PARENT SESSION
Poster Session # 15: Plant Ecology.

Wednesday, August 6 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

20,000 mm under the surface: Structure and function of deep tree roots in central Texas.

McElrone, Andrew^*,1, Pockman, Will², Addington, Robert¹, Jackson, Robert¹, ¹ Department of Biology, Durham, NC, USA² Department of Biology, Albuquerque, NM, USA

ABSTRACT- Deep roots often contribute substantially to whole tree water use through uptake of reliable deep water resources, particularly in water-limited environments. Despite their importance, most studies of deep roots have favored indirect methods over direct physiological measurements because of limited access. We used caves in the Edwards Plateau region of central Texas to access deep tree roots utilizing a perennial underground stream at approximately 20 meters depth. Daily and seasonal measurements of sap flow, water potential, and transpiration on Bumelia lanuginosa and Quercus fusiformis were combined with xylem anatomy and hydraulic conductivity (K_h) measurements to characterize the response of these species to changes in environmental conditions along the soil-plant-atmosphere continuum. Sap flow and water potential data in deep roots closely tracked similar changes in stem sap flow for both tree species. Changes in sap flow and water potential in all plant components were most strongly correlated with the changes in vapor pressure deficit and photosynthetically active radiation. Xylem pressure changes in stems propagated to deep roots in as little as 25-45 minutes, suggesting low water storage for the trees in this system. During whole-tree shade treatments repeated throughout the season on B. lanuginosa, stem sap flow was reduced 50-75% as a result of decreased transpiration. The concurrent reduction in deep root sap flow during these periods varied in magnitude depending on the availability of shallow soil water from recent precipitation events. Changes in xylem structure and function were detected for both species with mean vessel diameter and K_h largest in deep roots, intermediate in shallow roots, and smallest in branches. Such adjustments would reduce flow resistance in deep roots and maintain deep water use as environmental conditions change throughout the season. These data show that deep roots are tightly integrated with aboveground plant components, which may contribute to the persistence of woody species in water-limited habitats.

Key words: caves, deep tree roots, plant water relations