Tree growth can be influenced by many different variables.  Whether in a laboratory or in a forest, all trees need certain variables to grow.

 The dependent variable is determined by the values of the other variables present in the situation.  The value of the dependent variable is expected to change due to the changes in the independent variable.  The independent variable is the variable that is manipulated in an experiment and the variable that influences the dependent variable.  There is typically more than one independent variable in an experiment.  The dependent and independent variables can also be shown on graphs.  On a graph, the dependent variable is on the x-axis and the independent variable(s) is on the y-axis. 

As an example of these variables, if an investigation was conducted using tree growth as the dependent variable, then the independent variables would be anything that could influence the growth of a tree.  Independent variables could include sunlight, rainfall, soil nutrients, and topographic location.  Competition among trees could also be considered an independent variable.  Environmental disturbances such as construction, fire, insects, weather patterns, and storms can also greatly influence tree growth.