What is the difference between R-squared and Pearson correlation coefficient?

I am doing an investigation about the effect of altitude (IV) on the density of two tree species (DV). I want to prove whether there is a correlation between altitude and the density of tree species A, and altitude and the density of beech tree species B, not the correlation between the two tree species.

To prove this, I plotted my data in a scatter graph with lines of best fit. I obtained very low values for R-squared. Can I assume that there is a weak correlation between the independent variable and the dependent variable, or should I use Pearson correlation to prove this?
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