A recent study (open access) of grasslands in a Montana meadow found that productivity decreased 50% in 40 years (1969-2012), despite a 21% increase in atmospheric CO2.
Why? Because natural plants don't live in a tidy little greenhouse, where the CO2, temperature and precipitation aren't precisely controlled. From the paper in Nature Communications, by Brookshire and Weaver:
ANPP = above-ground net primary production. "Production" means how much plant mass is created from photosynthesis, measured in this paper in grams per square meter per year. "Primary" refers to plants, who fix carbon, as opposed to the secondary production of heterotrophs, who can't fix carbon and gain energy by eating other things. (So you're a heterotroph.)
Here's the measured decline in primary productivity in this Montana meadow (click to enlarge):
The Daily Climate has a nice article on this (click to enlarge). From it:
This is in-line with several papers I've seen that show CO2 isn't an unalloyed good when it comes to plants -- "Carbon dioxide, we call it life" claim is grossly simplistic, because plants must also content with temperature changes, precipitation changes, and more.
Some of these studies:
- “Nitrate assimilation is inhibited by elevated CO2 in field-grown wheat,” Arnold J. Bloom et al, Nature Climate Change, (April 6 2014). As Bloom said on Yale Climate Connections, “Higher CO2 tends to inhibit the ability of plants to make protein… And this explains why food quality seems to have been declining and will continue to decline as CO2 rises — because of this inhibition of nitrate conversion into protein..... “It’s going to be fairly universal that we’ll be struggling with trying to sustain food quality and it’s not just protein… it’s also micronutrients such as zinc and iron that suffer as well as protein.”
- “Global scale climate–crop yield relationships and the impacts of recent warming," David B Lobell and Christopher B Field, Environmental Research Letters Volume 2 Number 1 (2007). “For wheat, maize and barley, there is a clearly negative response of global yields to increased temperatures. Based on these sensitivities and observed climate trends, we estimate that warming since 1981 has resulted in annual combined losses of these three crops representing roughly 40 Mt or $5 billion per year, as of 2002.”
- "Increasing CO2 threatens human nutrition," Samuel S. Myers et al, Nature 510, 139–142 (05 June 2014). "Dietary deficiencies of zinc and iron are a substantial global public health problem. An estimated two billion people suffer these deficiencies, causing a loss of 63 million life-years annually. Most of these people depend on C3 grains and legumes as their primary dietary source of zinc and iron. Here we report that C3 grains and legumes have lower concentrations of zinc and iron when grown under field conditions at the elevated atmospheric CO2 concentration predicted for the middle of this century. C3 crops other than legumes also have lower concentrations of protein, whereas C4 crops seem to be less affected. Differences between cultivars of a single crop suggest that breeding for decreased sensitivity to atmospheric CO2 concentration could partly address these new challenges to global health."
- "Effect of warming temperatures on US wheat yields," Jesse Tack et al, PNAS, (April 20, 2015). "We also find that the overall effect of warming on yields is negative, even after accounting for the benefits of reduced exposure to freezing temperatures."
- "The fingerprint of climate trends on European crop yields," Frances C. Moorea and David B. Lobell, PNAS vol. 112 no. 9, 2670–2675 (2015). "Agriculture is one of the economic sectors most exposed to climate change impacts, but few studies have statistically connected long-term changes in temperature and rainfall with yields. Doing so in Europe is particularly important because yields of wheat and barley have plateaued since the early 1990s and climate change has been suggested as a cause of this stagnation. Here, we show that the impact of climate trends can be detected in the pattern of long-term yield trends in Europe. Although impacts have been large in some areas, the aggregate effect across the continent has been modest. Climate trends can explain 10% of the slowdown in wheat and barley yields, with changes in agriculture and environmental policies possibly responsible for the remainder."