How does rising atmospheric CO2 affect marine organisms?

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Does Elevated CO2 Promote Establishment of Woody Species in Grasslands?
Polley, H.W., Johnson, H.B. and Tischler, C.R.  2002.  Woody invasion of grasslands: evidence that CO2 enrichment indirectly promotes establishment of Prosopis glandulosaPlant Ecology 164: 85-94.

What was done
Representative grassland plants, dominated by the perennial C4 species Bothriochloa ischaemum, were grown in plastic tunnels having a continuous CO2 gradient from 550 to 200 ppm to determine how the air's CO2 concentration impacts grassland water use and subsequent soil moisture contents.  In a parallel study, honey mesquite (Prosopis glandulosa) seeds were germinated and seedling survivorship followed in grasslands where grass roots were excluded from the first 0.15 meters of soil or where supplemental irrigation was provided to study the effects of soil water content on these parameters.

What was learned
Although many results were derived from these studies, only two key observations will be discussed.  First, the presence of grasses (and their roots in the first 0.15 meters of soil) reduced average soil water content during the three months of the study.  Second, honey mesquite seedling survivorship was correlated with soil moisture content and increased from 1.5 % at 270 ppm to 15% at 360 ppm and 28% at 550 ppm.  Thus, atmospheric CO2 enrichment appeared to improve woody seedling survivorship by reducing grassland water use and thereby enhancing soil moisture content.

What it means
As the air's CO2 content continues to increase, it will likely reduce the water use of herbaceous grassland species.  This phenomenon should increase soil moisture contents, and consequently favor the germination and survivorship of honey mesquite seedlings in such areas.  Thus, an increase in the proliferation of this woody species on grasslands is likely to occur as the air's CO2 content continues to rise.  Indeed, the authors conclude that "by reducing water limitation to establishing seedlings, the continuing rise in the atmospheric CO2 concentration could facilitate mesquite establishment on dry grasslands."

Reviewed 22 January 2003