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Fig. 4. Regression tree analyses for forest floor and mineral soil to 15 cm (FF–15 cm) at the Yale-Myers Forest, Connecticut, USA for (a) C storage (kg m–2) and (b) N storage (g m–2). Explanatory variables used were stand composition (Cover), topography (Topo), and soil series (Soil). Each branch is labeled with the level of the explanatory variable that defines that branch, where Hw = hardwood, Hm = hemlock, HH = hemlock and hardwood, Pine = pine, PO = pine and other forest species, Oak = oak, top = top of slope, mid = midslope, bot = bottom of slope, var = variable slope, C = Charlton soil series, H = Hinckley soil series, P = Paxton soil series, and W = Woodbridge soil series. For example, the first split in Fig. 4a places all data defined by a Hw, Oak, or Pine forest composition type in the subset of data on the left of the split. The value in each oval represents the mean storage estimate (±1 SE) for the treatment combinations shown in the preceding branches of the tree. Similarly, the value in each ‘T’ in the tree represents the mean storage estimate for the levels of the classification listed in the previous branch. For example, plots with Hw, Oak, and Pine forest composition types stored 5.2 kg m–2. The length of each branch is proportional to the ratio of the sum of squares explained by that split and is shown as a percentage in parentheses in the body of each branch.
