Date of Graduation

7-2020

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor

Jill A. Marshall

Committee Member

Matthew Covington

Second Committee Member

Kusum Naithani

Keywords

Critical-zone, Ecohydrology, Roots, Sensors, Tree, Weathering

Abstract

The role of tree roots as stressors that contribute to physical weathering processes and thus soil generation remains an open question in critical zone science. While evidence suggests roots may be able to damage rock by accessing pre-existing fractures, where they can expand due to water uptake or generate forces on rock in response to wind gusts, these processes have not been investigated in temperate karst regions until now. I monitored forces at the root-rock interface for an American elm and Hackberry tree between September 2019 and May 2020. I used piezoelectric force sensors to determine if differences in species, tree size, the distance of roots from the tree, wind or precipitation conditions affected the frequency or magnitude of forces exerted by tree roots onto bedrock. I analyzed meteorological conditions in addition to root forces to examine the environmental controls on diurnal cycles of forces exerted on the bedrock and to identify how roots responded to wind gusts and rainfall events. Roots of both species exerted higher daily forces between the hours of 10:30 and 23:00, reaching daily maximum forces between 15:00 and 18:00, and exerting forces for approximately an extra hour during the fall and spring compared to the winter. I determined that temperature’s impact on vapor pressure deficit, which controls the rate of transpiration, was the primary driver of the timing of daily forces. Precipitation led to periods of higher forces, as the roots expanded due to water-uptake as well as reduced tree transpiration from lower VPD and solar radiation during rainfall events. Roots of greater size exerted increasing fluctuations in forces onto the bedrock in response to wind gusts and rainfall. American elm roots exerted forces on the bedrock more frequently during windy periods compared to the Hackberry roots. Variations in the root response to wind and precipitation events are hypothesized to be linked to contrasting rooting strategies between species and the specific functional role of individual roots in supporting the tree. My findings suggest that in warmer conditions, with more intense rainfall events, roots will exert greater forces on bedrock due to 1) increased temperature-controlled vapor pressure deficit and, 2) heavy rainfall-induced forces due to water uptake. These projected increase in forces suggest that in the karst landscape of Northwest Arkansas, tree roots may accelerate the physical weathering of bedrock.

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