By Brad Withrow-Robinson, OSU Forestry & Natural Resources Extension agent, Benton, Linn and Polk Counties
I got a call a while back from someone having trouble finding the seedlings she wanted and wondering if she could make do with something else.
The caller wanted large, bare root hemlock seedlings from her Coast Range seed zone, but all she could find was container stock from a Washington seed source, and wanted to know if that was an ok choice.
Given the current seedling supply situation, I am thinking many people may be facing a similar choice between the “right” planting stock type and the “right” seed source, if they have any choice of seedlings at all.
When is compromise a sound choice?
The seedling you buy reflects the way it was grown, resulting in a size and shape that makes it more or less suited to different conditions in the field (as well as more or less expensive), and is thus an important factor in successful establishment. We use terminology such as 1-1 or 2-0 to convey cultural history of different seedling types.
The seedling you buy also has a genetic heritage, usually described by its area and elevation of origin, which reflects its adaptation to particular environmental conditions. We commonly use seed zones as a guide to help assure adaptation of seedlings to their planting site. When not done right, we see unhappy trees that are often described as “off-site”.
So we are talking about different scales of impact, and so different scales of risk.
If you look at the seed zone map for western hemlock (from Sources of Native Forest Nursery Seedlings at right) you’ll see they are fairly generous in their north-south orientation and rather narrow east to west. This suggests that you may be able to move hemlock a fair distance north or south, but not as far east to west. A short distance one direction can be as important as a long distance in another. This is consistent with the experience of Rick Allen, a Forester for Starker Forests, who says that difference in behavior of coastal sources can be dramatic if planted inland no further than Blodgett or Burnt Woods, rather than near the coast. You can find more about that in the publication above as well as in this publication on selecting native plant materials. This issue is likely be of even more concern in the case of climate change.
Yes, people are experimenting with longer moves beyond the traditional seed zones, but unless being done with good bioclimatic information (likely with help of a computer model), it is more likely to be a roll of the dice than a sound management strategy.
So back to the type and scale of the risk. An inappropriate stock type may make the task of plant establishment harder (due to browsing or competition), and be quickly evident. But we can and often do address this culturally (with Vexar tubes or more rigorous weed control). An inappropriate seed source is likely to affect long term growth and survival, but not to be evident for years or even decades. Poor adaptation cannot be easily addressed afterwards.
All this seems to add up to an answer that it might be okay to buy a less-than-ideal type of planting stock if you think your management skills can provide for the shortcoming, but since we are talking about trees in the forest rather than tomatoes in the garden, you ought to be much more cautious about compromising on source of origin. In that case, you would likely be better off delaying the planting a year or so to get the genetics and stock type you want.
Which is just what the caller did.