Phenology, growth and physiological responses to photoperiod and elevated [CO2] of boreal white birch (Betula papyrifera Marsh): implications to climate change-induced northward migration
With the increase in average global temperature due to rising atmospheric carbon dioxide concentration ([CO2]), rapid shifts of climate envelopes are expected to lead to norward migration for boreal tree species. As a result, trees will be exposed to new photoperiod regimes and higher [CO2]. These factors will likely have interactive effects on the physiology, phenology, survival, and growth of tree species. The successful northward migration of trees will likely depend on their biological capacity (phenotypic plasticity) to cope with novel conditions. To better understand how boreal tree species will respond to changes in photoperiod and [CO2], it is critical to experimentally investigate the impact of potential changes on the physiology, phenology, and growth of individual species. The main objective of this thesis was to examine how the phenology, growth, and physiology of white birch (Betula papyrifera Marsh) would respond to photoperiod and elevated [CO2] in the context of climate change-induced tree migration. To achieve this target, seedlings were exposed to ambient (400 μmol mol−1) or elevated (1000 μmol mol−1) [CO2], and four photoperiod regimes corresponding to 48 (seed origin), 52, 55, and 58º N for two growing seasons. The results showed that a moderate increase in photoperiod increased growth and biomass, CO2 elevation stimulated photosynthetic carbon uptake (Pn). However, white birch did not benefit further from even longer photoperiods. The lack of Pn stimulation by CO2 elevation was associated with the downregulation of photosynthetic capacity, the maximum Rubisco activity (Vcmax) and maximum electron transport rate (Jmax). The seedlings under longer photoperiod advanced budburst under elevated CO2, whereas photoperiod had opposite effects on budburst under ambient CO2. Further, CO2 elevation lengthened the duration of the leaf senescence process due to an earlier onset and later completion of leaf senescence. Interestingly, longer photoperiods at the two high latitudes induced an earlier onset of leaf senescence and achieved a higher level of cold hardiness than at the two lower latitudes under elevated [CO2]. Further, the photoperiod at the latitude 10º N north of seed origin enhanced the maximum stem-specific hydraulic conductivity. The interactive effects of photoperiod and [CO2] on the phenology, growth, and physiology of white birch seedlings indicate at novel conditions at higher latitudes may play a critical role in the success of climate change-induced migration.