I fitted this lmer model:
m1 <- lmer(logR ~ N_g.m.2 * Year + (1|Wh/N_g.m.2), data = CO2_Ratio)
Rendering the attached qqplot.
What does it mean?
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The Q-Q plot is a probability plot of the standardized residuals against the values that would be expected under normality. If the model residuals are normally distributed then the points on this graph should fall on the straight line, if they don't, then you have violated the normality assumption.
Your plot shows that your model does does badly on low and high "outlier" values -- this was what I originally wrote, but consider also EdM's comments below; they supplements this answer, expand and offer more insight.