Long Term GrowthGrowth data from long term studies shows significant variability in growth at 32 years (Figure 11). The experimental area where all trees were grown is in central Alberta. Therefore we would expect that seeds from central Alberta sources would do very well, whereas trees from maritime locations, where the climate is considerably wetter, would not do as well. Yukon seeds are likely not used to the drier, warmer conditions in Alberta, and we can see this from the significantly lower average height. It is also possible that, although some individuals may be experiencing wetter conditions than their region of origin, they are unable to take advantage of the increase in precipitation even though it would benefit them. This is likely do to limitations caused by their physiology. It is interesting however to see that those trees sourced from Ontario grew nearly as well as Albertan native.
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Figure 11. Average height of individuals within each region at 32 years. Regions within Alberta are shown in green to compare growth within the province. Error bars represent standard error of the mean. A one-way ANOVA was used to determine significant differences between heights indicated by letters (a,b,c,d).
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Chlorophyll Fluorescence and Drought ToleranceThe effect of drought on seedlings is similar to what we expected. Those seeds that came from sources regions that are warmer and drier tended to have higher photosynthetic efficiency during drought. However, there are larger changes from drought to control than were expected. We see that northern Alberta seedlings show low efficiency in general but are not as strongly affected by drought, showing less change in efficiency (Figure 12).
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Figure 12. A comparison of photochemical efficiency, Fv/Fm, for control and drought treatments. Each treatment had 5 individuals per region. Error bars represent standard error of the mean. Stars (*) indicate significant differences between treatments. N.S. indicates non-significant differences between treatments. A two-way ANOVA was run to determine significant differences between drought and control treatments.
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As drought increased all seedlings showed decreasing photosynthetic efficiency. Some seedlings, those from southern and foothills Alberta showed high initial efficiency but similar amounts of decline as the other species (Figure 13). Seedlings from the regions that experience less drought, Yukon and maritime provinces, showed more decline with decreasing water potential values, illustrating their particular sensitivity to drought.
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Figure 13. Change in photochemical efficiency as drought stress increases. Drought increases as water potential becomes more negative.
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What do we do now?
Long term data coupled with our drought stress experiments show us that there are some regions that are very tolerant to drought stress, but show low rates of growth. Drought conditions seemed to have less detrimental effects on seedlings from southern Alberta than other regions, making it a seemingly good candidate for planting in the changing northern environments. However, it showed relatively low growth compared to other regions, which is not ideal for the forest industry. It seems that those seedlings which showed middle of the road drought tolerance, but showed high growth in long term trials are best suited for planting in warming regions. Central Alberta and Ontario seem to be the most likely regions to both survive drought conditions and maintain high levels of growth in the long run.
Further investigation is needed to fully advise planting strategies. Transitioning to warmer climates does not mean that every day is warmer, and therefore northern regions still experience extreme frost conditions which may not be tolerated by seedlings from warmer regions. An ideal planting stock with be drought tolerant as well as frost resistant. It must also show efficient long term growth. Other physiological factors could also be influenced to narrow down what causes some trees to maintain their photosynthetic efficiency during drought conditions. This is likely influenced by stomatal control and could potentially be investigated using isotope analysis.
Further investigation is needed to fully advise planting strategies. Transitioning to warmer climates does not mean that every day is warmer, and therefore northern regions still experience extreme frost conditions which may not be tolerated by seedlings from warmer regions. An ideal planting stock with be drought tolerant as well as frost resistant. It must also show efficient long term growth. Other physiological factors could also be influenced to narrow down what causes some trees to maintain their photosynthetic efficiency during drought conditions. This is likely influenced by stomatal control and could potentially be investigated using isotope analysis.