After Catherine's collapse when Edgar returns home from church to find her cradled in Heathcliff's arms in her bedroom, Heathcliff instructs Nelly to bring him word of Catherine's condition in the morning as he'll be in the garden under the larch trees.
Why did Bronte choose larch trees? Was there symbolic significance to larch trees? Was she just thinking of a group of larch trees she knew of and imagined it as a good shelter for waiting?
A quick Google search for the significance of larch trees brought me to a website called woodland trust.org.uk where I found that larch trees are significant in that both male and female flowers can be found on the same tree. Maybe Bronte did choose the larch to symbolize the inseparable souls of Heathcliff and Catherine.
Also, the larch tree was used to ward off evil spirits and enchantment. Since Heathcliff could be pretty evil, maybe that's why Nelly didn't find him under the larch trees after all, but leaning against an ash tree instead.
That same Google search also found a website called ancient-wisdom.com which has a page on tree lore that is pretty interesting. In ancient folklore, according to the site, trees were used as symbols of birth and death and spiritual ceremony and growth. The ash tree in particular is a Druid sacred tree associated with magic for, among other things, karmic law!
Is it coincidence or Bronte's deliberate choice that Heathcliff sacrifices his own blood on the trunk of the ash tree by smashing his head against it, more than once, as Nelly observes by the amount of blood, fresh and dried, as he prays aloud that as long as he is alive Catherine's soul, which he claims is one and the same as his, will never rest, but stay with him always?