Yes, yes, we hear it all the time: More CO2 is good because plants love CO2

That is a rather dumb thing to say for a number of reasons; nature is not simple. You don’t change one variable and expect other variables to respond as though we were turning a garden hose up or down. For example, while plant growth might be enhanced with more CO2 in the atmosphere, there is no reason to think this would be linear, or similar across all plants. You have to dance with the one who brung ya. The plants we have are the plants that have been under Darwinian selection optimizing growth and maintenance physiology for gazillions of plant generations. Changing a fundamental variable may have little effect (and in fact, CO2 increase only enhances growth somewhat, and for only some plants) and may even have negative effects.

A new paper out in Ecology looks at the nutritional value of plants in a Ugandan rainforest and finds that the nutritional value of the leaves eaten by some Colobine monkeys there has declined, because fibre has increased at the expense of usable protein. From the abstract:

Global change is affecting plant and animal populations and many of the changes are likely subtle and difficult to detect. Based on greenhouse experiments, changes in temperature and rainfall, along with elevated CO2, are expected to impact the nutritional quality of leaves. Here, we show a decline in the quality of tree leaves 15 and 30 years after two previous studies in an undisturbed area of tropical forest in Kibale National Park, Uganda. After 30 years in a sample of multiple individuals of ten tree species, the mature leaves of all but one species increased in fiber concentrations, with a mean increase of 10%; tagged individuals of one species increased 13% in fiber. After 15 years, in eight tree species the fiber of young leaves increased 15%, and protein decreased 6%. Like many folivores, Kibale colobus monkeys select leaves with a high protein-to-fiber ratio, so for these folivores declining leaf quality could have a major impact. Comparisons among African and Asian forests show a strong correlation between colobine biomass and the protein-to-fiber ratio of the mature leaves from common tree species. Although this model, predicts a 31% decline in monkey abundance for Kibale, we have not yet seen these declines.

Jessica M. Rothman, Colin A. Chapman, Thomas T. Struhsaker, David Raubenheimer, Dennis Twinomugisha, and Peter G. Waterman, 2014. Long term declines in nutritional quality of tropical leaves. Ecology