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Deciduous Forests Exert a Brake on Global Warming as They Lengthen Their Growing Seasons and Sequester More Carbon
One of the most simple ways by which earth's deciduous forests help to slow global warming is to leaf out progressively earlier each spring and remain photosynthetically active increasingly later every fall. This gradual lengthening of the growing season allows trees to remove more carbon dioxide from the atmosphere each succeeding year; and this phenomenon, in turn, reduces the annual rate of rise of the air's CO2 content, completing a negative feedback loop that slows the warming that originally set the whole process in motion.

Several aspects of this multi-stage phenomenon were investigated in detail by White et al. (1999). In a study that utilized 88 years of data, spanning the period 1900 to 1987, that were obtained from 12 different locations within the eastern U.S. deciduous forest - which stretched from Charleston, SC (32.8N latitude) to Burlington, VT (44.5N latitude) - they determined, first of all, that a 1C increase in mean annual air temperature increased the length of the forest's growing season by approximately five days. In addition, they demonstrated that this relationship was linear over the entire mean annual air temperature range investigated, which stretched from 7 to 19C and included growing seasons ranging in length from 150 to 210 days.

The second step of White et al.'s analysis was a bit more complicated, as they had to determine how much extra CO2 was removed from the air for each day's temperature-induced extension of the growing season. They began this process by using an ecosystem process model - which had previously been validated in a number of other studies - to predict carbon fluxes for the twelve different forest sites. Using daily meteorological data, the model first calculated annual dates of the appearance and disappearance of "greenness," i.e., the length of the photosynthetically-active growing season, for each of the 88 years of records at each site. Then, it calculated the net primary production (NEP = gross primary production minus the sum of autotrophic and heterotrophic respiration) for each day of the 88 growing seasons at each site and summed each year's results to obtain 88 annual NEP totals for each site.

Plotting these yearly totals of net CO2 removal from the atmosphere as functions of yearly growing season length for each site, White et al. determined that a one-day extension in growing season length increased the mean forest NEP of the 12 sites by 1.6%, with greater increases in the colder northern sites (1.9% for Burlington, VT) and smaller increases in the warmer southern sites (1.4% for Charleston, SC).

How significant are these numbers? Let us suppose, for the sake of illustration, that the earth were to experience a mean global warming of 1C. According to the first of the two relationships derived by White et al., this temperature increase would result in the eastern U.S. deciduous forest increasing the length of its growing season by approximately five days; while their second relationship suggests that this growing season expansion would lead to an 8% increase in total carbon sequestration (5 days x 1.6% per day = 8%).

It is possible, however, that this temperature-driven phenomenon may actually be much stronger than what is implied by this simple exercise. In a study of 30 years of phenological data derived from observations of identical clones of trees and shrubs maintained by the European network of the International Phenological Gardens - which network is located within the area bounded by latitudes 42 and 69 N and by longitudes 10 W and 27 E - Menzel and Fabian (1999) determined that the mean date of spring bud-break had increased by fully six days "since the early 1960s," while leaf senescence in the fall had been delayed by an average of 4.8 days over the same period. Using the northernmost NEP enhancement factor derived by White et al. - which in all likelihood is a conservative choice, as White et al.'s northernmost forest site is located at about the same latitude as the southernmost part of the latitudinal gradient spanned by Menzel and Fabian's study area - we thus calculate that the 10.8 extra warming-induced growing season days produce a 20.5% increase in annual carbon sequestration (10.8 days x 1.9% per day = 20.5%).

But the final result may well be larger still, for the temperature increase experienced over the 30-year period studied by Menzel and Fabian was considerably less than 1C. Using the Jones et al. (1999) data set - which is accessible at www.co2science.org - for example, we have calculated the amount of warming experienced over successive 30-year periods beginning in 1961, 62, 63 and 64 to approximately determine the 30-year regional mean annual air temperature increase that occurred, in the words of Menzel and Fabian, "since the early 1960s." The mean result suggests a region-wide warming of something on the order of 0.38C over this period; and since the study of White et al. indicates that the lengthening of the growing season is a linear function of mean annual air temperature, the data of Menzel and Fabian thus suggest that a 1C increase in this parameter could possibly boost carbon sequestration by deciduous trees and shrubs by fully 54% (1C/0.38C x 20.5% = 54%).

In view of these observations, we totally agree with White et al. that "persistent increases in growing season length may lead to long-term increases in carbon storage," as we also agree with Menzel and Fabian that "the lengthening of the growing season is likely to contribute to increased biomass formation." In fact, our analysis suggests that the impact of this incredibly simple negative-feedback phenomenon may well be much larger than anyone, including White et al. and Menzel and Fabian, has ever imagined.

Could it be that this analysis represents the long-sought answer to the enigmatic question of the 20th century's "missing carbon," which has exited the atmosphere and been sequestered in a large terrestrial sink that no one has yet been able to identify? With a calculated increase in woody-plant carbon sequestration that ranges from 8 to 54% for a 1C increase in mean annual air temperature, it is difficult to say. If the global mean is closer to the latter of these two numbers, however, the answer might very well be yes.

Dr. Sherwood B. Idso Dr. Keith E. Idso

Jones, P.D., Parker, D.E., Osborn, T.J. and Briffa, K.R. 1999. Global and hemispheric temperature anomalies - land and marine instrument records. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, TN, USA.

Menzel, A. and Fabian, P. 1999. Growing season extended in Europe. Nature 397: 659.

White, M.A., Running, S.W. and Thornton, P.E. 1999. The impact of growing-season length variability on carbon assimilation and evapotranspiration over 88 years in the eastern US deciduous forest. International Journal of Biometeorology 42: 139-145.