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Long-Term Studies (Woody Plants: Spruce) -- Summary
Limiting ourselves to experiments that have lasted a minimum of four years, only a few studies have produced results that may be pertinent to the question of spruce tree response to permanently-elevated concentrations of atmospheric carbon dioxide.

Spunda et al. (1998) grew fifteen-year-old Norway spruce (Picea abies) trees in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm for four years. At the end of this period they found current-year shoots of the trees growing in the CO2-enriched chambers displaying rates of net photosynthesis that were 78% greater than those exhibited by the current-year shoots of the trees growing in ambient air.

Also working with Norway spruce, Spinnler et al. (2003) grew seedlings originating from eight different provenances for four full years on a nutrient-poor acidic soil and a nutrient-rich calcareous soil placed in lysimeters located within open-top chambers that were maintained at CO2 concentrations of either 370 or 570 ppm.  When growing in the nutrient-poor soil, total spruce biomass increased by 9 to 38% across the eight different provenances, while in the nutrient-rich soil it increased by 10 to 74%.

In an experiment that ran for five years and focused on Sitka spruce (Picea sitchensis), Liu et al. (2002) grew seedlings in open-top chambers that were maintained for five years at CO2 concentrations of either 350 or 700 ppm.  For the first three years of this period, the seedlings were grown in well-watered and fertilized pots placed within the chambers, after which they were planted directly into native nutrient-deficient forest soil and maintained for two more years in larger open-top chambers, either with or without an extra supply of nitrogen (N).

After the first three years of growth in pots, the CO2-enriched trees were found to possess 11.6% more total biomass than the ambient-treatment trees.  At the end of the next two years of the study, the trees supplied with extra N possessed 15.6% more total biomass than their similarly-treated ambient-air counterparts, while those receiving no extra N had 20.5% more total biomass than their ambient-air counterparts.

Liu et al. make a point of noting that these CO2-induced increases in growth occurred in spite of a down-regulation of photosynthesis and a reduction of foliar Rubisco activity.  In addition, they report that "visual foliar N-deficiency symptoms (needle yellowing and chlorosis) were obvious on some of the saplings with no added N supply during the final year of the experiment."  However, they further note that "such N deficiency is common in many boreal forest sites, and therefore a growth response to rising atmospheric CO2 can be expected to occur in such forests," in support of which conclusion they say that "growth responses to elevated CO2 despite nutrient stress have been reported previously in Scots pine (Kellomaki and Wang, 1997), grass (Cannell and Thornley, 1998) and Sitka spruce (Centritto et al., 1999; Murray et al., 2000)."

In light of these several findings, as well as those cited by Liu et al., it would appear that long-term positive growth responses to the ongoing rise in the air's CO2 content have a good chance of being maintained indefinitely by earth's spruce trees (and other species), even when growing on nutrient-deficient soils.

Cannell, M.G.R. and Thornley, H.M.  1998.  N-poor ecosystems may respond more to elevated [CO2] than N-rich ones in the long term, a model analysis of grassland.  Global Change Biology 4: 101-112.

Centritto, M., Lee, H.S.J. and Jarvis, P.G.  1999.  Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce (Picea sitchensis). I. Plant growth, allocation and ontogeny.  Tree Physiology 19: 799-806.

Kellomaki, S. and Wang, K.Y.  1997.  Photosynthetic response of Scots pine to elevated CO2 and nitrogen supply: results of a branch-in-bag experiment.  Tree Physiology 17: 231-240.

Liu, S.R., Barton, C., Lee, H., Jarvis, P.G. and Durrant, D.  2002.  Long-term response of Sitka spruce (Picea sitchensis (Bong.) Carr.) to CO2 enrichment and nitrogen supply. I. Growth, biomass allocation and physiology.  Plant Biosystems 136: 189-198.

Murray, M.B., Smith, R.I., Friend, A. and Jarvis, P.G.  2000.  Effect of elevated [CO2] and varying nutrient application rates on physiology and biomass accumulation of Sitka spruce (Picea sitchensis).  Tree Physiology 20: 421-434.

Spinnler, D., Egli, P. and Korner, C.  2003.  Provenance effects and allometry in beech and spruce under elevated CO2 and nitrogen on two different forest soils.  Basic and Applied Ecology 4: 467-478.

Spunda, V., Kalina, J., Cajanek, M., Pavlickova, H. and Marek, M.V.  1998.  Long-term exposure of Norway spruce to elevated CO2 concentration induces changes in photosystem II mimicking an adaptation to increased irradiance.  Journal of Plant Physiology 152: 413-419.