Promotion of chemical weathering by higher plants: field observations on Hawaiian basalts

Abstract

An electron microprobe and petrographic microscope were used to study the biota-rock interface beneath higher-plant communities and lichens, as well as unvegetated surfaces, on Hawaiian basalt flows ranging from a few years to several thousand years in age and currently receiving moderate to extremely high annual rainfall. We find dramatic (order of magnitude minimum) acceleration of weathering rates beneath vascular plants on the youngest flows relative to adjacent non- or primitively-vegetated controls, notably rocks colonized by the common fruticose lichen Stereocaulon uulcani. Bulk dissolution of plagioclase accounts for most of the weathering observed beneath higher plants on these young basalts. Analysis of older flows under mature forests reveals that chemical weathering beneath vascular plant communities continues unabated for thousands of years after initial colonization. Moreover, such weathering is not limited to exterior basalt surfaces but is commonly found adjacent to joints and vesicles within the rock, where it is often associated with fine (sub-2 mm diameter) plant roots. Plagioclase dissolution was also observed on lichen-encrusted and unvegetated flows a few hundred to thousands of years in age, but it was typically much less advanced than beneath higher plants and rarely extended more than several millimeters from exterior rock surfaces. Chemical denudation rates on older flows with higher plants are at minimum ten times greater than those with only lichens or microbiota in the environments studied. If S. uulcani and associated microflora resemble the terrestrial precursors to vascular plant communities, then the results of studies such as this have implications for the history of atmospheric CO 2 and the Earth's climate.