Phenology and Dynamics of Root Growth of Three Cool Semi-Desert Shrubs Under Field Conditions

Abstract

Many cool arid land shrub species exhibit much higher root/shoot biomass ratios than many warm desert shrubs even though mean potential evaporation/precipitation ratios are usually less severe in these cooler areas (Barbour 1973; Rodin & Basilevich 1965; Shalyt & Zhivotenko 1968; Sveshnikova 1968; Bjerregaard 1971; Fernandez 1974). These high root/shoot biomass ratios are particularly apparent in regions where most of the annual precipitation occurs in the winter months (Rodin & Basilevich 1965; L. E. Rodin, personal communication; Bjerregaard 1971). Cool semi-desert communities in northern Utah exemplify this situation. Root/shoot biomass ratios are of the order of 9 for the dominant shrub species. Soil moisture recharge results primarily from winter precipitation in the form of snow. When the upper part of the soil profile thaws in early spring, the soil is recharged to a depth of approximately 80-90 cm. Subsequent summer season precipitation is of significance only for recharge of the uppermost soil layers. Therefore, during the course of the main growing season from March to September, there is a steady decline of soil moisture in most of the profile. Soil moisture potentials during the drier portion of the year of less than -70 atm are not uncommon in these soils (Moore & Caldwell 1972). Three prominent shrub species of this region, Atriplex confertifolia (Torr. and Frem.) S. Wats., Ceratoides lanata Nevski, and Artemisia tridentata Nutt. subsp. wyomingensis Beetle were studied as examples of cool semi-desert perennial species which possess profuse root systems composed almost entirely of fine root elements distributed in the upper metre of soil. The extensive root systems maintained by these species may be necessary in order to extract efficiently the available moisture infused throughout the upper metre of soil by the annual spring recharge. This study was undertaken to assess the seasonal root system dynamics and root morphology of such plants in the field in order to investigate the hypothesis that, even though these plants maintain an extensive and profuse root system throughout the zone of primary soil moisture recharge, only a small fraction of the roots are actively growing at any one time. A phased activity of a large root system should allow a prolonged period of root system growth during much of the year facilitating an efficient utilization of the annual soil moisture allotment.