re: The chemical effects were ruled out
"1. 'CHRISTELLE' case of 27/11/1979: Persistence of flattened grass several days after the observation. The samples taken and analyzed by a plant biology laboratory at Toulouse University did not give unequivocal evidence of chemical or biological disturbance of the samples taken from the marked area relative to controls. A study of the mechanical properties of grass tissue subjected to strong mechanical pressure showed that the duration is a more important factor than the mass.
from the link you gave
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