Can Carbon dioxide benefit plant growth? [Alan]
Plant photosynthesis involves consuming water and carbon dioxide in the presence of sunlight to generate carbohydrates – which comprise plant structure and food. It seems reasonable, therefore, to argue that increasing atmospheric carbon dioxide would benefit plants. But this is overly simplistic. For most plants, the limiting factors for growth is either Nitrogen (N), Phosphorus (P) or Potassium (K) and water which is why we have huge markets in NPK fertilizers and irrigation. Under greenhouse experimental conditions, when these other factors are in abundance, adding carbon dioxide can have a positive impact on growth, but it varies among plant species. In fact, weedy species (often exotic invasive species) are usually better able to take advantage than are crop plants – so we see an increased need for herbicides (think Round-up, for example). In natural communities the same generally applies, so weedy invasive species may well take over our natural plant communities.
In addition, some species, such as poison ivy (and probably poison oak) respond to increased carbon dioxide by increasing their toxins and become more urticaceous while others increase their pollen production – not beneficial for individuals with pollen allergies.
Finally, crop plants tend to adjust the nutritional composition of their tissues, to the detriment of consumers. This includes humans and herbivorous pests – which respond to the lower nutritional com position by consuming more – thus requiring increased pesticide use to protect yield.
While increased atmospheric carbon dioxide might benefit some species, we have to balance this against the ocean acidification, global warming and climate change costs.
Basically, then, the climate denier claim that carbon dioxide increase will benefit plant growth is based on ignorance of the complex responses that have been demonstrated.