Hydraulic Controls On Carbon Cycling and Water Exchange Rates of Southern Conifers.

Loblolly pines (Pinus taeda) represent one of the most widely planted species worldwide, and a species with very large distribution in the southeastern region of the U.S.  Pine plantations represent a mosaic of ever-changing leaf area index (LAI) and hydraulic resistance to water movement from the soil to the atmosphere.   At the leaf level, transpiration and photosynthesis are controlled by stomatal aperture regulated to maximize carbon gain for a given water loss rate. The transpired water is supplied to the leaves from the soil; it is controlled by the balance between xylem hydraulic conductivity and water potential gradient between soil and leaves. Large gradients provide a stronger driving force stimulating transpiration, but the accompanying low xylem water potential cause a reduction of hydraulic conductivity due to cavitation. This tradeoff between hydraulic efficiency and driving force results in maximum transpiration rates at intermediate values of water potential for a given stand age. Reaching this theoretical limit would allow plantations to fully exploit available water resources, maintaining leaves hydrated, and sustaining carbon dioxide uptake. Yet, aiming at water savings, stomatal regulation may prevent plants from reaching this limit. This project investigates whether pines actually reach the maximum theoretical transpiration rate under ambient or enriched atmospheric CO2. We address this question using a parsimonious model of soil-plant hydraulics, and show that indeed maximum transpiration rates are reached over a wide range of plant sizes and stand ages. These results suggest that plant xylem and stomata may have co-evolved to meet peak transpiration, thus sustaining carbon uptake while avoiding tissue damage. These results also explain the strong coordination between liquid- and gas-phase conductances reported across numerous ecosystems and biomes.