Effects of water availability, longer photoperiod, and elevated [CO2] on the potential of the northward migration in trembling aspen (populus tremuloides michx.)
Based on climate envelope models, boreal tree species may migrate to higher latitudes (approximately 10˚N toward the north) in response to rapid climate changes by the end of this century. Multiple factors will likely influence the success of the migration. However, the interactive impacts of these factors are poorly understood. The primary objective of this thesis was to investigate the interactive effect of soil moisture, photoperiod and [CO2] on the physiology, morphology, and phenology of trembling aspen (Populus tremuloides Michx.) in a context of northward migration. Seedlings were exposed to two [CO2] (AC: 400 vs. EC: 1000 μmol mol-1), four photoperiod regimes corresponding to 48 (seed origin), 52, 55, and 58°N latitudes and two soil moisture regimes (high soil moisture (HSM) vs. low soil moisture (LSM)) for two growing seasons in environment-controlled greenhouses. I assessed the responses of a suite of morphological, physiological and phenological responses. I found that the longest photoperiod regime (corresponding to 58˚N) had the greatest height growth and total biomass at the first growing season, but cavitation resistance significantly decreases in photoperiod at higher latitude under EC. The results suggest that when migrating to higher latitudes, trembling aspen may grow faster but will become less resistant to drought and more prone to hydraulic failure during a drought spell. There were also significant interactive effects of photoperiod, [CO2] and soil moisture on the tree physiology. Both stomatal conductance (gs) and its reduction in response to LSM declined with increasing photoperiod, which can have significant implications for the impact of soil moisture on the tree northward migration. Surprisingly, LSM resulted in an upregulation of photosynthesis as indicated by increases in the maximum rate of Rubisco carboxylation and maximum rate of electron transport for RuBP regeneration, and the magnitude of the upregulation increased with increasing photoperiod. The photosynthetic upregulation was accompanied by an increase in biomass allocation to roots and a reduction in gs under LSM. LSM increased the instantaneous water-use efficiency (iWUE), and the increase was greater under EC. While LSM increased photosynthesis in both AC and EC in July, the increase was significant under EC in August. EC increased iWUE in both soil moisture treatments in July, but the increase was significant only under LSM in August. Furthermore, EC enhanced net photosynthetic rate (Pn) only under LSM but not HSM. I also found that the significant interactive effects of photoperiod, [CO2] and soil moisture on phenology. The two longer photoperiods at 55 and 58˚N latitude significantly delayed growth cessation, bud set and the development of cold hardiness in the fall, particularly under the EC treatment. Although the longest photoperiod resulted in the highest height growth, photoperiod did not have a significant impact on diameter growth or biomass in the second growing season. LSM also delayed phenological events in the fall, but EC delayed bud break in the spring. The significant interactive effects of photoperiod, [CO2] and soil moisture on phenology, cold hardiness, growth and the length of the growing season indicate the complex nature of latitudinal tree migration that has been predicated under the scenario of climate change. The results of this thesis should advance our understanding of how soil moisture, photoperiod and [CO2] may interact each other and influence the northward migration of trembling aspen and should also have implications for the potential responses of other boreal tree species, particularly deciduous species. The results of this thesis indicate the importance of understanding interactions of multiple factors in affecting the potential migration of tree species and suggest that [CO2], photoperiod, and soil moisture should be considered in assessing the potential northward migration of boreal trees in general or human-assisted migration as a response strategy to climate change.