Biogeomorphological evolution of rocky hillslopes driven by roots in campos rupestres, Brazil

Abstract

The campos rupestres (rocky grassland) comprise an old-growth seasonally dry herbaceous ecosystem on mountaintops in central and eastern Brazil and in disjoint areas with sparse shrubs with high plant diversity and endemism. This ecosystem consists of sharp-edged quartzite landforms and rocky hillslopes with boulders, blocks, and sparse soil cover. The slopes in these environments have traditionally been viewed as a product of mechanical rock breakdown controlled by structural and lithological features of rocky hillslopes. In addition to the lithological effects on slope evolution, plant cover plays a pivotal role in the geomorphological process. We explored process–form relationships between plant cover and quartzite rocky hillslopes of campos rupestres, considering the functioning of root traits of rock dwelling endemic species of Velloziaceae. Velloziaceae is an iconic plant family in campos rupestres, and several species colonize quartzite rock with different biogeomorphic effects at different scales. We present a conceptual model of the evolution of quartzite hillslopes based on the arenization process driven by roots expressing a specialized nutrient-acquisition strategy: vellozioid roots. Our results show that at the outcrop scale, roots respond to previous lithological characteristics such as joints and fractures that allow root establishment, followed by the release of large quantities of carboxylates that lead to rock dissolution. The microscopic pattern of bioweathering is associated with enlargement of the quartzite secondary porosity through the formation of root microcracks. Roots that are about 100 μm thick form root mats that surround the grains and produce inter-mineral and intra-mineral porosity systems facilitating percolation of water and organic solutes increasing the weathering. This results in arenization of quartzite by grain-by-grain dissolution and reduces the rock strength, which leads to the formation of quartzite hillslopes, driven by a nutrient-acquisition strategy of vellozioid roots. The described biogeomorphic process determines trajectories of the development of landforms through time, slope-sediment production, slope morphology by the production of boulders and block fields, and sand patches that are reworked by surface runoff along the slope. The hillslopes and landforms that develop in campos rupestres are therefore products of self-reinforcing processes involving nutrient acquisition from bedrock by plants and denudation processes. Also, these positive feedbacks characterize the Velloziaceae species as ecosystem engineers.