If plants were able to respond in any possible way, what could plants have done to make plant productivity higher or at least less variable over the past 400,000 year time period?

Plants must have evolved in a manner where they require less water and nutrients as these two were the only limiting factor and carbon dioxide was in abundance

Explanation:

Some 400,000 years ago, the concentration of CO2 was very high and was near to 1000 ppm. At this high concentration of CO2 the photosynthetic activity of plants becomes very high and thus since 400,000 years ago plants could have evolved in an uncontrolled manner but this does not happened. So the reason behind this is limitation in terms of soil and its constituent’s nutrients availability and also limited supply of water.  

Therefore, if plants would have evolved themselves in a manner where they require less water and nutrients then they would have grown limitless.  

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.

not too sure but maybe in the hot springs.