From Selecting and Buying Quality Tree Seedlings by Amy Grotta, Glenn Ahrens, and Max Bennett
Trees are genetically adapted to the environment in their area of origin. Thus, it is important to plant seedlings grown from seed collected from a location where the environmental conditions closely match those in your area. When ordering trees, geographic seed collection zones and elevation of origin are useful criteria to help match environmental conditions of the seed source with those of your planting area. Seed zones have been established throughout the Pacific Northwest region; they identify areas where growing conditions are similar (Figures 1a–d). Seed zones are often defined by geographic features such as mountain ranges, river drainages, and major valleys. Elevation of origin is an important aspect of your seed source. Elevation is normally noted by 500-foot elevation bands within the seed zone.
The original seed zone map developed in the 1960s (Figure 1a) is still valid and widely used in the nursery industry. The newer, somewhat larger seed zones developed in the 1990s (Figures 1b-d) are species-specific and based on more recent genetic research. Research and experience show that the wider seed movement allowed by the new seed zones does not pose a reforestation risk from planting maladapted seedlings. Pacific Northwest tree species have different amounts of genetic variation among populations. For example, Douglas-fir has considerable genetic variation among populations in different locations and has 16 zones in western Oregon under the new system. On the other end of the spectrum, western redcedar has little genetic variation among populations and has only four zones in western Oregon.
When ordering seedlings, specify the seed zone and elevation band for the area of your planting. Nurseries in the Pacific Northwest generally identify the seed zone of origin for planting stock using a code numbering system (Figures 1a-d). It is essential to understand the seed zone system and code numbers when ordering seedlings. The seed zone and elevation should appear on the label of every batch of seedlings. Do not accept unidentified seedlings. The location of the seed source of origin, not the location of the nursery where the seedlings are grown, is the information that is important for ensuring that your trees are locally adapted.
Staying within appropriate seed zones is especially important in areas where conditions change rapidly over a short distance, such as in the Cascade Mountains or in southwest Oregon. The climate in the Pacific Northwest generally becomes warmer and drier moving inland from the coast. Consider this when planting trees east or west of their origin. Never plant trees from one side of the Cascades to the other. Trees from lower elevations or from more southerly latitudes usually start growing earlier in the season compared to trees from higher elevations or more northern latitudes. This may increase their susceptibility to frost damage or fungal disease if they are planted at higher elevations or farther north. Trees from warmer, drier climate zones are likely more resistant to heat and drought.
When seedlings are in short supply, you may face a tradeoff between planting from a source outside your seed zone versus delaying your planting until the right seedlings are available. You should not move seed from more than one seed zone or one elevation band away from your planting area. Also, seed zones are usually narrower from west to east versus from north to south. Moving seed from west to east across seed zone boundaries risks poor adaptation to heat and drought; conversely, moving seed from east to west across boundaries risks poor adaptation to mild and moist conditions. In general, movement north or south is less risky. Often, the most prudent choice is to delay planting until you have the right seedlings. Remember, to maintain forest health over the long term you have to plant seedlings that are genetically adapted to your site. Failure to do so creates stands of poor growth that are chronically hit with insect and disease problems.