Tree Seed Transfer Zones – What Are They, and Why Should I Care?

By Matt Provencher, Western Washington stewardship forester, Washington State Department of Natural Resources, matt.provencher@dnr.wa.gov 

Every year, hundreds of small forest landowners in Washington plant seedlings.

Some have harvested timber, some are trying to establish forests in fallow fields, some are trying to establish a shade-tolerant cohort, and some just like planting trees. But selecting a tree appropriate for your site is important for many reasons.

You’ll likely have fewer insect and disease problems, faster growth, better form, and longer life, among many other benefits. Many people already think about what tree species to plant, but many may not consider where the seed for those trees come from. This is where Tree Seed Transfer Zones, or what I will simply call “seed zones,” come into play.

What are seed zones?

Seed zones essentially split up a tree’s native range into areas where habitats are roughly similar.

They are typically large areas (with a few exceptions), but that’s not to say things are exactly the same in a particular seed zone. Each seed zone will have a range in climates, soils and topography, but in general will be similar enough that trees don’t tend to differ too much genetically within a particular zone.

Seed is also selected from sites located throughout the seed zone to help include some of the variability. Elevation bands are often established within a particular zone, so there could be a 0- to 1,000-foot band, a 1,000- to 2,000-foot band etc.

It’s important to know your elevation as well when selecting trees from a particular seed zone. These elevation bands may be even more important than seed zones (in many cases) than the seed zone itself in terms of moving seed from its source without consequence.

Why does it matter to me?

The theory behind seed zones is that the local population of a given tree species is best adapted to the site due to natural selection occurring over millennia. Thus, getting a tree grown from seed in your local area or region is likely to do better over the long term than a tree grown from seed located in a different part of a tree’s range.

As an example, let’s look at the current map of the seed zones for Douglas-fir used by tree-growing nurseries such as DNR’s Webster Forest Nursery near Olympia. We’ll want to focus on the modified seed zones shown on the map, which is the green shaded area, in which each particular seed zone is separated by a green line. For example, if our planting site is located somewhere on the San Juan Islands, we’d want to source our trees from the Islands Seed Zone. If we’re out in Aberdeen, then the Twin Harbors Seed Zone may be appropriate.

What if I’m near the boundary of a seed zone?

Seed zones are not an exact science. They are based on locations with similar climates and tree genetics. That being said, these changes are continuous, and abrupt changes in climates or genetics are rare. Except in cases other than ecologically separating geographic features such as the Cascade Crest, Coast Range Divide, or the Okanogan River population break, most seed zone lines were drawn based on conservative east/west moves and degrees of latitude of safe seed movement determined from genetic tests. Therefore, these boundaries are somewhat arbitrary in nature – so if you’re near a boundary, it’s reasonable to get a tree from your seed zone or an adjacent one.

How is climate change affecting seed zones?

The last update of the seed zones was done in 2002, so the data that they were based upon is almost 20 years old now.

We know certain things about moving trees from one seed zone to another: Moving trees east or west geographically or down in elevation is riskier than moving trees north or south or slightly up in elevation from their source.

So, again going to our map above, imagine we’re selecting trees from seed zones with climate change in mind and we’re planting in the Kitsap County area. Maybe it’s OK if our trees come from the Lower Columbia seed zone, located to the south of Kitsap seed zone (the zone we’re in if we’re in Kitsap County), but it wouldn’t be appropriate to source those tree from the Twin Harbors seed zone.

Sourcing trees from one seed zone south is probably alright, though I’d be skeptical about moving more than one at this time. Remember, even though the climate is changing, we still need the tree to be able to grow and thrive in the climate that exists today, tomorrow, and for the next several years. If you source a seed from too far south, thinking about what the climate might be like 50, 75, or 100 years from now, your tree may struggle in the environment that exists today.

What else do I need to know?

You can find the book on Washington Seed Transfer Zones here. Much of the information in this article came from this book.

When calling a tree nursery to inquire about seedlings, they should ask you where you will be planting the trees; they are trying to determine if they have the appropriate tree to sell you. If a nursery doesn’t ask that question, ask them where they source their seed from and ensure you are getting trees appropriate to your site.

If you don’t think you’re getting an appropriate tree, it may be best to find another nursery to source seedlings from, or delay planting until you can acquire the appropriate tree.

Webster Forest Nursery is a great resource for landowners, and seedling sales are currently open through the end of August.

As always, feel free to contact me with questions about this and Western Washington forest stewardship at matt.provencher@dnr.wa.gov.