The effect of low root temperature on water uptake in temperate tree species

Summary
Low temperature is one of the main drivers controlling plant growth, reproduction and species distribution range limits. As the limit for tree growth in general, most of studies focused on the general cold edge of the life-form tree at high elevations or latitudes, the fundamental range limits for a tree, but in fact, individual tree species that reach their distribution limits below treeline have their species-specific low temperature extremes as well. While our understanding of the direct cold temperature induced restrictions of cambial and meristematic activity has increased substantially over the last decades, other physiological effects that might also contribute to the cold temperature limit of tree growth have gained much less attention so far. Especially, the potential effects of restricted water uptake and deteriorated hydraulic relations at low root zone temperatures might be additional drivers for the cold limit of tree growth. A relatively limited number of studies delivered strong evidence that even in cold-tolerant tree species, cool root zone temperatures well above 0 °C can have significantly negative effects on the capacity of root water uptake and root hydraulic conductance. Negative cold soil effects on the hydraulic conductivity of trees can thus potentially amplify the direct effects of cold temperatures growth and contribute to the cold limit of temperate tree species. However, the assumption that the restricted water conductivity of roots at low soil temperature for grow limits can not be tested up to date.
As a consequence, a series of experimental studies in this dissertation aimed to provide new insights into the biophysiological mechanism responsible for the cold-temperature limits of temperate trees and the possible explanation for the species-specific cold limits of tree growth. The results of this work revealed that species-specific cold sensitivity in root water uptake might be a fundamental role for the cold range limits for temperate tree species behind the limiting of new cell growth at cold climate. The further study of this dissertation found the prolonged cold soil temperature can lead to the consistent reduction in root water uptake, inducing the occurrence of phenological signal that controlled the preparation for winter dormancy. Furthermore, the results also provide an experimental evidence that the long-term cold acclimation would potentially enhance the capacity of plant root water uptake in temperate trees, while can not improve the physiological threshold of trees controlling the sensitivity of root water uptake in general. Overall, this dissertation with different studies disentangled the physiological and ecological mechanisms behind the elevational or latitudinal cold limits of tree species below the treeline, and highlight the importance of species-specific physiological threshold controlling their sensitivity of root hydraulic conductance for tree growth and tree distribution models in nature.