Fig. 4

Most of the outcome variables used in quantitative synthesis in the systematic review (i.e., reported by at least five articles) referred to plants. The 47 outcome variables are ordered by the number of articles reporting each variable; number of studies is indicated to the right of the bars. The vertical dashed line denotes 10 articles, the threshold for including outcome variables in meta-regressions. Colours indicate broader classes of variables. The full list of outcome variables is presented in Additional file 4