Effects of CO2 elevation, photoperiod, and nutrient supply on growth and cold hardiness of black spruce (Picea mariana) seedlings

Background/Question/Methods

Climate Envelope models predict that boreal black spruce will migrate northward as the global warming associated with increasing atmospheric CO₂ and changes in precipitation progress. However, differences in photoperiod and nutrient supply at higher latitudes may influence the migration. This study examined the effect of photoperiod (equivalent 10^o northward migration) and nutrient supply (300 vs. 30 µmol mol^-1 N) on the growth and cold hardiness of black spruce (Picea mariana (Mill.) B.S.P.) seedlings under 360 and 720 µmol mol^-1CO₂ concentration. Seedlings were exposed to the treatments shortly after germination. The photoperiod treatment started with the value at the seed origin and 10^o northward and later ramped down to the corresponding fall values.  After 4 months of treatment, measurements were made on seedling height, root collar diameter, terminal bud size and biomass. The development of cold hardiness after treatment of fall photoperiod was assessed using electrolyte leakage method (ELM) in which measurements were made on shoot samples after exposure to -5, -15, -30 or -60 ^oC temperature for a 1-hour period (cooling rate -5 ^oC /hr). 

Results/Conclusions

It was found that seedling height, root collar diameter and biomass were greatest under the photoperiod at the higher latitude, high N supply and elevated CO₂. CO₂ concentration had insignificant effect on terminal bud development. However, cold hardiness was least under the low nitrogen supply and photoperiod corresponding to the higher latitude. The freezing injuries increased as temperature decreased from -5, -15, -30 to -60 ^oC.  CO₂ elevation, photoperiod and N supply had significant effects on freezing injury at -15 and -30 ^oC, but not at -5 or -60 ^oC. The injuries were greatest under the low nitrogen supply and photoperiod corresponding to the higher latitude. The CO₂ elevation did not significantly affect seedling cold hardiness. The results suggest that black spruce may grow faster if it migrates further north, but the risk of reduced cold hardiness and consequent freezing damages will increase and nutrient stress will likely exacerbate the problem.