Root hydraulic properties
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As a major component of whole-plant hydraulic conductance, without understanding root hydraulic properties, it would be impossible to understand hydraulic responses and growth of shoots to the changes in resource availability. The first study examined the impacts of atmospheric [CO2] and soil fertility on hydraulic conductivity (K) in stems, taproots and fine roots in Pinus taeda L. seedlings. The differences in K resulting from different resource availability were related to xylem anatomy. Higher stem K was associated with increased tracheid lumen diameter by high fertility. Taproot tracheid lumen diameter was larger than that in stems. Fine roots had larger individual tracheid lumen diameter but smaller root radius in the elevated [CO2] and high fertility treatment compared to other treatments. K of fine roots were more than 2-fold higher under elevated [CO2] and high fertility compared with other treatments, suggesting conductivity was primarily increased in fine roots by an increase in tracheid lumen diameter. Both stem and taproot K was positively related to most of the parameters we measured in tree growth and leaf gas exchange under different treatments. When plant species grow in natural ecosystems, intra- and interspecific root interaction is inevitable. In the second study, we measured k in the entire root system (kR) of two co-occurring plants. Root-specific hydraulic conductance (kS) was calculated by dividing kR by the biomass of entire root system. Pinus taeda and Quercus pagoda seedlings were planted in pots to create conditions of intra- or interspecific root interaction, or no interaction. High or low rates of fertilization were applied to each interaction treatment. The Q. pagoda seedlings under interspecific root interaction and low fertility increased kR by developing larger root systems, while P. taeda seedlings under interspecific interaction and high fertility increased kR by increasing kS. Higher kR of P. taeda seedlings under intraspecific interaction and high fertility was also associated with larger root systems. This is similar with the observation from mature P. taeda trees in the field: those growing under intraspecific interaction and high fertility had highest fine root k and growth rate compared to the other treatments.