Transpiration source water and geomorphological potential of root growth in the Boulder Creek Critical Zone Observatory, Colorado

Background/Question/Methods

The influence of vegetation on the hydrological cycle and the possible effect of roots in geomorphological processes are poorly understood. Gordon Gulch watershed in the Front Range of the Rocky Mountains, Colorado, is a montane climate ecosystem of the Boulder Creek Critical Zone Observatory whose study adds to the database of ecohydrological work in different climates. This work sought to identify the sources of water used by different tree species and to determine how trees growing in rock outcrops may contribute to the fracturing and weathering of rock.

Stable isotopes (¹⁸O and ²H) were analyzed from water extracted from soil and xylem samples. Pinus ponderosa  on the south-facing slope consumes water from deeper depths during dry periods and uses newly rain-saturated soils, after rainfall events. Tree roots growing in rock outcrops may contribute to long-term geomorphological processes by physically deteriorating the bedrock. Additionally, remote sensing data may provide the tools necessary to quantify the number of trees growing in rock outcrops in Gordon Gulch and therefore, quantifying how much contribution these trees have to the overall water balance of this catchment.

Results/Conclusions

Pinus contorta on the north -facing slope shows an expected response in water consumption, before and after rain. Two trees (Pinus ponderosa) growing within rock outcrops demonstrate water use from cracks replenished by new rains.The dominant roots of measured trees contributed approximately 30 - 80% of total water use, seen especially after rainfall events. Tree root growth rings indicates that root growth is capable of expanding rock outcrop fractures at an approximate rate of 0.6 – 1.0 mm per year. These results demonstrate the significant role roots play in tree physiological processes and in bedrock deterioration.