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".