You can find this in-training collected Larch in Bonsai from the Wild by Larch Master Nick Lenz (Stone Lantern Publishing). Good tapers on larches are hard to come by. This exceptionally strong taper is the result of what Nick calls a ‘swamp layer.’ For details see the chapter on larches in Bonsai from the Wild (THE book on collecting).
Planting two trunks together that diverge as they ascend is a good way to create the appearance of taper. This image is from an article by Nick Lenz entitled Larch with Taper, that appears in The Bonsai Journal.
Check out our previous post on larches: In Praise of the American Larch: aka the Tamarack