The basic graph of diameter distributions is similar to the previous version. Graphs now use a more distinctive color palette, and contain both solid and dashed lines depending on how many unique species are present.
Based on your feedback, we know that this basic graph can be difficult to read and interpret when a stand has a large number of unique species present. We’ve added a selection of other graphs to aid in visualization of stand structure, both for individual species and the dominant species in a stand.
There are two faceted graphs showing the distributions of individual species. One is black and white and one is in color; otherwise, they are identical. The color graph uses the same colors for each species as the overall graph, for continuity and so you can compare back-and-forth. The faceted graphs show the distribution of each species separately, without the overlapping that occurs when all are graphed on the same axis.
The example graph here shows that the faceted graph provides additional information about species that is hard to discern from the overall graph. Looking at the overall graph, it’s difficult to see the distributions in the middle and larger diameter classes. The faceted graph shows more easily that, for example, there are similar distributions of hickory and elm species in the middle diameter classes, with the largest number of trees per acre occurring around the 18-inch diameter class. You can also see that water oak shows a similarly-shaped distribution but with the largest number of trees per acre in approximately the 15-inch diameter class.
The next new graph shows only the distributions of the three most dominant species in the stand. Dominance is defined in this case by the three species with the highest basal area. This figure highlights those three species, showing their relative distributions, without being crowded with the additional curves for all the other species in the stand.
The final new graph is simply a breakdown of the species in the stand into broad hardwood and softwood categories. We’re using the USFS Forest Inventory and Analysis definitions for hardwood and softwood for North American species - softwoods are conifers, hardwoods are deciduous. Aspen species are “hardwood”.
You may also notice that there are smooth confidence intervals around the curves, gray in the faceted graphs and colored in the graphs of dominant species and major species group. Those are 90% confidence intervals (we'll explain how to interpret these in the next post). The width of the interval reflects the variability of the species between measured plots. We’ll do a future post showing the connection between plot- and stand-level data and those confidence intervals, to aid in their interpretation.
As before, the graphs reflect English units - trees per acre, and diameter in inches, and as before, please contact us if you are interested in different units or graphs! We’re always interested in hearing what you think about the new graph options, and if there’s anything else you’d like to see.
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