Why the range maps are wrong

A customer looks up a tree, finds a range map whose shading stops one county short of home, and gives up on it. Another walks past a wild stand of the same tree every day and decides it must have been planted, because the map says it does not grow there. Both are trusting a sketch as if it were a survey. Range maps are useful, they are wrong in small ways almost everywhere, and it helps to know why.

Where the maps come from

Nearly every native range map in circulation descends from one source: the Atlas of United States Trees, compiled by the botanist Elbert Little and published by the U.S. Forest Service in six volumes between 1971 and 1981. Little worked from herbarium specimens, published floras, forest surveys and field notes, and drew each range by hand at a scale where the whole country fits on a page. Decades later the maps were traced into digital form, and they have been redrawn, simplified and copied into field guides and plant pages ever since. That shaded blob is almost always Little’s pen at second or third remove, with each copyist smoothing the edges a little more.

Newer county-level maps, which light up each county as present or absent, are built from the same raw material: a preserved specimen or a published record from somewhere in that county at some date. They are more detailed, not necessarily more correct.

Why they are coarse

• Scale. On a page-sized map of the continent, the pen line itself covers miles of ground. Every edge is an approximation.
• Sampling. Specimens were collected where botanists happened to go: near colleges, along roads, in famous places. Long stretches of ordinary countryside were never collected, and on the map a gap in collecting looks exactly like a gap in the tree.
• Editorial choices. A cartographer drawing one clean line must decide what to do with a stand well outside it. Some outliers became a lone dot, some were left off, and records suspected of being planted were dropped even where the tree was there before the house.
• Time. A map freezes one moment. Ranges have shifted since, by clearing, by spread along highways and old fields, and by a warming climate that has pushed the edges around.
• County painting. A county map colors the whole county for one specimen on one ridge, and leaves the next county blank because nobody filed a sheet from it.

The kinds of errors you will meet

The errors come in types, and once you know the types you will find your own.

• Absent where it plainly grows. A species mapped as missing from a county where any woods walker can find it. Usually nobody collected it, or the specimen sits unrecorded in a cabinet.
• Disjunct populations left off. Relict stands far from the main range, on a bluff, in a bog, along one river, do not fit inside an outline. They vanish, or shrink to a dot that later copyists drop.
• Hardiness inferred from range. A range edge is set by history, dispersal, competition, soil and fire at least as much as by cold. Trees moved north behind the retreating glaciers, and some may still be catching up. Bald cypress grows well hundreds of miles north of its natural range when planted, and Oglethorpe oak has done well far north of its scattered native stands. The map records where a tree got to on its own, not where it can live.
• The coast and the mountains handled badly. Coastal-plain trees that live in scattered pockets, a swamp here and a sand ridge there, get mapped as one continuous band. Mountain trees that grow only above a certain elevation get painted across whole counties, and valley trees get painted up the slopes.

Native range and planting range are different questions

Native range is where a species arrived on its own and persisted. Planting range is where it will grow. A tree can be healthy and hardy well outside its native range, and it can be native to your state and doomed on your soil: a pin oak on limestone is native to the region and will be yellow and miserable for life. Soil, drainage, exposure and hardiness decide whether a tree thrives, and a map cannot see any of them. Our site check asks about the spot, not the county.

What it means for a native-plant gardener

The “native to your county” rule is a map artifact. Counties are surveyors’ lines; trees follow rivers, soils and slopes. A species that grows wild in the next county, on the same river and the same soil, is native to your watershed, and that is the better question: does it grow on its own in your physiographic region, in habitat like yours? A Piedmont clay ridge and a coastal-plain sand flat can sit in the same county and share almost no trees.

Seed source is the other half. Seed from a population at the cold edge of a range, near you, is more likely to handle your winters than seed from the center of the range far to the south; our cherrybark oak line comes from a parent tree at the northern edge of its range for that reason. Use the map to find the nearest wild population and get seed from something like it, and do not throw out a species because the shading stops one county short.

What to trust more

• Herbarium databases. Digitized specimen records show the actual collection, its locality and its date. You can judge the evidence yourself.
• State botanical atlases and county floras. Compiled by botanists who walk their own state and update as records come in. Many states now have one.
• Natural heritage programs. The state programs that track rare species keep the best records of the disjunct stands the maps drop.
• Your own eyes. A tree reproducing on its own in fitting habitat, with no sign of planting, is better evidence than any map. Photograph it, note the location and report it; the maps get corrected by people who did exactly that.

The best map is the woods, and the second best is the one whose sources you can see. Everything else is a sketch: a place to start, not a verdict on what belongs in your ground. The same site-first thinking applies to choosing among the oaks.