Palynological Studies in Brigalow (Acacia harpophylla F. Muell.) Communities: Pollen Analysis in Gilgai Soil Under the Primary Forest

Abstract

A pollen analysis of acid clays under brigalow is presented. The pollen fluxes in a first pair of soil profiles throw light upon the natural history of one locality as regards both its modern vegetation and the physical stability of gilgai. Fluxes in the relative pollen preservation indicate intensity of biological attack on the exine. In a profile representative of the higher level, preservation is good enough, erosion or fill on a micro-scale so slight, and any vertical down-wash so slow as to show up residual flux variations from successional change. But under this natural canopy, even though there is only a very slow wash of litter and soil down to the bottom of the depression, a faster tempo of decay than any in the higher ground, and physical incorporation down deeper cracks, maintain a balance that prevents accumulation on an almost bare surface. This shows up in deeper penetration of pollen grains, in even the residual pollen types being severely eroded, and in irregular fluxes in which the successional trace is lost. Presumably, old Myrtaceae types have washed out of the soil above. The pollen fluxes indicate a natural replacement of eucalypt forest by brigalow some time before clearing. The expanding fluxes at the immediate surface are taxa from outside the forest, particularly the Chenopodiaceae, that would have become more abundant after clearing. Insight into soil stability comes from the marked contrast in the distribution and the preservation of the pollen grains across the microrelief. The higher level is physically more stable than the bottom of the adjacent depression. Depressions are surprisingly dry in natural forest, but these pollen fluxes suggest that they are occas- ionally the wettest microenvironment.