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Experiments Testing the Causes of Namibian Fairy Circles.

Tschinkel WR - PLoS ONE (2015)

Bottom Line: Limitation of plant growth due to micronutrient depletion within fairy circles was tested by supplementing circles with a micronutrient mixture, but did not result in differences in plant seedling density and growth.These four experiments provided evidence that fairy circles were not caused by subterranean vapors, that fairy circle soil per se did not inhibit plant growth, and that the circles were not caused by micronutrient deficiency.Landscape-scale vegetative self-organization is discussed as a more likely cause of fairy circles.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Science, Florida State University, Tallahassee, Florida, United States of America.

ABSTRACT
The grasslands on the sandy soils of the eastern edge of the Namib Desert of Namibia are strikingly punctuated by millions of mostly regularly-spaced circular bare spots 2 to 10 m or more in diameter, generally with a margin of taller grasses. The causes of these so called fairy circles are unknown, but several hypotheses have been advanced. In October 2009, we set up experiments that specifically tested four hypothesized causes, and monitored these 5 times between 2009 and 2015. Grass exclusion in circles due to seepage of subterranean vapors or gases was tested by burying an impermeable barrier beneath fairy circles, but seedling density and growth did not differ from barrier-less controls. Plant germination and growth inhibition by allelochemicals or nutrient deficiencies in fairy circle soils were tested by transferring fairy circle soil to artificially cleared circles in the grassy matrix, and matrix soil to fairy circles (along with circle to circle and matrix to matrix controls). None of the transfers changed the seedling density and growth from the control reference conditions. Limitation of plant growth due to micronutrient depletion within fairy circles was tested by supplementing circles with a micronutrient mixture, but did not result in differences in plant seedling density and growth. Short-range vegetation competitive feedbacks were tested by creating artificially-cleared circles of 2 or 4 m diameter located 2 or 6 m from a natural fairy circle. The natural circles remained bare and the artificial circles revegetated. These four experiments provided evidence that fairy circles were not caused by subterranean vapors, that fairy circle soil per se did not inhibit plant growth, and that the circles were not caused by micronutrient deficiency. There was also no evidence that vegetative feedbacks affected fairy circles on a 2 to 10 m scale. Landscape-scale vegetative self-organization is discussed as a more likely cause of fairy circles.

No MeSH data available.


Related in: MedlinePlus

Seedling density and height in the micronutrient experiment.Addition of micronutrients had no effect (Table 3) on the density of either grass or herb seedlings, nor on plant growth, although all changed greatly over time. The addition of micronutrients had no significant effect on plant height, although height differed significantly by date. The fraction of fairy circles with the general plant health indicated by the color is shown by the circles in the lowest panel. In some cases, data were missing for one replicate.
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pone.0140099.g008: Seedling density and height in the micronutrient experiment.Addition of micronutrients had no effect (Table 3) on the density of either grass or herb seedlings, nor on plant growth, although all changed greatly over time. The addition of micronutrients had no significant effect on plant height, although height differed significantly by date. The fraction of fairy circles with the general plant health indicated by the color is shown by the circles in the lowest panel. In some cases, data were missing for one replicate.

Mentions: Addition of micronutrients to fairy circles did not significantly change either seedling density or plant growth (ANOVA: grass, F1,3 = 0.74; p>0.4; herbs, F1,3 = 0.22; p>0.4; n.s.) (Fig 8), although all measures differed significantly across check dates (grass, F1,3 = 24.7; herbs, F1,3 = 9.28; both p< 0.0001) (Table 3). Height growth between February and May of 2011 was especially apparent (F1,3 = 9.9; p<0.001), but such growth was similar for treatments and controls (p> 0.9). In the images of the nine pairs of treated circles, no visible differences in the growth of vegetation between treatment and controls are apparent, even in the high growth year of 2011 (Fig 9). Of the 18 circles in the experiment, plant health varied from poor to good but appeared unrelated to treatment (Fig 8, bottom panel). Five years after supply of micro-nutrients, there were no differences between treated and control circles.


Experiments Testing the Causes of Namibian Fairy Circles.

Tschinkel WR - PLoS ONE (2015)

Seedling density and height in the micronutrient experiment.Addition of micronutrients had no effect (Table 3) on the density of either grass or herb seedlings, nor on plant growth, although all changed greatly over time. The addition of micronutrients had no significant effect on plant height, although height differed significantly by date. The fraction of fairy circles with the general plant health indicated by the color is shown by the circles in the lowest panel. In some cases, data were missing for one replicate.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4625006&req=5

pone.0140099.g008: Seedling density and height in the micronutrient experiment.Addition of micronutrients had no effect (Table 3) on the density of either grass or herb seedlings, nor on plant growth, although all changed greatly over time. The addition of micronutrients had no significant effect on plant height, although height differed significantly by date. The fraction of fairy circles with the general plant health indicated by the color is shown by the circles in the lowest panel. In some cases, data were missing for one replicate.
Mentions: Addition of micronutrients to fairy circles did not significantly change either seedling density or plant growth (ANOVA: grass, F1,3 = 0.74; p>0.4; herbs, F1,3 = 0.22; p>0.4; n.s.) (Fig 8), although all measures differed significantly across check dates (grass, F1,3 = 24.7; herbs, F1,3 = 9.28; both p< 0.0001) (Table 3). Height growth between February and May of 2011 was especially apparent (F1,3 = 9.9; p<0.001), but such growth was similar for treatments and controls (p> 0.9). In the images of the nine pairs of treated circles, no visible differences in the growth of vegetation between treatment and controls are apparent, even in the high growth year of 2011 (Fig 9). Of the 18 circles in the experiment, plant health varied from poor to good but appeared unrelated to treatment (Fig 8, bottom panel). Five years after supply of micro-nutrients, there were no differences between treated and control circles.

Bottom Line: Limitation of plant growth due to micronutrient depletion within fairy circles was tested by supplementing circles with a micronutrient mixture, but did not result in differences in plant seedling density and growth.These four experiments provided evidence that fairy circles were not caused by subterranean vapors, that fairy circle soil per se did not inhibit plant growth, and that the circles were not caused by micronutrient deficiency.Landscape-scale vegetative self-organization is discussed as a more likely cause of fairy circles.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Science, Florida State University, Tallahassee, Florida, United States of America.

ABSTRACT
The grasslands on the sandy soils of the eastern edge of the Namib Desert of Namibia are strikingly punctuated by millions of mostly regularly-spaced circular bare spots 2 to 10 m or more in diameter, generally with a margin of taller grasses. The causes of these so called fairy circles are unknown, but several hypotheses have been advanced. In October 2009, we set up experiments that specifically tested four hypothesized causes, and monitored these 5 times between 2009 and 2015. Grass exclusion in circles due to seepage of subterranean vapors or gases was tested by burying an impermeable barrier beneath fairy circles, but seedling density and growth did not differ from barrier-less controls. Plant germination and growth inhibition by allelochemicals or nutrient deficiencies in fairy circle soils were tested by transferring fairy circle soil to artificially cleared circles in the grassy matrix, and matrix soil to fairy circles (along with circle to circle and matrix to matrix controls). None of the transfers changed the seedling density and growth from the control reference conditions. Limitation of plant growth due to micronutrient depletion within fairy circles was tested by supplementing circles with a micronutrient mixture, but did not result in differences in plant seedling density and growth. Short-range vegetation competitive feedbacks were tested by creating artificially-cleared circles of 2 or 4 m diameter located 2 or 6 m from a natural fairy circle. The natural circles remained bare and the artificial circles revegetated. These four experiments provided evidence that fairy circles were not caused by subterranean vapors, that fairy circle soil per se did not inhibit plant growth, and that the circles were not caused by micronutrient deficiency. There was also no evidence that vegetative feedbacks affected fairy circles on a 2 to 10 m scale. Landscape-scale vegetative self-organization is discussed as a more likely cause of fairy circles.

No MeSH data available.


Related in: MedlinePlus