Very briefly, this growth-promoting phenomenon is believed to be the result of better light penetration into plant canopies when the incoming flux of solar radiation is not as unidirectional as it is when fewer aerosols are present.  The greater multi-directional reception of the solar flux that occurs in high-aerosol situations reduces the degree of shade experienced by the lower leaves of many plant canopies and enhances their photosynthetic rates.  In all of the situations so far studied, however, the source of the enhanced aerosol concentration has been a singular significant event, such as a massive volcanic eruption.  So what happens under more normal conditions?  This is the new and important question asked in the study of Niyogi et al. (2004): "can we detect the effect of relatively routine aerosol variability on field measurements of CO2 fluxes, and if so, how does the variability in aerosol loading affect CO2 fluxes over different landscapes?"

To answer this question, the group of sixteen researchers used CO2 flux data from the AmeriFlux network (Baldocchi et al., 2001) together with cloud-free aerosol optical depth data from the NASA Robotic Network (AERONET; Holben et al., 2001) to assess the effect of aerosol loading on the net assimilation of CO2 by three types of vegetation: trees (broadleaf deciduous forest and mixed forest), crops (winter wheat, soybeans and corn) and grasslands.  Their work revealed that an aerosol-induced increase in diffuse radiative-flux fraction [DRF = ratio of diffuse (Rd) to total or global (Rg) solar irradiance] increased the net CO2 assimilation of trees and crops, making them larger carbon sinks, but that it decreased the net CO2 assimilation of grasslands, making them smaller carbon sinks.

How significant were the effects observed by Niyogi et al.?  For a summer mid-range Rg flux of 500 W m^-2, going from the set of all DRF values between 0.0 and 0.4 to the set of all DRF values between 0.6 and 1.0 resulted in an approximate 50% increase in net CO2 assimilation by a broadleaf deciduous forest located in Tennessee, USA.  Averaged over the entire daylight period, they further determined that the shift from the lower to the higher set of DRF values "enhances photosynthetic fluxes by about 30% at this study site."  Similar results were obtained for the mixed forest and the conglomerate of crops studied.  Hence, they concluded that natural variability among commonly-present aerosols can "routinely influence surface irradiance and hence the terrestrial CO2 flux and regional carbon cycle."  And for these types of land-cover (forests and agricultural crops), that influence is to significantly increase the assimilation of CO2 from the atmosphere.

In the case of grasslands, however, the effect was found to be just the opposite, with greater aerosol loading of the atmosphere leading to less CO2 assimilation, due most likely, in the estimation of Niyogi et al., to grasslands' significantly different canopy architecture.  With respect to the planet as a whole, however, the net effect is decidedly positive, as earth's trees are the primary planetary players in the sequestration of carbon.  Post et al. (1990), for example, note that woody plants account for approximately 75% of terrestrial photosynthesis, which comprises about 90% of the global total (Sellers and McCarthy (1990); and those numbers make earth's trees and shrubs responsible for fully two thirds (0.75 x 90% = 67.5%) of the planet's net primary production.

What is especially exciting about these real-world observations is that much of the commonly-present aerosol burden of the atmosphere is plant-derived.  Hence, it can be appreciated that earth's woody plants are themselves responsible for emitting to the air that which ultimately enhances their photosynthetic prowess.  In other words, earth's trees significantly control their own destiny, i.e., they alter the atmospheric environment in a way that directly enhances their opportunities for greater growth.

Man helps too, in this regard; for as he pumps ever more CO2 into the atmosphere, the globe's woody plants quickly respond to its aerial fertilization effect, becoming ever more productive, which leads to even more plant-derived aerosols being released to the atmosphere, which stimulates this positive feedback cycle to a still greater degree.  Stated another way, earth's trees use some of the CO2 emitted to the atmosphere by man to alter the aerial environment so as to enable them to remove even more CO2 from the air.  The end result is that earth's trees and humanity are working hand-in-hand to significantly increase the productivity of the biosphere; and it is happening in spite of all other insults to the environment that work in opposition to enhanced biological activity.

References
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