Tree root systems have evolved multidimensional functioning, specializing in water and nutrient acquisition via different strategies. Root functional strategies vary among tree species and are adapted to the abiotic and biotic soil environment. This dissertation explores how three facets of root system functional strategies – morphology and chemistry, biotic associations with fungi, and respiration rates – vary within and among tree species along environmental gradients. Chapter one examines how root system morphology varies with forest succession and soil environment in a tropical forest of Hainan, China, finding that root systems had larger diameters and fewer root tips in the younger forest, and that soil phosphorus and base saturation affected intraspecific root trait variation. Chapter two compares root and leaf tissue chemistry within the same forest, finding that root systems function to maintain tree elemental homeostasis despite variation in morphology and soil environment. Chapter three links morphological variation of roots and leaves from chapter one to forest compositional turnover at the landscape-scale using existing plot data, finding that trait variation negatively co-varied viii with species turnover to influence community-weighted mean trait estimates. Chapter four assesses how root-associated fungal communities are structured by the soil environment and root traits in three tropical forest plots of varying age and soil fertility in Xishuangbanna, China. Variation in the soil environment strongly influenced rootassociated fungal communities, outweighing the effects of root morphological traits, although root system architecture and root calcium were important secondary determinants. Chapter five explores how root functional strategies shape the contribution of root system respiration to soil respiration among eight temperate tree species. Root system tissue respiration rates were greater with root system length and root tissue nitrogen concentrations. The root-associated fraction of soil respiration increased with specific root length, tip abundance, but decreased with root tissue density. Lastly, chapter six chronicles a warming experiment with the model tree, Populus trichocarpa, finding that acclimation of root respiration was less than leaf respiration and was mediated by soil nitrogen availability. Interspecific differences in root functional strategies evidence evolved trade-offs in nutrient and water uptake strategies among species; however, the environment exerts considerable influence on root morphology, chemistry, physiology, and root-soil interactions.
