In Celebration of Diversity in the USA

To mark the inauguration of the 46th American President Joe Biden I thought it fitting to celebrate the amazing diversity of the USA’s forests.

I don’t tend to comment on politics, and in any case, who would care what a small-time forest researcher and author in the UK might have to say about one of the most dangerous men ever to be allowed to roam uncontrolled in the corridors of a world superpower. All I will say is that 2021 is now starting to look up, especially with the USA rejoining the 2015 Paris Agreement which after all aims to secure a future for all life on Earth.

I’ve written before about Britain’s depauperate tree species diversity, having only 35 common native trees (stretched to 60 if you include minor species) – read more. The USA meanwhile has many hundreds of native tree species. In fact the Native Tree Society of the USA lists 777 native tree species.

Overall, the USA has 33.9% forest cover by area, according to the United Nation’s Food and Agriculture Organisation (FAO). This places the USA somewhat in the middle of forest cover compared to European countries, which range from 74% in Finland to 13% in the UK – read more. In terms of trees per person, the citizens of the USA are lucky to have 716 trees each, compared to just 47 trees per person for those living in the UK – read more about trees per person around the world.

The USDA Forest Service provides excellent inventory data on trees by states, including some really attractive maps. I found it a little harder to track down some basic facts and figures for the USA as a whole, which is perhaps unsurprising given the huge diversity of landscapes and the scale of the country. A citation for the data I have used in preparing the following table and figure is provided at the bottom of this page.

First of all, if we look at data available for 200km plots, one way to consider dominance is to look at basal area. Basal Area (BA) is the cross-sectional area of a tree at breast height (at 1.3m above ground level), and is normally described as tree stem area per hectare (m² ha^-1) (or this case, as it’s the US, basal area is measured in square feet per acre). Basal Area provides an indication of the productivity of the land, and the growth rate of the trees when one or basal area estimates are compared. Read more about Basal Area here.

The table below lists the top 20 tree species by basal area, showing that when they occur in a forest, various mesquite species are the most dominant. Be aware however, this means that these are the dominant species only in plots where they exist. In other words, while mesquite species cover 113.8 ft2 per acre, they only appear in 0.001% of measured plots.

Common name	Scientific name	Plots (%)	Basal Area (ft2/acre)
mesquite spp.	Prosopis spp.	0.001	113.8
Shasta red fir	Abies shastensis	0.060	86.3
California red fir	Abies magnifica	0.216	82.1
Pacific silver fir	Abies amabilis	0.182	65.2
mountain hemlock	Tsuga mertensiana	0.259	64.7
Utah juniper	Juniperus osteosperma	1.920	60.6
plains cottonwood	Populus deltoides	0.038	58.0
Douglas-fir	Pseudotsuga menziesii	3.382	53.7
white fir	Abies concolor	1.060	53.7
lodgepole pine	Pinus contorta	1.707	50.1
tanoak	Lithocarpus densiflorus	0.078	48.6
loblolly pine	Pinus taeda	5.396	44.1
western hemlock	Tsuga heterophylla	0.317	44.0
water tupelo	Nyssa aquatica	0.088	44.0
oneseed juniper	Juniperus monosperma	0.358	43.8
Engelmann spruce	Picea engelmannii	1.330	43.0
Jeffrey pine	Pinus jeffreyi	0.333	42.6
giant sequoia	Sequoiadendron giganteum	0.002	42.2
western redcedar	Thuja plicata	0.238	41.5
ponderosa pine	Pinus ponderosa	2.817	41.1

Another way to look at the data is to report the frequency of occurrence, i.e. how many plots that a tree species appears across the US. The chart below includes the top 100 species ordered by frequency recorded in the 200km plots. The graph is interactive so if you move your mouse or finger over it, it will reveal more information. You can also scroll to the right to reveal more data..

The top ten tree species in the USA ordered by frequency of occurrence shows that red maple (Acer rubrum) is the most frequent species, followed by white oak (Quercus alba) and sweetgum (Liquidambar styraciflua).

Rank	Common name	Scientific name
1	red maple	Acer rubrum
2	white oak	Quercus alba
3	sweetgum	Liquidambar styraciflua
4	loblolly pine	Pinus taeda
5	black cherry	Prunus serotina
6	yellow-poplar	Liriodendron tulipifera
7	blackgum	Nyssa sylvatica
8	northern red oak	Quercus rubra
9	sugar maple	Acer saccharum
10	American elm	Ulmus americana

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Citation

Wilson, Barry Tyler; Lister, Andrew J.; Riemann, Rachel I.; Griffith, Douglas M. 2013. Live tree species basal area of the contiguous United States (2000-2009). Newtown Square, PA: USDA Forest Service, Rocky Mountain Research Station. https://doi.org/10.2737/RDS-2013-0013