I have uploaded a PDF copy of a clinic handout to the freight shed that describes a method for making fake conifer trees from yucca stalks and furnace filter material. I gave this clinic back in 2000 at one of the Sn3 Symposiums in Denver, Colorado. It is based on an article I published in the Sn3 Modeler magazine back in 1999.
Although the article was intended for S scale modelers (1/64 scale), the method can be used to make realistic conifers in just about any scale. Many of the ones I made for my Sn3 layout were over two feet tall.
I’m not sure if the web links in the handout are still valid, and I know that the e-mail address listed for me is definitely outdated, but the contents are still good. Here is the link to the pdf file in the freight shed.