The increased CO₂ concentration in the atmosphere increases the assimilation rate of plants. This results in increased growth rates (the so-called CO₂ fertilisation effect), higher nutrient efficiency, lower respiration and stomata conductivity (Ainsworth & Long 2005).

One consequence of the increased CO₂ values is that the C/N ratio increases and thus the feed quality decreases (Essl & Rabitsch, 2013).

However, the fertiliser effect does not always lead to increased ecosystem productivity in the long term. Growth is often limited by other factors such as nutrient limitation, preferably nitrogen (Essl & Rabitsch, 2013).

Another limiting factor is often water. (Essl & Rabitsch, 2013).

In summary: There is the CO₂ fertilisation effect. However, factors such as water and nutrient availability play a decisive role in whether the effect can materialise or whether the factors mentioned have a limiting effect.

Sources:

Ainsworth EA, Long SP (2005) What have we learnt from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2 . New Phytol 165: 351-372

Essl, Franz, and Wolfgang Rabitsch, eds. 2013. Biodiversity and climate change: impacts and options for nature conservation in Central Europe. Berlin, Heidelberg: Springer. P. 37 ff, 141 ff