</p