TRANSPIRATION SOURCE WATER AND GEOMORPHOLOGICAL POTENTIAL OF ROOT GROWTH IN THE BOULDER CREEK CRITICAL ZONE OBSERVATORY, COLORADO

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 whose study adds to the database of ecohydrological work in different climates. The watershed is a mid-elevation catchment of the Boulder Creek Critical Zone Observatory. This work sought to identify the sources of water used by different tree species and to determine how trees growing on soil-free 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 collected from two species on opposing slopes (Pinus ponderosa on south-facing slopes and Pinus contorta on north-facing slopes). Additionally, samples for isotope analysis were obtained from two trees growing within rock outcrops. We anticipate that isotopic analysis will determine if the water consumed by the trees corresponds to seasonal precipitation rather than deeper recharge provided by snowmelt.

An underexplored question in geomorphology is whether tree roots in the soil-free growth environment of rock outcrops contribute to long-term geomorphological processes by physically deteriorating the bedrock. Measurements of water flux, determined from sapflow sensors, through the apparent dominant root of two selected trees growing in the extreme environment of rock outcrop fractures demonstrated that those roots responded quickly to large rainfall events. The dominant roots contributed approximately 30 - 80% of total water use with large variability between the two measured trees. Preliminary analysis of 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.