I am analyzing nutrient concentrations in wood, and I have found that in all my samples the distribution of gallium and barium are closely related. As a related phenomenon, the presence of these elements also sometimes correlates to that of another pair of nutrients, calcium and strontium, but not always. For example, in samples from a humid site $\ce{Ba}$, $\ce{Ga}$, $\ce{Ca}$ and $\ce{Sr}$ are always closely related, but in samples from a dry site $\ce{Ba}$ and $\ce{Ga}$ remain correlated to each other, but are generally not strongly correlated with $\ce{Ca}$ and $\ce{Sr}$.
I would like to know what aspects of their respective chemistries might cause $\ce{Ga}$ and $\ce{Ba}$ to have this strongly correlated relationship in wood samples. For example, is there some sort of similarity in how their valence electrons participate in bonding?