Articles | Volume 14, issue 23
Technical note
30 Nov 2017
Technical note |  | 30 Nov 2017

Technical note: Application of geophysical tools for tree root studies in forest ecosystems in complex soils

Ulises Rodríguez-Robles, Tulio Arredondo, Elisabeth Huber-Sannwald, José Alfredo Ramos-Leal, and Enrico A. Yépez

Abstract. While semiarid forests frequently colonize rocky substrates, knowledge is scarce on how roots garner resources in these extreme habitats. The Sierra San Miguelito Volcanic Complex in central Mexico exhibits shallow soils and impermeable rhyolitic-rock outcrops, which impede water movement and root placement beyond the soil matrix. However, rock fractures, exfoliated rocks and soil pockets potentially permit downward water percolation and root growth. With ground-penetrating radar (GPR) and electrical resistivity tomography (ERT), two geophysical methods advocated by Jayawickreme et al. (2014) to advance root ecology, we advanced in the method development studying root and water distribution in shallow rocky soils and rock fractures in a semiarid forest. We calibrated geophysical images with in situ root measurements, and then extrapolated root distribution over larger areas. Using GPR shielded antennas, we identified both fine and coarse pine and oak roots from 0.6 to 7.5 cm diameter at different depths into either soil or rock fractures. We also detected, trees anchoring their trunks using coarse roots underneath rock outcroppings. With ERT, we tracked monthly changes in humidity at the soil–bedrock interface, which clearly explained spatial root distribution of both tree species. Geophysical methods have enormous potential in elucidating root ecology. More interdisciplinary research could advance our understanding in belowground ecological niche functions and their role in forest ecohydrology and productivity.

Short summary
The approach we present has the potential to contribute to the understanding of several types of plant interactions such as coexistence, competition and niche extent. By combining geophysical exploration techniques GPR and ERT we provide experimental evidence of horizontal roots located under exfoliated rocks and in water reservoirs. We also study how the roots access water retained in the weathered rock during droughty periods and the implications for survival and coexistence of forest species.
Final-revised paper