An interesting study, and I find it surprising that goldenrod (a classic "weed") would do worse under higher CO2 when most plants thrive. Generally, low CO2 is worse for plant growth, and indoor gardeners sometimes intentionally boost the CO2 level for higher yields.

The paper asserts that the protein content of goldenrod pollen goes down as CO2 concentration rises. Since bees depend on goldenrod pollen for protein, implication is that higher CO2 is bad for bees. As evidence, they point to both pollen collected when CO2 levels were lower, as well as pollen collected from plants grown under controlled CO2 conditions.

Maybe when exposed to high CO2, the goldenrod produces more pollen, but with a lower protein percentage, similar to the way that water stress can result in fruit with higher sugar concentrations? Plausible, but the paper seems to say no: "S. canadensis biomass production was not correlated with Ca for either year (p=0.26 and 0.87 in 2012 and 2014, respectively)." I'm assuming they are giving the p-value for "is correlated" and using it's high value as evidence of "not correlated", but I'm not sure because they don't give the actual data or cite an outside source here.

So what's happening here? Why doesn't goldenrod thrive with increasing CO2? Well, maybe the issue is that something else thrives better, and crowds out the goldenrod? Mentioned in the paper is that the study site represents a number of species representative of highgrass prairie. In two of the sites, the goldenrod was outcompeted regardless of CO2, but in one of them (the high clay "Austin" soil) both the goldenrod (Solidago canadensis) and indiangrass (Sorghastrum nutans) were codominant.

That's about all the information given in this paper, but digging deeper into earlier studies we find more relevant information. Earlier studies show that in the presence of higher CO2, the total biomass on the Austin/clay soil increases dramatically. Estimating from figure 3B here (http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=14...) it looks like the biomass goes from about 200 g/m^2 to 300 g/m^2 as the CO2 goes from 300 ppm to 500 ppm.

More interestingly, in that earlier study it's noted that "The covariance matrix of species biomass suggested there were negative associations among several species. The strongest negative covariance was between S. nutans and S. canadensis (Figure 4B), indicating that an increase in S. nutans was strongly associated with a decrease in S. canadensis. This negative association was apparent in the biomass values (Table 4)."

Assuming the new paper is correct that there is no correlation between CO2 and goldenrod biomass, this makes it sound like the goldenrod is getting pushed out by the indiangrass at higher levels of atmospheric CO2. Or more exactly, indiangrass thrives and goldenrod is unchanged. Maybe, but I can't confirm from the tables and figures. They don't seem to give a before and after CO2 treatment that is specific to both species and soil type.

One interesting point they do make is that "The strong negative covariances among Bouteloua, Sorghastrum, Solidago, and Schizachyrium imply that CO2 and soil effects on plant productivity and species composition will be mediated primarily through interactions among these dominant species. We expect to see different trajectories of community change among the soil types in response to the CO2 gradient because of differential effects of CO2 on the water budgets of these three soils. For example, we expect CO2 enrichment to favor the C3 component of the experimental communities, especially S. canadensis, more on the sandy Bastrop soil than on the clay soils".

They also say "Total % N declined from 2002 to 2005 by about 10% below 10 cm in Austin soils, but was unchanged in Bastrop and Houston profiles." Is it coincidental that decreasing soil nitrogen would correspond to decreasing nitrogen content in the pollen?

So what does it all mean? I think it means that increasing CO2 levels will benefit some plants more than others, and which plants that get the most benefit from increasing CO2 differ depending on the base environment. I continue to be worried about the future of honeybees (we lost our backyard hives over the winter), but I'm doubtful that increasing CO2 levels are among the major risk factors.

I'm also bothered that the new paper doesn't seem to seem to discuss this, and that it doesn't even mention the effect of the increased CO2 on the plants competing for the shared resources. I'd guess that the headline of "Increasing CO2 helps native prairie grass outcompete famous allergen" didn't have much appeal. I'm friends with people who've done fieldwork for one of the authors, and everything they say implies she's a good scientist, but this paper seems more opportunistic than fundamental.