Tracking the Natural Range of Trees by Annual Precipitation Rates

When I started preparing for my 2014 tree care presentations earlier this year, I realized I needed to rework one of the most important parts: explaining why irrigation is so important for Boise's urban forest. Many folks simply cannot grasp that the vast majority of these trees would not be here without supplemental water (whether or not this is sustainable, good, bad or meaningless is a rich topic for a future blog).

I blame this on the suspiciously apocryphal story of how Boise got it's name. It goes something like this: French fur traders exclaimed "Les Bois!" as they crested over a rise and saw the poplar lined Boise river below. When I hear this I can't stop imagining Herve Villechaize in a bearskin cloak, shouting "De trees! De trees!

My presentation strategy this year is to shock and awe my audience with a spread sheet. That's right, I'm going to Blind Them With Science! 

The idea is pretty straight forward. Most of the trees in our urban forest are native to the hardwood forests of the eastern Unites States. Those forests start to disappear past the Mississippi right along a magical line of demarcation where the precipitation drops below 20 inches a year. Here are the facts of life: Mountain Home gets an average of 7.62" of precipitation, Meridian gets 10.94", Boise 11.73". No water. No trees. *

Growing up first as a Marine Corp brat and then later a Forest Service brat, our family slowly moved westward from North Carolina, to Indiana, on to North Dakota and then zig-zagged across the basin and range interior west. It was a slow motion lesson in how precipitation shapes ecology.

For my presentation, I speed up this lesson by picking a common shade tree, the Silver Maple, and track it's natural range. By finding the western edge of the range and comparing that to the annual precipitation of that edge, we can infer the low end of the Silver Maple's annual water needs.

So why just pick a couple data points? I decided to find towns and cities with the highest and lowest precipitation in every state (courtesy of weatherDB). The data backs up what we know to be true, that is, precipitation begins to taper off in Nebraska, Minnesota, Kansas and Oklahoma. West Mineral in eastern Kansas, for instance, has an annual precipitation rate of 46.89", while Coolidge on the far western edge gets 16.82".

So take a look at the numbers. Basin and range topography starts to mess with the gradual decrease in precipitation as you move west**, and there are a couple unexpected anomalies (what's with Bell City, Louisiana 30 miles from the Gulf of Mexico?), but the numbers support my assertion that an urban forest in the arid west composed primarily of eastern hardwoods is by definition an artificial construct, albeit a beautiful one.





Alabama

13.54” (Jones)
70.73” (Brooklyn)





Alaska

3.8” (Prudhoe Bay)
156” (Whittier)





Arizona

3.3” (Yuma)
23.91” (Superior)





Arkansas

41.83” (Peel)
58.82” (Sulphur Springs)





California

2.85” (Anza)
84.36” (Smith River)





Colorado

7.04” (Monte Vista)
30.82” (Idaho Springs)





Connecticut

42.74” (Bridgeport)
58.83” (West Hartford)





Delaware

41.59” (Little Creek)
45.91” (Cheswold)





Florida

42.97” (Salem)
70.73” (Century)





Georgia

43.57” (Keysville)
80.96” (Pine Mountain)





Hawaii

11.96” (Kailua Kona)
240.02” (Volcano)





Idaho

7.62” (Mountain Home)
               42.43” (Elk River)





Illinois

33.35” (Arlington Hills)
50.21” (Joppa)





Indiana

36.58” (Buck Creek)
51.17” (Rome)





Iowa

26.38” (Larchwood)
40.91” (Russell)





Kansas

16.82” (Coolidge)
46.89” (West Mineral)





Kentucky

38.64” (Flatwoods)
57.73” (Cosplint)





Louisiana

14.93” (Bell City)
67.26” (St. Bernard)





Maine

33.6” (Saint Agatha)
56.72” (Cherryfield)





Maryland

37.36” (Flintstone)
50.2” (Oakland)





Massachusetts

42.55” (Orange)
53.42” (Avon)





Michigan

41.14” (Lacota)
23.82” (Iron Mountain)





Minnesota

20.65” (Kennedy)
36.5” (Whalan)





Mississippi

50.22” (Lake Cormorant)
68.88” (Wiggins)





Missouri

33.57” (Mound City)
52.26” (Lesterville)





Montana

7.2” (Red Lodge)
37.96” (Saltese)





Nebraska

14.21” (Henry)
34.84” (Douglas)





Nevada

3.66” (Luning)
23.74” (Glenbrook)





New Hampshire

38.03” (Plainfield)
98.87” (Randolph)





New Jersey

40.01” (Ocean City)
52.39” (Dover)





New Mexico

8.28” (Sanostee)
30.28” (Mescalero)





New York

30.8” (Springwater)
62.69” (Bearsville)





North Carolina

39.96” (Wagram)
90.51” (Balsam Grove)





North Dakota

13.98” (Grenora)
25.2” (Abercrombie)





Ohio

31.48” (Farmer)
47.71” (Harrison)





Oklahoma

15.79” (Felt)
56.14” (Broken Bow)





Oregon

8.92” (Boardman)
122.28” (Depoe Bay)





Pennsylvania

33.05” (Tioga)
54.21” (Brownfield)





Rhode Island

43.58” (Block Island)
51.42” (Smithfield)





South Carolina

39.96” (Tatum)
80.96” (Mountain Rest)





South Dakota

14.52” (Ludlow)
31.34” (Jefferson)





Tennessee

73.49” (Gatlinburg)
41.01” (Kingsport)





Texas

8.93” (Fort Hancock)
64.2” (Orange)





Utah

5.23” (Monument Valley)
34.42” (Brian Head)





Vermont

33.64” (North Hero)
55.84” (Manchester)





Virginia

35.28” (Timberville)
52.78” (Tyro)





Washington

7.42” (Royal City)
115.62” (Amanda Park)





West Virginia

33.02” (Kirby)
59.87” (Snowshoe)





Wisconsin

23.82” (Dunbar)
37.86” (Belmont)





Wyoming

6.48” (Big Piney)
22.11” (Moose)














*OK, that's a bit too simplistic. We do have riparian trees that cling to the banks of our waterways like a mercy seat, but the only native, non-riparian, deciduous tree in our area is the shrubby, brutish Celtis reticulata.

**Idaho City, for example, is 36 miles northeast and 1200 feet higher than Boise and receives 27.95" of precip. compared to Boise at 11.73".