Biplot scores from canonical correspondence analysis
Canonical correlation analysis (CCA) not correspondence canonical analysis. Biplots are the same as in PCA: the representation in a new space of the samples and the relative position of the variables on the new space. 

However, CCA seeks the values of one block that maximize the correlation with the other block. Here you don't have an independent variable that you maximize from the variables you introduced on the CCA. So the interpretation is much trickier as there isn't a direction: WaterC and FallenTwigs show the opposite effect than BarSand and CoverMoss, but there is a group of variables that explain what is the effect of water on the plants(?)