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Old Trees Doing it Better than Young Trees: Responding to CO2
Volume 14, Number 8: 23 February 2011

In introducing their important new study, Knapp and Soule (2011) write that "atmospheric CO2 concentrations have increased by over 27% since the early 20th century, resulting in enhanced radial tree growth in natural environments for numerous tree species in a variety of climatic regions (e.g., LaMarche et al., 1984; Knapp et al., 2001; Soule and Knapp, 2006; Voelker et al., 2006; Wang et al., 2006; Koutavas, 2008)." They additionally note that "the principal benefit of elevated CO2 for radial growth has been linked to increased intrinsic water-use efficiency (iWUE), which is the ratio of net CO2 assimilation through leaf stomata to leaf stomatal conductance." And they report that "increases in iWUE based on carbon isotope chronologies have been identified for trees growing in both controlled (e.g., Leavitt et al., 2003) and natural environments (e.g., Bert et al., 1997; Feng, 1999; Tang et al., 1999; Arneth et al., 2002; Saurer et al., 2004; Waterhouse et al., 2004; Liu et al., 2007)."

In light of these well established facts, the two researchers, as they describe it, "examined radial growth responses of ponderosa pine (Pinus ponderosa var. ponderosa) between 1905-1954 and 1955-2004 to determine if the effects of increased intrinsic water-use efficiencies caused by elevated atmospheric CO2 concentrations were age-specific," working with 209 cores collected from mature trees (ranging in age from at least 100 to over 450 years) from five different sites in the USA's northern Rocky Mountains, while additionally calculating iWUE using carbon isotope data from 1850 to 2004.

This work, according to Knapp and Soule, revealed that "(1) responses to elevated atmospheric CO2 in old-growth ponderosa forests are age-specific; (2) radial growth increases in older trees coincided with increased iWUE; (3) ponderosa had increased growth rates in their third, fourth, and fifth centuries of life; and (4) age-specific growth responses during 1955-2004 are unique since at least the mid-16th century." In addition, they report that "increases in iWUE during 1955-2004 were 11% greater than during 1905-1954."

In discussing their impressive findings, Knapp and Soule say they "demonstrate that old-growth ponderosa pine forests of the northern Rockies have likely benefited from the effects of increased atmospheric CO2 since the mid-20th century and that the benefits increase with tree age." And since the CO2-induced radial growth increases in the older trees "were significantly associated with rising iWUE," they say the "accelerated growth rates are likely caused by more efficient water use in the semiarid environment where the trees were sampled."

In concluding, the two scientists rightly state that "old-growth trees can be highly responsive to environmental changes," especially, we would add, that of the ongoing rise in the air's CO2 content, as their work clearly demonstrates to be the case. In fact, they note that even what many might call ancient trees "are capable of increased growth rates several hundred years after establishment," further citing in this regard the work of McDowell et al. (2003) and Martinez-Vilalta et al. (2007).

Sherwood, Keith and Craig Idso

Arneth, A., Lloyd, J., Santruckova, H., Bird, M., Grigoryev, S., Kalaschnikov, Y.N., Sukachev, V.N., Gleixner, G. and Schulze, E.-D. 2002. Response of central Siberian Scots pine to soil water deficit and long-term trends in atmospheric CO2 concentration. Global Biogeochemical Cycles 16: 10.1029/2000GB001374.

Bert, D., Leavitt, S.W. and Dupouey, J.-L. 1997. Variations of wood δ13C and water-use efficiency of Abies alba during the last century. Ecology 78: 1588-1596.

Feng, X. 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: a response to atmospheric concentration. Geochimica et Cosmochimica Acta 63: 1891-1903.

Knapp, P.A. and Soule, P.T. 2011. Increasing water-use efficiency and age-specific growth responses of old-growth ponderosa pine trees in the Northern Rockies. Global Change Biology 17: 631-641.

Knapp, P.A., Soule, P.T. and Grissino-Mayer, H.D. 2001. Detecting the potential regional effects of increased atmospheric CO2 on growth rates of western juniper. Global Change Biology 7: 903-917.

Koutavas, A. 2008. Late 20th century growth acceleration in Greek firs (Abies cephalonica) from Cephalonia Island, Greece: a CO2 fertilization effect? Dendrochronologia 26: 13-19.

LaMarche Jr., V.C., Graybill, D.A., Fritts, H.C. and Rose, M.R. 1984. Increasing atmospheric carbon dioxide: Tree ring evidence for growth enhancement in natural vegetation. Science 223: 1019-1021.

Leavitt, S.W., Idso, S.B., Kimball, B.A., Burns, J.M., Sinha, A. and Stott, L. 2003. The effect of long-term atmospheric CO2 enrichment on the intrinsic water-use efficiency of sour orange trees. Chemosphere 50: 217-222.

Liu, X., Shao, X., Liang, E., Zhao, L., Chen, T., Qin, D. and Ren, J. 2007. Species dependent responses of juniper and spruce to increasing CO2 concentration and to climate in semi-arid and arid areas of northwestern China. Plant Ecology 193: 195-209.

Martinez-Vilalta, J., Vanderklein, D. and Mencuccini, M. 2007. Tree height and age-related decline in growth in Scots pine (Pinus sylvestris L.). Oecologia 150: 529-544.

McDowell, N., Brooks, J.R., Fitzgerald, S.A. and Bond, B.J. 2003. Carbon isotope discrimination and growth response of old Pinus ponderosa trees to stand density reductions. Plant, Cell and Environment 26: 631-644.

Saurer, M.S., Siegwolf, R.T.W. and Schweingruber, F. 2004. Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years. Global Change Biology 10: 2109-2120.

Soule, P.T. and Knapp, P.A. 2006. Radial growth rate increases in naturally-occurring ponderosa pine trees: a late 20th century CO2 fertilization effect? New Phytologist 171: 379-390.

Tang, K., Feng, X. and Funkhouser, G. 1999. The δ13C of trees in full-bark and strip-bark bristlecone pines in the White Mountains of California. Global Change Biology 5: 33-40.

Voelker, S.L., Muzika, R., Guyette, R.P. and Stambaugh, M.C. 2006. Evidence for historic CO2 enhancement of tree-ring growth shows a decline through age in Quercus velutina, Quercus coccinea and Pinus echinata. Ecological Monographs 76: 549-564.

Wang, G.G., Chhin, S. and Baurle, W.L. 2006. Effect of natural atmospheric CO2 fertilization suggested by open-grown white spruce in a dry environment. Global Change Biology 12: 601-610.

Waterhouse, J.S., Switsur, V.R., Barker, A.C., Carter, A.H.C., Hemming, D.L., Loader, N.J. and Robertson, I. 2004. Northern European trees show a progressively diminishing response to increasing atmospheric carbon dioxide concentrations. Quaternary Science Reviews 23: 803-810.