By way of introduction, I am a tree farmer in zone 3, in Alberta, Canada.  What with climate change, I'm always pushing the envelope a bit to bring different ornamentals into the area.

This week I had a chance to pick up a box (300) of western larch seedlings (L. occidentalis) 

I'm aware that these are marginal for my zone -- they are not native here, but are native close to here.  Much of the time the reason for a plant not being non-native is that it doesn't compete well with local vegetation while young. 

I already grow Tamarack (L. larcina) for the naturalization community.  I grow Siberian Larch (L. sibirica) and recommend it over Tamarack for any site that is less than soggy, or for people who want a more symmetrical, fuller tree.  

From my reading, Western Larch grows faster, has a larger size at maturity, but other than that, what reason would someone recommend one over the other?

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Hmm.  I hope that Mr. Python's exposition is not typical of this group's knowledge base. <grin>

Western larch is more frost sensitive than its cousins. Early frosts will damage it while other larix species tend to start hardening off in mid-summer

Much of the time the reason for a plant not being non-native is that it doesn't compete well with local vegetation while young. 

No, that is not so. There are a number of limiting factors for plants: soils, drainage, exposure, competing species,differences in microclimate,  lack of suitable pollinators, lack of suitable seedling conditions, etc., etc., etc.

Didn't say all of the time, and it may not be true in other regions.  We have about 16-20 inches of precip a year here.  Lots of species here havea problem with competition with grass and forbs while they are young.  Competition for both water and nutrients.  Get them to the 2 foot stage and they are fine from that point on.

You are correct that there are many factors.  But when a plant is native in very similar circumstances, then the most common reason so far in my experience for it not being native locally is juvenile mortality.  Limiting factors are more limiting on small specimens with limited internal resources.

Eastern white pine is not native here.  Planted as a 1 yr plug in partial shade, and watered once mid summer the first year the 5 yr survival rate is 90+%  Colorado spruce is not native here.  Planted as a 12" 4 yr old and ignore it does fine.  Concolor fir is not native here.  Planted as a 2 yr old in a wind protected spot it does fine.  

Sugar Maple is not native for a different reason:  Until 100 years ago the prairies regularly burned.  Sugar maple has little fire resistance at juvenile and pole stages.  Introduced as a poplar bush maintenance species it's doing well.

Balsam poplar is native but rare -- normally found only on north facing slopes of ravines and coulees.  Does well as an understory tree in poplar bush.

Tamarack locally grows only in bogs.  It's good at dealing with nutrient poor environments.  But it does well in wet meadowlands too -- if you transplant in as a 4 foot tree.

Dogwood here normally is found either in shade or where there is additional water.  But it can be transplanted into full sun and does fine.

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