Effects of anabolic and catabolic nutrients on woody plant encroachment after long-term experimental fertilization in a South African savanna - Fig 2

<p><b>(a) Satellite image showing the approximate position of the experimental plots at Towoomba Agricultural Development Centre, South Africa; and (b) a photograph taken in September 2014 of two experimental plots.</b> Black borders in (a) depict the plots shown in (b). Foreground in (b): a plot with no woody encroachment that received the maximum applications of AS (1166 kg ha^-1 yr^-1) and SP (466 kg ha^-1 yr^-1). Background in (b): a plot with intense woody encroachment by <i>Vachellia karroo</i> that received minor applications of AS (146 kg ha^-1 yr^-1) and moderate applications of SP (233 kg ha^-1 yr^-1). [Depicted is the author Antoni Milewski who confirmed consent for publication].</p

Abundance and cumulative height of trees according to experimental treatment at Towoomba in 2011/12.

<p>Box-plots show medians (dark bar), upper and lower quartiles (tops and bottoms of boxes), and ranges (upper and lower whiskers). AS = ammonium sulphate; SP = superphosphate. p values show the results of the Kruskal-Wallis testing for individual effects of AS and SP after Friedman testing for differences across all treatments.</p

Significant effects for ammonium sulphate (AS) and superphosphate (SP) treatments.

<p>Significant effects for ammonium sulphate (AS) and superphosphate (SP) treatments.</p

Tree abundance (categorised as 0–1, 2–4, 5–8 or >8 individual trees per plot) relative to nutrient concentrations, pH (KCl) and acid saturation.

<p>Data points are depicted as grey circles. Means ± standard errors are depicted with black circles and error bars. Asterisks show significant differences in tree abundance according to Kruskal-Wallis rank sum tests (*** p<0.0001; ** p = 0.001–0.009; * p = 0.01–0.05). Different letters designate significant differences between means (p<0.05).</p

Tree abundance (individual trees per plot) relative to nutrient concentrations, nutrient ratios, pH (KCl) and acid saturation.

<p>Dashed lines depict boundary lines identified by boundary line analysis (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179848#pone.0179848.s004" target="_blank">S4 Table</a>). In the bottom row, four categories of tree abundance (i.e. 0–1, 2–4, 5–8 or >8 individual trees per plot) are shown relative to nutrient ratios, with data points depicted as grey circles, and means ± standard errors depicted with black circles and error bars. Asterisks show significant differences in tree abundance according to Kruskal-Wallis rank sum tests (*** p<0.0001; ** p = 0.001–0.009; * p = 0.01–0.05). Different letters designate significant differences between means (p<0.05).</p

Principal component analysis of tree abundance in relation to all soil properties.

<p>Left: arrows indicate the strength of the correlation between each element and the first two axes produced by PCA. Right: points show the scores for the samples on axes 1 and 2 of the PCA, and ellipses summarize the variation in soil samples taken from plots containing 3 or fewer woody plant individuals per plot (black) and those containing 4 or more woody plant individuals per plot (grey).</p

Location of Towoomba Agricultural Development Centre in the southern Springbok Flats of Limpopo Province in South Africa.

<p>This map was generated based on datasets from the following sources: [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179848#pone.0179848.ref035" target="_blank">35</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179848#pone.0179848.ref036" target="_blank">36</a>].</p

Concentrations of soil properties according to experimental treatment at Towoomba in 2014.

<p>Box-plots show medians (dark bar), upper and lower quartiles (tops and bottoms of boxes), and ranges (upper and lower whiskers). AS = ammonium sulphate; SP = superphosphate. p values show the results of the Kruskal-Wallis testing for individual effects of AS and SP after Friedman testing for differences across all treatments.</p

Theoretical relationships between biotic response and nutrient concentration for various plant species occurring in the same environment.

<p>The response of plants in terms of vigour to a particular nutrient are invariably bell-shaped curves (a) as opposed to distinct steps (b) delineating zones where concentrations of a particular nutrient are deficient, optimal or toxic.</p