vertical CO2 gradients

[teixeirak]

@NidhiVinod , where /who did the following text come from (assuming a coauthor):

"Vertical CO2 concentration gradients can drive gradients in the ratio of leaf intercellular CO2 to ambient CO2 (c_i/c_a) calculated from δ^13 C, and in rates of refixation of respired CO2 vertically, both of which tend to be higher at lower heights (Table 1). Particularly, understory seedlings benefit from elevated CO2 by having higher rates of CO2 refixation (Brooks et al., 1997). "

I'm not sure it fits, and I think it's referring to c_i, not c_a. The first sentence would only make sense (in terms of direction of pattern) if referring to c_i.

[teixeirak]

I looked at the Brooks paper and revised this paragraph according to my best understanding. @NidhiVinod , please check the changes (see commit flagging this issue) and make sure it looks okay to you.

Cut text: Vertical CO~2~ concentration gradients can drive gradients in the ratio of leaf intercellular CO~2~ to ambient CO~2~ ($c{i}/c{a}$) calculated from $\delta^{13}C$, and in rates of refixation of respired CO~2~ vertically, both of which tend to be higher at lower heights (Table 1). Particularly, understory seedlings benefit from elevated CO~2~ by having higher rates of CO~2~ refixation [@brooksVerticalGradientsPhotosynthetic1997].

[NidhiVinod]

@teixeirak, this sounds good. We didn't have this text before but I included it because of R2's suggestion: refixation of respired CO2 in the understory isn’t mentioned here, but there have been a number of studies, particularly involving 13C, that have looked at this. Please mention along with at least 1 reference. --but I think in the text you included, we address this so should be fine.

[teixeirak]

Great, thanks! We'll need to be sure to update this in the response to reviews.