Re: Why are C3 plants dominant compared to C4 plants?

Good question. There are several reasons why C3 plants dominate the landscape
rather than C4 plants.

1. There are many more C3 species than C4 species.

2. C4 plants are mainly tropical and subtropical grasses such as corn, crabgrass
and sugarcane. C4 plants are more efficient in photosynthesis than C3 plants
under hot, dry conditions but not under cool, moist conditions. Most of the land
on earth does not have hot, dry conditions. C4 plants do not do very well under
very dry conditions, where survival rather than fast growth is important. CAM
plants and a variety of C3 species are often better adapted for very dry
conditions than C4 plants.  

3. Most of our cultivated crops are not C4 plants, so in agriculture we grow
mainly C3 crops such as rice, wheat, soybeans, potatoes, beans, fruit trees, nut
trees, vegetables and forests. 

4. Despite their higher photosynthetic rates, C4 grasses cannot usually
outcompete trees, which grow taller than grasses and shade them out. There are
so many different environments and plant adaptations that one superior trait
will not necessarily make a species dominant.  

5. Lack of photorespiration has often been considered a major advantage of C4
plants over C3 plants. However, recent research (Rachmilevitch et al. 2004)
indicates that photorespiration in C3 plants has an essential role in nitrate
assimilation so a comparison between C3 and C4 might not be as simple as it once
seemed.

6. In biology, newer is not always better. Primitive trees, such as conifers,
still dominate huge areas in the Northern Hemsiphere such as the Taiga biome.
Primitive ferns and mosses are still widespread as well.

References

Rachmilevitch, S., Cousins, A.B. and Bloom, A.J. 2004. Nitrate
assimilation in plant shoots depends on photorespiration. 
Proc. Nat. Acad. Sci. 101(31): 11506-11510.

Re: Why hasn't C4 Photosynthesis taken over as the dominant process?

Re: What is the difference between C3 and C4 plants and could you give examples