The main difference is that in C4 plants, CO2 can't enter directly into the mesophyll to interact with Rubisco because of a feature called the Kranz Anatomy. CO2 is in the bundle sheath and then PEP carboxylase binds CO2 and through a process, transfers it into the mesophyll. This saves Rubisco (because it is equally attracted to O2 and CO2, whereas PEP carboxylase is only attracted to CO2). I should say though that depending on the specific plant, there will be different specifics.

The names C3 and C4 come from the number of carbons in the 1st stable compound in the cycle.

From an evolutionary perspective, C3 plants likely evolved during a time when CO2 was more abundant in the atmosphere (so Rubisco was entirely fine being able to bind to CO2 or O2 because there wasn't much O2), whereas now, O2 levels are much higher, so C3 plants tend to "waste" energy when Rubisco binds O2 instead of CO2. C4 (and CAM) plants take care of this problem by isolating the Calvin Cycle (particularly Rubisco) from CO2/O2 mixture. There are other physiological advantages too.

Attached here is a good book chapter on the topic starting at about page 66.

I hope this was somewhat helpful.

Despite are quite clear the differences between C3 and C4 metabolism, I guess this is not enough to answer to the true question. The question was related with the term “vegetation”. Vegetation, in a phytosociological approach, commonly used in Europe, is how the plant species living inside a given area are associated each other in the different habitats occurring in that area. I never heard or read something like “C3 vegetation” before now but I think it’s an interesting approach to offer a simple description of plant environment for large areas of the planet. I guess this definition (e.g. C3 vegetation) should be linked to the dominant species in a certain habitat as a response to specific ecological factors. In other words, at a general level, so at large scale, if I try to imagine a C4 vegetation I think to tropical and subtropical environments, where C4 species are usually more efficient than C3. On the other hand in temperate climates C3 species generally prevail. And what about CAM? A good example of CAM vegetation should be the north American desert where Carnegia gigantea (saguaro) dominates the landscape and many other Crassulaceae (CAM means Crassulacean Acid Metabolism) occur. Should exist a problem of scale. At narrow scale this approach couldn’t work.

@Gluseppe Caruso - thank you for elaborating; I apologize for misunderstanding the question. Interesting exploration!

No es adecuado hablar de vegetació C3 o C4, se debe hablar de especies con metabolismo del carbono C3 o C4.

I think at Plant Physiology my opiniien like with Bryan Runck · University of Minnesota if The difference iplant sp in C4 plants, CO2 can't enter directly into the mesophyll to interact with Rubisco because of a feature called the Kranz Anatomy. CO2 is in the bundle sheath and then PEP carboxylase binds CO2 and through a process, transfers it into the mesophyll. This saves Rubisco (because it is equally attracted to O2 and CO2, whereas PEP carboxylase is only attracted to CO2). I should say though that depending on the specific plant, there will be different specifics.

The names C3 and C4 come from the number of carbons in the 1st stable compound in the cycle.

From an evolutionary perspective, C3 plants likely evolved during a time when CO2 was more abundant in the atmosphere (so Rubisco was entirely fine being able to bind to CO2 or O2 because there wasn't much O2), whereas now, O2 levels are much higher, so C3 plants tend to "waste" energy when Rubisco binds O2 instead of CO2. C4 (and CAM) plants take care of this problem by isolating the Calvin Cycle (particularly Rubisco) from CO2/O2 mixture. There are other physiological advantages too.

En ambientes con temperaturas altas, alta incidencia de radiación solar y deficit hídrico estacioanal, las plantas C4 son más eficientes en la utilización de los recursos que las C3. Como ejemplo : la caña de azúcar, maíz y varias especies de árboles tropicales.