A preliminary test of catabolic nutrients in explanation of the puzzling treelessness of grassland in mesic Australia

The puzzling exclusion of native trees from grassland under mesic climates can hypothetically be explained by nutritional regimes that make grasses competitively superior to trees. One hypothesis holds that where nutrient concentrations in soils allow catabolism to be as rapid as anabolism at the scale of the whole plant, photosynthates are respired instead of being allocated to stems, resulting in dominance of herbaceous over woody growth-forms. This matching of catabolic and anabolic rates would depend on the ratios of catabolic to anabolic nutrient elements in soils at a particular site. We consequently investigated elemental concentrations of soils near boundaries between treeless grassland and eucalypt woodland in New South Wales, Australia. Soils were analysed for Be, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Cd, Ba and Pb. Based on our preliminary classification of nutrients as mainly catabolic (e.g. Cu) or mainly anabolic (e.g. Mn), the results showed that, relative to adjacent woodland, the soils under treeless grassland were naturally enriched in anabolic nutrients, but even more so in catabolic nutrients. Furthermore, the greater concentration of Mo - which can induce deficiency of the catabolic nutrient Cu - in soils under woodland than in soils under treeless grassland suggests that catabolic dystrophy allows eucalypt seedlings to prevail in competition with grasses. The results support the theory that where ratios of catabolic to anabolic nutrients are sufficient grasses will achieve competitive superiority over woody seedlings. Our study indicates that soil nutrient concentrations may partly explain the incidence of natural patches of treeless grassland in Australian landscapes otherwise dominated by woody plants

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

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

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

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