Effects of Photoperiod, Soil Moisture and Soil Temperature on the Northward Migration Potential of Jack Pine under the Scenario of Doubled Atmospheric CO2 Concentration

Abstract

Climate envelope models predict that boreal trees can potentially migrate northward by as much as 10º by 2100 as a consequence of the predicted climate change associated with the doubling of atmospheric CO2 concentration, which might expose them to a new set of environmental conditions. The ability of the migrating trees to acclimate to the new set of environmental conditions may be critical for the actual scope and success of their migration or seed transfer. The changes in photoperiod, soil temperature and soil moisture associated with the migration will likely affect tree’s ecophysiological traits. Furthermore, the combined effects of several environmental factors may be substantially different from the total of their individual effects. In this study, I investigated the interactive effects of (1) photoperiod and soil temperature and (2) photoperiod and soil moisture on the ecophysiological responses of jack pine (Pinus banksiana Lamb.) seedlings to elevated [CO2]. In the first set of experiments, jack pine seedlings were exposed to two [CO2] (400 and 950 μmol mol-1), two soil temperatures (soil temperature at seed origin and 5º C warmer) and three photoperiod regimes (photoperiod at seed origin, 5o and 10o north of the seed origin). It was found that the photoperiod regime associated with a 10º northward migration advanced the timing of budburst by 10 days under the doubled [CO2] and current soil temperature at the seed origin. Also the photoperiod regimes at higher latitudes prolonged the process of bud setting. Photoperiod regimes at latitudes higher than the seed origin increased seedling height growth but did not have significant impact on seedling biomass. The elevated [CO2] increased the total leaf area per seedling, but reduced shoot to root ratio. Elevated [CO2] also increased the photosynthetic rate and photosynthetic water use efficiency (WUE). The maximum rate of carboxylation (Vcmax) and triose phosphate utilization (TPU) were affected by interactions involving CO2 and photoperiod but no meaningful pattern could be discerned. Both CO2 elevation and soil warming reduced the cold hardiness of jack pine seedlings as indicated by the injury index tested at -15 and -30º C testing temperatures. The lack of photoperiod effects on cold hardiness suggests that jack pine may be plastic enough to acclimate to the new photoperiod regime associated with climate change induced northward migration. However, advanced budburst associated with long distance migration (e.g. 10º north) will likely expose the species to late-spring frost damage. In the second set of experiments, seedlings were exposed to two [CO2] (400 and 950 μmol mol-1), two soil moistures (60–70% and 30–40% of field capacity) and three photoperiod regimes (photoperiod at seed origin, 5o and 10o north of the seed origin). The results suggest that the responses of jack pine to climate change will become complicated under the interactive effects of the longer growing season photoperiod and faster rate of change in day length at higher latitudes, and soil moisture stress under elevated [CO2]. Longer photoperiods at higher latitudes advanced budburst at both high and low soil moisture regime, which will likely increase the risk of late spring frosts damage prior to and during budburst. Longer summer photoperiods with northward migration increased the WUE under elevated [CO2] and low soil moisture regime. However, the significant 2- and 3-way interactions suggest that drought and longer photoperiods associated with northward migration will limit the positive effects of elevated [CO2] on growth and physiological processes in the species. Hydraulic conductivity in jack pine seedlings was significantly increased under elevated [CO2] while it was reduced at low soil moisture regime. The interactions of [CO2] and photoperiod had significant effects on the stem xylem vulnerability to cavitation. Tendency to embolize was significantly greater in the seedlings grown under elevated [CO2] with the photoperiod regime 10º north of the seed origin compared to those grown under ambient [CO2] with photoperiod 10º north of the seed origin and elevated [CO2] at the photoperiod regime of the seed origin. This result suggests that 10º northward migration under elevated CO2 will affect the hydraulic behavior of the species and make it vulnerable to xylem cavitation.